vercel/ai · error · MCPClientError

Unsupported method

Error message

Unsupported method: ${method}

What it means

MCPClientError thrown by the default branch of assertCapability when a request method is passed that the client does not recognize as one of the supported MCP methods (tools/*, resources/*, prompts/*). It is a client-side guard indicating a programming error or an unsupported/newer MCP method name rather than a server problem.

Solutions

1. Use the documented public APIs (listTools, callTool, listResources, readResource, listPrompts, etc.) instead of the generic request path
2. Fix the method name spelling/namespace (e.g. 'tools/list' not 'tool/list')
3. Check the SDK version supports the method; upgrade packages/mcp if a newer MCP spec method is needed
4. Add a switch/mapping in your code that only dispatches known-supported methods

Example fix

// before
class, do not hand-roll: await (client as any).request({ method: 'tool/list', params: {} });

// after
const tools = await client.listTools();

const SUPPORTED_METHODS = ['tools/list','tools/call','resources/list','resources/read','resources/templates/list','prompts/list','prompts/get'];
if (!SUPPORTED_METHODS.includes(method)) {
  throw new Error(`Method not supported by this SDK: ${method}`);
}

function isSupportedMethod(m: string): m is 'tools/list' | 'tools/call' | 'resources/list' | 'resources/read' | 'resources/templates/list' | 'prompts/list' | 'prompts/get' {
  return ['tools/list','tools/call','resources/list','resources/read','resources/templates/list','prompts/list','prompts/get'].includes(m);
}

try {
  await client.request({ method, params });
} catch (error) {
  if (MCPClientError.isInstance(error) && error.message.startsWith('Unsupported method:')) {
    // fix method name or use public API wrappers
  } else {
    throw error;
  }
}

When it happens

Trigger: Calling client.request() (or an internal helper) with a method string outside the supported set, e.g. a typo like 'tool/list', a new spec method the SDK has not implemented (sampling/logging), or an empty method value.

Common situations: Hand-rolling an MCP method call through the client's generic request path; upgrading the MCP spec and using a newly introduced method against an older SDK; typos in method names when writing custom tooling around the client.

Related errors

• Server does not support prompts
• Server does not support resources
• Server does not support tools
• ACP MCP server must be configured with an object value.
• ACP MCP server name "ai-sdk-harness-tools" is reserved for…

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/dc338a87f47d5486. Report an issue: GitHub.

Appendix: source

Thrown at packages/mcp/src/tool/mcp-client.ts:747

      case 'resources/list':
      case 'resources/read':
      case 'resources/templates/list':
        if (!this.serverCapabilities.resources) {
          throw new MCPClientError({
            message: `Server does not support resources`,
          });
        }
        break;
      case 'prompts/list':
      case 'prompts/get':
        if (!this.serverCapabilities.prompts) {
          throw new MCPClientError({
            message: `Server does not support prompts`,
          });
        }
        break;
      default:
        throw new MCPClientError({
          message: `Unsupported method: ${method}`,
        });
    }
  }

  private async request<T extends z.ZodType<object>>({
    request,
    resultSchema,
    options,
  }: {
    request: Request;
    resultSchema: T;
    options?: RequestOptions;
  }): Promise<z.infer<T>> {
    return new Promise((resolve, reject) => {
      if (this.isClosed) {
        return reject(
          new MCPClientError({

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