An MCP connection error does not automatically mean the server is down. The failure may happen while a local process starts, while a remote hostname or TLS connection is being resolved, when HTTP authorization or host checks reject a request, during protocol negotiation, or later while waiting for a response. Start by identifying the transport—local stdio or remote HTTP—and then use the raw error, HTTP response, and server or proxy logs to locate the failing layer.
First identify how the MCP client connects
Local and remote MCP connections take different paths, so the same generic error can point to different causes.
- Local stdio: The host launches a process and exchanges protocol messages over its standard input and output. Check that the intended process starts, stays running, and writes only protocol messages to stdout; diagnostic text there can corrupt the exchange.
- Remote HTTP: The client connects to a network endpoint. Determine whether it uses Streamable HTTP or a legacy HTTP+SSE transport. The TypeScript SDK recommends Streamable HTTP for remote servers and describes HTTP+SSE as deprecated for backward compatibility; verify the exact client and server implementation before applying SDK-specific behavior. TypeScript SDK documentation
For local failures, capture the exact launch command, process exit code, stderr, selected server module, and stdout. For HTTP, preserve the endpoint, status code, response headers and body, and relevant server or proxy logs.
Diagnose the error at the layer that produced it
For HTTP, investigate in order: name resolution and reachability, TLS, HTTP routing and authorization, then MCP protocol behavior. A generic SDK exception may hide a useful refusal: the Python SDK documents MCPError: Server returned an error response when an HTTP response cannot be parsed as JSON-RPC. Python SDK documentation
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DNS and endpoint reachability
Confirm that the configured hostname resolves in the environment where the MCP client runs, and that the endpoint and route are the intended ones. A hostname that resolves does not prove that the HTTP request reached the right service: DNS, proxies, load balancers, and virtual-host routing can each affect the path. Collect the client’s exact resolver or connection error and check proxy and server logs rather than treating every failure as an MCP protocol problem. The reviewed MCP sources do not establish a universal DNS error catalog across platforms.
TLS certificate or handshake errors
When the client reports a TLS exception, preserve the exact exception and inspect the endpoint hostname, certificate chain, trust store, and any TLS-terminating proxy. Do not infer that a certificate is invalid from a generic timeout or connection error. MCP documentation does not define one cross-platform mapping from TLS alerts to client error strings, so the useful details depend on the client runtime and network path.
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HTTP 421 or “Invalid Host header”
A 421 Misdirected Request with Invalid Host header can mean the server’s Host-header or DNS-rebinding protection rejected the request—not that DNS lookup failed. The Python SDK’s default Streamable HTTP protection accepts only localhost unless configured; a reverse proxy forwarding a public hostname can therefore trigger the check. Configure an allowlist for the actual public hostname where appropriate, and retain the protection rather than disabling it indiscriminately. The TypeScript SDK also documents localhost DNS-rebinding protection and custom host validation. Python SDK documentation · TypeScript SDK documentation
HTTP 401 and 403
Treat these statuses as authorization evidence. A 401 commonly indicates missing or invalid credentials; a 403 indicates a refusal based on authorization or permission, though precise behavior depends on the server and authentication challenge. Check whether credentials are present and current, and verify their audience or resource and scopes against the server’s authentication design. Inspect the challenge and server logs. MCP recommends its Authorization framework for HTTP transports; for stdio, implementations should retrieve credentials from the environment instead. MCP authorization specification
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Tell timeouts apart from negotiation failures
A timeout says that no response arrived within the configured interval; it does not identify why. The request may not have reached the server, a proxy may have blocked or delayed a response, or the server may be slow. Record which phase timed out—connection, initialization, or a later request—and whether server logs show that the request arrived.
Timeout behavior during version negotiation is implementation-specific. In its v2 guidance, the TypeScript SDK treats silence from an HTTP negotiation probe as an outage and rejects with a timeout, while silence on stdio may be treated as a legacy server and followed by an initialize attempt. Other SDKs can use different timeout settings and fallback behavior; consult the implementation in use. TypeScript SDK documentation
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Check protocol compatibility only after accounting for transport and HTTP evidence. Compare the client and server SDK versions and the protocol revisions they support. A 5xx is evidence of a server failure; 401 or 403 is authorization evidence. Neither status alone establishes a protocol-version mismatch. TypeScript SDK documentation · PHP SDK documentation
Use the symptom to decide what evidence to collect
| Symptom | Collect | Investigate |
|---|---|---|
| Local server is absent or appears empty | Launch command, process exit code and stderr, selected server module, stdout output | Startup or configuration failure, wrong server instance, or protocol corruption from non-protocol stdout output. Python SDK documentation |
| Generic “server returned an error response” | Raw HTTP status, body, content type, and server or proxy logs | An HTTP refusal that the SDK could not parse as JSON-RPC. Python SDK documentation |
421 / Invalid Host header |
Request Host header, proxy-forwarded Host, and server security logs | DNS-rebinding protection or host-validation allowlist. Python SDK documentation · TypeScript SDK documentation |
HTTP 401 |
Authentication challenge, credential presence and expiry, authentication callback or logs | Missing or invalid authentication; do not infer protocol incompatibility from this status. TypeScript SDK documentation |
HTTP 403 |
Challenge, scope or permission configuration, and server logs | Authorization refusal or insufficient scope; exact semantics depend on the server and challenge. TypeScript SDK documentation |
| TLS certificate or handshake exception | Exact TLS exception, endpoint hostname, certificate chain, trust store, TLS-terminating proxy | Certificate validation or TLS negotiation; the reviewed sources do not establish universal platform-independent error mappings. |
| Timeout | Transport, connection phase, configured timeout, server and proxy logs, and whether the request arrived | Unreachable or slow endpoint, blocked response, server delay, or transport-specific negotiation behavior. TypeScript SDK documentation · PHP SDK documentation |
| Version negotiation failure | Client and server SDK versions, supported protocol revisions, HTTP status, structured error | Protocol incompatibility after excluding authorization and server failures. TypeScript SDK documentation · PHP SDK documentation |
Retry only when repeating the operation is safe
Handshake retries and tool-call retries are not interchangeable. The PHP SDK documents retries for failed connection handshakes but sends individual tool calls once because they may not be idempotent. A repeated call can duplicate a side effect; retry only when the client’s behavior and the operation’s semantics make replay safe. PHP SDK documentation
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