Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsFind the earliest step that fails: process launch, transport connection, protocol negotiation, capability discovery, tool listing, or tool execution. For a local stdio server, start with the executable and launch environment. For a remote server, check the endpoint, transport, HTTP status, and authorization. Once connected, inspect advertised capabilities and the actual tool list before investigating a tool handler.
Start by locating the failure
Record the client and server SDK names and versions, configured transport, endpoint or launch command, and the first error returned. Then place the failure in the connection sequence:
- The client launches a local process or reaches a remote endpoint.
- The selected transport exchanges protocol messages.
- Client and server negotiate a compatible protocol version.
- The server advertises capabilities and returns its tools list.
- The client calls a listed tool, and the server validates and runs it.
An error at one stage does not establish a problem at a later stage. For example, a missing executable is not a protocol-version mismatch, and an authorization response is not evidence that tool registration failed.
Debug local stdio launch errors
In a stdio setup, the client transport launches and owns the server child process, then communicates with it using JSON-RPC over stdin and stdout. If the client is configured to spawn the server, do not start a second copy independently; troubleshoot the process the client actually launches. The TypeScript SDK’s connection guide documents the transport’s child-process lifecycle.
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When the error is spawn npx ENOENT
This means npx could not be found as an executable on the launching process’s PATH. Check the executable name, PATH, working directory, environment, and arguments in the same context that starts the MCP client. A command that works in your interactive shell may not resolve the same way in an IDE, service, container, or other client launch environment.
Keep protocol output separate from diagnostics
For stdio transport, stdout carries protocol messages. Do not print banners, debug messages, or other incidental text there; use the host’s supported logging channel. The TypeScript SDK example forwards the child process’s stderr as a banner. If a connection fails after the child starts, close the client in a finally block so the transport can clean up the process rather than leave it running.
Check HTTP endpoint and transport compatibility
For a remote server, verify the exact endpoint path and the transport it implements. Streamable HTTP and the older HTTP+SSE transport are distinct; a client configured for one should not be assumed to work with a server that only supports the other. The TypeScript SDK connection guide uses StreamableHTTPClientTransport for remote servers.
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If Streamable HTTP fails and you suspect the server is legacy SSE-only, the guide’s compatibility approach is to create a fresh client and retry with SSEClientTransport. Treat that as a transport compatibility check, not a fix for failed authentication, an unavailable service, or a broken endpoint.
Interpret HTTP errors before changing protocol settings
HTTP responses and timeouts offer different clues. The TypeScript SDK’s protocol-version guide describes how its negotiation handles these cases; exact behavior can vary by SDK version.
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- 401 or 403: investigate authentication or permissions. These statuses do not, by themselves, show that the server uses an older protocol.
- 5xx: investigate a server-side failure.
- Timeout: treat it as a reachability or availability problem rather than silently classifying the server as legacy.
- Unusable 2xx response: a successful status with a body the client cannot use is not valid evidence of an older protocol.
- Browser CORS exception: investigate browser or gateway policy. The SDK guide identifies this as a special compatibility case, so check the actual client version and environment.
If a reverse proxy or gateway sits between client and server, check that it preserves the request method, relevant MCP headers, response content type, and streaming behavior required by the chosen transport and SDK. The SDK guidance establishes that negotiation depends on valid responses and transport behavior, but it does not define one universal proxy configuration.
Verify protocol-version negotiation
MCP protocol behavior depends on the revisions and negotiation modes supported by both sides. The TypeScript SDK documentation describes a legacy flow based on the initialize handshake and a 2026-era flow using server/discover; its modern automatic negotiation can fall back to the older handshake when appropriate. The Python SDK protocol-version guide likewise documents discovery followed by an initialize fallback when discovery fails or a server does not support the latest version.
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Check the client and server SDK versions, their supported protocol revisions, and the negotiation mode actually in use. Do not treat every failed HTTP request as a negotiation failure: first rule out authorization, server errors, timeouts, malformed successful responses, and transport mismatch.
When the client connects but shows no tools
Use the client’s list operation and inspect the returned tool names, descriptions, and input schemas. The list response is more useful than assuming that a successful connection means the server registered tools.
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The tool list is empty
Check server-side registration and capability declarations. According to the TypeScript SDK migration guide, the high-level McpServer installs handlers for declared primitive capabilities, while users of the low-level Server must register handlers themselves. A high-level server can declare tools yet return an empty list if none were registered.
The list operation itself fails
Check whether the server registered or advertised the relevant capability, then compare the client and server SDK versions and protocol support. A connection can succeed while a capability needed for tool listing is absent or incompatible.
Separate a missing tool from a failing tool call
Compare the requested tool name exactly with the names in the list response. The TypeScript SDK’s first-client example distinguishes an unregistered tool name, which is a protocol-level failure, from a handler exception or arguments that fail the input schema. In that example, handler and input errors are returned as a tool result with isError: true.
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- If the name is absent from the list, investigate registration and capability setup.
- If it is listed, validate the call arguments against its advertised input schema.
- If the arguments match the schema but the call returns an error, investigate the handler and its server-side logs.
Collect evidence for a useful bug report
Capture the information that identifies the first failing stage, while redacting credentials and other secrets:
- Client and server SDK names and versions.
- Configured transport and protocol revision or negotiation mode, if available.
- Launch command or endpoint path.
- Exact error text, HTTP status, and relevant client and server logs.
- Whether connection completed, the capability response, and the raw tool list.
- For stdio, whether the launching process can see the expected executable and environment.
- For HTTP, whether the endpoint uses Streamable HTTP or legacy SSE, and whether authentication or a gateway interrupts the exchange.
These details help distinguish process, transport, authorization, negotiation, registration, and execution failures instead of grouping them under a single “MCP connection” problem.
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