mastra-ai/mastra · error · Error

MCPClientServerProxy does not support Hono SSE transport

Error message

MCPClientServerProxy does not support Hono SSE transport

What it means

startHonoSSE() serves an MCP server over SSE inside a Hono app; MCPClientServerProxy is a client-side wrapper and cannot host any HTTP endpoint, so it throws unconditionally. It exists only to satisfy the MCPServer interface shape.

Source

Thrown at packages/mcp/src/client/server-proxy.ts:200

    const result = await client.resources.read(uri);
    return {
      contents: (result.contents ?? []).map((c: any) => ({
        uri: c.uri ?? uri,
        ...(c.text !== undefined ? { text: c.text } : {}),
        ...(c.blob !== undefined ? { blob: c.blob } : {}),
      })),
    };
  }

  // Transport methods — not applicable for client proxies
  public async startStdio(): Promise<void> {
    throw new Error('MCPClientServerProxy does not support stdio transport');
  }
  public async startSSE(_options: MCPServerSSEOptions): Promise<void> {
    throw new Error('MCPClientServerProxy does not support SSE transport');
  }
  public async startHonoSSE(_options: MCPServerHonoSSEOptions): Promise<Response | undefined> {
    throw new Error('MCPClientServerProxy does not support Hono SSE transport');
  }
  public async startHTTP(_options: MCPServerHTTPOptions): Promise<void> {
    throw new Error('MCPClientServerProxy does not support HTTP transport');
  }
  public async close(): Promise<void> {
    this.cachedClient = null;
  }

  public getServerInfo(): ServerInfo {
    return {
      id: this.id,
      name: this.name,
      description: this.description,
      version_detail: {
        version: this.version,
        release_date: this.releaseDate,
        is_latest: this.isLatest,
      },

View on GitHub (pinned to 75dd419e61)

Solutions

Call startHonoSSE() only on MCPServer instances
Skip client proxies via instanceof check before mounting routes
Remove the mount call — remote servers are already reachable via the proxy client

Example fix

// before
app.route('/mcp', createRoute(proxy.startHonoSSE.bind(proxy)));
// after
if (!(proxy instanceof MCPClientServerProxy)) proxy.startHonoSSE({ path: '/mcp' });

Defensive patterns

Strategy: validation

Validate before calling

if (proxy instanceof MCPClientServerProxy) throw new Error('startHonoSSE is only valid on MCPServer instances');

Type guard

function isHonoMountable(s: unknown): s is MCPServer {
  return !(s instanceof MCPClientServerProxy);
}

Try / catch

try {
  await maybeServer.startHonoSSE(opts);
} catch (e) {
  if (e instanceof Error && e.message.includes('does not support Hono SSE transport')) return undefined;
  throw e;
}

Prevention

Only mount genuine MCPServer instances as Hono routes
Skip MCPClientServerProxy entries when iterating a mixed registry
Keep server hosting and client consumption in separate code paths

When it happens

Trigger: Calling startHonoSSE(options) on an MCPClientServerProxy, typically when uniformly registering all servers (real and proxied) as Hono routes.

Common situations: Auto-mounting every entry of MCPClients.getStandardServers()/registry onto a Hono app; mixing client connections with server hosting in shared bootstrap code.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/bcefe2ce93282f75. Report an issue: GitHub.