From 3bebcd4c1d4bbff467de76b8589e7a12c19d5ca0 Mon Sep 17 00:00:00 2001
From: Stehen Jeremiah Haruna <141843972+Ugasutun@users.noreply.github.com>
Date: Sat, 17 Oct 2026 12:19:14 +0100
Subject: [PATCH 1/4] feat(mcp): gate paid_call behind an operator host
 allow-list (#404)
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paid_call passed any URL the agent named to a fetch wrapper that answers a
402 with USDC from a live mainnet key, so a single sentence in a page, issue
comment or search result could choose where the money went.

The payable origins are server configuration (MCP_PAID_CALL_ALLOWED_HOSTS),
never a tool argument, and the default is an empty list that pays nobody.
A refused URL fails with ERR_HOST_NOT_ALLOWED naming its host, before any
request is sent, so an unlisted host is never contacted. Matching is
origin-exact — case, root dot and punycode folded, port pinned when the
entry names one — plus an optional path prefix enforced at a segment
boundary, which refuses subdomain look-alikes, url userinfo, homoglyphs and
sibling paths. An entry this cannot read is dropped with a warning rather
than guessed at, because a typo should cost an allowed host, not a blocked
one.

Bounds a spend this tool can make; the ceiling (#210) and the payment log
(#211) cover what happens to hosts that are allow-listed.
---
 .changeset/mcp-paid-call-allowlist.md |  12 ++
 .env.example                          |  16 +++
 examples/mcp-server/agent.ts          |  12 +-
 examples/mcp-server/run.ts            |  17 ++-
 src/mcp/codes.ts                      |   1 +
 src/mcp/paidCallAllowlist.ts          | 184 ++++++++++++++++++++++++++
 src/mcp/server.ts                     |  35 ++++-
 tests/mcp-paid-call-allowlist.test.ts | 166 +++++++++++++++++++++++
 tests/mcp.test.ts                     |   8 +-
 9 files changed, 441 insertions(+), 10 deletions(-)
 create mode 100644 .changeset/mcp-paid-call-allowlist.md
 create mode 100644 src/mcp/paidCallAllowlist.ts
 create mode 100644 tests/mcp-paid-call-allowlist.test.ts

diff --git a/.changeset/mcp-paid-call-allowlist.md b/.changeset/mcp-paid-call-allowlist.md
new file mode 100644
index 00000000..9977f27a
--- /dev/null
+++ b/.changeset/mcp-paid-call-allowlist.md
@@ -0,0 +1,12 @@
+---
+'lens': minor
+---
+
+Gate `paid_call` behind an operator allow-list of hosts (#404). The tool pays
+whatever URL it is given, up to `MCP_MAX_PAYMENT_PRICE`, from a live key — and
+an agent that reads attacker-influenced text is one sentence from being told
+which URL. `paidCallAllowlist` (server config, `MCP_PAID_CALL_ALLOWED_HOSTS`)
+lists origins or URL prefixes; an unlisted, non-http(s) or credential-bearing
+URL is refused with `ERR_HOST_NOT_ALLOWED` naming the host, before any request
+is sent, and the list cannot be widened by a tool argument. The default is
+empty, which pays nobody.
diff --git a/.env.example b/.env.example
index 41e4465f..cc444c42 100644
--- a/.env.example
+++ b/.env.example
@@ -184,6 +184,22 @@ FACILITATOR_SECRET_KEY=
 FACILITATOR_FEE_STROOPS_TESTNET=50000
 FACILITATOR_FEE_STROOPS_MAINNET=50000
 
+# --- MCP server (npm run mcp:server) ---
+# The agent's own x402 payer keys, used to answer a 402 challenge when paid_call
+# fetches a gated resource. These spend money, so keep them out of any other
+# process and fund only what you are willing to lose.
+MCP_AGENT_SECRET_KEY_TESTNET=
+MCP_AGENT_SECRET_KEY_MAINNET=
+# Ceiling per paid_call, enforced by the x402 fetch wrapper.
+MCP_MAX_PAYMENT_PRICE=$1.00
+# Comma-separated origins (or URL prefixes) paid_call may pay for, e.g.
+# "https://api.lens.example,localhost:3000". An entry with no scheme matches
+# both http and https; an entry with a path only allows that path and what sits
+# under it. UNSET IS EMPTY, AND AN EMPTY LIST PAYS NOBODY: every paid_call is
+# refused with ERR_HOST_NOT_ALLOWED before a request is sent. This is a
+# prompt-injection guard — text an agent reads must not choose where money goes.
+MCP_PAID_CALL_ALLOWED_HOSTS=
+
 
 # ─── Snapshot retention ────────────────────────────────────────────────────
 # Rows removed by one DELETE during a retention run. The job drains in batches
diff --git a/examples/mcp-server/agent.ts b/examples/mcp-server/agent.ts
index 7d47263d..1e4408b9 100644
--- a/examples/mcp-server/agent.ts
+++ b/examples/mcp-server/agent.ts
@@ -52,10 +52,20 @@ async function main() {
 
   console.log('[agent] Starting MCP server child process...');
   
-  // Start the MCP server as a child process
+  // Start the MCP server as a child process.
+  // paid_call spends real USDC, so the server only pays origins the operator
+  // allow-lists (MCP_PAID_CALL_ALLOWED_HOSTS, empty by default). This demo
+  // spends against the mock server above, so it names that origin and nothing
+  // else. A real deployment lists its own endpoints and leaves the rest refused.
+  const demoAllowlist = 'localhost:3000';
+
   const transport = new StdioClientTransport({
     command: process.platform === 'win32' ? 'npx.cmd' : 'npx',
     args: ['tsx', path.join(__dirname, 'run.ts')],
+    env: {
+      ...process.env as Record<string, string>,
+      MCP_PAID_CALL_ALLOWED_HOSTS: process.env.MCP_PAID_CALL_ALLOWED_HOSTS || demoAllowlist,
+    },
   });
 
   const client = new Client(
diff --git a/examples/mcp-server/run.ts b/examples/mcp-server/run.ts
index 2b971525..ef9da239 100644
--- a/examples/mcp-server/run.ts
+++ b/examples/mcp-server/run.ts
@@ -33,13 +33,26 @@ async function main() {
     ]
   });
 
+  // paid_call spends from the keys above, so the hosts it may pay for are the
+  // operator's decision. Unset means no host is payable at all (#404).
+  const paidCallAllowlist = (process.env.MCP_PAID_CALL_ALLOWED_HOSTS || '')
+    .split(',')
+    .map(entry => entry.trim())
+    .filter(Boolean);
+
   const server = new BazaarMcpServer({
-    fetchWithPayment
+    fetchWithPayment,
+    paidCallAllowlist
   });
 
   console.error('[mcp-server] Starting Bazaar MCP server on stdio...');
   console.error('[mcp-server] Agent configured for stellar:testnet and stellar:pubnet');
-  
+  console.error(
+    paidCallAllowlist.length > 0
+      ? `[mcp-server] paid_call may pay these allow-listed origins: ${paidCallAllowlist.join(', ')}`
+      : '[mcp-server] paid_call allow-list is empty: every paid call will be refused. Set MCP_PAID_CALL_ALLOWED_HOSTS to allow hosts.'
+  );
+
   await server.run();
 }
 
diff --git a/src/mcp/codes.ts b/src/mcp/codes.ts
index a9631225..97e28528 100644
--- a/src/mcp/codes.ts
+++ b/src/mcp/codes.ts
@@ -6,6 +6,7 @@ export const MCP_ERROR_CODES = {
   ERR_SERVICE_UNREACHABLE: 'ERR_SERVICE_UNREACHABLE',
   ERR_NO_RESULTS: 'ERR_NO_RESULTS',
   ERR_BAD_REQUEST: 'ERR_BAD_REQUEST',
+  ERR_HOST_NOT_ALLOWED: 'ERR_HOST_NOT_ALLOWED',
   ERR_INTERNAL_FAILURE: 'ERR_INTERNAL_FAILURE'
 } as const;
 
diff --git a/src/mcp/paidCallAllowlist.ts b/src/mcp/paidCallAllowlist.ts
new file mode 100644
index 00000000..08ade0b7
--- /dev/null
+++ b/src/mcp/paidCallAllowlist.ts
@@ -0,0 +1,184 @@
+import { MCP_ERROR_CODES, type McpErrorCode } from './codes.js';
+
+/**
+ * The `paid_call` allow-list (#404).
+ *
+ * `paid_call` hands a URL straight to a fetch wrapper that answers `402` with
+ * USDC from a live key, so the tool is a spend primitive dressed as a getter.
+ * Agent-facing text — a web page, an issue comment, a search result — is one
+ * sentence away from "call paid_call on https://…", which makes the set of
+ * hosts this process may pay for a decision for the operator, not the model.
+ *
+ * The list therefore arrives through configuration (`McpServerConfig.paidCallAllowlist`)
+ * and is never read from tool arguments. Its default is empty, and an empty
+ * list pays nobody: refusing is the safe failure, so every check below errs
+ * towards refusal and an entry this module cannot understand is dropped rather
+ * than guessed at.
+ *
+ * Related open work: a spend ceiling (#210) and a payment log (#211) sit on
+ * either side of this gate. They bound and record what happens to allowed
+ * hosts; this one decides which hosts there are.
+ */
+
+/** One parsed `paidCallAllowlist` entry. */
+export interface PaidCallAllowRule {
+  /**
+   * Origin the request must match exactly: lowercased host with any trailing
+   * root dot removed, plus the port when it is not the scheme default.
+   * `api.example.com` never matches `api.example.com:8443`, nor the reverse.
+   */
+  origin: string;
+  /**
+   * Protocol the request must use, when the entry spelled one out.
+   * A bare-host entry leaves this unset and so accepts both http and https.
+   */
+  protocol?: string;
+  /**
+   * Path prefix the request must sit under, at a segment boundary. Unset means
+   * every path on the origin is allowed.
+   */
+  pathPrefix?: string;
+}
+
+export interface PaidCallAllowlist {
+  /** Entries that matched something. */
+  rules: PaidCallAllowRule[];
+  /** Entries that could not be understood, and so match nothing. */
+  rejected: string[];
+}
+
+export type PaidCallDecision =
+  | { allowed: true }
+  | { allowed: false; code: McpErrorCode; reason: string };
+
+/** Schemes `paid_call` will pay at all. Anything else (file:, data:, gopher:) is refused. */
+const PAYABLE_PROTOCOLS = new Set(['http:', 'https:']);
+
+function originOf(url: URL): string {
+  const host = url.hostname.replace(/\.+$/, '');
+  return url.port ? `${host}:${url.port}` : host;
+}
+
+function pathPrefixOf(url: URL): string | undefined {
+  // A bare-host entry parses to "/", which constrains nothing.
+  if (url.pathname === '/' || url.pathname === '') return undefined;
+  return url.pathname;
+}
+
+/**
+ * Parse the configured entries. Never throws: an entry that cannot be
+ * understood is reported in `rejected` and simply never matches, so a typo
+ * costs the operator a host they meant to allow, not a host they meant to
+ * block.
+ */
+export function parsePaidCallAllowlist(entries: readonly string[]): PaidCallAllowlist {
+  const rules: PaidCallAllowRule[] = [];
+  const rejected: string[] = [];
+
+  for (const raw of entries) {
+    const entry = raw.trim();
+    if (!entry) continue;
+
+    const hasProtocol = /^[a-z][a-z0-9+.-]*:\/\//i.test(entry);
+    let parsed: URL;
+    try {
+      // Bare "host[:port][/path]" entries are parsed as http so that URL gives
+      // us IDN-to-punycode and case folding for free; the protocol is only
+      // pinned when the entry named one itself.
+      parsed = new URL(hasProtocol ? entry : `http://${entry}`);
+    } catch {
+      rejected.push(entry);
+      continue;
+    }
+
+    const origin = originOf(parsed);
+    const hasLeftovers = parsed.search || parsed.hash;
+    if (!origin || parsed.username || parsed.password || hasLeftovers || !PAYABLE_PROTOCOLS.has(parsed.protocol)) {
+      // A query or fragment in an entry would silently drop the constraint the
+      // operator thought they wrote, so it is rejected rather than ignored.
+      rejected.push(entry);
+      continue;
+    }
+
+    rules.push({
+      origin,
+      protocol: hasProtocol ? parsed.protocol : undefined,
+      pathPrefix: pathPrefixOf(parsed),
+    });
+  }
+
+  return { rules, rejected };
+}
+
+function matches(rule: PaidCallAllowRule, url: URL): boolean {
+  if (rule.origin !== originOf(url)) return false;
+  if (rule.protocol && rule.protocol !== url.protocol) return false;
+  if (!rule.pathPrefix) return true;
+
+  const { pathname } = url;
+  if (rule.pathPrefix.endsWith('/')) {
+    // "prefix/" allows the prefix itself and anything below it, never a sibling.
+    return pathname === rule.pathPrefix.slice(0, -1) || pathname.startsWith(rule.pathPrefix);
+  }
+  // "prefix" allows exactly that segment path and anything under it, so
+  // "/pricexyz" does not ride in on an allow-listed "/price".
+  return pathname === rule.pathPrefix || pathname.startsWith(`${rule.pathPrefix}/`);
+}
+
+/**
+ * Decide whether `paid_call` may spend against `url`. Call this before any
+ * network I/O: a refused host must never be contacted, paid or not, and the
+ * wrapper that pays runs inside the fetch.
+ *
+ * The URL is parsed here rather than trusted from the caller, so a relative or
+ * nonsense target is refused instead of being resolved by some later layer.
+ */
+export function checkPaidCallTarget(
+  url: string,
+  rules: readonly PaidCallAllowRule[],
+): PaidCallDecision {
+  const refuse = (code: McpErrorCode, detail: string): PaidCallDecision => ({
+    allowed: false,
+    code,
+    reason: `paid_call refused; no request was sent and no payment was made. ${detail} Configure the allow-list with MCP_PAID_CALL_ALLOWED_HOSTS.`,
+  });
+
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return refuse(
+      MCP_ERROR_CODES.ERR_BAD_REQUEST,
+      'the url is not an absolute http(s) URL.',
+    );
+  }
+
+  const origin = originOf(parsed);
+  const label = origin || 'the url';
+
+  if (!PAYABLE_PROTOCOLS.has(parsed.protocol)) {
+    return refuse(
+      MCP_ERROR_CODES.ERR_HOST_NOT_ALLOWED,
+      `${label} uses ${parsed.protocol || 'an unknown protocol'}, which this tool does not pay for.`,
+    );
+  }
+
+  if (parsed.username || parsed.password) {
+    // "https://api.example.com@evil.test" reads as example.com to a human and
+    // resolves to evil.test, so credentials are never a legitimate part of a
+    // payable url.
+    return refuse(
+      MCP_ERROR_CODES.ERR_HOST_NOT_ALLOWED,
+      `${label} is behind url userinfo, which is not allowed.`,
+    );
+  }
+
+  if (!rules.some(rule => matches(rule, parsed))) {
+    return refuse(
+      MCP_ERROR_CODES.ERR_HOST_NOT_ALLOWED,
+      `${label} is not in the paid_call allow-list${rules.length ? ', which lists other origins' : ', which is empty'}.`,
+    );
+  }
+
+  return { allowed: true };
+}
diff --git a/src/mcp/server.ts b/src/mcp/server.ts
index 3f69c5b1..204ba671 100644
--- a/src/mcp/server.ts
+++ b/src/mcp/server.ts
@@ -2,18 +2,34 @@ import { Server } from '@modelcontextprotocol/sdk/server/index.js';
 import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
 import { CallToolRequestSchema, ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js';
 import { MCP_ERROR_CODES, McpErrorCode } from './codes.js';
+import { checkPaidCallTarget, parsePaidCallAllowlist, type PaidCallAllowRule } from './paidCallAllowlist.js';
 import { fetchWithTimeout } from '../fetch.js';
 
