The Tool Desk
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First identify how the client connects
MCP has different setup paths for local and remote servers. A local integration commonly starts a child process and exchanges protocol messages through standard input and output (stdio). A remote integration uses HTTP; determine whether it uses Streamable HTTP or the older HTTP+SSE transport before applying SDK-specific advice. The TypeScript SDK documentation recommends stdio for local process-spawned integrations and Streamable HTTP for remote servers, and describes HTTP+SSE as deprecated for backward compatibility. Transport support and behavior still depend on the client and server implementation.
- For stdio: Record the exact launch command, process exit code, and standard error. Check that the intended server process starts and that standard output contains only protocol messages; diagnostic text on stdout can corrupt the exchange.
- For HTTP: Verify the configured hostname and endpoint, then retain the raw HTTP status, response headers and body, plus relevant client, server, and proxy logs. A generic SDK exception can obscure a refusal that was not returned as JSON-RPC.
The Python SDK documents the literal message MCPError: Server returned an error response; that wording alone does not reveal whether the cause was networking, security middleware, or something else.
Use the observed error to find the failing layer
| Symptom | Evidence to collect | Likely area |
|---|---|---|
| Local server is missing or appears empty | Launch command, process exit code and stderr, selected server module, and stdout output | Startup or configuration, the wrong server instance, or stdout polluted with non-protocol output |
| Generic “server returned an error response” | Raw HTTP status, body, content type, and server/proxy logs | An HTTP refusal the SDK could not parse as JSON-RPC |
421 or Invalid Host header |
Request Host header, proxy-forwarded Host, and security logs | Host validation or DNS-rebinding protection |
HTTP 401 |
Authorization challenge, credential presence and expiry, and authentication logs | Authentication; do not treat the status by itself as evidence of protocol incompatibility |
HTTP 403 |
Challenge, scope and permission configuration, and server logs | Authorization or insufficient permission; exact meaning depends on the server and challenge |
| TLS certificate or handshake exception | Exact TLS exception, endpoint hostname, certificate chain and trust store, and any TLS-terminating proxy | Certificate validation or TLS negotiation; there is no universal MCP-wide TLS error catalog |
| Timeout | Transport, connection phase, configured timeout, server/proxy logs, and whether the request arrived | Unreachable or slow endpoint, blocked response, server delay, or transport-specific negotiation behavior |
| Version negotiation failure | Client/server SDK versions, supported protocol revisions, HTTP status, and structured error | Potential protocol incompatibility, after network, authorization, and server failures have been ruled out |
Diagnose HTTP routing, DNS, and TLS
DNS and endpoint reachability
Check that the configured hostname resolves as expected and that the client is reaching the intended endpoint. A DNS or connection failure happens before MCP can exchange protocol messages. Preserve the exact resolver or connection error rather than translating every failure into “server down”; proxy routing and the endpoint configuration can also be involved.
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Host-header rejection is not necessarily a DNS failure
A 421 Misdirected Request with Invalid Host header can mean the server received the request but rejected its Host header under a DNS-rebinding defense. The Python SDK documentation says its default Streamable HTTP protection accepts only localhost unless configured. A reverse proxy that forwards a public hostname can therefore trigger the check even when DNS resolution succeeds. Configure an allowlist for the actual public hostname when appropriate; do not disable host protections indiscriminately. The TypeScript SDK documentation also describes localhost DNS-rebinding protection and custom host validation.
TLS errors need the underlying exception
Inspect the raw TLS exception and the hostname the client used. Check whether the certificate chain is trusted and whether a proxy terminates TLS. An MCP connection error label does not identify a universal TLS cause, and the reviewed SDK material does not establish common TLS alert mappings across platforms.
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Interpret authentication responses before blaming protocol negotiation
An HTTP 401 is evidence of an authentication challenge, commonly because credentials are absent or invalid. A 403 indicates the request was refused for authorization or permission reasons, although precise semantics depend on the server and its challenge. Current TypeScript SDK v2 guidance treats 401 and 403 responses during version probing as authorization outcomes, not proof of protocol-era incompatibility.
Check that the credential is present, unexpired, intended for the correct audience or resource, and carries the scopes the server expects. Use the server’s authentication design, challenge, and logs to determine the required remedy. The MCP specification recommends its Authorization framework for HTTP transports; for stdio, it says implementations should retrieve credentials from the environment instead.
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Check version negotiation and timeouts in context
Version negotiation
Clients and servers need compatible protocol behavior, but not every failure during connection setup is a version mismatch. A 5xx response indicates a server failure; a 401 or 403 is authorization evidence. First examine network and HTTP evidence, then compare the actual client and server SDK versions and supported protocol revisions. SDKs can differ in negotiation and fallback behavior, so consult documentation for the implementation in use.
Timeouts
A timeout means a response did not arrive within the configured interval; it does not identify why. The TypeScript SDK v2 negotiation guidance treats silence over HTTP as an outage and rejects with a timeout, but may interpret silence on stdio as a legacy server and fall back to initialize. Other SDKs have their own connection, initialization, and request timeout settings. Record which phase timed out and whether the server received the request.
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Retry only when replaying the operation is safe
Connection-handshake retries do not automatically make it safe to replay a later tool call. The PHP SDK documentation describes retries for failed connection handshakes and says individual tool calls are sent once because they may not be idempotent. That is SDK-specific behavior, but the practical caution applies broadly: a retried operation with side effects could run twice. Check the client’s retry policy and the operation’s semantics before enabling retries.
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A practical troubleshooting order
- Identify the transport: determine whether the client uses local stdio, Streamable HTTP, or legacy HTTP+SSE, and confirm the exact SDK and version.
- For stdio, verify startup: check the launch command, process exit status, stderr, selected server module, and whether stdout contains only protocol traffic.
- For HTTP, separate reachability from protocol: check hostname resolution and endpoint routing, then inspect TLS exceptions directly.
- Capture the response: preserve the HTTP status, headers, and body, along with proxy and server logs. Do not infer the cause from a generic client exception alone.
- Follow the evidence: investigate Host validation for a 421, credentials and permissions for 401/403, server logs for 5xx, and the timed-out phase for a timeout.
- Check protocol compatibility last: compare supported revisions after excluding transport, server, and authorization failures.
- Review retry safety: distinguish handshake retries from replaying a tool or other operation that may have side effects.
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