ResponseCachingMiddleware breaks imported/mounted server prefixes #2300
Description
Description
When using ResponseCachingMiddleware, prefixes of imported/mounted MCP servers are not returned in response to tools/list.
The tools still require the prefix in order to be called though.
Everything works fine when not using ResponseCachingMiddleware.
// List tools
curl -X POST http://localhost:8000/mcp \
-H "Content-Type: application/json" \
-H "Accept: text/event-stream, application/json" \
-d '{
"jsonrpc": "2.0",
"id": "1",
"method": "tools/list",
"params": {}
}' \
| grep '^data: ' \
| sed 's/^data: //' \
| jq '.result'
// Response
{
"tools": [
{
"name": "example_user_tool",
"inputSchema": {
"properties": {},
"type": "object"
},
"_meta": {
"_fastmcp": {
"tags": []
}
}
},
{
"name": "example_incident_tool",
"inputSchema": {
"properties": {},
"type": "object"
},
"_meta": {
"_fastmcp": {
"tags": []
}
}
},
{
"name": "example_knowledgebase_tool",
"inputSchema": {
"properties": {},
"type": "object"
},
"_meta": {
"_fastmcp": {
"tags": []
}
}
}
]
}
// Call example_user_tool
curl -X POST http://localhost:8000/mcp \
-H "Content-Type: application/json" \
-H "Accept: text/event-stream, application/json" \
-d '{
"jsonrpc": "2.0",
"id": "1",
"method": "tools/call",
"params": {
"name": "example_user_tool"
}
}' \
| grep '^data: ' \
| sed 's/^data: //' \
| jq '.result.content[0].text'
// Response
"Unknown tool: example_user_tool"
// Call user_example_user_tool
curl -X POST http://localhost:8000/mcp \
-H "Content-Type: application/json" \
-H "Accept: text/event-stream, application/json" \
-d '{
"jsonrpc": "2.0",
"id": "1",
"method": "tools/call",
"params": {
"name": "user_example_user_tool"
}
}' \
| grep '^data: ' \
| sed 's/^data: //' \
| jq '.result.content[0].text'
// Response
"This is an example user tool."Example Code
import asyncio
import uvicorn
from fastmcp import FastMCP
from fastmcp.server.middleware.caching import ResponseCachingMiddleware
mcp = FastMCP(name="Main MCP", stateless_http=True) # Stateless to make the cURL demo easier, same symptoms without this.
user_mcp = FastMCP(name="User MCP")
incident_mcp = FastMCP(name="Incident MCP")
knowledgebase_mcp = FastMCP(name="Knowledgebase MCP")
@user_mcp.tool
def example_user_tool():
return "This is an example user tool."
@incident_mcp.tool
def example_incident_tool():
return "This is an example incident tool."
@knowledgebase_mcp.tool
def example_knowledgebase_tool():
return "This is an example knowledgebase tool."
async def import_servers():
await mcp.import_server(user_mcp, prefix="user")
await mcp.import_server(incident_mcp, prefix="incident")
await mcp.import_server(knowledgebase_mcp, prefix="knowledgebase")
mcp.add_middleware(ResponseCachingMiddleware())
http_app = mcp.http_app()
if __name__ == "__main__":
asyncio.run(import_servers())
uvicorn.run(http_app, host="0.0.0.0", port=8000)Version Information
FastMCP version: 2.13.0.2
MCP version: 1.19.0
Python version: 3.13.9
Platform: Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.39
FastMCP root path: /home/seanmcgrath/servicenow-mcp/.venv/lib/python3.13/site-packages