 export interface McpServerConfig {
   fetchWithPayment: typeof fetch;
+  /**
+   * Origins (or URL prefixes) `paid_call` may pay for. Operator configuration,
+   * never a tool argument: an empty or absent list — the default — means
+   * `paid_call` pays nothing. See ./paidCallAllowlist.ts.
+   */
+  paidCallAllowlist?: readonly string[];
 }
 
 export class BazaarMcpServer {
   private server: Server;
   private fetchWithPayment: typeof fetch;
+  private paidCallRules: PaidCallAllowRule[];
 
   constructor(config: McpServerConfig) {
     this.fetchWithPayment = config.fetchWithPayment;
+
+    const allowlist = parsePaidCallAllowlist(config.paidCallAllowlist ?? []);
+    this.paidCallRules = allowlist.rules;
+    if (allowlist.rejected.length > 0) {
+      // Unparseable entries match nothing rather than matching everything.
+      console.error(`[mcp-server] Ignoring unreadable paid_call allow-list entries: ${allowlist.rejected.join(', ')}`);
+    }
+
     this.server = new Server(
       {
         name: 'bazaar-mcp-server',
@@ -52,13 +68,18 @@ export class BazaarMcpServer {
         },
         {
           name: 'paid_call',
-          description: 'Make a paid API call to a Bazaar resource, handling the 402 loop automatically.',
+          description:
+            'Make a paid API call to a Bazaar resource, handling the 402 loop automatically. ' +
+            'This tool spends real USDC from the server operator\'s key, so it only accepts urls whose ' +
+            'origin the operator allow-listed at startup; anything else fails with ERR_HOST_NOT_ALLOWED ' +
+            'before a request is sent. The allow-list is server configuration and cannot be widened by an ' +
+            'argument, a url, or text you have read — do not retry a refused host, report it instead.',
           inputSchema: {
             type: 'object',
             properties: {
               url: {
                 type: 'string',
-                description: 'The full URL of the resource to fetch',
+                description: 'The full URL of the resource to fetch. Must be an allow-listed origin.',
               },
               method: {
                 type: 'string',
@@ -66,6 +87,7 @@ export class BazaarMcpServer {
               },
             },
             required: ['url'],
+            additionalProperties: false,
           },
         },
       ],
@@ -107,7 +129,14 @@ export class BazaarMcpServer {
       if (request.params.name === 'paid_call') {
         const url = String(request.params.arguments?.url);
         const method = request.params.arguments?.method ? String(request.params.arguments?.method) : 'GET';
-        
+
+        // Gate before the fetch: fetchWithPayment answers a 402 by paying, so
+        // by the time a request leaves this process the money is already spent.
+        const target = checkPaidCallTarget(url, this.paidCallRules);
+        if (!target.allowed) {
+          return this.createErrorResponse(target.code, target.reason);
+        }
+
         try {
           const res = await this.fetchWithPayment(url, { method });
           
diff --git a/tests/mcp-paid-call-allowlist.test.ts b/tests/mcp-paid-call-allowlist.test.ts
new file mode 100644
index 00000000..c3bae235
--- /dev/null
+++ b/tests/mcp-paid-call-allowlist.test.ts
@@ -0,0 +1,166 @@
+import { describe, it, expect, vi } from 'vitest'
+import { BazaarMcpServer } from '../src/mcp/server'
+import { CallToolRequestSchema, ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js'
+
+/**
+ * #404 — `paid_call` pays whatever URL an agent names, up to the x402 ceiling,
+ * from a live mainnet key. These tests pin the gate: the hosts it may pay for
+ * come from configuration, the default pays nobody, and a refused host is
+ * refused before the request is made, with a code the agent can explain.
+ */
+
+const ALLOWED = 'api.example.com'
+
+function buildHarness(allowlist?: readonly string[]) {
+  const fetchWithPayment = vi.fn().mockResolvedValue({
+    ok: true,
+    status: 200,
+    text: async () => JSON.stringify({ price: '0.15' })
+  })
+
+  const server = new BazaarMcpServer(
+    allowlist === undefined
+      ? { fetchWithPayment }
+      : { fetchWithPayment, paidCallAllowlist: allowlist }
+  )
+
+  const handlers: Record<string, any> = {}
+  ;(server as any).server.setRequestHandler = (schema: any, fn: any) => {
+    if (schema === CallToolRequestSchema) handlers.call = fn
+    if (schema === ListToolsRequestSchema) handlers.list = fn
+  }
+  ;(server as any).setupHandlers()
+
+  const callPaid = (arguments_: Record<string, unknown>) =>
+    handlers.call({ params: { name: 'paid_call', arguments: arguments_ } })
+
+  return { fetchWithPayment, callPaid, listTools: () => handlers.list() }
+}
+
+async function paidCall(allowlist: readonly string[] | undefined, url: string) {
+  const h = buildHarness(allowlist)
+  const response = await h.callPaid({ url })
+  return { response, body: JSON.parse(response.content[0].text), fetches: h.fetchWithPayment }
+}
+
+describe('paid_call host allow-list', () => {
+  it('pays an allow-listed host', async () => {
+    const { response, body, fetches } = await paidCall([ALLOWED], `https://${ALLOWED}/price/XLM/USDC`)
+
+    expect(response.isError).toBeFalsy()
+    expect(body.price).toBe('0.15')
+    expect(fetches).toHaveBeenCalledTimes(1)
+  })
+
+  it('refuses a host that is not allow-listed, before any request is made', async () => {
+    const { response, body, fetches } = await paidCall([ALLOWED], 'https://evil.test/price')
+
+    expect(response.isError).toBe(true)
+    expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(body.reason).toContain('evil.test')
+    expect(body.reason).toMatch(/no request was sent and no payment was made/i)
+    expect(fetches).not.toHaveBeenCalled()
+  })
+
+  it('pays nobody when the operator configures nothing', async () => {
+    // The default failure mode: an unconfigured server is a server that cannot
+    // be talked into spending.
+    for (const url of ['https://api.example.com/price', 'http://localhost:3000/price']) {
+      const { response, body, fetches } = await paidCall(undefined, url)
+
+      expect(response.isError).toBe(true)
+      expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+      expect(fetches).not.toHaveBeenCalled()
+    }
+  })
+
+  it('refuses a subdomain look-alike of an allow-listed host', async () => {
+    // The host an agent sees is not the host that answers.
+    const { body, fetches } = await paidCall([ALLOWED], `https://${ALLOWED}.evil.test/price`)
+
+    expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(fetches).not.toHaveBeenCalled()
+  })
+
+  it('refuses a path that merely starts with an allow-listed prefix', async () => {
+    const prefix = `https://${ALLOWED}/price`
+
+    const allowed = await paidCall([prefix], `${prefix}/XLM/USDC`)
+    expect(allowed.response.isError).toBeFalsy()
+
+    const nearMiss = await paidCall([prefix], `https://${ALLOWED}/prices-for-free`)
+    expect(nearMiss.body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(nearMiss.fetches).not.toHaveBeenCalled()
+  })
+
+  it('refuses a host hidden behind url userinfo', async () => {
+    const { body, fetches } = await paidCall([ALLOWED], `https://${ALLOWED}@evil.test/price`)
+
+    expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(fetches).not.toHaveBeenCalled()
+  })
+
+  it('refuses a homograph of an allow-listed host', async () => {
+    // "api.exаmplе.com" with a Cyrillic а and е: a different registered name, a
+    // different payment, and text an agent cannot tell apart from the list.
+    const { body, fetches } = await paidCall([ALLOWED], 'https://api.ex\u0430mpl\u0435.com/price')
+
+    expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(fetches).not.toHaveBeenCalled()
+  })
+
+  it('pins the port when the entry names one, and only then', async () => {
+    const wrongPort = await paidCall([ALLOWED], `https://${ALLOWED}:8443/price`)
+    expect(wrongPort.body.code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(wrongPort.fetches).not.toHaveBeenCalled()
+
+    const rightPort = await paidCall([`${ALLOWED}:8443`], `https://${ALLOWED}:8443/price`)
+    expect(rightPort.response.isError).toBeFalsy()
+  })
+
+  it('refuses protocols this tool does not pay for, and unreadable urls', async () => {
+    for (const url of ['file:///etc/passwd', 'not-a-url', 'https://']) {
+      const { body, fetches } = await paidCall([ALLOWED], url)
+
+      expect(['ERR_HOST_NOT_ALLOWED', 'ERR_BAD_REQUEST']).toContain(body.code)
+      expect(fetches).not.toHaveBeenCalled()
+    }
+  })
+
+  it('drops an entry it cannot read rather than letting it match everything', async () => {
+    for (const entry of ['not a valid entry %%', 'https://api.example.com/price?free=1', 'ftp://api.example.com']) {
+      const { body, fetches } = await paidCall([entry], `https://${ALLOWED}/price`)
+
+      expect(body.code).toBe('ERR_HOST_NOT_ALLOWED')
+      expect(fetches).not.toHaveBeenCalled()
+    }
+  })
+
+  it('takes the allow-list from configuration, never from the call', async () => {
+    const h = buildHarness([ALLOWED])
+
+    // An argument cannot widen the list — the only inputs the tool accepts are
+    // url and method.
+    const smuggled = await h.callPaid({
+      url: 'https://evil.test/price',
+      allowed_hosts: ['evil.test'],
+      paidCallAllowlist: ['evil.test']
+    })
+    expect(JSON.parse(smuggled.content[0].text).code).toBe('ERR_HOST_NOT_ALLOWED')
+    expect(h.fetchWithPayment).not.toHaveBeenCalled()
+
+    const tools = (await h.listTools()).tools
+    const paidCall = tools.find((t: any) => t.name === 'paid_call')
+    expect(Object.keys(paidCall.inputSchema.properties).sort()).toEqual(['method', 'url'])
+    expect(paidCall.description).toMatch(/allow-list/i)
+  })
+
+  it('matches the allow-listed host regardless of case or root dot', async () => {
+    const upper = await paidCall([ALLOWED], `https://API.EXAMPLE.COM/price`)
+    expect(upper.response.isError).toBeFalsy()
+
+    // A trailing dot is the same name, written absolutely, not a new host.
+    const dotted = await paidCall([ALLOWED], `https://${ALLOWED}./price`)
+    expect(dotted.response.isError).toBeFalsy()
+  })
+})
diff --git a/tests/mcp.test.ts b/tests/mcp.test.ts
index 8b979ad6..0d3748e5 100644
--- a/tests/mcp.test.ts
+++ b/tests/mcp.test.ts
@@ -5,7 +5,7 @@ import { CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
 describe('BazaarMcpServer', () => {
   it('should map signature expiration ledger error to ERR_EXPIRED_AUTHORISATION', async () => {
     const mockFetch = vi.fn().mockRejectedValue(new Error('Transaction signature expired. Signature expiration ledger 123 is less than current ledger 125'));
-    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch });
+    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch, paidCallAllowlist: ['example.com'] });
 
     // The SDK server doesn't expose the router directly without a client, but we can test the handlePaidCallError method directly,
     // or by overriding setRequestHandler to capture our handler.
@@ -35,7 +35,7 @@ describe('BazaarMcpServer', () => {
 
   it('should map missing trustline error to ERR_MISSING_TRUSTLINE', async () => {
     const mockFetch = vi.fn().mockRejectedValue(new Error('op_no_trust: The account does not have a trustline for this asset'));
-    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch });
+    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch, paidCallAllowlist: ['example.com'] });
 
     let handler: any = null;
     (server as any).server.setRequestHandler = (schema: any, fn: any) => {
@@ -66,7 +66,7 @@ describe('BazaarMcpServer', () => {
       text: async () => 'Payment Required'
     });
     
-    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch });
+    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch, paidCallAllowlist: ['example.com'] });
 
     let handler: any = null;
     (server as any).server.setRequestHandler = (schema: any, fn: any) => {
@@ -97,7 +97,7 @@ describe('BazaarMcpServer', () => {
       text: async () => JSON.stringify({ success: true, data: "hello" })
     });
     
-    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch });
+    const server = new BazaarMcpServer({ fetchWithPayment: mockFetch, paidCallAllowlist: ['example.com'] });
 
     let handler: any = null;
     (server as any).server.setRequestHandler = (schema: any, fn: any) => {

From aa2a795e681671e99d2515b9bd504cb57d2e99af Mon Sep 17 00:00:00 2001
From: Stehen Jeremiah Haruna <141843972+Ugasutun@users.noreply.github.com>
Date: Sat, 17 Oct 2026 21:35:05 +0100
Subject: [PATCH 2/4] test(x402): drive the successful 402 payment loop end to
 end (#423)

Existing 402 tests mocked fetchWithPayment, so the branch that actually moves
money - 402 -> construct -> sign -> submit -> resource returned - was never
exercised. This drives the real @x402/fetch transport and a real Soroban
transfer signed with @stellar/stellar-sdk against a stubbed resource + fake
facilitator. Only the RPC/Horizon and the facilitator are stubbed; both
upgrade-sensitive surfaces (x402 loop wiring, Stellar SDK signing) stay real.

