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Fix an MCP failure by locating the layer that broke: process startup, transport, initialization, or tool execution. A local stdio server can fail because it wrote a log line to stdout; a remote server can return 404 because the client is using the wrong HTTP transport; a tool can return a valid error result without throwing an exception. Capture the exact symptom first, then work through transport, handshake, protocol shape, and tool-result checks in that order.
Start by identifying where the failure occurs
“MCP server connection failed” is not one diagnostic category. The same client message can hide a dead subprocess, a wrong URL, a protocol-version mismatch, an authentication refusal, or an application error inside a tool. Classify the failure by the last stage that succeeded:
- Startup: the client cannot launch the local process, or the process exits immediately.
- Transport: bytes cannot move between client and server, or the HTTP endpoint rejects the request.
- Initialization: the connection exists, but capability and protocol negotiation does not complete.
- Discovery: initialization succeeds, but listing tools fails.
- Execution: a specific tool call reaches the server and returns an error.
Write down the client and server names and versions, transport type, endpoint or launch command, time of failure, exact error text, and whether the problem occurs at startup, handshake, tool discovery, or one method. For HTTP, record the status code, content type, relevant response headers, and a sanitized response body. Remove access tokens, API keys, cookies, authorization headers, environment secrets, and sensitive tool arguments before sharing a log.
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| Evidence | What it can prove | What it cannot prove |
|---|---|---|
| Process exit code and stderr | Whether a local server started, crashed, or was terminated | That its stdout was valid MCP traffic |
| Raw stdout or captured stdio bytes | Whether newline-delimited JSON-RPC framing was preserved | That a tool’s business logic succeeded |
| HTTP status, headers, and body | Whether a proxy, route, authentication layer, or server rejected the HTTP request | That an MCP tool result exists |
| JSON-RPC response | Whether the request was parsed and how the protocol represented success or failure | That the underlying HTTP request was accepted if no response exists |
| Server and client logs | Which side received the request and what it did next | That secrets are safe to publish; sanitize them first |
An HTTP refusal may be an HTML or plain-text body, not JSON-RPC. Conversely, a tool can return a valid JSON-RPC result with an error flag. Always inspect both channels.
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Local stdio: fix process and framing failures first
With stdio, the client launches the server as a child process. MCP messages travel through the child’s standard input and standard output as one newline-delimited JSON-RPC message per line. Standard output is therefore a protocol channel, not a console.
Verify the launch contract
- Confirm the executable or interpreter path is correct and available to the client’s user account.
- Check every argument, working directory, and environment variable. A command that works in your terminal can fail when launched from a different directory with a smaller environment.
- Run the server directly with the same command and inspect its exit status. An immediate exit usually indicates a missing dependency, invalid configuration, permission problem, or startup exception.
- Check that the process remains alive while the client connects. If it exits, preserve the exit code and stderr output before restarting it.
Keep stdout protocol-only
The server must not write banners, debug prints, stack traces, progress bars, or shell prompts to stdout. One stray line can make the client report an apparently unrelated parse or connection error because framing is no longer valid. Send diagnostics to stderr or to a structured logging sink instead. The MCP stdio specification’s rule is absolute: the server must not write anything to stdout that is not a valid MCP message.
Use a separate stderr capture when testing. A client may capture, suppress, or display stderr differently, so stderr text by itself does not establish that the protocol failed. The decisive checks are valid newline-delimited messages on stdout, a live child process, and a completed initialization exchange.
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Restart the process after correcting the startup cause, then repeat initialization rather than replaying arbitrary messages. If the process died during a write or tool execution, retry only operations that are safe to repeat; a tool may have completed on the server before the client lost the response.
Remote HTTP: distinguish Streamable HTTP from legacy SSE
For a remote server, verify the complete URL, route, port, TLS settings, authentication, proxy configuration, and the transport the server actually implements. MCP deployments commonly use Streamable HTTP or the older server-sent events (SSE) transport. They are not interchangeable client settings.
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Read 404, 405, and content-type clues
- 404 Not Found: the path may be wrong, the reverse proxy may not forward the MCP route, or the client may be calling an endpoint that exists only for another transport.
