diff --git a/CHANGELOG.md b/CHANGELOG.md
index fdc2848ce..b2db4f284 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,6 +48,12 @@ refused in every mode, including `allow_all`, and the browser's WebRTC traffic n
 filter instead of around it. A computer run without an API server can set
 `EGRESS_POLICY_REQUIRED=0` to keep the old behaviour.
 
+### Provider and Bot lookups ignore inherited object properties
+
+Unknown names such as `constructor` and `__proto__` no longer return an inherited
+JavaScript object as a provider or Bot entry. Unknown providers return no spec, and
+missing Bots raise the existing startup error. Configured providers and Bots are unchanged.
+
 ### A malformed `%` in a stream URL no longer returns a 500
 
 A request to `/api/computers/<id>/stream` whose id held a broken percent-escape, such as `%zz`, made the server throw and answer 500. It is now treated as not matching the stream route and goes through normal routing. Valid ids behave as before.
diff --git a/shared/model-providers.test.ts b/shared/model-providers.test.ts
index 6f42fde3e..ac71102fe 100644
--- a/shared/model-providers.test.ts
+++ b/shared/model-providers.test.ts
@@ -319,3 +319,18 @@ describe("what a Bot runs from the spec file", () => {
     );
   });
 });
+
+describe("registry lookups ignore inherited object properties", () => {
+  test("prototype names are not providers", () => {
+    for (const name of ["constructor", "__proto__", "toString", "valueOf"]) {
+      expect(providerSpec(name)).toBeUndefined();
+      expect(keyVariableFor(name)).toBeUndefined();
+      expect(baseUrlVariableFor(name)).toBeUndefined();
+    }
+  });
+  test("prototype names are not Bot entries", () => {
+    for (const name of ["constructor", "__proto__", "toString", "valueOf"]) {
+      expect(() => botSettings(name, {})).toThrow(`bots has no ${name} entry`);
+    }
+  });
+});
diff --git a/shared/model-providers.ts b/shared/model-providers.ts
index ab1ca34dd..ac7a7aa70 100644
--- a/shared/model-providers.ts
+++ b/shared/model-providers.ts
@@ -166,7 +166,9 @@ export function providerSpec(
   provider: string | undefined,
 ): ProviderSpec | undefined {
   const normalized = provider?.trim().toLowerCase() || "openai";
-  return MODEL_PROVIDERS[normalized as ModelProviderId];
+  return Object.hasOwn(MODEL_PROVIDERS, normalized)
+    ? MODEL_PROVIDERS[normalized as ModelProviderId]
+    : undefined;
 }
 
 /** The environment variable this provider's key arrives in, or nothing for a provider unknown. */
@@ -248,7 +250,9 @@ export function botSettings(
   env: Readonly<Record<string, string | undefined>> = process.env,
   pinnedProvider?: string,
 ): BotSettings {
-  const entry = BOT_ENTRIES[botId];
+  const entry = Object.hasOwn(BOT_ENTRIES, botId)
+    ? BOT_ENTRIES[botId]
+    : undefined;
   if (!entry) {
     fail(
       `bots has no ${botId} entry. Add one before this Bot reads the spec file.`,