Asserts on what was signed - amount, destination and network passphrase
(valid on testnet, fails on mainnet) - not just the response body, and covers
amount above maxPrice, an unregistered network, and an expired signature.
Throwaway testnet keys only; deterministic, no network access.
---
 .../x402-successful-payment-loop-test.md      |  10 +
 tests/x402-paid-call-loop.test.ts             | 412 ++++++++++++++++++
 2 files changed, 422 insertions(+)
 create mode 100644 .changeset/x402-successful-payment-loop-test.md
 create mode 100644 tests/x402-paid-call-loop.test.ts

diff --git a/.changeset/x402-successful-payment-loop-test.md b/.changeset/x402-successful-payment-loop-test.md
new file mode 100644
index 00000000..17fd48fc
--- /dev/null
+++ b/.changeset/x402-successful-payment-loop-test.md
@@ -0,0 +1,10 @@
+---
+'lens': patch
+---
+
+Add an end-to-end test for the successful x402 402 payment loop (#423). Drives the real
+`@x402/fetch` transport against a stubbed resource + fake facilitator, signing a real Soroban
+`transfer` with `@stellar/stellar-sdk` using throwaway testnet keys. Asserts on what was signed —
+amount, destination and network passphrase (valid on testnet, invalid on mainnet) — not just the
+response body, and covers the failure branches: amount above maxPrice refused before submitting,
+a network the client is not registered for, and an expired signature that the facilitator rejects.
diff --git a/tests/x402-paid-call-loop.test.ts b/tests/x402-paid-call-loop.test.ts
new file mode 100644
index 00000000..627d2d3b
--- /dev/null
+++ b/tests/x402-paid-call-loop.test.ts
@@ -0,0 +1,412 @@
+import { describe, it, expect } from 'vitest';
+import { CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
+import { BazaarMcpServer } from '../src/mcp/server';
+import {
+  Account,
+  Address,
+  Keypair,
+  Networks,
+  Operation,
+  TransactionBuilder,
+  authorizeInvocation,
+  nativeToScVal,
+  scValToNative,
+  xdr,
+  hash,
+} from '@stellar/stellar-sdk';
+
+// @ts-ignore - @x402 packages ship ESM-only types incompatible with the repo's
+// commonjs moduleResolution (same @ts-ignore the production code uses).
+import { wrapFetchWithPaymentFromConfig } from '@x402/fetch';
+
+/**
+ * End-to-end coverage for the *successful* x402 payment loop (#423).
+ *
+ * The pre-existing 402 tests in tests/mcp.test.ts mock `fetchWithPayment`, so
+ * they only ever exercise a rejected payment mapping to ERR_PAYMENT_REJECTED.
+ * Nothing drove the branch that actually moves money: 402 -> construct a
+ * payment -> sign it -> submit it -> resource returned. That branch is the one
+ * that would break silently on an x402 or Stellar SDK upgrade.
+ *
+ * Here the whole loop runs for real:
+ *   - the x402 client + transport (`wrapFetchWithPaymentFromConfig`) is the
+ *     production one, so header encoding / requirement parsing / payment
+ *     selection / spend-control enforcement are all exercised.
+ *   - the payment is signed with the real `@stellar/stellar-sdk` primitives
+ *     (`authorizeInvocation`, Soroban `transfer` invocation, network
+ *     passphrase), so an SDK change to any of those surfaces fails the test.
+ *
+ * Only two things are stubbed, and both are stubs on purpose:
+ *   - the *resource + facilitator*, replaced by an in-process fake that demands
+ *     payment, decodes the submitted `PAYMENT-SIGNATURE`, verifies it against
+ *     the Stellar testnet network passphrase, and only then releases the
+ *     resource; and
+ *   - the Soroban RPC / Horizon that `ExactStellarScheme.createPaymentPayload`
+ *     would otherwise call for `getLatestLedger` + `simulateTransaction`.
+ *     Driving the real scheme needs those network round-trips, which would
+ *     break "deterministic and needs no network access", so the scheme client
+ *     builds the transfer invocation directly. The cryptography it runs is the
+ *     SDK's own, which is exactly what the upgrade risk is about.
+ *
+ * All keys are generated per-test and are throwaway testnet accounts: never a
+ * real secret, never a mainnet account, never committed.
+ */
+
+// A Stellar testnet Stellar Asset Contract id. The bytes are irrelevant to an
+// offline signing test; it only needs to be a well-formed C-address.
+const SAC_CONTRACT = 'CBIELTK6YBZJU5UP2WWQEUCYKLPU6AUNZ2BQ4WWFEIE3USCIHMXQDAMA';
+const RESOURCE_URL = 'http://stub.test/paid';
+
+interface SignedPayment {
+  functionName: string;
+  from: string;
+  to: string;
+  amount: string;
+  expirationLedger: number;
+  credentialAddress: string;
+  sigValidOnTestnet: boolean;
+  sigValidOnMainnet: boolean;
+}
+
+/** Read the payment back out of the signed XDR exactly as a facilitator would. */
+function decodeSignedPayment(transactionXdr: string): SignedPayment {
+  const parsed = TransactionBuilder.fromXDR(transactionXdr, Networks.TESTNET) as any;
+  const op = parsed.operations[0];
+  const contractFn = op.func.invokeContract();
+
+  const args: any[] = contractFn.args();
+  const entry = op.auth[0];
+  const credentials = entry.credentials().address();
+
+  // Recompute the auth preimage for each candidate network and check which one
+  // the payer's signature actually covers. A payment signed for testnet must
+  // NOT verify under the mainnet passphrase - that is the "network" assertion.
+  const signatureBytes = signatureBytesFrom(credentials);
+  const payerKey = Keypair.fromPublicKey(Address.fromScAddress(credentials.address()).toString());
+  const verify = (passphrase: string) => {
+    const networkId = hash(Buffer.from(passphrase));
+    const preimage = xdr.HashIdPreimage.envelopeTypeSorobanAuthorization(
+      new xdr.HashIdPreimageSorobanAuthorization({
+        networkId,
+        nonce: credentials.nonce(),
+        invocation: entry.rootInvocation(),
+        signatureExpirationLedger: credentials.signatureExpirationLedger(),
+      }),
+    );
+    return payerKey.verify(Buffer.from(hash(preimage.toXDR())), signatureBytes);
+  };
+
+  return {
+    functionName: contractFn.functionName().toString(),
+    from: scValToNative(args[0]) as string,
+    to: scValToNative(args[1]) as string,
+    amount: (scValToNative(args[2]) as bigint).toString(),
+    expirationLedger: credentials.signatureExpirationLedger(),
+    credentialAddress: Address.fromScAddress(credentials.address()).toString(),
+    sigValidOnTestnet: verify(Networks.TESTNET),
+    sigValidOnMainnet: verify(Networks.PUBLIC),
+  };
+}
+
+function signatureBytesFrom(addressCredentials: any): Buffer {
+  const vec = addressCredentials.signature(); // scvVec of Ed25519 signature entries
+  const mapEntries = vec.value()[0].value(); // [{key: ScSymbol, val: ScVal}, ...]
+  const signatureEntry = mapEntries.find(
+    (entry: any) => entry.key().sym().toString() === 'signature',
+  );
+  return Buffer.from(signatureEntry.val().bytes());
+}
+
+/**
+ * Minimal `SchemeNetworkClient` that signs a real Soroban `transfer` offline.
+ * It mirrors what `ExactStellarScheme.createPaymentPayload` produces
+ * (`{ x402Version, payload: { transaction } }`) minus the RPC round-trips.
+ */
+function createSigningClient(deps: {
+  payer: Keypair;
+  source: Keypair;
+  expirationLedger: number | ((req: any) => number);
+  networkPassphrase: string;
+  paymentCalls: { count: number };
+}) {
+  return {
+    scheme: 'exact' as const,
+    async createPaymentPayload(x402Version: number, req: any) {
+      deps.paymentCalls.count += 1;
+      const validUntil =
+        typeof deps.expirationLedger === 'function' ? deps.expirationLedger(req) : deps.expirationLedger;
+
+      const invokeArgs = new xdr.InvokeContractArgs({
+        contractAddress: new Address(SAC_CONTRACT).toScAddress(),
+        functionName: 'transfer',
+        args: [
+          nativeToScVal(deps.payer.publicKey(), { type: 'address' }),
+          nativeToScVal(req.payTo, { type: 'address' }),
+          nativeToScVal(req.amount, { type: 'i128' }),
+        ],
+      });
+      const authorizedFn = xdr.SorobanAuthorizedFunction.sorobanAuthorizedFunctionTypeContractFn(invokeArgs);
+      const invocation = new xdr.SorobanAuthorizedInvocation({ function: authorizedFn, subInvocations: [] });
+
+      const entry = await authorizeInvocation(
+        deps.payer,
+        validUntil,
+        invocation,
+        deps.payer.publicKey(),
+        deps.networkPassphrase,
+      );
+
+      const hostFn = xdr.HostFunction.hostFunctionTypeInvokeContract(invokeArgs);
+      const op = Operation.invokeHostFunction({ func: hostFn, auth: [entry] });
+      const account = new Account(deps.source.publicKey(), '1');
+      const tx = new TransactionBuilder(account, {
+        fee: '100',
+        networkPassphrase: deps.networkPassphrase,
+      })
+        .addOperation(op)
+        .setTimeout(300)
+        .build();
+      tx.sign(deps.source);
+
+      return { x402Version, payload: { transaction: tx.toXDR() } };
+    },
+  };
+}
+
+function base64Json(value: unknown): string {
+  return Buffer.from(JSON.stringify(value)).toString('base64');
+}
+
+function paymentRequiredHeader(network: string, amount: string, payTo: string) {
+  return base64Json({
+    x402Version: 2,
+    resource: { url: RESOURCE_URL, description: 'paid resource', mimeType: 'application/json' },
+    accepts: [
+      {
+        scheme: 'exact',
+        network,
+        amount,
+        asset: SAC_CONTRACT,
+        payTo,
+        maxTimeoutSeconds: 60,
+        extra: { areFeesSponsored: true },
+      },
+    ],
+  });
+}
+
+interface FakeResourceOptions {
+  /** Ledger the fake facilitator treats as "now" when checking expiry. */
+  currentLedger: number;
+  /** Called with the decoded payment when the facilitator inspects it. */
+  onPayment?: (payment: SignedPayment) => void;
+}
+
+/**
+ * The stubbed resource + facilitator: it answers an unpaid request with 402 +
+ * PAYMENT-REQUIRED, and answers a paid request only after decoding and
+ * verifying the submitted payment. It returns 200 + PAYMENT-RESPONSE on a valid
+ * payment and 402 (never the resource) when verification fails.
+ */
+function createFakeResource(opts: FakeResourceOptions & {
+  network?: string;
+  amount?: string;
+  payTo: string;
+}) {
+  const network = opts.network ?? 'stellar:testnet';
+  const amount = opts.amount ?? '1000000';
+  const calls: { submitted: boolean }[] = [];
+  let paid: SignedPayment | null = null;
+
+  const fetch: typeof globalThis.fetch = async (input: any) => {
+    const request = input as Request;
+    const signatureHeader = request.headers.get('PAYMENT-SIGNATURE');
+
+    if (!signatureHeader) {
+      calls.push({ submitted: false });
+      return new Response(JSON.stringify({ error: 'payment required' }), {
+        status: 402,
+        headers: { 'PAYMENT-REQUIRED': paymentRequiredHeader(network, amount, opts.payTo) },
+      });
+    }
+
+    calls.push({ submitted: true });
+    const payload = JSON.parse(Buffer.from(signatureHeader, 'base64').toString('utf-8'));
+    const payment = decodeSignedPayment(payload.payload.transaction);
+    paid = payment;
+    opts.onPayment?.(payment);
+
+    const expired = payment.expirationLedger <= opts.currentLedger;
+    if (expired || !payment.sigValidOnTestnet || payment.sigValidOnMainnet) {
+      // A real facilitator refuses without releasing the resource.
+      return new Response(JSON.stringify({ error: 'payment verification failed' }), {
+        status: 402,
+        headers: { 'PAYMENT-REQUIRED': paymentRequiredHeader(network, amount, opts.payTo) },
+      });
+    }
+
+    return new Response(JSON.stringify({ secret: 'the-paywalled-value' }), {
+      status: 200,
+      headers: {
+        'PAYMENT-RESPONSE': base64Json({ success: true, transaction: 'fake-tx-hash', network }),
+      },
+    });
+  };
+
+  return { fetch, get calls() { return calls; }, get paid() { return paid; } };
+}
+
+// authorizeInvocation / TransactionBuilder take the network passphrase
+// explicitly, so no global network configuration is required.
+
+describe('x402 successful payment loop (end to end, stubbed facilitator)', () => {
+  it('drives 402 -> sign -> submit -> resource, verifying what was actually signed', async () => {
+    const payer = Keypair.random();
+    const source = Keypair.random();
+    const payTo = Keypair.random().publicKey(); // fake facilitator's receiving account
+    const currentLedger = 1_000_000;
+    const expirationLedger = currentLedger + 12;
+    const amount = '1000000'; // 0.1 USDC at 7 decimals
+
+    const paymentCalls = { count: 0 };
+    const resource = createFakeResource({ currentLedger, payTo, amount });
+    const client = createSigningClient({ payer, source, expirationLedger, networkPassphrase: Networks.TESTNET, paymentCalls });
+
+    const fetchWithPayment = wrapFetchWithPaymentFromConfig(resource.fetch as unknown as typeof fetch, {
+      schemes: [{ network: 'stellar:testnet', client: client as any }],
+      spendControls: {
+        allowedAssets: [{ network: 'stellar:testnet', asset: SAC_CONTRACT, maxAmountPerPayment: '5000000' }],
+      },
+    });
+
+    const response = await fetchWithPayment(RESOURCE_URL);
+    const body = await response.json();
+
+    // 1. The loop reached the resource only after paying.
+    expect(response.status).toBe(200);
+    expect(body).toEqual({ secret: 'the-paywalled-value' });
+    expect(resource.calls.map(c => c.submitted)).toEqual([false, true]);
+    expect(paymentCalls.count).toBe(1);
+
+    // 2. The submitted payment was signed with the exact terms - not merely a
+    //    200 body. Asserting here would fail if the client paid a different
+    //    amount or a different destination while still returning a body.
+    const paid = resource.paid!;
+    expect(paid.functionName).toBe('transfer');
+    expect(paid.amount).toBe(amount);                       // amount
+    expect(paid.to).toBe(payTo);                            // destination
+    expect(paid.from).toBe(payer.publicKey());             // payer
+    expect(paid.credentialAddress).toBe(payer.publicKey()); // signer address
+    // 3. Bound to the Stellar testnet network passphrase, and provably NOT mainnet.
+    expect(paid.sigValidOnTestnet).toBe(true);
+    expect(paid.sigValidOnMainnet).toBe(false);
+    expect(paid.expirationLedger).toBe(expirationLedger);
+  });
+
+  it('refuses to pay when the demanded amount is above maxPrice, without submitting', async () => {
+    const payer = Keypair.random();
+    const source = Keypair.random();
+    const payTo = Keypair.random().publicKey();
+    const paymentCalls = { count: 0 };
+
+    // The resource demands 9.0000000 USDC but the cap is 5.0000000 atomic max.
+    const resource = createFakeResource({ currentLedger: 1_000_000, payTo, amount: '9000000' });
+    const client = createSigningClient({ payer, source, expirationLedger: 1_000_012, networkPassphrase: Networks.TESTNET, paymentCalls });
+
+    const fetchWithPayment = wrapFetchWithPaymentFromConfig(resource.fetch as unknown as typeof fetch, {
+      schemes: [{ network: 'stellar:testnet', client: client as any }],
+      spendControls: {
+        allowedAssets: [{ network: 'stellar:testnet', asset: SAC_CONTRACT, maxAmountPerPayment: '5000000' }],
+      },
+    });
+
+    await expect(fetchWithPayment(RESOURCE_URL)).rejects.toThrow(/spendControls\.allowedAssets maxAmountPerPayment/);
+
+    // Only the unpaid 402 was requested; nothing was signed and no payment moved.
+    expect(resource.calls.map(c => c.submitted)).toEqual([false]);
+    expect(paymentCalls.count).toBe(0);
+    expect(resource.paid).toBeNull();
+  });
+
+  it('refuses to pay for a network the client is not configured for', async () => {
+    const payer = Keypair.random();
+    const source = Keypair.random();
+    const payTo = Keypair.random().publicKey();
+    const paymentCalls = { count: 0 };
+
+    // The resource asks for mainnet while only testnet is registered.
+    const resource = createFakeResource({ currentLedger: 1_000_000, payTo, network: 'stellar:pubnet' });
+    const client = createSigningClient({ payer, source, expirationLedger: 1_000_012, networkPassphrase: Networks.TESTNET, paymentCalls });
+
+    const fetchWithPayment = wrapFetchWithPaymentFromConfig(resource.fetch as unknown as typeof fetch, {
+      schemes: [{ network: 'stellar:testnet', client: client as any }],
+      spendControls: { allowedAssets: true },
+    });
+
+    await expect(fetchWithPayment(RESOURCE_URL)).rejects.toThrow(/No network\/scheme registered/);
+    expect(resource.calls.map(c => c.submitted)).toEqual([false]);
+    expect(paymentCalls.count).toBe(0);
+  });
+
+  it('does not release the resource when the signature is already expired', async () => {
+    const payer = Keypair.random();
+    const source = Keypair.random();
+    const payTo = Keypair.random().publicKey();
+    const currentLedger = 1_000_000;
+    const paymentCalls = { count: 0 };
+
+    // Signed for a ledger already in the past: the payment is constructed and
+    // submitted, but a verifying facilitator must reject it.
+    const resource = createFakeResource({ currentLedger, payTo });
+    const client = createSigningClient({ payer, source, expirationLedger: currentLedger - 1, networkPassphrase: Networks.TESTNET, paymentCalls });
+
+    const fetchWithPayment = wrapFetchWithPaymentFromConfig(resource.fetch as unknown as typeof fetch, {
+      schemes: [{ network: 'stellar:testnet', client: client as any }],
+      spendControls: { allowedAssets: true },
+    });
+
+    const response = await fetchWithPayment(RESOURCE_URL);
+
+    // A payment was submitted (so this genuinely tested the verification path),
+    // the signature was expired, and the paywalled value was NOT returned.
+    expect(resource.calls.map(c => c.submitted)).toEqual([false, true]);
+    expect(paymentCalls.count).toBe(1);
+    expect(resource.paid!.expirationLedger).toBeLessThanOrEqual(currentLedger);
+    expect(response.status).toBe(402);
+    const body = await response.json();
+    expect(body).not.toHaveProperty('secret');
+  });
+
+  it('serves the resource through BazaarMcpServer.paid_call once the loop settles', async () => {
+    const payer = Keypair.random();
+    const source = Keypair.random();
+    const payTo = Keypair.random().publicKey();
+    const currentLedger = 1_000_000;
+    const paymentCalls = { count: 0 };
+
+    const resource = createFakeResource({ currentLedger, payTo });
+    const client = createSigningClient({ payer, source, expirationLedger: currentLedger + 12, networkPassphrase: Networks.TESTNET, paymentCalls });
+    const fetchWithPayment = wrapFetchWithPaymentFromConfig(resource.fetch as unknown as typeof fetch, {
+      schemes: [{ network: 'stellar:testnet', client: client as any }],
+      spendControls: { allowedAssets: true },
+    });
+
+    // Drive the real tool handler, wired to the real payment-enabled fetch.
+    const server = new BazaarMcpServer({ fetchWithPayment, paidCallAllowlist: ['stub.test'] });
+    let handler: any = null;
+    (server as any).server.setRequestHandler = (schema: any, fn: any) => {
+      if (schema === CallToolRequestSchema) handler = fn;
+    };
+    (server as any).setupHandlers();
+
+    const result = await handler({
+      params: { name: 'paid_call', arguments: { url: RESOURCE_URL } },
+    });
+
+    expect(result.isError).toBeFalsy();
+    expect(JSON.parse(result.content[0].text)).toEqual({ secret: 'the-paywalled-value' });
+    // The money path ran for real: the payment was signed and submitted.
+    expect(paymentCalls.count).toBe(1);
+    expect(resource.paid!.to).toBe(payTo);
+  });
+});