- 405 Method Not Allowed: the route exists but the client used the wrong HTTP method, often because a health probe or SSE client is being sent to a POST-oriented endpoint.
- Unexpected
text/event-stream: the server may be SSE-only while the client expects Streamable HTTP, or the reverse may be true. - HTML or plain text instead of JSON: a proxy, authentication gateway, framework error page, or platform layer rejected the request before MCP processing.
Confirm the transport in the server documentation and configure the matching client transport. Do not “fix” a 404 by changing protocol methods until the route and transport agree. Microsoft’s MCP troubleshooting guidance likewise treats a transport mismatch as a primary cause of 404, 405, and unexpected event-stream responses.
Check deployment-specific HTTP behavior
Hosted services can reject a request before it reaches MCP. Inspect the Host header accepted by the platform, TLS termination, authentication middleware, proxy route, and listening port. A generic client error after deployment often means the platform returned a non-MCP response that the client did not expose in detail.
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Health probes deserve special attention. A probe that sends a GET request to a POST-only MCP route can mark a healthy container as unhealthy and trigger repeated restarts. Configure a dedicated health endpoint or a probe method and path that the hosting platform supports, rather than probing the MCP operation endpoint itself.
Complete initialization before testing methods
Do not diagnose tools/list or tools/call until the normal SDK connection flow has completed. Initialization negotiates the protocol revision and exposes server capabilities. Record the negotiated version and capabilities when the SDK provides them.
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Use the SDK’s connect sequence
- Create the client with the transport that matches the server.
- Invoke the SDK’s normal connect or initialize operation instead of sending an individual method manually.
- Wait for the initialization response and capability exchange.
- Only then request tool discovery or call a tool.
The MCP specification describes probing modern servers and falling back to an older initialization flow for legacy servers under defined conditions. A client should not infer that a server is legacy from one hard-coded error code. Let the SDK perform the compatibility behavior, and verify the version it actually negotiated.
Diagnose -32601 method not found
A method-not-found response is often blamed on a missing tool, but it can have several causes:
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- The request was sent before initialization finished.
- The method name is misspelled or uses the wrong namespace. Standard examples include
tools/listandtools/call. - The client and server support incompatible protocol behavior or revisions.
- The selected transport is not the one that exposes the method.
- The request reached a proxy or another service rather than the MCP endpoint.
Check these in order: completed handshake, exact spelling and case, negotiated revision, transport, and the endpoint that actually received the request.
Validate the JSON-RPC envelope for parse and bad-request errors
For -32700, HTTP 400, or an equivalent parse complaint, inspect the complete envelope rather than only the human-readable message. A request should contain the expected JSON-RPC version, a request identifier, a method, and a parameters object with the shape required by that method.
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{
"jsonrpc": "2.0",
"id": 7,
"method": "tools/call",
"params": {
"name": "example_tool",
"arguments": {}
}
}The example shows the envelope shape, not a universal tool name or argument schema. Use the schema advertised by the server for the selected tool. Check that the body is valid JSON, that exactly one newline-delimited message is sent over stdio, and that an HTTP request uses the content type required by the server.
Do not treat a malformed request and an application-level tool failure as the same problem. A malformed envelope is rejected before the method runs; a tool-level failure is returned after the method has been selected.
Separate tool execution errors from transport errors
Once a call reaches the server, inspect the result object even when the client library did not throw an exception. SDKs can represent a failed tool execution with an isError or is_error field. Code that catches exceptions only can therefore report a false success or hide the useful error details.
Handle a protocol-level tool error
- Check whether the call returned a normal JSON-RPC result.
- Read the result’s error flag and structured content.
- Display the tool’s message and any retry or remediation data.
- Decide whether the operation is safe to retry; do not blindly repeat side-effecting calls.
Recognize a refusal that never became an MCP result
If the HTTP server returns a refusal before an MCP response exists, there may be no JSON-RPC result or tool error to inspect. Record the HTTP status, content type, response headers, and sanitized body. Authentication failures, rejected Host headers, proxy rules, route errors, and rate limits commonly occur at this layer.
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Handle rate limiting correctly
An HTTP 429 is separate from a tool result that carries a rate-limit error. For an HTTP refusal, honor the server’s Retry-After value and relevant rate-limit headers before retrying. If the MCP call itself returned successfully with a structured rate-limit result, follow the retry information in that result instead. GitLab documents these as distinct cases, along with deployment-specific authorization and protocol-version behavior.
