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fix: throw SdkError for unsupported capabilities in assertCapability() #1676

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fix: throw SdkError for unsupported capabilities in assertCapability() #1676

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490293c
fix: throw SdkError with CapabilityNotSupported code instead of gener…
kai-agent-free Mar 13, 2026
95e2a96
Add changeset for capability error fix
kai-agent-free Mar 13, 2026
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5 changes: 5 additions & 0 deletions .changeset/fix-capability-error.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/sdk": patch
---

fix: throw SdkError for unsupported capabilities in assertCapability()
2 changes: 1 addition & 1 deletion packages/client/src/client/client.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -434,7 +434,7 @@ export class Client extends Protocol<ClientContext> {

protected assertCapability(capability: keyof ServerCapabilities, method: string): void {
if (!this._serverCapabilities?.[capability]) {
throw new Error(`Server does not support ${capability} (required for ${method})`);
throw new SdkError(SdkErrorCode.CapabilityNotSupported, `Server does not support ${capability} (required for ${method})`);
}
}

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6 changes: 6 additions & 0 deletions test/integration/test/client/client.test.ts
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Expand Up @@ -418,6 +418,12 @@ test('should respect server capabilities', async () => {
argument: { name: 'test', value: 'test' }
})
).rejects.toThrow('Server does not support completions');

// Verify that unsupported capability errors are SdkError with CapabilityNotSupported code
await expect(client.listPrompts()).rejects.toThrow(SdkError);
await expect(client.listPrompts()).rejects.toMatchObject({
code: SdkErrorCode.CapabilityNotSupported
});
});

/***
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