From cbb74b63daba6f066422c282fce5f9f68a51f446 Mon Sep 17 00:00:00 2001
From: Stehen Jeremiah Haruna <141843972+Ugasutun@users.noreply.github.com>
Date: Sat, 17 Oct 2026 23:56:33 +0100
Subject: [PATCH 3/4] feat(mcp): serve Lens's own aggregated prices through
 lens_get_price (#408)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The MCP server exposed bazaar_search and paid_call, so an agent connected to
Lens could pay a third party for a price but could not ask Lens for one. The
service aggregates SDEX, AMM, Soroswap and Aquarius into a VWAP and serves it
over REST, GraphQL and a websocket — none of that was reachable over MCP.

lens_get_price closes that by delegating to the same getAggregatedPrice and
getBestRoute the HTTP surfaces use, rather than growing a second price
implementation the tool could drift from.

The tool takes the same issuer discipline REST was fixed to in 41b1f62, and
GraphQL still lacks in #357: assets are CODE or CODE:ISSUER, the issuer binds
whenever both the request and the watched pair name one, and a code that
matches under a different issuer is refused with ERR_ASSET_ISSUER_MISMATCH
naming the issuer this network actually prices. Answering about the wrong
asset is the one outcome a price feed cannot be allowed, so a bare code still
resolves but the result echoes the issuers it priced.

Also: the network is selectable and defaults to the one the server runs; the
answer is structuredContent so an agent reads the number instead of parsing
English; staleness travels with the price via confidence,
lastTradeAgeSeconds and stale; and a pair with no ingested trade, or a zero
from every venue, is refused as ERR_NO_RESULTS instead of being quoted as a
price of 0.
---
 .changeset/mcp-lens-get-price.md        |  11 +
 examples/mcp-server/agent.ts            |  21 ++
 examples/mcp-server/run.ts              |   7 +-
 src/mcp/codes.ts                        |   3 +
 src/mcp/priceQueries.ts                 |  42 +++
 src/mcp/priceTool.ts                    | 439 +++++++++++++++++++++++
 src/mcp/server.ts                       |  53 ++-
 tests/mcp-get-price-aggregation.test.ts | 127 +++++++
 tests/mcp-get-price.test.ts             | 440 ++++++++++++++++++++++++
 9 files changed, 1138 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/mcp-lens-get-price.md
 create mode 100644 src/mcp/priceQueries.ts
 create mode 100644 src/mcp/priceTool.ts
 create mode 100644 tests/mcp-get-price-aggregation.test.ts
 create mode 100644 tests/mcp-get-price.test.ts

diff --git a/.changeset/mcp-lens-get-price.md b/.changeset/mcp-lens-get-price.md
new file mode 100644
index 00000000..af49820e
--- /dev/null
+++ b/.changeset/mcp-lens-get-price.md
@@ -0,0 +1,11 @@
+---
+'lens': minor
+---
+
+Add the `lens_get_price` MCP tool, so an agent connected over MCP can ask Lens for
+an aggregated price instead of only searching and paying other people's Bazaar
+nodes. Assets are `CODE` or `CODE:ISSUER`, the issuer binds when both sides name
+one (a same-named asset is refused with `ERR_ASSET_ISSUER_MISMATCH`, never
+priced), the network defaults to the one the server runs, and the answer is
+structured content carrying the price, venues, source count, confidence, the age
+of the last trade and the best route.
diff --git a/examples/mcp-server/agent.ts b/examples/mcp-server/agent.ts
index 1e4408b9..0a6c2e7c 100644
--- a/examples/mcp-server/agent.ts
+++ b/examples/mcp-server/agent.ts
@@ -86,6 +86,27 @@ async function main() {
 
   const BAZAAR_URL = 'http://localhost:3000';
 
+  // Step 0 is the one that needs no Bazaar node and no payment: Lens answering
+  // for its own data. It reads the ingestion database, so a machine without one
+  // gets a deterministic refusal instead of a number.
+  console.log('\n[agent] 0. Asking Lens directly for a price...');
+
+  const priceResult = await client.callTool({
+    name: 'lens_get_price',
+    arguments: { assetA: 'native', assetB: 'USDC' },
+  });
+
+  if (priceResult.isError) {
+    console.log('[agent] lens_get_price refused:', JSON.parse(priceResult.content[0].text).reason);
+  } else {
+    const price: any = priceResult.structuredContent ?? JSON.parse(priceResult.content[0].text);
+    console.log(
+      `[agent] ${price.pairKey} on ${price.network}: ${price.price} ${price.quote.code} per ${price.base.code} ` +
+        `(confidence ${price.confidence}, ${price.sources} source(s), last trade ${price.lastTradeAgeSeconds}s ago, ` +
+        `route ${price.bestRoute?.route ?? 'unknown'})`
+    );
+  }
+
   console.log(`\n[agent] 1. Discovering resources at ${BAZAAR_URL}...`);
   console.log(`[agent] Executing tool: bazaar_search`);
   
diff --git a/examples/mcp-server/run.ts b/examples/mcp-server/run.ts
index ef9da239..7108a3a8 100644
--- a/examples/mcp-server/run.ts
+++ b/examples/mcp-server/run.ts
@@ -1,5 +1,7 @@
 import 'dotenv/config';
 import { BazaarMcpServer } from '../../src/mcp/server';
+import { createLensPriceQueries } from '../../src/mcp/priceQueries';
+import { activeNetwork } from '../../src/config';
 import { wrapFetchWithPaymentFromConfig } from '@x402/fetch';
 import { ExactStellarScheme } from '@x402/stellar/exact/client';
 import { createEd25519Signer } from '@x402/stellar';
@@ -42,11 +44,14 @@ async function main() {
 
   const server = new BazaarMcpServer({
     fetchWithPayment,
-    paidCallAllowlist
+    paidCallAllowlist,
+    // Lens's own aggregated prices, read from the same Postgres the API serves.
+    priceQueries: createLensPriceQueries(),
   });
 
