Copilot CLI couldn't handle BigInt in structured MCP response

[xj-ms]

Describe the bug

When a MCP server returns big numbers in its response, Copilot CLI cannot handle it and fails with the following error. All ongoing tasks will be aborted

Execution failed: TypeError: Do not know how to serialize a BigInt

Affected version

1.0.74-0

Steps to reproduce the behavior

Requires dotnet 10.

1. Clone the repo https://github.com/xj-ms/StructuredContentBigIntMcp
2. Launch Copilot CLI in the repo. The MCP server should be already configured with the local .mcp.json
3. Ask Copilot to call the return_big_numbers tool
4. Observe the failure

Expected behavior

No response

Additional context

No response

[gokhanarkan]

Triage summary

Thank you for the minimal reproduction. Current code shows this is a known failure shape the codebase already has a fix pattern for: src/runtime/src/mcp/transport_content.rs's own documentation states the TypeScript caller is expected to JSON.stringify(structuredContent) itself specifically so it can catch BigInt/cycle failures and fall back to text-only -- and src/tools/utils/safeJsonStringify.ts already implements exactly that safe try/catch-with-fallback pattern, currently used for tool-input serialization.

The crash you are hitting indicates the call site that stringifies a tool's structuredContent RESULT (feeding shape_invoke_result/shape_task_executor_return) is using a raw, unguarded JSON.stringify instead of that existing safe wrapper, so a BigInt value throws synchronously instead of degrading to text-only content.

Recommended fix: route that call site through safeJsonStringify (or an equivalent try/catch fallback) so structuredContent with BigInt/cyclic values falls back to text-only instead of aborting the task, and add a regression test based on your reproduction repo.

Route: agent-ready.

This triage summary was prepared with GitHub Copilot and reviewed before posting.

[Szachta]

This bug really ruins work with MCPs, in both CLI and Github Copilot App. Looking forward to have it fixed, the sooner the better.

[pranoytez]

I can reproduce this exact issue in GitHub Copilot app v1.1.7 on Windows:

TypeError: Do not know how to serialize a BigInt

It occurs during normal use of managed MCP tools and stops the active agent workflow. The same app-side failure was also reported in github/app#2504 against v1.1.2, so it remains unresolved in a newer release.

This is severely disrupting MCP tool usage. Since the issue is already linked to GitHub’s internal tracker, please prioritize the fix and release it as soon as possible.

[EricVentor]

Agree, why not just pass the execution failed message to the LLM and let it re-asses? This ends up pausing and delaying work.

[danield137]

Still reproduces on 1.0.81-0 (Windows, Node v24.18.1), so it survived from 1.0.74-0.

Real-world trigger, no custom server needed: microsoft-fabric-rti-mcp (Python, stdio) querying Azure Data Explorer. Any Kusto long beyond 2^53-1 hits it — including datetime/timespan tick values, which are pervasive in ADX.

The threshold is exactly Number.MAX_SAFE_INTEGER:

value returned by the tool	outcome
87400004999940	works
3074457345618258432	Execution failed: TypeError: Do not know how to serialize a BigInt

One detail that may matter for the fix: the MCP server never sends a BigInt — it sends a plain JSON number. The BigInt is produced by the CLI's own lossless JSON parse in the shipped app.js, which upgrades out-of-safe-range integers using the JSON.parse reviver's context.source:

JSON.parse(t, (n, r, o) => {
  let s = typeof r == "number" && (r > Number.MAX_SAFE_INTEGER || r < Number.MIN_SAFE_INTEGER),
      a = o && /^-?\d+$/.test(o.source);
  return s && a ? BigInt(o.source) : /* ... */;
})

So the triage above matches what's observable from the outside: the parse deliberately creates BigInt, and an unguarded JSON.stringify downstream throws. Two small notes on the proposed fix:

• Falling back to text-only via try/catch would still discard the numeric fidelity the parser went out of its way to preserve. The same bundle already contains a BigInt-safe stringifier built on JSON.rawJSON; routing the result path through that would keep the exact value instead of degrading it.
• The blast radius is bigger than a failed tool call — the whole turn aborts and in-flight agent work is lost.

Minimal repro, no MCP server or dotnet needed:

const parse = (text) =>
  JSON.parse(text, (key, value, ctx) =>
    typeof value === "number" &&
    (value > Number.MAX_SAFE_INTEGER || value < Number.MIN_SAFE_INTEGER) &&
    /^-?\d+$/.test(ctx?.source)
      ? BigInt(ctx.source)
      : value);

JSON.stringify(parse('{"v":3074457345618258432}'));
// TypeError: Do not know how to serialize a BigInt

Workaround for anyone blocked on ADX/Eventhouse in the meantime: cast large longs to string in the query itself, e.g. project tostring(x).

[copilot-cli-bot]

This appears to be resolved as of GitHub Copilot CLI v1.0.86. Please update to that release or a newer stable version. We're closing this as completed. If it still happens, please comment with your version and reproduction steps.