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The message “Failed to connect to MCP server” is a generic failure notice, not a diagnosis. In Open WebUI, the cause may be an incorrect integration type, a URL that the backend cannot reach, mismatched authentication, an incomplete OAuth flow, an unsupported transport, or a server that takes longer than the initialization timeout. Start with the configuration checks below, then inspect backend logs and confirm your Open WebUI version before applying a fix from an issue report.
1. Confirm the MCP connection type and URL
Choose MCP (Streamable HTTP), not OpenAPI
Open WebUI’s official documentation states that “Native MCP support in Open WebUI is Streamable HTTP only.” In Settings > Admin > Integrations, add the server as MCP (Streamable HTTP). Do not paste an mcpServers JSON configuration into an OpenAPI connection, and do not register an MCP endpoint as OpenAPI. The documentation warns that this mismatch can produce a crash or an infinite loading screen. See the Open WebUI MCP guide and the repository documentation.
Use an address reachable from the backend
The URL is tested by the Open WebUI backend, not necessarily by the browser where you opened the interface. If Open WebUI runs in Docker and the MCP server runs directly on the host, localhost points to the container itself. The Open WebUI guide recommends a host address such as http://host.docker.internal:PORT for that topology.
- Both services on the same Docker network: use the MCP container’s service name and port, for example
http://mcp-server:8000/mcp. - MCP server on the host: use
host.docker.internaland the listening port. On Linux, this name may require Docker’s host-gateway mapping. - Remote server: use its HTTPS URL and verify that firewalls, reverse proxies and allow-lists permit requests from the Open WebUI host.
From the Open WebUI container, test DNS and the endpoint with the tools available in your image (for example, curl). A successful request from your laptop does not prove that the container can resolve or reach the same address.
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2. Match authentication to what the server expects
None means no authorization header
If the MCP server does not require a token, set authentication to None. Selecting Bearer without entering a key can send an empty Authorization: Bearer header, which many servers reject. Remove accidental whitespace and make sure the token belongs to the server and environment you are calling.
Bearer and custom credentials
For a bearer-protected endpoint, select Bearer and enter the complete key in the credential field. Check reverse-proxy logs for HTTP 401 or 403 responses. A 401 usually means the credential is absent or invalid; a 403 can indicate a valid identity without permission to use the MCP endpoint.
OAuth 2.1 needs an interactive authorization
OAuth authorization requires a browser redirect and user consent. Do not pre-enable an OAuth 2.1 tool as a model default and expect authorization to begin during a chat completion. Enable the tool manually in the chat so Open WebUI can start the flow first. For the community-contributed Notion integration, toggling the tool off and on in a chat can restart authorization after an expired session; follow the Notion integration instructions.
For Docker deployments, the repository documentation identifies WEBUI_SECRET_KEY as a prerequisite for OAuth-connected tools to survive container restarts or recreation. Set it to a persistent secret and redeploy rather than generating a new value each time.
3. Do not confuse OAuth discovery with an MCP connectivity test
For OAuth connections, Check OAuth Discovery only fetches and parses the authorization-server discovery document. It does not contact the MCP server, authenticate a tool call, or list tools. Therefore, a successful discovery check can coexist with “Failed to connect to MCP server” during actual use.
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- Run discovery to confirm that the authorization server metadata is readable.
- Complete consent for the intended user account.
- Enable the MCP tool manually in a chat.
- Watch the Open WebUI backend and MCP server logs while invoking a simple tool.
If discovery succeeds but invocation fails, focus on the redirect URI, token audience or scope, expired sessions, and the MCP endpoint’s authorization response—not on discovery DNS alone.
4. Check initialization time and the function filter
Allow for cold starts and large tool catalogs
Open WebUI must establish a session and complete session.initialize() before tools can be used. A cold-starting server, a sleeping hosted process, or a server exposing many tools can exceed the initialization timeout. The troubleshooting documentation lists a default of 10 seconds and advises raising MCP_INITIALIZE_TIMEOUT when initialization legitimately needs longer.
Increase the setting only after confirming that the server eventually responds. A longer timeout cannot repair a wrong URL, an HTTP 401, or a server that never completes initialization. Restart Open WebUI after changing the environment setting and then retry with a clean chat.
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Try the documented empty-filter workaround
The troubleshooting page suggests entering a comma in an empty Function Name Filter List when an empty field coincides with a connection error. Treat this as a targeted diagnostic, not a universal setting: save the integration, reconnect, and remove the comma if it makes no difference.
