HTTP MCP: "Error POSTing to endpoint" despite server returning a valid tools/list result (Google Docs MCP docsmcp.googleapis.com)

### Environment

- **Claude Code version:** 2.1.259 (native install)
- **OS:** Windows 11
- **MCP server:** `google-docs` → `https://docsmcp.googleapis.com/mcp/v1` (Google's official Docs MCP), configured in project `.mcp.json` as `"type": "http"` with an OAuth desktop client.

### What happens

1. OAuth completes successfully (browser flow, consent granted).
2. `/mcp` reports: **"Reconnected to google-docs, but fetching tools failed (detail withheld on this connection)."**
3. No `mcp__google-docs__*` tools are ever exposed. `claude mcp list` shows the server as `! Needs authentication` even right after a successful auth.

### Log evidence

From `~/AppData/Local/claude-cli-nodejs/Cache//mcp-logs-google-docs/*.jsonl`:

```
"debug": "Connection error: Error POSTing to endpoint: {"id":1,"jsonrpc":"2.0","result":{"tools":[ ... ]}}"
"error": "Failed to fetch tools: Error POSTing to endpoint: {"id":1,"jsonrpc":"2.0","result":{"tools":[{"name":"read_doc",...},{"name":"update_doc",...}]}}"
```

The payload attached to the error is a **complete, well-formed JSON-RPC `result`** containing the full tool catalog (`read_doc`, `update_doc`). The transport layer rejects the response anyway. Note there is **no `(HTTP nnn)`** in the error string, unlike the usual `StreamableHTTPError` path — so it does not look like a plain non-2xx rejection.

### Likely cause

The client appears to mishandle how Google's endpoint frames the `tools/list` response — e.g. an SSE-framed (`text/event-stream`) body where plain JSON is expected on that code path, or a 2xx response whose `Content-Type` the POST handler doesn't accept. The server side is clearly returning a valid result.

### Expected

Tools registered from `result.tools`; `mcp__google-docs__read_doc` / `mcp__google-docs__update_doc` available.

### Repro

1. Add Google's Docs MCP as an `http` server in `.mcp.json`.
2. Authenticate via `/mcp`.
3. Run `/mcp` again → "fetching tools failed".

Reproduce the failure using the project .mcp.json entry for Google's HTTP server, then inspect the /mcp flow and the mcp-logs-google-docs/*.jsonl files under the stated cache path. Compare the successful tools/list JSON-RPC result with the response framing and content type; done means read_doc and update_doc are exposed as MCP tools after authentication.

索引モデルが issue の本文から書いたものです。