An MCP “Connection closed” error means the client lost its server process or transport, but the message alone does not identify the cause. First determine whether the server uses local stdio, Streamable HTTP, or SSE, and note exactly when the close occurs: during process launch, initialization and protocol negotiation, or after a working session. Then follow the matching diagnostic path below.
Start with the exact error and transport
Record the complete message, client and version, server command or endpoint, transport, and timing. Messages such as “Connection closed immediately after launch,” “Server works in Inspector but not in Claude Desktop,” and “SSE stream disconnected: TypeError: terminated” point to different branches. The official TypeScript SDK troubleshooting guide organizes remedies by the verbatim error rather than treating every close as a timeout.
- Launch-time close: the process may exit, fail to find its executable, lack an environment variable, or write invalid data to the protocol stream.
- Initialization or negotiation failure: client and server may not support the same protocol era or transport behavior.
- Established HTTP/SSE session closes: investigate authentication, proxies, server errors, network interruptions, and keepalive behavior.
Fix a local stdio server that closes immediately
1. Run the configured command outside the host
Copy the exact command from the MCP host configuration into a terminal. If the process exits immediately, capture its exit code and stderr. A command that stays alive in the terminal but closes in the host usually has a different environment, working directory, or executable lookup path.
- Use an absolute path for the runtime or executable when the host has a different
PATH. - Confirm every required API key and other environment variable is present in the host configuration, not only in your interactive shell.
- Use the same working directory that the server expects for relative imports, configuration files, and virtual environments.
- Check file permissions and runtime versions for the account that launches the host.
Do not assume that a successful manual launch proves the host is configured correctly. Installation guidance highlights host-versus-terminal differences as a common reason a server works in one environment but not another (MCP server installation guide).
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2. Keep stdout strictly for MCP messages
With stdio, the host parses stdout as JSON-RPC. A startup banner, debug print, progress message, or dependency warning on stdout can corrupt the stream and make the host report a closed connection. Send human-readable diagnostics to stderr instead.
For example, in a TypeScript server use:
console.error("Server starting");Do not use console.log for diagnostics on a stdio server. Also inspect libraries and shell wrappers: a package warning printed before your server starts has the same effect as your own log line. Keep stdout limited to the SDK’s protocol responses.
3. Verify that the process remains alive
A server can launch successfully and still terminate while handling initialization. Watch stderr and the host’s MCP log while connecting. Look for missing modules, invalid configuration, uncaught exceptions, a rejected authentication check, or a startup script that reaches its end instead of entering the server loop.
Compare the host’s command, arguments, environment, current directory, and user account with the values used in your terminal and in MCP Inspector. Correct the launch configuration before changing protocol settings.
Investigate protocol negotiation and transport compatibility
The TypeScript SDK guide distinguishes negotiation errors from ordinary network failures. A client may request a pinned protocol version that the server does not offer, the two sides may belong to incompatible protocol eras, or the server may exit during the negotiation probe.
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Use the error-specific remedy
- If the client is pinned to a version the server does not advertise, use automatic negotiation where that SDK and host support it.
- If a deployment must temporarily interoperate with an older implementation, restore a supported older protocol version rather than guessing at a newer one.
- If a custom transport fails during pre-initialize probing, test the SDK’s base stdio transport.
These are SDK-specific options, not universal settings. Check the versions and documentation for your client and server before applying them. A refused TCP connection, HTTP 5xx response, proxy failure, or dropped stream should be diagnosed as connectivity or deployment trouble, not automatically labeled a protocol-version mismatch.