   console.error('[mcp-server] Starting Bazaar MCP server on stdio...');
   console.error('[mcp-server] Agent configured for stellar:testnet and stellar:pubnet');
+  console.error(`[mcp-server] lens_get_price serves aggregated prices on ${activeNetwork}`);
   console.error(
     paidCallAllowlist.length > 0
       ? `[mcp-server] paid_call may pay these allow-listed origins: ${paidCallAllowlist.join(', ')}`
diff --git a/src/mcp/codes.ts b/src/mcp/codes.ts
index 97e28528..767169a3 100644
--- a/src/mcp/codes.ts
+++ b/src/mcp/codes.ts
@@ -7,6 +7,9 @@ export const MCP_ERROR_CODES = {
   ERR_NO_RESULTS: 'ERR_NO_RESULTS',
   ERR_BAD_REQUEST: 'ERR_BAD_REQUEST',
   ERR_HOST_NOT_ALLOWED: 'ERR_HOST_NOT_ALLOWED',
+  ERR_INVALID_ASSET: 'ERR_INVALID_ASSET',
+  ERR_PAIR_NOT_WATCHED: 'ERR_PAIR_NOT_WATCHED',
+  ERR_ASSET_ISSUER_MISMATCH: 'ERR_ASSET_ISSUER_MISMATCH',
   ERR_INTERNAL_FAILURE: 'ERR_INTERNAL_FAILURE'
 } as const;
 
diff --git a/src/mcp/priceQueries.ts b/src/mcp/priceQueries.ts
new file mode 100644
index 00000000..c7f823c9
--- /dev/null
+++ b/src/mcp/priceQueries.ts
@@ -0,0 +1,42 @@
+import { getAggregatedPrice } from '../aggregator/vwap'
+import { getBestRoute } from '../aggregator/bestRoute'
+import { activeNetwork, getNetworkConfig } from '../config'
+import type { NetworkName } from '../config'
+import type { LensPriceQueries } from './priceTool.js'
+
+/**
+ * Order size the route is computed for. REST's `/price` and `/route` default to
+ * the same 1000 units, so an agent reading the MCP tool and an operator reading
+ * the HTTP response see one number rather than two.
+ */
+const ROUTE_AMOUNT = 1000
+
+/**
+ * The data layer behind `lens_get_price`, built from the same aggregation the
+ * REST and GraphQL surfaces serve.
+ *
+ * Keeping this in its own module is what lets `server.ts` stay free of the
+ * database: the tool's logic is testable with a stub, and only a process that
+ * opts into serving Lens's own prices pays for importing `pg` and Prisma.
+ */
+export function createLensPriceQueries(): LensPriceQueries {
+  return {
+    activeNetwork,
+
+    // The declared pair list for that network, not the process-wide runtime
+    // registry: entries added through POST /pairs are registered against
+    // whichever network the API process runs, so reading them here would let a
+    // mainnet request resolve a pair that was only ever added on testnet.
+    watchedPairs(network: NetworkName) {
+      return getNetworkConfig(network).pairs
+    },
+
+    aggregatedPrice(pairKey: string, network: NetworkName) {
+      return getAggregatedPrice(pairKey, network)
+    },
+
+    bestRoute(assetA, assetB, pairKey, network) {
+      return getBestRoute(assetA, assetB, pairKey, ROUTE_AMOUNT, network)
+    },
+  }
+}
diff --git a/src/mcp/priceTool.ts b/src/mcp/priceTool.ts
new file mode 100644
index 00000000..3f5d3104
--- /dev/null
+++ b/src/mcp/priceTool.ts
@@ -0,0 +1,439 @@
+import { MCP_ERROR_CODES } from './codes.js'
+import type { McpErrorCode } from './codes.js'
+import type { AssetId, RouteInfo, WatchedPair } from '../types.js'
+import type { NetworkName } from '../config.js'
+
+/**
+ * #408 — the MCP server was a client for other people's Bazaar nodes and had no
+ * way to answer the one question Lens exists to answer. Everything below is the
+ * pure half of that tool: argument parsing, pair resolution, and the shape of
+ * the answer. The data access lives in ./priceQueries.ts and reaches this
+ * module only through `LensPriceQueries`, so the tool can be exercised without
+ * a database and cannot accidentally grow its own price maths.
+ */
+
+export const LENS_PRICE_TOOL_NAME = 'lens_get_price'
+
+/** What `getAggregatedPrice` returns; mirrored here to keep this module data-layer-free. */
+export interface AggregatedPriceFields {
+  price: number
+  sdexPrice: number
+  ammPrice: number
+  volume24h: number
+  sdexVolume24h: number
+  ammVolume24h: number
+  vwap1m: number
+  vwap5m: number
+  vwap1h: number
+  vwap24h: number
+  priceChange24h: number
+  sources: number
+  confidence: 'high' | 'medium' | 'low' | 'unknown'
+  lastTradeAgeSeconds: number | null
+  stale: boolean
+}
+
+/** The three reads `lens_get_price` needs, supplied by the process that hosts the tool. */
+export interface LensPriceQueries {
+  /** Network the caller gets when they name none — the deployment's own default. */
+  readonly activeNetwork: NetworkName
+  /** Pairs watched on that network, issuers intact. */
+  watchedPairs(network: NetworkName): readonly WatchedPair[]
+  aggregatedPrice(pairKey: string, network: NetworkName): Promise<AggregatedPriceFields>
+  bestRoute(assetA: AssetId, assetB: AssetId, pairKey: string, network: NetworkName): Promise<RouteInfo>
+}
+
+/** An asset as the caller wrote it, after shape validation. */
+export interface AssetRef {
+  code: string
+  issuer: string | null
+}
+
+/**
+ * A type alias rather than an interface, so it carries an implicit index
+ * signature and satisfies the MCP `structuredContent` record type.
+ */
+export type LensPriceResult = {
+  network: NetworkName
+  pairKey: string
+  /** The watched pair's base side, with the issuer that actually defines it. */
+  base: AssetRef
+  /** The watched pair's quote side. `price` is units of this per one unit of base. */
+  quote: AssetRef
+  /** Whether the pair was matched in the order asked, or the other way round. */
+  direction: 'as-requested' | 'reversed'
+  /** Constant, so the number cannot be read backwards by an agent that ignores base/quote. */
+  orientation: 'quote-per-base'
+  price: number
+  sources: number
+  confidence: AggregatedPriceFields['confidence']
+  lastTradeAgeSeconds: number | null
+  stale: boolean
+  vwap1m: number
+  vwap5m: number
+  vwap1h: number
+  vwap24h: number
+  priceChange24h: number
+  volume24h: number
+  sdexPrice: number
+  ammPrice: number
+  sdexVolume24h: number
+  ammVolume24h: number
+  bestRoute: RouteInfo | null
+  lastUpdated: string
+}
+
+export type PriceToolFailure = { code: McpErrorCode; reason: string }
+export type PriceToolOutcome =
+  | { ok: true; result: LensPriceResult }
+  | { ok: false; error: PriceToolFailure }
+
+const VALID_NETWORKS: readonly NetworkName[] = ['testnet', 'mainnet']
+
+/**
+ * `native`, or an uppercase alphanumeric code with an optional full strkey
+ * issuer. Codes are matched case-sensitively because that is how Stellar stores
+ * them — "usdc" and "USDC" are different strings on the ledger — so a lowercase
+ * code is refused rather than quietly folded.
+ */
+const ASSET_PATTERN = '^(?:native|[A-Z][A-Z0-9]{0,11}(?::G[A-Z2-7]{55})?)$'
+const ASSET_RE = new RegExp(ASSET_PATTERN)
+const ISSUER_RE = /^G[A-Z2-7]{55}$/
+
+/** How many pair keys an unknown-pair refusal lists back to the caller. */
+const PAIR_HINT_LIMIT = 20
+
+export const lensPriceToolDefinition = () => ({
+  name: LENS_PRICE_TOOL_NAME,
+  description:
+    'Aggregated Lens price for one watched asset pair on one Stellar network: volume-weighted price ' +
+    'across SDEX and AMM venues, with the venue split, source count, confidence, the age of the last ' +
+    'trade and the best route for a 1000-unit order. Read-only — it spends nothing and needs no ' +
+    'allow-listed host, so use this instead of paid_call whenever the question is "what is the price". ' +
+    'Assets are CODE or CODE:ISSUER (use "native" or "XLM" for lumens). Always give the issuer for a ' +
+    'token: asset codes are not unique on Stellar, anyone can issue "USDC", and a bare code matches ' +
+    'whichever pair of that code this network watches. If the code is watched under a different ' +
+    'issuer this tool answers ERR_ASSET_ISSUER_MISMATCH naming the issuer it does watch — it never ' +
+    'prices a same-named asset. The result states which pair matched (base/quote with issuers) and the ' +
+    'orientation: price is units of quote per one base. ' +
+    'Network is "testnet" or "mainnet", defaulting to the network this server runs. ' +
+    'A price is only returned when the pair has data: confidence is high under 30s of last-trade age, ' +
+    'medium under 300s, low at or beyond 300s (stale: true), and unknown when nothing has been ' +
+    'ingested; check lastTradeAgeSeconds and stale before treating a price as current. Refusals are ' +
+    'deterministic: ERR_INVALID_ASSET, ERR_PAIR_NOT_WATCHED, ERR_ASSET_ISSUER_MISMATCH, ' +
+    'ERR_NO_RESULTS. Report those rather than retrying — a refused pair is not a transient failure.',
+  inputSchema: {
+    type: 'object',
+    properties: {
+      assetA: {
+        type: 'string',
+        description: 'First asset: CODE, CODE:ISSUER, or "native" for lumens. Codes are uppercase.',
+        pattern: ASSET_PATTERN,
+        examples: ['XLM', 'USDC:GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'],
+      },
+      assetB: {
+        type: 'string',
+        description: 'Second asset, same form as assetA. Order does not matter; the pair is matched both ways.',
+        pattern: ASSET_PATTERN,
+        examples: ['USDC:GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'],
+      },
+      network: {
+        type: 'string',
+        enum: [...VALID_NETWORKS],
+        description: 'Stellar network to quote. Defaults to the network this server is configured for.',
+      },
+    },
+    required: ['assetA', 'assetB'],
+    additionalProperties: false,
+  },
+  outputSchema: {
+    type: 'object',
+    properties: {
+      network: { type: 'string', enum: [...VALID_NETWORKS] },
+      pairKey: { type: 'string' },
+      base: codeIssuerSchema(),
+      quote: codeIssuerSchema(),
+      direction: { type: 'string', enum: ['as-requested', 'reversed'] },
+      orientation: { type: 'string', enum: ['quote-per-base'] },
+      price: { type: 'number' },
+      sources: { type: 'integer' },
+      confidence: { type: 'string', enum: ['high', 'medium', 'low', 'unknown'] },
+      lastTradeAgeSeconds: { type: ['number', 'null'] },
+      stale: { type: 'boolean' },
+      vwap1m: { type: 'number' },
+      vwap5m: { type: 'number' },
+      vwap1h: { type: 'number' },
+      vwap24h: { type: 'number' },
+      priceChange24h: { type: 'number' },
+      volume24h: { type: 'number' },
+      sdexPrice: { type: 'number' },
+      ammPrice: { type: 'number' },
+      sdexVolume24h: { type: 'number' },
+      ammVolume24h: { type: 'number' },
+      bestRoute: {
+        description: 'Best route for a 1000-unit order, or null when no venue could price the pair.',
+        anyOf: [routeSchema(), { type: 'null' }],
+      },
+      lastUpdated: { type: 'string' },
+    },
+    required: [
+      'network', 'pairKey', 'base', 'quote', 'direction', 'orientation', 'price', 'sources',
+      'confidence', 'lastTradeAgeSeconds', 'stale', 'vwap1m', 'vwap5m', 'vwap1h', 'vwap24h',
+      'priceChange24h', 'volume24h', 'sdexPrice', 'ammPrice', 'sdexVolume24h', 'ammVolume24h',
+      'bestRoute', 'lastUpdated',
+    ],
+    additionalProperties: false,
+  },
+})
+
+function codeIssuerSchema() {
+  return {
+    type: 'object',
+    properties: {
+      code: { type: 'string' },
+      issuer: { type: ['string', 'null'] },
+    },
+    required: ['code', 'issuer'],
+    additionalProperties: false,
+  }
+}
+
+function routeSchema() {
+  return {
+    type: 'object',
+    properties: {
+      route: { type: 'string', enum: ['SDEX', 'AMM', 'SPLIT', 'UNKNOWN'] },
+      sdexPrice: { type: 'number' },
+      ammPrice: { type: 'number' },
+      estimatedOutput: { type: 'number' },
+      slippagePct: { type: 'number' },
+      recommendation: { type: 'string' },
+    },
+    required: ['route', 'sdexPrice', 'ammPrice', 'estimatedOutput', 'slippagePct', 'recommendation'],
+    additionalProperties: false,
+  }
+}
+
+/**
+ * Validate one asset argument. `native` is accepted in any case because it names
+ * lumens and cannot be reissued by anyone; everything else must match the
+ * declared shape exactly, since folding case or padding an issuer is how a
+ * request ends up priced about the wrong asset.
+ */
+export function parseAssetRef(raw: unknown, field: string): { asset: AssetRef } | PriceToolFailure {
+  if (typeof raw !== 'string' || raw.length === 0) {
+    return {
+      code: MCP_ERROR_CODES.ERR_INVALID_ASSET,
+      reason: `${field} must be a non-empty string: CODE, CODE:ISSUER, or "native"`,
+    }
+  }
+
+  if (raw.toLowerCase() === 'native') return { asset: { code: 'XLM', issuer: null } }
+
+  if (!ASSET_RE.test(raw)) {
+    const issuer = raw.split(':')[1]
+    if (raw !== raw.toUpperCase() && ASSET_RE.test(raw.toUpperCase())) {
+      return {
+        code: MCP_ERROR_CODES.ERR_INVALID_ASSET,
+        reason:
+          `${field}="${raw}" is not accepted: Stellar asset codes are case-sensitive, so a lowercase ` +
+          `code is a different asset. Resend it uppercase ("${raw.toUpperCase()}") if that is the code you mean.`,
+      }
+    }
+    if (issuer !== undefined && !ISSUER_RE.test(issuer)) {
+      return {
+        code: MCP_ERROR_CODES.ERR_INVALID_ASSET,
+        reason:
+          `${field}="${raw}" carries an issuer that is not a Stellar account id. An issuer must be the ` +
+          `full G... public key of the issuing account; a partial or abbreviated one is refused.`,
+      }
+    }
+    return {
+      code: MCP_ERROR_CODES.ERR_INVALID_ASSET,
+      reason:
+        `${field}="${raw}" is not an asset reference. Expected "native", an uppercase alphanumeric ` +
+        `code of up to 12 characters, or CODE:ISSUER with the issuer's full G... account id.`,
+    }
+  }
+
+  const [code, issuer] = raw.split(':')
+  return { asset: { code, issuer: issuer ?? null } }
+}
+
+function codeMatches(q: AssetRef, side: AssetRef): boolean {
+  return q.code === side.code.toUpperCase()
+}
+
+/**
+ * REST's issuer rule, kept identical: the issuer binds only when both the
+ * request and the watched pair name one. XLM has no issuer, and a caller who
+ * passes a bare code has asserted nothing about which issuer they meant.
+ */
+function sideMatches(q: AssetRef, side: AssetRef): boolean {
+  if (!codeMatches(q, side)) return false
+  if (!q.issuer || !side.issuer) return true
+  return q.issuer === side.issuer
+}
+
+function assetLabel(side: AssetRef): string {
+  return side.issuer ? `${side.code}:${side.issuer}` : side.code
+}
+
+/**
+ * Resolve a request against the pairs one network watches.
+ *
+ * A code that matches but an issuer that does not is reported as its own
+ * failure, naming the issuer this network actually prices. That is deliberate:
+ * "not watched" would send the agent hunting for a pair that is there, and the
+ * alternative — matching on code alone — is the failure that made this repo
+ * quote testnet USDC for a mainnet USDC request.
+ */
+export function resolveWatchedPair(
+  assetA: AssetRef,
+  assetB: AssetRef,
+  network: NetworkName,
+  pairs: readonly WatchedPair[],
+): { pair: WatchedPair; direction: 'as-requested' | 'reversed' } | PriceToolFailure {
+  const asRef = (a: AssetId): AssetRef => ({ code: a.code, issuer: a.issuer ?? null })
+
+  const forward = (p: WatchedPair) =>
+    sideMatches(assetA, asRef(p.assetA)) && sideMatches(assetB, asRef(p.assetB))
+  const backward = (p: WatchedPair) =>
+    sideMatches(assetA, asRef(p.assetB)) && sideMatches(assetB, asRef(p.assetA))
+
+  const matched = pairs.find(p => forward(p) || backward(p))
+  if (matched) {
+    return { pair: matched, direction: forward(matched) ? 'as-requested' : 'reversed' }
+  }
+
+  const byCode = (p: WatchedPair) =>
+    (codeMatches(assetA, asRef(p.assetA)) && codeMatches(assetB, asRef(p.assetB))) ||
+    (codeMatches(assetA, asRef(p.assetB)) && codeMatches(assetB, asRef(p.assetA)))
+
+  const nameClash = pairs.find(byCode)
+  if (nameClash) {
+    return {
+      code: MCP_ERROR_CODES.ERR_ASSET_ISSUER_MISMATCH,
+      reason:
+        `On ${network} this deployment watches ${assetLabel(nameClash.assetA)}/${assetLabel(nameClash.assetB)}, ` +
+        `but the request named ${assetLabel(assetA)}/${assetLabel(assetB)}. Those issuers identify different ` +
+        `assets, so no price is returned for them. Ask for the pair exactly as watched, or register the ` +
+        `pair you mean.`,
+    }
+  }
+
+  const watched = pairs.map(p => p.pairKey).slice(0, PAIR_HINT_LIMIT)
+  return {
+    code: MCP_ERROR_CODES.ERR_PAIR_NOT_WATCHED,
+    reason:
+      `Pair ${assetLabel(assetA)}/${assetLabel(assetB)} is not watched on ${network}. ` +
+      (watched.length
+        ? `Watched on this network: ${watched.join(', ')}${pairs.length > PAIR_HINT_LIMIT ? ', ...' : ''}`
+        : `This deployment watches no pairs on ${network}; none can be priced.`),
+  }
+}
+
+function failure(code: McpErrorCode, reason: string): PriceToolOutcome {
+  return { ok: false, error: { code, reason } }
+}
+
+/**
+ * Run one `lens_get_price` request. Everything the tool can get wrong by
+ * accident — an unknown argument, a folded case, a network that is not this
+ * deployment's, an issuer that names a different asset — is settled here, before
+ * any query runs, so a refusal costs the database nothing.
+ */
+export async function readLensPrice(
+  args: unknown,
+  queries: LensPriceQueries,
+): Promise<PriceToolOutcome> {
+  if (typeof args !== 'object' || args === null || Array.isArray(args)) {
+    return failure(MCP_ERROR_CODES.ERR_BAD_REQUEST, 'lens_get_price expects an object of arguments')
+  }
+
+  const given = args as Record<string, unknown>
+
+  // The low-level MCP server hands the caller's object straight to the handler,
+  // so `additionalProperties: false` is only a promise until something enforces
+  // it. Without this, a typo'd or smuggled key is silently ignored and the agent
+  // believes it asked for something it did not.
+  const allowed = ['assetA', 'assetB', 'network']
+  const unexpected = Object.keys(given).filter(k => !allowed.includes(k))
+  if (unexpected.length > 0) {
+    return failure(
+      MCP_ERROR_CODES.ERR_BAD_REQUEST,
+      `Unknown argument(s): ${unexpected.join(', ')}. lens_get_price accepts only ${allowed.join(', ')}.`,
+    )
+  }
+
+  const parsedA = parseAssetRef(given.assetA, 'assetA')
+  if ('code' in parsedA) return failure(parsedA.code, parsedA.reason)
+  const parsedB = parseAssetRef(given.assetB, 'assetB')
+  if ('code' in parsedB) return failure(parsedB.code, parsedB.reason)
+
+  let network = queries.activeNetwork
+  if (given.network !== undefined) {
+    if (typeof given.network !== 'string' || !(VALID_NETWORKS as readonly string[]).includes(given.network)) {
+      return failure(
+        MCP_ERROR_CODES.ERR_BAD_REQUEST,
+        `network must be one of ${VALID_NETWORKS.join(' or ')}, got ${JSON.stringify(given.network)}`,
+      )
+    }
+    network = given.network as NetworkName
+  }
+
+  const resolved = resolveWatchedPair(parsedA.asset, parsedB.asset, network, queries.watchedPairs(network))
+  if ('code' in resolved) return failure(resolved.code, resolved.reason)
+  const { pair, direction } = resolved
+
+  const agg = await queries.aggregatedPrice(pair.pairKey, network)
+
+  // No ingested trade, or a price of zero from every venue, is not a quote. A
+  // `0` served as a price is the confident wrong number this project has been
+  // burned by before, so it is refused with the data that explains it.
+  if (agg.lastTradeAgeSeconds === null || !(agg.price > 0)) {
+    return failure(
+      MCP_ERROR_CODES.ERR_NO_RESULTS,
+      `${pair.pairKey} is watched on ${network} but has no aggregated price: ` +
+        `last trade ${agg.lastTradeAgeSeconds === null ? 'never' : `${agg.lastTradeAgeSeconds}s ago`}, ` +
+        `price ${agg.price} across ${agg.sources} source(s).`,
+    )
+  }
+
+  // Routing consults Horizon as well as the database. A price that is already
+  // known should not be withheld because the route leg failed, so the failure is
+  // reported in the payload rather than thrown away or turned into an error.
+  const route = await queries.bestRoute(pair.assetA, pair.assetB, pair.pairKey, network).catch(() => null)
+
+  const base = { code: pair.assetA.code, issuer: pair.assetA.issuer ?? null }
+  const quote = { code: pair.assetB.code, issuer: pair.assetB.issuer ?? null }
+
+  return {
+    ok: true,
+    result: {
+      network,
+      pairKey: pair.pairKey,
+      base,
+      quote,
+      direction,
+      orientation: 'quote-per-base',
+      price: agg.price,
+      sources: agg.sources,
+      confidence: agg.confidence,
+      lastTradeAgeSeconds: agg.lastTradeAgeSeconds,
+      stale: agg.stale,
+      vwap1m: agg.vwap1m,
+      vwap5m: agg.vwap5m,
+      vwap1h: agg.vwap1h,
+      vwap24h: agg.vwap24h,
+      priceChange24h: agg.priceChange24h,
+      volume24h: agg.volume24h,
+      sdexPrice: agg.sdexPrice,
+      ammPrice: agg.ammPrice,
+      sdexVolume24h: agg.sdexVolume24h,
+      ammVolume24h: agg.ammVolume24h,
+      bestRoute: route,
+      lastUpdated: new Date().toISOString(),
+    },
+  }
+}
diff --git a/src/mcp/server.ts b/src/mcp/server.ts
index 204ba671..6a264a89 100644
--- a/src/mcp/server.ts
+++ b/src/mcp/server.ts
@@ -3,6 +3,7 @@ import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js'
 import { CallToolRequestSchema, ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js';
 import { MCP_ERROR_CODES, McpErrorCode } from './codes.js';
 import { checkPaidCallTarget, parsePaidCallAllowlist, type PaidCallAllowRule } from './paidCallAllowlist.js';
+import { LENS_PRICE_TOOL_NAME, lensPriceToolDefinition, readLensPrice, type LensPriceQueries } from './priceTool.js';
 import { fetchWithTimeout } from '../fetch.js';
 
