vercel/ai · error · MCPClientError

Server does not support tools

Error message

Server does not support tools

What it means

MCPClientError thrown by DefaultMCPClient.assertCapability when a request method ('tools/list' or 'tools/call') is invoked but the MCP server did not advertise the `tools` capability during the initialize handshake. The client checks `this.serverCapabilities.tools` (captured from the initialize result at mcp-client.ts:660) before sending the request. It fails fast locally instead of sending a request the server will reject.

Source

Thrown at packages/mcp/src/tool/mcp-client.ts:724

      : this.transport.send(message, options);
  }

  private assertCapability(method: string): void {
    switch (method) {
      case 'initialize':
      case 'server/discover':
        break;
      case 'completion/complete':
        if (!this.serverCapabilities.completions) {
          throw new MCPClientError({
            message: `Server does not support completions`,
          });
        }
        break;
      case 'tools/list':
      case 'tools/call':
        if (!this.serverCapabilities.tools) {
          throw new MCPClientError({
            message: `Server does not support tools`,
          });
        }
        break;
      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`,

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Verify the MCP server actually supports tools by inspecting the capabilities object in its initialize response
2. Ensure client.init() is awaited before calling tools/list or tools/call so serverCapabilities is populated
3. Switch to a server implementation that advertises tool support, or remove the tool calls if the server only offers resources/prompts
4. If behind a proxy, check that it forwards the full initialize result unchanged

Example fix

// before: assumes tools exist
const client = new DefaultMCPClient({ url: mcpUrl });
const tools = await client.tools();

// after: initialize first and handle unsupported capability
const client = new DefaultMCPClient({ url: mcpUrl });
await client.init();
try {
  const tools = await client.tools();
} catch (error) {
  if (MCPClientError.isInstance(error) && error.message.includes('does not support tools')) {
    // fall back to a non-tool integration
  }
}

await client.init();
// inspect negotiated capabilities if exposed, or track them from the initialize result
const toolsSupported = !!initResult.capabilities?.tools;
if (!toolsSupported) throw new Error('Server does not support tools');

function supportsTools(error: unknown): error is MCPClientError {
  return MCPClientError.isInstance(error) && error.message.includes('does not support tools');
}

try {
  const tools = await client.tools();
} catch (error) {
  if (MCPClientError.isInstance(error) && error.message.includes('does not support tools')) {
    // fallback: use resources/prompts or surface a capability error
  } else {
    throw error;
  }
}

When it happens

Trigger: Calling client.listTools(), client.tools(), or a tool call ('tools/call') against a server whose initialize result omits `capabilities.tools`. Also occurs when the client was never initialized properly so serverCapabilities remains the empty object {} (mcp-client.ts:424).

Common situations: Connecting to an MCP server that only exposes resources or prompts (e.g. a documentation server); pointing at an older/limited MCP server implementation that predates tool support; forgetting to call init() before using tools so capabilities were never negotiated; proxies/gateways that strip the capabilities object from the initialize response.

Related errors

• Server does not support resources
• Server does not support prompts
• The ACP implementation did not load the active harness-owned
• Unsupported method: ${method}
• Tool "${toolName}" returned structuredContent that does not

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/e3bfb3dc5f513869. Report an issue: GitHub.