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This guide covers local subprocesses, Streamable HTTP, legacy SSE, SDK and protocol-version mismatches, authentication and proxy issues, hosted health checks, rate limits, and the evidence you should capture for a reproducible bug report.
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Identify the layer that is failing
MCP errors become much easier to diagnose when you classify the first failed boundary. Do not treat “MCP server connection failed” as one universal condition. Ask where the exchange stopped:
- Startup or reachability: the local child process exits, the executable cannot be launched, DNS or TLS fails, or an HTTP server returns before MCP sees a request.
- Transport: bytes arrive on the wrong channel, a stdio stream is not newline-delimited JSON, or a client uses SSE against a Streamable HTTP route (or the reverse).
- Initialization: the client sends a method before negotiation, the protocol revisions are incompatible, or capabilities were not established.
- Tool discovery: the connection is open but
tools/listfails, is misspelled, or is sent to a server that does not expose tools. - Tool execution:
tools/callreaches the server and returns a valid result that reports an execution error.
The distinction matters because an HTTP refusal may never produce a JSON-RPC result, while a failed tool can be a normal MCP result with an error flag. Record the first boundary that fails instead of jumping straight to application code.
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Know which MCP transport you are using
Transport determines the wire format, endpoint behavior, and the evidence you should inspect. Confirm it in both the client and server documentation before editing protocol code.
| Transport | How it carries MCP | Checks that matter most |
|---|---|---|
| Local stdio | The client launches a server subprocess. JSON-RPC messages travel through the child process’s standard input and standard output, one newline-delimited message per line. | Executable path, arguments, working directory, environment, process lifetime, exact stdout bytes, and exit status. Diagnostics belong on stderr. |
| Streamable HTTP | MCP requests use the server’s modern HTTP route; responses and any streaming behavior follow that transport’s contract. | Exact URL and route, HTTP method, content type, TLS, authentication, proxy behavior, Host header, and whether the deployment supports the expected state model. |
| Legacy SSE | The older server-sent-events transport uses event-stream behavior and a different client setup from Streamable HTTP. | Whether the server is SSE-only, whether the client explicitly selects SSE, route and method expectations, and unexpected text/event-stream responses. |
Microsoft’s MCP troubleshooting guidance specifically identifies 404, 405, and unexpected text/event-stream responses as signs to check transport and route compatibility. A valid URL is not enough if the client and server speak different transports.
Capture the failure before changing anything
A precise record prevents a generic retry from hiding the cause and gives maintainers enough information to reproduce the problem. Capture these items in one place:
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- Identify both ends: record the client name and version, server name and version, SDK language and version, MCP protocol revision if exposed, operating system or container image, and the deployment region when it affects routing.
- Describe the path: note whether the server is local stdio, Streamable HTTP, or SSE; include the endpoint path without secrets; and record any proxy, gateway, service mesh, or authentication layer in front of it.
- Mark the stage: say whether the failure occurs when the process starts, when the connection opens, during initialization, during tool discovery, or on one particular tool call.
- Save the complete symptom: preserve the exact exception text, JSON-RPC error object, HTTP status, response content type, relevant headers, and a sanitized response body. A plain-text HTTP error and a JSON-RPC error are not interchangeable.
- Record timing: include the UTC time, whether the failure is repeatable, and whether it began after a deployment, SDK upgrade, protocol-version change, or infrastructure change.
- Sanitize safely: remove API keys, bearer tokens, cookies, environment secrets, personal data, and sensitive tool arguments. Redact values while keeping field names, data types, and the overall request shape.
Do not infer that stderr text alone proves a server failure. Some clients capture or ignore stderr, and a server can emit harmless informational messages there. Pair logs with process status or a protocol response.
Repair local stdio connections
With stdio, the client owns the subprocess lifecycle. The server is not listening on a network port; it is a child process whose stdin and stdout form the protocol channel.
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Verify launch settings
- Use the intended executable or interpreter path, not a path that exists only in an interactive shell.
- Check every argument, the working directory, and required environment variables. A relative path can resolve differently when launched by an IDE, service manager, or container.
- Confirm the child process remains alive after startup. An immediate exit usually points to a missing dependency, invalid configuration, permission problem, or startup exception.
- Inspect the process exit code and the client’s captured stderr. Restart the process after correcting the cause; retry in-flight work only when the operation is safe to repeat.
Keep stdout protocol-only
The MCP stdio specification states: “The server MUST NOT write anything to its stdout that is not a valid MCP message.” A startup banner, debug print, progress message, stack trace, or shell warning on stdout can corrupt framing before the client reads the first response.
- Send diagnostics to stderr or to a structured logger configured for stderr.
- Ensure each JSON-RPC message is encoded as one complete line with a newline terminator. Do not pretty-print multi-line JSON onto the protocol stream.
- Check wrappers and launch scripts. A shell that echoes commands, a runtime warning, or a package manager notice can contaminate stdout even when the server code is correct.
- If the server must emit human-readable output, route it through the logging facility rather than a standard-output call.