 export interface McpServerConfig {
@@ -13,15 +14,23 @@ export interface McpServerConfig {
    * `paid_call` pays nothing. See ./paidCallAllowlist.ts.
    */
   paidCallAllowlist?: readonly string[];
+  /**
+   * Lens's own aggregated prices, for `lens_get_price`. Hosts that can read the
+   * price data pass {@link createLensPriceQueries}; leaving it out simply does
+   * not advertise the tool, so an agent is never offered a price it cannot get.
+   */
+  priceQueries?: LensPriceQueries;
 }
 
 export class BazaarMcpServer {
   private server: Server;
   private fetchWithPayment: typeof fetch;
   private paidCallRules: PaidCallAllowRule[];
+  private priceQueries?: LensPriceQueries;
 
   constructor(config: McpServerConfig) {
     this.fetchWithPayment = config.fetchWithPayment;
+    this.priceQueries = config.priceQueries;
 
     const allowlist = parsePaidCallAllowlist(config.paidCallAllowlist ?? []);
     this.paidCallRules = allowlist.rules;
@@ -46,8 +55,8 @@ export class BazaarMcpServer {
   }
 
   private setupHandlers() {
-    this.server.setRequestHandler(ListToolsRequestSchema, async () => ({
-      tools: [
+    this.server.setRequestHandler(ListToolsRequestSchema, async () => {
+      const tools: Record<string, unknown>[] = [
         {
           name: 'bazaar_search',
           description: 'Search for resources in a Stellar Bazaar node',
@@ -90,10 +99,46 @@ export class BazaarMcpServer {
             additionalProperties: false,
           },
         },
-      ],
-    }));
+      ];
+
+      // #408: without this the server could spend money on someone else's price
+      // feed while being unable to state Lens's own.
+      if (this.priceQueries) {
+        tools.push(lensPriceToolDefinition());
+      }
+
+      return { tools };
+    });
 