Use Inspector and logging to observe the actual exchange
The MCP Inspector is designed to test a server and inspect traffic. Use it to determine whether the failure reproduces outside your application. If the Inspector succeeds while your application fails, compare transport settings, initialization order, authentication, and request shape rather than changing the server blindly.
Capture both sides of the conversation
- For stdio, capture the exact launch command, process status, exit code, stderr, and raw protocol messages.
- For HTTP, capture method, route, status, content type, selected headers, and sanitized body.
- Record the JSON-RPC request ID so a request can be matched with its response in interleaved logs.
- Record initialization and capability responses before the failing tool call.
The MCP Go SDK documentation describes a logging transport for stdio and middleware for logging HTTP request bodies. Equivalent logging facilities exist in other SDKs; enable them at the narrowest useful scope and avoid writing credentials or sensitive arguments to persistent logs.
Build a reproducible bug report
A useful report contains SDK and protocol versions, transport, a sanitized endpoint, exact reproduction steps, method and sanitized arguments, the complete JSON-RPC request and response when available, HTTP status and headers, client and server logs, and whether a minimal Inspector or SDK client reproduces the issue. State whether the failure is deterministic and whether it began after a version, route, authentication, or hosting change.
Symptom-to-check map
| Symptom | First checks | Most likely direction |
|---|---|---|
| Local process launches, but the client cannot parse messages | Look for non-JSON stdout, broken newline framing, and child-process exit | Keep stdout protocol-only; send logs to stderr; verify command, environment, and process status |
| HTTP 404, 405, or unexpected event-stream content | Confirm route, HTTP method, and Streamable HTTP versus SSE on both sides | Correct the endpoint or select the matching transport |
-32601 method not found |
Check completed initialization, exact method spelling, transport, and negotiated revision | Negotiate first and use a method the server supports |
| HTTP 400 or JSON-RPC parse error | Validate JSON-RPC version, ID, method, parameters, encoding, and content type | Repair the envelope or request serialization |
| Tool returns an error but no exception is caught | Inspect isError/is_error and structured result content |
Handle protocol-level tool errors as well as thrown exceptions |
| Generic remote connection error after deployment | Inspect server logs, Host header, authentication, proxy, and route | Find the HTTP refusal that occurred before MCP processing |
| HTTP 429 | Read status, Retry-After, and rate-limit headers |
Wait as directed and distinguish HTTP refusal from a tool-level rate-limit result |
| Container repeatedly restarts or fails health checks | Check probe path, method, configured port, and startup logs | Use a dedicated health endpoint instead of probing a POST-only MCP route |
Stateful and stateless deployment considerations
Transport troubleshooting is not complete when a request reaches one container. If the deployment keeps connection or callback state in memory, a proxy or load balancer that sends later requests to a different instance can break an otherwise valid session. Check whether the server expects a stateful connection, whether the platform preserves routing as required, and whether scaling or restarts discard session data. A stateless design avoids some affinity problems but may require every request to carry the context the server needs. Use the server and hosting documentation for the deployment’s exact model rather than assuming all MCP servers behave alike.
A practical recovery sequence
When you need a short, repeatable procedure, use this order:
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- Save the exact error, timestamp, versions, transport, endpoint, and failure stage.
- Sanitize secrets and capture the full HTTP or JSON-RPC evidence available.
- For stdio, verify command, environment, process lifetime, and stdout purity.
- For HTTP, verify route, method, port, TLS, authentication, Host header, proxy, and transport type.
- Run the SDK’s normal initialization flow and record the negotiated revision and capabilities.
- Validate the JSON-RPC envelope before changing tool code.
- Call a known, low-risk tool and inspect both exceptions and the result error flag.
- Reproduce with MCP Inspector or a minimal SDK client to isolate application code.
- Check platform-specific rate limits, health probes, scaling, and restart behavior.
- Retry only operations whose side effects are understood, then attach the sanitized exchange to a focused bug report.
This sequence prevents a common waste of time: changing tool implementation while the client is still speaking the wrong transport or sending messages before negotiation.
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