Check framing and shutdown behavior
Capture the raw exchange only with a tool that does not alter bytes. Look for a non-JSON line, truncated output, multiple JSON objects on one line, or a process that closes stdout before sending a response. A client that cannot parse the first message is failing before MCP method handling begins.
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Repair remote HTTP and SSE connections
Confirm the exact route and method
Check the complete URL, path prefix, port, TLS certificate, authentication scheme, and proxy rewrites. A reverse proxy can expose /mcp while the application listens on another internal path; a missing prefix commonly produces 404. A route that accepts only POST can produce 405 when a health probe or manual browser request sends GET.
Unexpected text/event-stream content is a strong clue that the server is speaking legacy SSE while the client expects Streamable HTTP, or that a proxy is routing to an SSE endpoint. Select the transport explicitly in the client and use the route documented for that transport. Microsoft guidance summarizes the rule: “Ensure your client and server use the same transport.”
Inspect status, headers, and body together
For every failed HTTP request, preserve the status code, content type, request method, relevant response headers, and sanitized body. A gateway may return an HTML 404 page, a load balancer may return plain text, and an authentication service may return JSON that is not an MCP response. None of those bodies can be parsed as a JSON-RPC result.
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- 404: verify the path, deployment base URL, proxy rewrite, and whether the selected transport has a separate endpoint.
- 405: verify the HTTP method and whether a health checker or browser sent GET to a POST-only MCP route.
- 401 or 403: check credentials, token audience and scope, clock skew, forwarded authorization headers, and the identity expected by the hosting layer.
- TLS or connection refusal: verify certificate trust, hostname, port exposure, firewall rules, and whether the service is listening on the configured interface.
- Content-type mismatch: compare the response’s content type with the transport selected by the client. Do not force a parser to accept an event stream, HTML page, or plain-text gateway message as JSON-RPC.
Account for state and callbacks
Some servers keep session state or make callbacks during a request. A load balancer that sends successive requests to different instances can break such a session unless affinity or shared state is configured. Stateless deployments avoid that dependency but may not support workflows that require server-side session context. Consult the server’s deployment documentation before changing scaling or routing behavior.
Complete initialization before calling methods
Do not diagnose an individual tool until the MCP handshake has succeeded. Use the SDK’s normal connect flow so it can send the required initialization messages, negotiate a compatible protocol revision, and learn server capabilities.
What to verify
- Record the negotiated protocol version and the capabilities object when the SDK exposes them.
- Confirm that the client waits for initialization to complete before sending
tools/list,tools/call, or another method. - Check that the server’s advertised capabilities include the feature you are trying to use.
- When supporting older servers, use the SDK’s documented compatibility behavior. The MCP specification describes probing modern servers and falling back to the older initialization flow under defined conditions; clients should not label a server “legacy” from one hard-coded error code alone.
Interpret common JSON-RPC errors
-32601 means method not found, but it does not identify one cause. Check, in order, that initialization finished, the method name is exact, the server advertises or implements that method, the transport is correct, and the client and server revisions support compatible behavior. Common method names include tools/list and tools/call; a spelling or slash error is enough to trigger the same code.
-32700 indicates a parse error. For an HTTP 400 or parse failure, inspect the complete JSON-RPC envelope: the JSON-RPC version field, request ID, method, and params shape must be valid JSON and match the method contract. A proxy that truncates or rewrites a body can create the same symptom as a bug in the client serializer.
Use the request envelope as evidence
{"jsonrpc":"2.0","id":7,"method":"tools/list","params":{}}The example shows the fields you should expect to see, not a universal request for every method. Compare the captured envelope with the SDK documentation for the method being called. Preserve the request ID so you can match a response to the request when several calls are in flight.
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Separate a tool execution error from a transport failure
Once a call reaches the server, the server can return a valid MCP result that reports an execution problem. The Python SDK guide documents this with a result-level is_error flag (often represented as isError in protocol data). Code that catches exceptions only can therefore report success or silently discard the real tool failure.
Handle both result-level and exception-level failures
- Catch transport, authentication, timeout, and serialization exceptions.
- After a successful protocol exchange, inspect the result’s error flag before treating the call as successful.
- Read the returned content and structured fields for a server-provided explanation, validation detail, or retry information.
- Log the method, sanitized arguments, request ID, and result status so a tool error can be reproduced without exposing secrets.
The opposite case is equally important: an HTTP refusal can happen before MCP produces any result. If the body is non-JSON or the status is 4xx/5xx, investigate the HTTP and hosting layers rather than looking for isError in a response that does not exist.
Validate arguments separately
If discovery succeeds but one tool fails, compare the supplied arguments with the tool schema returned by tools/list. Check required fields, exact field names, data types, enum values, and whether a value is valid for the server’s current environment. A minimal call with the smallest safe argument set helps distinguish schema rejection from a downstream service failure. Never use destructive or irreversible arguments merely to test connectivity.
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The MCP Go SDK troubleshooting guide recommends MCP Inspector for testing and inspecting server traffic. It is useful because it can show whether initialization, discovery, and the tool request actually occurred, rather than relying on a client UI that collapses several failures into “connection error.”