     this.server.setRequestHandler(CallToolRequestSchema, async (request) => {
+      if (request.params.name === LENS_PRICE_TOOL_NAME) {
+        if (!this.priceQueries) {
+          return this.createErrorResponse(
+            'ERR_INTERNAL_FAILURE',
+            'lens_get_price is not available: this server has no Lens price source configured.'
+          );
+        }
+
+        try {
+          const outcome = await readLensPrice(request.params.arguments, this.priceQueries);
+
+          if (!outcome.ok) {
+            return this.createErrorResponse(outcome.error.code, outcome.error.reason);
+          }
+
+          // structuredContent is what the agent reads; the text block keeps the
+          // same payload visible to clients that only render content.
+          return {
+            content: [{ type: 'text', text: JSON.stringify(outcome.result, null, 2) }],
+            structuredContent: outcome.result,
+          };
+        } catch (e: any) {
+          // Prices come from Postgres and Horizon, so a throw here is the data
+          // layer being unreachable — never an answer about the pair.
+          return this.createErrorResponse('ERR_SERVICE_UNREACHABLE', e.message || 'Price lookup failed.');
+        }
+      }
+
       if (request.params.name === 'bazaar_search') {
         const url = String(request.params.arguments?.url);
         const query = request.params.arguments?.query ? String(request.params.arguments?.query) : undefined;
diff --git a/tests/mcp-get-price-aggregation.test.ts b/tests/mcp-get-price-aggregation.test.ts
new file mode 100644
index 00000000..fc1b4a12
--- /dev/null
+++ b/tests/mcp-get-price-aggregation.test.ts
@@ -0,0 +1,127 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js'
+import { BazaarMcpServer } from '../src/mcp/server'
+
+/**
+ * `lens_get_price` has to be the same number REST and GraphQL serve, computed in
+ * one place. These tests stub the two aggregator modules and check the MCP tool
+ * delegates to them with the pair and network it resolved — so the tool cannot
+ * quietly grow its own price maths or read the wrong chain's rows.
+ */
+
+const TESTNET_USDC = 'GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5'
+const CIRCLE_MAINNET_USDC = 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'
+
+const mocks = vi.hoisted(() => ({
+  getAggregatedPrice: vi.fn(),
+  getBestRoute: vi.fn(),
+}))
+
+vi.mock('../src/aggregator/vwap', () => ({ getAggregatedPrice: mocks.getAggregatedPrice }))
+vi.mock('../src/aggregator/bestRoute', () => ({ getBestRoute: mocks.getBestRoute }))
+
+const AGG = {
+  price: 0.18, sdexPrice: 0.18, ammPrice: 0.18,
+  volume24h: 10, sdexVolume24h: 10, ammVolume24h: 0,
+  vwap1m: 0.18, vwap5m: 0.18, vwap1h: 0.18, vwap24h: 0.18,
+  priceChange24h: 0, sources: 2,
+  confidence: 'high' as const, lastTradeAgeSeconds: 5, stale: false,
+}
+
+const ROUTE = {
+  route: 'SDEX' as const, sdexPrice: 0.18, ammPrice: 0,
+  estimatedOutput: 180, slippagePct: 0, recommendation: 'Only SDEX liquidity available',
+}
+
+async function loadPriceQueries() {
+  vi.resetModules()
+  const { createLensPriceQueries } = await import('../src/mcp/priceQueries')
+  return createLensPriceQueries()
+}
+
+beforeEach(() => {
+  process.env.STELLAR_NETWORK = 'testnet'
+  process.env.WATCHED_PAIRS_TESTNET = `USDC:${TESTNET_USDC}/XLM`
+  mocks.getAggregatedPrice.mockReset().mockResolvedValue(AGG)
+  mocks.getBestRoute.mockReset().mockResolvedValue(ROUTE)
+})
+
+describe('the MCP price source is the shared aggregator', () => {
+  it('reads watched pairs from the named network, not the active one', async () => {
+    const queries = await loadPriceQueries()
+
+    expect(queries.activeNetwork).toBe('testnet')
+    expect(queries.watchedPairs('testnet').map(p => p.pairKey)).toEqual([`USDC:${TESTNET_USDC}/XLM`])
+    // Mainnet carries its own default, pinned to Circle's issuer.
+    expect(queries.watchedPairs('mainnet').map(p => p.pairKey)).toEqual([`USDC:${CIRCLE_MAINNET_USDC}/XLM`])
+  })
+
+  it('passes the resolved pair key and network to the aggregation query', async () => {
+    const queries = await loadPriceQueries()
+
+    await queries.aggregatedPrice(`USDC:${TESTNET_USDC}/XLM`, 'mainnet')
+    expect(mocks.getAggregatedPrice).toHaveBeenCalledWith(`USDC:${TESTNET_USDC}/XLM`, 'mainnet')
+  })
+
+  it('routes with the same 1000-unit order size REST uses', async () => {
+    const queries = await loadPriceQueries()
+    const pair = queries.watchedPairs('testnet')[0]
+
+    await queries.bestRoute(pair.assetA, pair.assetB, pair.pairKey, 'testnet')
+    expect(mocks.getBestRoute).toHaveBeenCalledWith(pair.assetA, pair.assetB, pair.pairKey, 1000, 'testnet')
+  })
+
+  it('serves the aggregated number through the tool end to end', async () => {
+    const queries = await loadPriceQueries()
+    const server = new BazaarMcpServer({ fetchWithPayment: vi.fn(), priceQueries: queries })
+
+    let handler: any = null
+    ;(server as any).server.setRequestHandler = (schema: any, fn: any) => {
+      if (schema === CallToolRequestSchema) handler = fn
+    }
+    ;(server as any).setupHandlers()
+
+    const response = await handler({
+      params: { name: 'lens_get_price', arguments: { assetA: 'native', assetB: `USDC:${TESTNET_USDC}` } },
+    })
+
+    expect(response.isError).toBeFalsy()
+    expect(response.structuredContent).toMatchObject({
+      network: 'testnet',
+      pairKey: `USDC:${TESTNET_USDC}/XLM`,
+      price: 0.18,
+      confidence: 'high',
+      lastTradeAgeSeconds: 5,
+      stale: false,
+      bestRoute: ROUTE,
+    })
+    // Both aggregator legs were asked about the same pair, on the same network.
+    expect(mocks.getAggregatedPrice).toHaveBeenCalledWith(`USDC:${TESTNET_USDC}/XLM`, 'testnet')
+    expect(mocks.getBestRoute).toHaveBeenCalledWith(
+      { code: 'USDC', issuer: TESTNET_USDC },
+      { code: 'XLM', issuer: null },
+      `USDC:${TESTNET_USDC}/XLM`,
+      1000,
+      'testnet',
+    )
+  })
+
+  it('refuses a mainnet USDC request on this testnet deployment without querying', async () => {
+    const queries = await loadPriceQueries()
+    const server = new BazaarMcpServer({ fetchWithPayment: vi.fn(), priceQueries: queries })
+
+    let handler: any = null
+    ;(server as any).server.setRequestHandler = (schema: any, fn: any) => {
+      if (schema === CallToolRequestSchema) handler = fn
+    }
+    ;(server as any).setupHandlers()
+
+    const response = await handler({
+      params: { name: 'lens_get_price', arguments: { assetA: 'native', assetB: `USDC:${CIRCLE_MAINNET_USDC}` } },
+    })
+
+    expect(response.isError).toBe(true)
+    expect(JSON.parse(response.content[0].text).code).toBe('ERR_ASSET_ISSUER_MISMATCH')
+    expect(mocks.getAggregatedPrice).not.toHaveBeenCalled()
+  })
+})
diff --git a/tests/mcp-get-price.test.ts b/tests/mcp-get-price.test.ts
new file mode 100644
index 00000000..9c013cfa
--- /dev/null
+++ b/tests/mcp-get-price.test.ts
@@ -0,0 +1,440 @@
+import { describe, it, expect, vi } from 'vitest'
+import { CallToolRequestSchema, ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js'
+import { Client } from '@modelcontextprotocol/sdk/client/index.js'
+import { InMemoryTransport } from '@modelcontextprotocol/sdk/inMemory.js'
+import { BazaarMcpServer } from '../src/mcp/server'
+import type { AggregatedPriceFields, LensPriceQueries } from '../src/mcp/priceTool'
+import type { NetworkName } from '../src/config'
+import type { RouteInfo, WatchedPair } from '../src/types'
+
+/**
+ * #408 — an agent connected over MCP could reach other people's Bazaar nodes but
+ * not Lens's own prices. These tests pin the other half of that: the tool answers
+ * about the asset that was actually asked about (issuer included), refuses a
+ * same-named asset instead of pricing it, defaults to the server's network but
+ * takes another one when named, and reports the age of what it returned.
+ */
+
+const TESTNET_USDC = 'GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5'
+
+/** Every tool result here is one JSON text block; the SDK types content loosely. */
+const jsonOf = (result: unknown) =>
+  JSON.parse((result as { content: Array<{ text: string }> }).content[0].text)
+const MAINNET_USDC = 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'
+
+const testnetPair = (): WatchedPair => ({
+  assetA: { code: 'USDC', issuer: TESTNET_USDC },
+  assetB: { code: 'XLM', issuer: null },
+  pairKey: `USDC:${TESTNET_USDC}/XLM`,
+})
+
+const mainnetPair = (): WatchedPair => ({
+  assetA: { code: 'USDC', issuer: MAINNET_USDC },
+  assetB: { code: 'XLM', issuer: null },
+  pairKey: `USDC:${MAINNET_USDC}/XLM`,
+})
+
+const aggregated = (over: Partial<AggregatedPriceFields> = {}): AggregatedPriceFields => ({
+  price: 0.185,
+  sdexPrice: 0.184,
+  ammPrice: 0.186,
+  volume24h: 1200,
+  sdexVolume24h: 900,
+  ammVolume24h: 300,
+  vwap1m: 0.1851,
+  vwap5m: 0.1852,
+  vwap1h: 0.185,
+  vwap24h: 0.184,
+  priceChange24h: -1.2,
+  sources: 3,
+  confidence: 'high',
+  lastTradeAgeSeconds: 12,
+  stale: false,
+  ...over,
+})
+
+const route = (): RouteInfo => ({
+  route: 'SDEX',
+  sdexPrice: 0.184,
+  ammPrice: 0.186,
+  estimatedOutput: 184,
+  slippagePct: 0,
+  recommendation: 'AMM offers 1.09% better rate',
+})
+
+interface HarnessOptions {
+  pairs?: Partial<Record<NetworkName, readonly WatchedPair[]>>
+  activeNetwork?: NetworkName
+  agg?: AggregatedPriceFields | Error
+  route?: RouteInfo | Error
+}
+
+function buildHarness(options: HarnessOptions = {}) {
+  const pairs: Record<NetworkName, readonly WatchedPair[]> = {
+    testnet: [testnetPair()],
+    mainnet: [mainnetPair()],
+    ...options.pairs,
+  }
+
+  const settle = <T,>(value: T | Error | undefined, fallback: T): Promise<T> =>
+    value instanceof Error ? Promise.reject(value) : Promise.resolve(value ?? fallback)
+
+  const watchedPairs = vi.fn((network: NetworkName) => pairs[network] ?? [])
+  const aggregatedPrice = vi.fn(() => settle(options.agg, aggregated()))
+  const bestRoute = vi.fn(() => settle(options.route, route()))
+
+  const priceQueries: LensPriceQueries = {
+    activeNetwork: options.activeNetwork ?? 'testnet',
+    watchedPairs,
+    aggregatedPrice,
+    bestRoute,
+  }
+
+  const server = new BazaarMcpServer({
+    fetchWithPayment: vi.fn(),
+    priceQueries,
+  })
+
+  const handlers: Record<string, any> = {}
+  ;(server as any).server.setRequestHandler = (schema: any, fn: any) => {
+    if (schema === CallToolRequestSchema) handlers.call = fn
+    if (schema === ListToolsRequestSchema) handlers.list = fn
+  }
+  ;(server as any).setupHandlers()
+
+  const call = async (arguments_: unknown) => {
+    const response = await handlers.call({ params: { name: 'lens_get_price', arguments: arguments_ } })
+    return {
+      response,
+      body: JSON.parse(response.content[0].text),
+      structured: response.structuredContent,
+    }
+  }
+
+  return {
+    call,
+    listTools: () => handlers.list(),
+    watchedPairs,
+    aggregatedPrice,
+    bestRoute,
+    priceQueries,
+  }
+}
+
+/** The same tool over a server that was never given a price source. */
+function buildServerWithoutPrices() {
+  const server = new BazaarMcpServer({ fetchWithPayment: vi.fn() })
+  const handlers: Record<string, any> = {}
+  ;(server as any).server.setRequestHandler = (schema: any, fn: any) => {
+    if (schema === CallToolRequestSchema) handlers.call = fn
+    if (schema === ListToolsRequestSchema) handlers.list = fn
+  }
+  ;(server as any).setupHandlers()
+  return {
+    listTools: () => handlers.list(),
+    call: async (arguments_: unknown) =>
+      handlers.call({ params: { name: 'lens_get_price', arguments: arguments_ } }),
+  }
+}
+
+describe('lens_get_price', () => {
+  it('returns the aggregated price of a known pair as structured content', async () => {
+    const h = buildHarness()
+    const { response, structured } = await h.call({
+      assetA: 'XLM',
+      assetB: `USDC:${MAINNET_USDC}`,
+      network: 'mainnet',
+    })
+
+    expect(response.isError).toBeFalsy()
+    expect(structured.price).toBe(0.185)
+    expect(structured.network).toBe('mainnet')
+    expect(structured.pairKey).toBe(`USDC:${MAINNET_USDC}/XLM`)
+    expect(structured.sources).toBe(3)
+    expect(structured.confidence).toBe('high')
+    expect(structured.bestRoute.route).toBe('SDEX')
+    // The pair is watched USDC/XLM and asked XLM/USDC: the answer says which way
+    // round it is stated, so the number cannot be read as its own reciprocal.
+    expect(structured.direction).toBe('reversed')
+    expect(structured.orientation).toBe('quote-per-base')
+    expect(structured.base).toEqual({ code: 'USDC', issuer: MAINNET_USDC })
+    expect(structured.quote).toEqual({ code: 'XLM', issuer: null })
+  })
+
+  it('queries the resolved pair, not the string the caller typed', async () => {
+    const h = buildHarness()
+    await h.call({ assetA: 'native', assetB: 'USDC' })
+
+    expect(h.watchedPairs).toHaveBeenCalledWith('testnet')
+    expect(h.aggregatedPrice).toHaveBeenCalledWith(`USDC:${TESTNET_USDC}/XLM`, 'testnet')
+    expect(h.bestRoute).toHaveBeenCalledWith(
+      { code: 'USDC', issuer: TESTNET_USDC },
+      { code: 'XLM', issuer: null },
+      `USDC:${TESTNET_USDC}/XLM`,
+      'testnet',
+    )
+  })
+
+  it('names the issuer it priced when the caller passes a bare code', async () => {
+    const { structured } = await buildHarness().call({ assetA: 'native', assetB: 'USDC' })
+
+    // Bare codes still resolve — XLM has no issuer and single-issuer assets are
+    // quoted by code — but the answer must say which USDC it is about.
+    expect(structured.base).toEqual({ code: 'USDC', issuer: TESTNET_USDC })
+  })
+
+  it('does not answer a mainnet USDC request from the testnet pair', async () => {
+    const h = buildHarness()
+    const { response, body } = await h.call({
+      assetA: 'native',
+      assetB: `USDC:${MAINNET_USDC}`,
+      network: 'testnet',
+    })
+
+    expect(response.isError).toBe(true)
+    expect(body.code).toBe('ERR_ASSET_ISSUER_MISMATCH')
+    // The refusal is actionable: it names the issuer this network does price,
+    // so the agent can correct the request instead of guessing.
+    expect(body.reason).toContain(TESTNET_USDC)
+    expect(h.aggregatedPrice).not.toHaveBeenCalled()
+  })
+
+  it('prices the same code correctly when the network that watches it is named', async () => {
+    const h = buildHarness()
+    const { response, structured } = await h.call({
+      assetA: 'native',
+      assetB: `USDC:${MAINNET_USDC}`,
+      network: 'mainnet',
+    })
+
+    expect(response.isError).toBeFalsy()
+    expect(structured.pairKey).toBe(`USDC:${MAINNET_USDC}/XLM`)
+  })
+
+  it('refuses a pair nothing watches, and says what is watched instead', async () => {
+    const h = buildHarness()
+    const { response, body } = await h.call({ assetA: 'XLM', assetB: 'ABCD' })
+
+    expect(response.isError).toBe(true)
+    expect(body.code).toBe('ERR_PAIR_NOT_WATCHED')
+    expect(body.reason).toContain(`USDC:${TESTNET_USDC}/XLM`)
+    expect(h.aggregatedPrice).not.toHaveBeenCalled()
+  })
+
+  it('defaults to the network the server runs and takes another when named', async () => {
+    const h = buildHarness({ activeNetwork: 'mainnet' })
+
+    const defaulted = await h.call({ assetA: 'native', assetB: 'USDC' })
+    expect(defaulted.structured.network).toBe('mainnet')
+
+    const named = await h.call({ assetA: 'native', assetB: 'USDC', network: 'testnet' })
+    expect(named.structured.network).toBe('testnet')
+    expect(named.structured.pairKey).toBe(`USDC:${TESTNET_USDC}/XLM`)
+  })
+
+  it('refuses a network that does not exist before reading anything', async () => {
+    const h = buildHarness()
+    for (const network of ['public', 'pubnet', 'Mainnet', '', 3]) {
+      const { response, body } = await h.call({ assetA: 'native', assetB: 'USDC', network })
+
+      expect(response.isError).toBe(true)
+      expect(body.code).toBe('ERR_BAD_REQUEST')
+      expect(body.reason).toMatch(/testnet or mainnet/)
+    }
+    expect(h.watchedPairs).not.toHaveBeenCalled()
+  })
+
+  it('reports staleness rather than presenting an old price as current', async () => {
+    const h = buildHarness({
+      agg: aggregated({ lastTradeAgeSeconds: 4200, stale: true, confidence: 'low', price: 0.17 }),
+    })
+    const { response, structured } = await h.call({ assetA: 'native', assetB: 'USDC' })
+
+    expect(response.isError).toBeFalsy()
+    expect(structured.lastTradeAgeSeconds).toBe(4200)
+    expect(structured.stale).toBe(true)
+    expect(structured.confidence).toBe('low')
+  })
+
+  it('refuses to quote a pair with no ingested trade, or a price of zero', async () => {
+    for (const agg of [
+      aggregated({ lastTradeAgeSeconds: null, confidence: 'unknown', sources: 0 }),
+      aggregated({ price: 0 }),
+    ]) {
+      const h = buildHarness({ agg })
+      const { response, body } = await h.call({ assetA: 'native', assetB: 'USDC' })
+
+      expect(response.isError).toBe(true)
+      expect(body.code).toBe('ERR_NO_RESULTS')
+      // A 0 is not a price; it is the absence of one, and an agent that trades on
+      // it loses money.
+      expect(body.reason).toMatch(/no aggregated price/)
+    }
+  })
+
+  it('still returns the price when only the route leg fails', async () => {
+    const h = buildHarness({ route: new Error('No pricing data available') })
+    const { response, structured } = await h.call({ assetA: 'native', assetB: 'USDC' })
+
+    expect(response.isError).toBeFalsy()
+    expect(structured.price).toBe(0.185)
+    expect(structured.bestRoute).toBeNull()
+  })
+
+  it('reports an unreachable data source as a service failure, not a price', async () => {
+    const h = buildHarness({ agg: new Error('connection refused') })
+    const { response, body } = await h.call({ assetA: 'native', assetB: 'USDC' })
+
+    expect(response.isError).toBe(true)
+    expect(body.code).toBe('ERR_SERVICE_UNREACHABLE')
+  })
+
+  describe('rejects an asset it cannot identify', () => {
+    const badAssets = [
+      ['usdc', /case-sensitive/],
+      [`USDC:${TESTNET_USDC.slice(0, 20)}`, /issuer that is not a Stellar account id/],
+      ['USDC:', /issuer that is not a Stellar account id/],
+      ['', /non-empty string/],
+      ['TOO.LONG-CODE', /not an asset reference/],
+      ['1USDC', /not an asset reference/],
+      [42, /non-empty string/],
+    ] as const
+
+    for (const [assetA, pattern] of badAssets) {
+      it(`refuses ${JSON.stringify(assetA)} without asking the database`, async () => {
+        const h = buildHarness()
+        const { response, body } = await h.call({ assetA, assetB: 'XLM' })
+
+        expect(response.isError).toBe(true)
+        expect(body.code).toBe('ERR_INVALID_ASSET')
+        expect(body.reason).toMatch(pattern)
+        expect(h.watchedPairs).not.toHaveBeenCalled()
+      })
+    }
+
+    it('requires both sides of the pair', async () => {
+      const h = buildHarness()
+      const { body } = await h.call({ assetA: 'native' })
+
+      expect(body.code).toBe('ERR_INVALID_ASSET')
+      expect(body.reason).toContain('assetB')
+    })
+
+    it('prices a native request written either way', async () => {
+      for (const native of ['native', 'Native', 'XLM']) {
+        const { structured } = await buildHarness().call({ assetA: native, assetB: 'USDC' })
+        expect(structured.quote).toEqual({ code: 'XLM', issuer: null })
+      }
+    })
+  })
+
+  describe('argument strictness', () => {
+    it('refuses an argument the tool does not accept', async () => {
+      const h = buildHarness()
+      const { response, body } = await h.call({
+        assetA: 'native',
+        assetB: 'USDC',
+        issuer: MAINNET_USDC,
+        network: 'mainnet',
+      })
+
+      expect(response.isError).toBe(true)
+      expect(body.code).toBe('ERR_BAD_REQUEST')
+      expect(body.reason).toContain('issuer')
+      expect(h.aggregatedPrice).not.toHaveBeenCalled()
+    })
+
+    it('refuses arguments that are not an object', async () => {
+      for (const args of [undefined, null, 'native/USDC', ['native', 'USDC']]) {
+        const { body } = await buildHarness().call(args)
+        expect(body.code).toBe('ERR_BAD_REQUEST')
+      }
+    })
+  })
+
+  describe('tool surface', () => {
+    it('accepts exactly the pair and the network', async () => {
+      const tools = (await buildHarness().listTools()).tools
+      const price = tools.find((t: any) => t.name === 'lens_get_price')
+
+      expect(Object.keys(price.inputSchema.properties).sort()).toEqual(['assetA', 'assetB', 'network'])
+      expect(price.inputSchema.required).toEqual(['assetA', 'assetB'])
+      expect(price.inputSchema.additionalProperties).toBe(false)
+      expect(price.inputSchema.properties.network.enum).toEqual(['testnet', 'mainnet'])
+      expect(price.outputSchema.required).toEqual(expect.arrayContaining(['price', 'confidence', 'stale']))
+      expect(price.description).toMatch(/issuer/i)
+      expect(price.description).toMatch(/stale/i)
+    })
+
+    it('is not advertised, and says so if called anyway, when no price source is configured', async () => {
+      const server = buildServerWithoutPrices()
+
+      const tools = (await server.listTools()).tools
+      expect(tools.map((t: any) => t.name)).not.toContain('lens_get_price')
+
+      const response = await server.call({ assetA: 'native', assetB: 'USDC' })
+      expect(response.isError).toBe(true)
+      expect(JSON.parse(response.content[0].text).code).toBe('ERR_INTERNAL_FAILURE')
+    })
+  })
+})
+
+/**
+ * The handler-level tests above drive the server the way its own code does. This
+ * one drives it the way an agent does — a real MCP client over a linked
+ * transport — because the point of #408 is that a connected agent could not get
+ * a price at all. It catches a declaration the SDK will not accept, or a result
+ * shape a client will not parse, which neither of the other layers can see.
+ */
+describe('lens_get_price through an MCP client', () => {
+  async function connect(queries: LensPriceQueries) {
+    const server = new BazaarMcpServer({ fetchWithPayment: vi.fn(), priceQueries: queries })
+    const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair()
+    await (server as any).server.connect(serverTransport)
+
+    const client = new Client({ name: 'test-agent', version: '1.0.0' }, { capabilities: {} })
+    await client.connect(clientTransport)
+    return client
+  }
+
+  it('is advertised, called, and answered with structured content', async () => {
+    const { priceQueries } = buildHarness()
+    const client = await connect(priceQueries)
+
+    const tools = await client.listTools()
+    const declared = tools.tools.find(t => t.name === 'lens_get_price')
+    expect(declared).toBeDefined()
+    expect(declared!.outputSchema).toBeDefined()
+
+    const result = await client.callTool({
+      name: 'lens_get_price',
+      arguments: { assetA: 'native', assetB: `USDC:${TESTNET_USDC}` },
+    })
+
+    expect(result.isError).toBeFalsy()
+    expect((result.structuredContent as any).price).toBe(0.185)
+    // A client that only renders text still reads the same answer.
+    expect(jsonOf(result).pairKey).toBe(`USDC:${TESTNET_USDC}/XLM`)
+
+    const refused = await client.callTool({
+      name: 'lens_get_price',
+      arguments: { assetA: 'native', assetB: `USDC:${MAINNET_USDC}` },
+    })
+    expect(refused.isError).toBe(true)
+    expect(jsonOf(refused).code).toBe('ERR_ASSET_ISSUER_MISMATCH')
+  })
+
+  it('refuses an argument the schema does not declare', async () => {
+    const { priceQueries, aggregatedPrice } = buildHarness()
+    const client = await connect(priceQueries)
+
+    const result = await client.callTool({
+      name: 'lens_get_price',
+      arguments: { assetA: 'native', assetB: 'USDC', pairKey: 'XLM/USDC' },
+    })
+
+    expect(result.isError).toBe(true)
+    expect(jsonOf(result).code).toBe('ERR_BAD_REQUEST')
+    expect(aggregatedPrice).not.toHaveBeenCalled()
+  })
+})

