MCPinspector fails to show Tools #1064
Description
Inspector Version
v0.19.0 (or latest via npx @modelcontextprotocol/inspector@latest)
Describe the bug
When connecting to a FastMCP(v2.14.5)
server running as a Proxy (aggregating other MCP servers) over Streamable HTTP transport, the Inspector establishes a session (returns 200 OK and 202 Accepted in logs) but fails to display any tools.
Manual testing via a Python MCP Client confirms the tools are present and the server is healthy. However, the Inspector seems unable to complete the tools/list handshake or handle the session-based requirement of the latest Streamable HTTP spec, often resulting in a blank UI or a "Missing Session ID" error when trying to force the request.
To Reproduce
Steps to reproduce the behavior:
Start a FastMCP server in a Docker container using mcp.run(transport="http", host="0.0.0.0", port=8000).
Configure the server as a proxy for other remote MCP servers (e.g., Mercado Pago MCP).
Launch the inspector: npx @modelcontextprotocol/inspector http://localhost:8000/mcp.
Click on 'Connect'.
See that the connection status is green (Connected), but the Tools list remains empty even after clicking the "List Tools" button.
Server logs show successful POST /mcp 200 OK and GET /mcp 200 OK, but no data is rendered in the UI.
Expected behavior
The Inspector should successfully negotiate the Session ID, maintain the persistent HTTP stream, and display the tools aggregated by the proxy server.
Environment (please complete the following information):
OS: Windows 11 (Running Docker Desktop)
Browser: Chrome / Edge
Server Environment: Docker (Python 3.11 + FastMCP)
Additional context
The issue seems specific to the V1 Inspector's handling of the new Streamable HTTP transport. While the Python SDK fastmcp.Client can successfully list tools by managing the session lifecycle, the Inspector appears to drop the session or fail to include the required session headers/Accept types (application/json, text/event-stream) in subsequent tools/list requests.
Version Consideration
This bug is critical for MCP spec compliance regarding the new Streamable HTTP transport introduced in late 2024/2025.