5. Verify transport compatibility
Native Open WebUI MCP support accepts Streamable HTTP. A server that exposes only stdio or legacy SSE needs a compatible bridge. The official documentation names mcpo, an open-source proxy that translates stdio or SSE servers into OpenAPI-compatible endpoints. Configure the bridge according to its documentation, then add the resulting endpoint using the connection type it exposes; do not pretend that a raw stdio command is a Streamable HTTP URL.
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6. Read logs before changing more settings
The browser message hides the failure stage. Collect logs from all three points while reproducing the error:
- Open WebUI backend: look for DNS errors, connection refusals, TLS failures, timeout messages, authorization status codes, and tool-initialization exceptions.
- Reverse proxy or firewall: verify whether a request arrived, which path and method were used, and whether it was blocked.
- MCP server: check startup completion, protocol negotiation, authentication, and the time required to answer initialization.
Record the installed Open WebUI version, deployment method, integration type, transport, authentication mode, endpoint hostname, and the exact timestamp. Redact tokens before sharing logs.
7. Treat GitHub issue reports as version-specific clues
A reported discussion describes the symptom where a connection test works but chat tool use fails. A separate May 2026 issue discusses a 10-second initialization timeout and a configurable setting while reporting an overlay on v0.9.5. Those details are not a guarantee about every release. Check your version, current environment variables, and logs before copying a proposal.
The same June 2026 report mentions failures for names with leading or trailing whitespace when ENABLE_FORWARD_USER_INFO_HEADERS is enabled; it is closed, so verify current release behavior before using a whitespace workaround. Upgrade or downgrade only with a documented compatibility reason and a backup of your configuration.
8. A failure-stage decision checklist
| Observed stage | Most useful checks |
|---|---|
| Integration will not save or keeps loading | Use MCP (Streamable HTTP), not OpenAPI; validate URL syntax and required fields. |
| Connection refused or DNS error | Test from the backend; replace container localhost with the correct service name or host.docker.internal. |
| 401/403 response | Match None, Bearer or OAuth to the server; remove empty bearer headers; verify scopes and account. |
| OAuth discovery succeeds but tools fail | Complete interactive consent, manually enable the tool, and inspect token and MCP-server logs. |
| Timeout during initialization | Check cold-start time and tool count; raise MCP_INITIALIZE_TIMEOUT only after confirming eventual responsiveness. |
| Server is stdio or SSE only | Use a bridge such as mcpo; native MCP expects Streamable HTTP. |
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“Connection test works, chat fails”
A test may validate only URL reachability or OAuth metadata. Manually enable the tool, inspect initialization and invocation logs, and verify that the chat user has the required scope.
“Connection refused”
The process is stopped, listening on another interface or hidden behind the wrong Docker hostname. Confirm the listening address and test from the Open WebUI container.
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Replace an empty or stale key, or choose None if the server is unauthenticated. Check proxy logs for the actual authorization header status.
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Timeout with no server error
Measure startup and initialization time. Increase MCP_INITIALIZE_TIMEOUT only when the server eventually completes; otherwise fix startup, routing or protocol compatibility.
OAuth works until Docker is recreated
Persist WEBUI_SECRET_KEY, reauthorize the intended account, and retry the tool manually.
Security checks administrators should not skip
MCP integrations are admin-only in Open WebUI and are stateful and capability-rich. Add only servers you trust, use HTTPS for remote endpoints, grant the least necessary OAuth scopes, and avoid posting access tokens in issue reports. Secure the Open WebUI host and the MCP server independently; making the endpoint reachable is not the same as making it safe.
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Frequently Asked Questions
Does a successful OAuth discovery check prove the MCP server is working?
No. It only retrieves and parses authorization-server metadata; it does not call the MCP endpoint or list tools.
Can Open WebUI connect directly to a stdio MCP server?
Not through native MCP support, which is Streamable HTTP only. Use a compatible bridge such as mcpo for stdio or SSE servers.
Why does localhost fail when Open WebUI runs in Docker?
Inside a container, localhost refers to that container. A host-running MCP server generally requires host.docker.internal and its listening port.
Should OAuth tools be enabled as model defaults?
No. OAuth 2.1 requires an interactive browser redirect and consent, so enable the tool manually in each chat when authorization is needed.
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Work from the failure stage: correct the integration type, prove backend reachability, match authentication, complete OAuth interactively, verify Streamable HTTP transport, and allow for initialization time. Confirm your Open WebUI version and logs before treating any reported issue as a universal fix.
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