MCP 'tool not found' right after the server starts
Some MCP servers add tools a few seconds after they start. MUI MCP listed 2 tools at first and 3 about 8 s later. Our first call to the late tool failed with Tool useMuiDocs not found. After waiting, both calls we made passed.
Wait a few seconds after the server starts, call tools/list again, then call the tool. A client that handles the notifications/tools/list_changed message refreshes its list on its own when the server sends one.
Last tested 2026-11-11
How we test: methodology. Context cost: lowest context cost MCP servers. Not tested by us: security, output quality, and behaviour with real credentials.
Symptom
The server starts and lists tools, but a tool named in its README is missing, or calling it right away returns "tool not found". A few seconds later the same tool works.
The exact error we saw
| Server | What it printed (verbatim) | Where we saw it | Tested |
|---|---|---|---|
MUI MCP (official)@mui/mcp 0.1.9 | Tool useMuiDocs not found | first tools/call, right after start (our recorded test note) | 2026-11-11 |
Cause
The server registers some tools only after a startup step finishes. Our test note says MUI's useMuiDocs appears only after its docs catalog loads. The MCP spec lets a server declare listChanged and then send notifications/tools/list_changed when its tools change, so clients can list them again (MCP spec, tools). We did not record whether @mui/mcp declares listChanged.
Fix
- Wait and list again. Call
tools/lista few seconds after start, then call the tool. This worked in our test: after waiting, useMuiDocs and fetchDocs both passed. - Use a client that refreshes on
notifications/tools/list_changed, or reconnect the server if your client keeps the first list it saw. not tested by us
Affected listings
- MUI MCP (official) (
@mui/mcp, tested 2026-11-11, status working)
More troubleshooting: all troubleshooting pages · MCP error -32000: Connection closed · MCP servers that need an API key or sign-in. Quotes are verbatim from our own test logs (colour codes removed; a trailing … marks a shortened line). We have not run these servers inside Claude Code, Claude Desktop or Cursor.
FAQ
Why does an MCP tool show as not found right after start?
Some servers register tools after a startup step. In our test MUI MCP listed 2 tools at first and 3 about 8 s later, and the first call to the late tool failed with 'Tool useMuiDocs not found'.
How do clients learn about tools added later?
A server that declares the listChanged capability should send notifications/tools/list_changed, and the client lists tools again. Otherwise, calling tools/list again or reconnecting picks up the new tools.