From 2f64a080316d0cb336ce6ab476ff8078ea86f4d4 Mon Sep 17 00:00:00 2001
From: Stehen Jeremiah Haruna <141843972+Ugasutun@users.noreply.github.com>
Date: Mon, 19 Oct 2026 09:49:05 +0100
Subject: [PATCH 4/4] fix(mcp): repair the main merge that broke the server
 class head
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The merge of main into this branch left two conflict-resolution artefacts
behind: the opening `/**` of the bazaarFetch doc comment was eaten and my
old field/constructor fragment was kept alongside main's
`constructor(config_)`, so the class had two constructors and tsc stopped
at the first line of the body. codes.ts kept ERR_HOST_NOT_ALLOWED twice —
once from each side — which is TS1117.

Both are restored to main's shape, with the lens_get_price additions
reapplied on top.
---
 src/mcp/codes.ts  |  1 -
 src/mcp/server.ts | 10 +++-------
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/src/mcp/codes.ts b/src/mcp/codes.ts
index 29d74d47..cacfe590 100644
--- a/src/mcp/codes.ts
+++ b/src/mcp/codes.ts
@@ -6,7 +6,6 @@ export const MCP_ERROR_CODES = {
   ERR_SERVICE_UNREACHABLE: 'ERR_SERVICE_UNREACHABLE',
   ERR_NO_RESULTS: 'ERR_NO_RESULTS',
   ERR_BAD_REQUEST: 'ERR_BAD_REQUEST',
-  ERR_HOST_NOT_ALLOWED: 'ERR_HOST_NOT_ALLOWED',
   ERR_INVALID_ASSET: 'ERR_INVALID_ASSET',
   ERR_PAIR_NOT_WATCHED: 'ERR_PAIR_NOT_WATCHED',
   ERR_ASSET_ISSUER_MISMATCH: 'ERR_ASSET_ISSUER_MISMATCH',
diff --git a/src/mcp/server.ts b/src/mcp/server.ts
index 7adf273b..6f311f0c 100644
--- a/src/mcp/server.ts
+++ b/src/mcp/server.ts
@@ -22,6 +22,7 @@ export interface McpServerConfig {
    * not advertise the tool, so an agent is never offered a price it cannot get.
    */
   priceQueries?: LensPriceQueries;
+  /**
    * Outbound fetch policy for `bazaar_search`. Falls back to the operator's
    * shared `OUTBOUND_ALLOWED_HOSTS` allowlist. `lookup`/`fetchImpl` exist so
    * tests can drive DNS resolution and the transport without real sockets.
@@ -33,17 +34,12 @@ export class BazaarMcpServer {
   private server: Server;
   private fetchWithPayment: typeof fetch;
   private paidCallRules: PaidCallAllowRule[];
-  private priceQueries?: LensPriceQueries;
-
-  constructor(config: McpServerConfig) {
-    this.fetchWithPayment = config.fetchWithPayment;
-    this.priceQueries = config.priceQueries;
-
-    const allowlist = parsePaidCallAllowlist(config.paidCallAllowlist ?? []);
   private bazaarFetchOptions: SsrfFetchOptions;
+  private priceQueries?: LensPriceQueries;
 
   constructor(config_: McpServerConfig) {
     this.fetchWithPayment = config_.fetchWithPayment;
+    this.priceQueries = config_.priceQueries;
 
     // Two independent gates, deliberately separate: this one decides which
     // hosts may be PAID (#404), the one below decides which addresses may be