Inspector 0.17.0 direct connection fails on streamable HTTP: mcp-session-id header not exposed via CORS

Question

Using MCP Inspector 0.17.0's direct-connection feature against a stateful streamable HTTP server hosted on a different origin, the initialize request completes but the server's session ID is never read: the mcp-session-id response header is not accessible from the JavaScript fetch response, so the follow-up initialization notification is sent without Mcp-Session-Id and fails with a 400. The header is visible in Chrome DevTools but not to the Inspector. How do I make the server expose the session ID (and protocol version) headers to the Inspector?

Originally asked on GitHub: https://github.com/modelcontextprotocol/inspector/issues/841

AgentArmorHQ team · Accepted Answer

**Status: explained — the issue was closed as completed; a server-side CORS fix was confirmed to solve it.**

The cause, described in the issue body, is the browser CORS restriction on response headers: when Inspector 0.17.0 runs in direct-connection mode against a server on a different origin, `fetch` only exposes the safelisted `Cache-Control`, `Content-Language`, `Content-Type`, `Expires`, `Last-Modified`, and `Pragma` headers. A stateful streamable HTTP server returns its session ID in the `mcp-session-id` response header, so the Inspector never reads it, the session ID stays null, and the follow-up request goes out without `Mcp-Session-Id`, producing a 400.

The fix confirmed in the thread is to expose the header via CORS in your server or reverse proxy. The Caddy example that worked was:

```
localhost:8080 {
    header {
        Access-Control-Allow-Origin *
        Access-Control-Allow-Credentials true
        Access-Control-Allow-Methods *
        Access-Control-Allow-Headers *
        Access-Control-Expose-Headers "mcp-session-id"
        defer
    }
    root * public/
    file_server
    log
    php_server
}
```

The same contributor also added `mcp-protocol-version` to the exposed headers, since the Inspector reads that value to resolve the protocol version, and proposed an Express `cors()` middleware example (`exposedHeaders: ["mcp-session-id", "mcp-protocol-version"]`) for the TypeScript SDK — a proposal, not confirmed tested in the thread. Other commenters recommended documenting the requirement rather than changing SDK defaults; one noted that setting permissive CORS as a default is a security issue, so configure it deliberately in your own proxy or CDN. The thread also reports that the basic reference server (`npx @modelcontextprotocol/server-everything streamableHttp`) no longer works with Inspector direct mode.

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

Connected Inspector 0.17.0's direct-connection mode to a stateful streamable HTTP server on a different origin. The mcp-session-id response header was not exposed to the fetch response, the session ID stayed null, and the initialization notification was rejected with 400 because no Mcp-Session-Id header was sent.

Source: https://github.com/modelcontextprotocol/inspector/issues/841

AgentArmorHQ team · Answer

**What @soyuka tried on GitHub** (Worked)

Served the server behind a Caddy reverse proxy with Access-Control-Expose-Headers "mcp-session-id" plus permissive CORS headers. A contributor confirmed this configuration solves the issue.

Source: https://github.com/modelcontextprotocol/inspector/issues/841#issuecomment-3406171982

AgentArmorHQ team · Answer

**What @fredericbarthelet tried on GitHub** (Outcome unclear)

The contributor added mcp-protocol-version to the exposed headers and proposed an Express cors() middleware example (exposedHeaders: ["mcp-session-id", "mcp-protocol-version"]) as a possible SDK default. The Express snippet was not confirmed tested in the thread.

Source: https://github.com/modelcontextprotocol/inspector/issues/841#issuecomment-3406824036

AgentArmorHQ team · Answer

**What @greedy52 tried on GitHub** (Didn't work)

Also tried the basic reference server with `npx @modelcontextprotocol/server-everything streamableHttp` and reported that it no longer works with Inspector direct mode.

Source: https://github.com/modelcontextprotocol/inspector/issues/841#issuecomment-3407861500

Inspector 0.17.0 direct connection fails on streamable HTTP: mcp-session-id header not exposed via CORS

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Inspector 0.17.0 direct connection fails on streamable HTTP: mcp-session-id header not exposed via CORS

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