vercel/ai · error · MCPClientError

Server does not support the requested protocol version: ${th

Error message

Server does not support the requested protocol version: ${this.protocolVersion}

What it means

Thrown by applyDiscoverResult during modern protocol discovery when the server's server/discover result does not list the client's requested protocolVersion in supportedVersions. The client refuses to proceed because the server explicitly does not support the negotiated version.

Source

Thrown at packages/mcp/src/tool/mcp-client.ts:643

      this.applyDiscoverResult(result);
      return true;
    } catch (error) {
      if (
        MCPClientError.isInstance(error) &&
        error.code != null &&
        MODERN_PROTOCOL_ERROR_CODES.includes(error.code)
      ) {
        throw error;
      }

      return false;
    }
  }

  private applyDiscoverResult(result: DiscoverResult): void {
    if (!result.supportedVersions.includes(this.protocolVersion)) {
      throw new MCPClientError({
        message: `Server does not support the requested protocol version: ${this.protocolVersion}`,
      });
    }

    const serverInfo = result._meta?.['io.modelcontextprotocol/serverInfo'];
    if (
      serverInfo != null &&
      typeof serverInfo === 'object' &&
      'name' in serverInfo &&
      typeof serverInfo.name === 'string' &&
      'version' in serverInfo &&
      typeof serverInfo.version === 'string'
    ) {
      this._serverInfo = serverInfo as Configuration;
    }

    this.serverCapabilities = result.capabilities;
    this._serverInstructions = result.instructions;

View on GitHub (pinned to 69428b1f8b)

Solutions

Upgrade @ai-sdk/mcp so the client requests a protocol version the server supports.
Upgrade the MCP server so it supports the client's requested protocol version.
Check the server's serverInfo/_meta in the discover result to see which versions are supported and align deployments.
Disable protocol version discovery if the server only supports the legacy handshake.

Example fix

// before (server discover result)
{ supportedVersions: ['2025-03-26'], _meta: {...} }
// after (server supports client's requested version)
{ supportedVersions: ['2025-06-18', '2025-03-26'], _meta: {...} }

Defensive patterns

Strategy: try-catch

Validate before calling

// probe discovery manually
const res = await fetch(mcpUrl, { method: 'POST', body: JSON.stringify({ jsonrpc: '2.0', id: 1, method: 'server/discover' }) });
const { result } = await res.json();
if (!result?.supportedVersions?.includes('2025-06-18')) {
  console.warn('Server does not support client protocol version; upgrade server or SDK');
}

Type guard

function supportsVersion(r: unknown, v: string): boolean {
  return !!r && typeof r === 'object' && Array.isArray((r as any).supportedVersions) && (r as any).supportedVersions.includes(v);
}

Try / catch

try {
  const client = await createMCPClient({ transport });
} catch (e) {
  if (MCPClientError.isInstance(e) && e.message.startsWith('Server does not support the requested protocol version')) {
    // align server/SDK versions or fall back to legacy handshake
  } else throw e;
}

Prevention

Keep @ai-sdk/mcp and the MCP server on versions supporting the same protocol versions
Check server supportedVersions after each MCP spec release
Pin server versions in deployment to avoid silent drift

When it happens

Trigger: Client attempts protocol version discovery ('server/discover'); the server responds successfully but its supportedVersions array omits the version the client sent (LATEST_PROTOCOL_VERSION).

Common situations: Connecting to a newer/older MCP server that supports different protocol versions; mixed client/server SDK versions after an MCP spec release; proxies that advertise discovery but with restricted versions.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/c5210b2ef78acd78. Report an issue: GitHub.