vercel/ai · error · MCPClientError

Server sent invalid initialize result

Error message

Server sent invalid initialize result

What it means

Thrown during MCPClient.init when the 'initialize' request resolves with undefined — i.e. the server's response failed to validate against InitializeResultSchema or the request produced no result. This indicates a malformed or non-compliant server response to the MCP handshake.

Source

Thrown at packages/mcp/src/tool/mcp-client.ts:568

      this.protocolEra = 'legacy';
      this.protocolVersion = LATEST_LEGACY_PROTOCOL_VERSION;
      this.setTransportProtocolVersion(this.protocolVersion);

      const result = await this.request({
        request: {
          method: 'initialize',
          params: {
            protocolVersion: LATEST_LEGACY_PROTOCOL_VERSION,
            capabilities: this.clientCapabilities,
            clientInfo: this.clientInfo,
          },
        },
        resultSchema: InitializeResultSchema,
        options: { signal },
      });

      if (result === undefined) {
        throw new MCPClientError({
          message: 'Server sent invalid initialize result',
        });
      }

      this.applyInitializeResult(result);

      // Complete initialization handshake:
      await this.notification(
        {
          method: 'notifications/initialized',
        },
        { signal },
      );

      return this;
    } catch (error) {
      try {
        await waitForAbort(this.transport.close({ signal }), signal);

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Inspect the raw initialize response from the server (curl the endpoint) to see what is actually returned.
2. Update the MCP server to a spec-compliant version that returns protocolVersion, capabilities, and serverInfo.
3. Verify you are pointing at the correct MCP endpoint URL and transport, not an HTML page or wrong route.
4. Update @ai-sdk/mcp in case the schema expectations changed.

Example fix

// before (server response missing fields)
{ jsonrpc: '2.0', id: 1, result: { serverInfo: { name: 'x' } } }
// after (spec-compliant)
{ jsonrpc: '2.0', id: 1, result: { protocolVersion: '2025-06-18', capabilities: {}, serverInfo: { name: 'x', version: '1.0' } } }

const res = await fetch(mcpUrl, { method: 'POST', headers: { 'content-type': 'application/json' }, body: JSON.stringify({ jsonrpc: '2.0', id: 1, method: 'initialize', params: { protocolVersion: '2025-06-18', capabilities: {}, clientInfo: { name: 'probe', version: '0' } } }) });
const body = await res.json();
if (!body?.result?.protocolVersion || !body?.result?.serverInfo) throw new Error('Endpoint is not a compliant MCP server');

function isInitializeResult(r: unknown): r is { protocolVersion: string; capabilities: object; serverInfo: { name: string; version: string } } {
  return !!r && typeof r === 'object' && 'protocolVersion' in r && 'capabilities' in r && 'serverInfo' in r;
}

try {
  const client = await createMCPClient({ transport });
} catch (e) {
  if (MCPClientError.isInstance(e) && e.message === 'Server sent invalid initialize result') {
    // inspect raw server response / verify endpoint URL
  } else throw e;
}

When it happens

Trigger: The server replies to the initialize request with a payload missing required fields (protocolVersion, capabilities, serverInfo) or with invalid shapes, causing schema validation to fail and result to be undefined.

Common situations: Connecting to a non-MCP or partially-implemented HTTP/SSE endpoint; a proxy returning HTML error pages; protocol version drift where the server speaks an incompatible response format; server bugs after upgrades.

Related errors

• ACP session initialization did not produce a session.
• Server does not support the requested protocol version: ${th
• Server's protocol version is not supported: ${result.protoco
• CODE_MODE_PROTOCOL_ERROR
• CODE_MODE_PROTOCOL_ERROR

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/e648a81fefc68c0b. Report an issue: GitHub.