If an MCP client intermittently shows an empty tool list, first find out whether the server launched, completed initialization, and returned tools. A running process alone proves none of those things. The documented npx workflow can add first-run prompts and version drift, but the available documentation does not establish that npx startup time itself causes an empty list.
Diagnose the failure in order
Check each layer separately. This prevents you from changing server code when the client is hiding tools—or blaming npx when the process never completed its handshake.
- Reproduce the client’s exact launch. Run the same command with the same arguments, environment variables, and working directory. Verify that the executable resolves, paths are valid, permissions and dependencies are present, and the server does not exit early. A GUI client may have a different PATH or environment from your terminal. GitHub’s MCP server debugging guide documents invoking npx through
cmd /cin a .NET configuration example on Windows. - Test initialization without calling a tool. Use MCP Inspector’s
initializeprobe. Confirm that the server connects and completes the handshake, then note the reported protocol version, server information, and capabilities. The server also needs to handlenotifications/initialized; check that the client and server use compatible protocol versions. - Request the tool list directly. Probe
tools/listand check whether the expected names appear. If the response is empty, investigate server-side registration, startup configuration, and filters. If the response contains tools but the application shows none, investigate the client’s enabled and trusted state, filters, configuration, and cached list. - Check the protocol stream and logs. With stdio, stdout carries protocol messages; debug output belongs on stderr. Look for debug text on stdout, malformed JSON, encoding or BOM problems, and incorrect message framing. Turn on the client’s available logging and inspect the server output.
- Make failures bounded and repeatable. Set a finite connection timeout and record whether the probe reached initialization or the tool-list request. This distinguishes a launch or connection delay from a successfully initialized server that returned an empty list.
What the “npx startup tax” does—and does not—explain
The official Inspector documentation describes three practical sources of variability in an npx workflow: a first-run package-install prompt, package-version drift when using a range or a moving tag such as latest, and differences between a developer’s shell and a GUI client’s launch environment. It does not report a measured npx startup delay or show that delay as a cause of empty tool lists. Treat npx as a cause only if logs show that the process timed out, failed, or did not initialize.
For unattended runs, the Inspector CLI smoke-testing guide recommends using --yes to avoid a first-run installation prompt and pinning an exact Inspector version so a later run does not silently resolve a different version. Its documented default connection timeout for ad-hoc target or URL runs is 15,000 ms; when using a config file, the file-level timeout applies. The version 2.5.0 below is the guide’s pinning example, not a claim that it is the newest release.
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Probe initialization and tools with Inspector
Run these examples from the project directory and substitute the exact server command and arguments your client uses. The first checks the handshake; the second requests the tool list as JSON.
npx --yes @modelcontextprotocol/inspector@2.5.0 --cli node build/index.js --method initialize
npx --yes @modelcontextprotocol/inspector@2.5.0 --cli node build/index.js --method tools/list --format jsonUse the Inspector version pinned in your own workflow. For CI, assert that the JSON includes the expected tool names and fail with a clear message if any are missing; the guide shows jq -e as one way to make a missing name return a nonzero status. Keep the connection timeout finite so an unreachable process cannot stall the check indefinitely.
The GitHub debugging guide also describes a manual JSON-RPC sequence: send initialize, then notifications/initialized, then request tools/list. Its example uses protocol version 2024-11-05; use the version appropriate to your server and client rather than copying the sample blindly.
Use the symptoms to identify the failing layer
| What you observe | Likely layer to check | Next check |
|---|---|---|
| The process does not start, or exits immediately | Command launch or environment | Compare the exact command, arguments, PATH, working directory, permissions, and dependencies with the client configuration. |
The process starts, but Inspector cannot complete initialize |
Connection or protocol handshake | Inspect initialization errors, protocol-version compatibility, and handling of notifications/initialized. |
Initialization succeeds, but direct tools/list is empty |
Server implementation or startup configuration | Check tool registration, whether tools are exposed, and any server-side filters. |
Direct tools/list contains tools, but the client shows none |
Client configuration or display state | Check enabled and trusted state, allowlists or filters, tool selection, and cached tools. |
| Logs show parse or framing errors | Stdio stream | Keep stdout reserved for protocol messages; move diagnostics to stderr and inspect JSON, encoding, BOM, and framing. |
| The probe stops at a timeout | Launch or connection timing | Use a bounded timeout and log the last completed stage. A timeout is not the same as an initialized server returning an empty list. |
Check server status and cached tools in VS Code
VS Code’s MCP server management documentation provides the MCP: List Servers command to inspect status, show output, restart, or manage a server. If Chat reports an error, read the MCP output log. Confirm that the server is enabled and trusted and that workspace trust permits its configuration. If the server’s tools changed but VS Code still shows an older list, run MCP: Reset Cached Tools.
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Do not assume a single autostart setting controls every host. The current VS Code documentation notes that Agent Host and Copilot SDK sessions manage processes independently of VS Code’s autostart pass.
Quick Recap
Make the result reproducible
- Record the launch command, arguments, working directory, and relevant environment used by the client.
- Run separate checks for
initializeandtools/list, and save their outputs. - In stdio servers, keep protocol messages on stdout and diagnostic logs on stderr.
- For npx-based CI checks, suppress first-run prompts, pin the Inspector version, use a finite timeout, and assert the expected tool names.
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