Environment: Windows 11, Claude Code + Unity MCP (v 9.4.0)

Description

When launching the Unity MCP server after Claude Code is already running, Claude encounters a Session not found error (JSON-RPC code -32600) whenever it attempts to call any MCP functions (Refresh, Read Console, or Manage Editor).

Steps to Reproduce

1. Launch Claude Code.
2. Start the Unity MCP server/plugin.
3. In Claude Code, attempt to trigger an MCP action (e.g., Refresh Unity).

Expected Result

Claude should establish a handshake with the Unity MCP server and execute the requested tool.

Actual Result

The request fails with a Streamable HTTP error:

 Error: Streamable HTTP error: Error POSTing to endpoint:
     {"jsonrpc":"2.0","id":"server-error","error":{"code":-32600,"message":"Session not
     found"}}

Suspected Cause

The Unity MCP server likely expects a specific initialization sequence or session ID that is lost or not established if the server starts after the client (Claude) is already polling for it.

Workaround

Use /mcp in Claude Code and reconnect.