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fix mcp: improve error code detection when listing tools fails #787

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fix mcp: improve error code detection when listing tools fails #787

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59de4ac
fix(mcp): handle wrapped JSON-RPC Method not found errors in capabili…
Jan 14, 2026
21a3ce1
Reuse existing error handling.
Jan 16, 2026
f6a503b
Create tall-mirrors-provide.md
threepointone Feb 2, 2026
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5 changes: 5 additions & 0 deletions .changeset/tall-mirrors-provide.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"agents": patch
---

fix mcp: improve error code detection when listing tools fails
15 changes: 11 additions & 4 deletions packages/agents/src/mcp/client-connection.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@ import { Emitter, type Event } from "../core/events";
import type { MCPObservabilityEvent } from "../observability/mcp";
import type { AgentsOAuthProvider } from "./do-oauth-client-provider";
import {
getErrorCode,
isTransportNotImplemented,
isUnauthorized,
toErrorMessage
Expand Down Expand Up @@ -663,17 +664,23 @@ export class MCPClientConnection {
}

private _capabilityErrorHandler<T>(empty: T, method: string) {
return (e: { code: number }) => {
// server is badly behaved and returning invalid capabilities. This commonly occurs for resource templates
if (e.code === -32601) {
return (e: unknown) => {
// Server is badly behaved and returning invalid capabilities. This commonly occurs for resource templates.
// Check both the error code property and the error message for -32601 (Method not found)
const errorCode = getErrorCode(e);
const errorMessage = toErrorMessage(e);
const isMethodNotFound =
errorCode === -32601 || errorMessage.includes('"code":-32601');
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@mattzcarey mattzcarey Feb 2, 2026

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On second thoughts this feels like it will be too broad. Do you have an example of a server which breaks this? I'd prefer to fix this on the MCP SDK side.


if (isMethodNotFound) {
const url = this.url.toString();
this._onObservabilityEvent.fire({
type: "mcp:client:discover",
displayMessage: `The server advertised support for the capability ${method.split("/")[0]}, but returned "Method not found" for '${method}' for ${url}`,
payload: {
url,
capability: method.split("/")[0],
error: toErrorMessage(e)
error: errorMessage
},
timestamp: Date.now(),
id: nanoid()
Expand Down
2 changes: 1 addition & 1 deletion packages/agents/src/mcp/errors.ts
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Expand Up @@ -2,7 +2,7 @@ export function toErrorMessage(error: unknown): string {
return error instanceof Error ? error.message : String(error);
}

function getErrorCode(error: unknown): number | undefined {
export function getErrorCode(error: unknown): number | undefined {
if (
error &&
typeof error === "object" &&
Expand Down
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