What to inspect on each side
- Client: selected transport, endpoint, launch command, protocol revision, outgoing method, request ID, timeout, and the exact exception or response.
- Server: process startup, authentication decision, route handling, initialization state, tool validation, downstream calls, and shutdown reason.
- HTTP infrastructure: proxy access log, upstream status, rewritten path, forwarded headers, timeout, and response size. Some clients replace a plain-text gateway refusal with a generic connection exception, so the proxy log may be the only place showing the real status.
For stdio, SDK logging or a logging transport can capture messages without polluting stdout. For HTTP, middleware that logs request bodies and a general HTTP inspection tool can reveal a malformed envelope or an unexpected content type. Redact credentials before storing or sharing these traces.
Build a minimal reproduction
- Start the server with the smallest configuration that still fails.
- Connect with MCP Inspector or a minimal SDK client instead of the full application.
- Complete initialization, list tools, and call one harmless tool with sanitized arguments.
- Compare the minimal trace with the failing application’s trace. If the minimal client works, focus on application middleware, authentication injection, concurrency, or argument construction.
- If both fail, attach the sanitized trace and server logs to the issue report.
A useful bug report contains SDK and protocol versions, transport, sanitized endpoint, reproduction steps, exact method and arguments, full JSON-RPC request and response when available, HTTP status and content type, relevant headers, client and server logs, and whether Inspector or a minimal SDK client reproduces the fault.
Handle hosted-service and platform failures
Host headers, proxies, and authentication
Hosted MCP services can reject a request before it reaches the application. Check whether the platform requires a particular Host header, canonical hostname, forwarded-proto value, or authorization format. Confirm that the proxy forwards the HTTP method, path, body, content type, and authentication headers unchanged. A successful TLS handshake proves only that a certificate was accepted; it does not prove that the request reached the MCP route.
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Container platforms often probe a configured path and method. Pointing a GET health probe at a POST-only MCP endpoint can generate repeated 405 responses and cause the platform to restart a healthy process. Configure a dedicated health endpoint that implements the probe method, or use the platform’s documented probe settings. Also verify the application listens on the port the platform exposes, not only on a loopback interface or a different internal port.
Rate limits
GitLab documents two distinct rate-limit cases: an HTTP 429 refusal and a tool result that carries a rate-limit error. Treat them differently. For HTTP 429, inspect Retry-After and rate-limit headers, wait the directed interval, and reduce request concurrency. For a tool-level rate-limit result, inspect the structured result and its retry information; the MCP session itself may still be healthy. Do not retry non-idempotent operations automatically unless the tool’s contract makes repetition safe.
Deployment state and scaling
Record whether the service is running one instance or many, whether sessions are sticky, and whether callbacks can reach the public route. A deployment that scales out without shared session state may pass initialization and then fail when a later request lands on another instance. Conversely, a stateless server may require every request to carry context that a stateful client assumed would persist.
Symptom-to-check reference
| Observed symptom | Check first | Most likely direction |
|---|---|---|
| Process launches but the client cannot parse messages | Raw stdout, newline framing, child exit status | Remove all non-MCP output from stdout; send logs to stderr and correct the launch environment. |
| HTTP 404, 405, or event-stream content | Endpoint path, HTTP method, and selected transport | Match Streamable HTTP with Streamable HTTP or explicitly select SSE for an SSE-only server. |
-32601 method not found |
Initialization completion, exact method spelling, protocol revision | Negotiate first and use a method supported by that server and transport. |
HTTP 400 or -32700 |
JSON-RPC version, ID, method, params, and body encoding | Correct the envelope or find the proxy that is rewriting or truncating it. |
| Tool reports failure but no exception is caught | Result isError/is_error |
Handle protocol-level tool errors in addition to thrown exceptions. |
| Generic remote connection error after deployment | Proxy and server logs, Host header, auth, route, and status | Determine whether the platform rejected HTTP before MCP returned a result. |
| HTTP 429 | Retry-After and rate-limit headers |
Honor the delay and distinguish an HTTP refusal from a tool-level rate-limit result. |
| Container restarts during health checks | Probe path, method, and configured listening port | Use a dedicated health endpoint instead of probing a POST-only MCP route. |
Final recovery checklist
- Write down the exact stage, timestamp, versions, transport, endpoint, and complete sanitized symptom.
- For stdio, verify command, environment, process lifetime, newline framing, and protocol-only stdout.
- For HTTP or SSE, verify route, method, content type, TLS, authentication, proxy rewrites, and transport selection.
- Complete initialization and record the negotiated protocol revision and capabilities before testing methods.
- Validate the exact JSON-RPC envelope and method name; treat
-32601and-32700as protocol clues, not universal diagnoses. - After a call reaches the server, inspect the result error flag as well as exceptions.
- Reproduce with MCP Inspector or a minimal SDK client, then compare client, server, and proxy logs.
- For hosted services, check Host-header rules, health probes, ports, session state, callbacks, and rate-limit headers.
Following that order keeps a transport problem from being mistaken for a tool bug and keeps a valid tool error from being hidden behind a generic connection message.
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