Diagnose Streamable HTTP and SSE disconnects
Read the HTTP evidence first
For a remote endpoint, record the HTTP status, response body, request headers, server logs, and any proxy or load-balancer message. Separate these cases:
| Evidence | Likely branch | Next action |
|---|---|---|
| 401 or 403 | Authentication or authorization | Check the token, required header, scope, and endpoint; verify that the host actually sends the credential. |
| 404 or a routing error | Wrong endpoint or deployment path | Confirm the MCP URL and route exposed by the server. |
| 5xx | Server, proxy, or upstream failure | Inspect server and proxy logs and retry after correcting the deployment fault. |
| Connection refused or DNS/TLS error | Network reachability | Test DNS, firewall, certificate, proxy, and outbound network policy from the host machine. |
| SSE stream terminates after connecting | Keepalive, proxy idle handling, server crash, or network interruption | Compare client and server keepalive settings and inspect the disconnect timestamp in both logs. |
Account for SSE keepalives
For the documented TypeScript SDK SSE transport, idle streams receive keepalive comments every 15 seconds by default, and the transport exposes a keepAliveMs setting. This behavior is implementation guidance for that SDK; another client, server, or intermediary may use different defaults. If a reverse proxy closes an idle stream, configure compatible keepalives and proxy idle timeouts, then verify that comments or events reach the client.
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Do not generalize one timeout report
Claude Code issue #85625, opened August 10, 2026, reports a clean HTTP close after 420 seconds followed by reconnection in that environment, including local stdio, local Streamable HTTP, and a remote Atlassian MCP connection. Treat that as a dated client report and a possible reconnect pattern, not as a universal MCP timeout or protocol rule (issue #85625).
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Use MCP Inspector to isolate the failing side
MCP Inspector is a diagnostic client for testing an MCP server. Connect to the server with the same command or endpoint, arguments, environment variables, working directory, and credentials used by the host. Capture stderr, initialization messages, negotiated capabilities, and any HTTP response details.
- Run the server through Inspector using the documented launch method.
- Confirm whether initialization completes and tools or resources are listed.
- Repeat with the host and compare command, executable path, environment, current directory, and user account.
- If Inspector and the host both fail, fix the server, transport, or deployment. If Inspector succeeds, concentrate on the host configuration, version, credential injection, and executable lookup.
Inspector success is evidence about that particular launch environment; it does not prove that a desktop host has identical settings. The SDK’s per-request transport API is documented at the official API reference.
A practical decision checklist
- Stage: launch, initialization/negotiation, or an established session?
- Transport: stdio, Streamable HTTP, or SSE?
- Process: does the exact configured command stay alive outside the host?
- Output: is stdout free of every non-JSON-RPC line?
- Environment: are paths, variables, permissions, and working directory identical?
- Protocol: do client and server advertise a compatible version and transport?
- HTTP: what status, headers, body, proxy message, and server log correspond to the close?
- Reproduction: does Inspector reproduce the issue with the same launch settings?
Common symptoms and targeted fixes
“Connection closed immediately after launch”
Run the command manually, inspect stderr and exit status, replace relative paths with absolute ones, supply missing environment variables, and remove startup output from stdout. If the process exits only in the host, compare its working directory and user account.
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“Works in Inspector but not in Claude Desktop”
Compare the two launch environments rather than rewriting the server. Check executable lookup, inherited variables, current directory, permissions, arguments, and the host’s MCP logs. Reproduce with the host’s exact command.
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“SSE stream disconnected: TypeError: terminated”
Inspect the HTTP status and network logs, then check server crashes, proxy idle limits, TLS interruptions, and keepalive compatibility. For the TypeScript SDK SSE transport, review the 15-second default and keepAliveMs; do not assume another implementation shares it.
Negotiation fails before tools appear
Read the precise negotiation error, check advertised and pinned protocol versions, and test automatic negotiation or a supported legacy version only when your SDK documents that option. A network refusal or 5xx requires connectivity or deployment work instead.
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FAQ
Is “Connection closed” proof that MCP is down?
No. It is a transport-level symptom. The process may have exited, stdout may be malformed, negotiation may fail, or an HTTP/SSE path may have disconnected.
Should I increase a timeout first?
No. Identify the transport and collect the exact process or HTTP evidence first. A timeout change cannot repair invalid stdout, missing credentials, or an incompatible protocol.
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 errorsIs the 420-second close a standard MCP limit?
No. It is reported in one Claude Code issue opened August 10, 2026, with reconnection afterward; it is not a published universal limit.
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