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First identify which connection is failing: the local LLM client reaching Home Assistant’s MCP server, Home Assistant authenticating to Google Calendar, Home Assistant authenticating to Google Mail, or Home Assistant fetching a remote calendar feed. These routes use different endpoints, credentials, and permissions. Test the failing boundary from the machine or container that actually makes the request; fixing Google OAuth will not repair an unreachable MCP endpoint, and exposing MCP will not grant Gmail access.
Start by locating the failed connection
“The model cannot connect” is not precise enough to choose a fix. In a tool-enabled setup, the HTTP request may come from an MCP client, an integration inside Home Assistant, a proxy, or a separate automation service rather than from the model process itself. Record these details before changing settings:
- The LLM application and version, plus the MCP or tool connector it uses.
- Where each component runs: the Home Assistant host, the same computer, a container, another LAN device, or a remote server.
- The exact hostname, scheme (
httporhttps), port, and path being requested. - Which application owns the credential: an MCP token, Home Assistant OAuth client, Google Application Credential, feed password, or another token.
- The complete error text, HTTP status, and timestamp.
Use the boundary that matches the symptom:
| What is failing | Endpoint or service to inspect | Credential to verify |
|---|---|---|
| LLM client cannot use Home Assistant tools | Home Assistant MCP Server at /api/mcp |
Home Assistant authentication token and, where applicable, OAuth client settings |
| Home Assistant cannot load Google Calendar | Google Calendar integration and its authorization flow | Google developer project, enabled Calendar API, and Home Assistant Application Credentials |
| Home Assistant cannot perform Google Mail actions | Google Mail integration | Google project with Gmail API enabled and an authorized account |
| Home Assistant cannot fetch an iCalendar URL | Remote Calendar integration | Public URL or HTTP Basic Authentication, if the feed requires it |
| Home Assistant IMAP mail setup fails | IMAP server connection | IMAP credentials supported by that integration; OAuth2 is not supported |
Do not assume a successful connection at one boundary proves the others work. A local LLM may reach Home Assistant while Home Assistant is denied access to Google, or Google Calendar may work while the LLM has not been given a Home Assistant tool that exposes calendar entities.
Verify Home Assistant’s MCP endpoint
Home Assistant documents its MCP Server at /api/mcp. The client must authenticate when calling it. The official documentation states: “The Home Assistant MCP server is exposed as /api/mcp and requires the client to provide an authentication token.” See the Home Assistant MCP Server documentation for the supported setup.
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Confirm the integration and selected API
- Open Home Assistant and confirm that the MCP Server integration is configured.
- Check the MCP client’s configured base URL and path. The normal path is
/api/mcp. - If your Home Assistant installation exposes more than one LLM API, verify that the client is selecting an existing API-specific path. A nonexistent API ID returns HTTP 404.
- Confirm that the token supplied by the client is a Home Assistant token accepted by the server and that it has not been revoked.
A 404 narrows the search to the server integration, path, or API identifier; it does not demonstrate that the local model is defective. A connection refusal or timeout instead points toward DNS, routing, firewall, port, or proxy problems. An authentication status points toward the token or OAuth configuration.
Test from the MCP client’s network location
Run reachability tests from the host or container where the MCP client runs, not only from the Home Assistant machine. Resolve the configured hostname, verify that the TCP port is reachable, and check that the TLS certificate matches the hostname when using HTTPS. An example diagnostic request is:
curl -i https://home-assistant.example/api/mcpUse the exact URL configured in the client and supply authentication only according to your client’s documented method. A response showing an HTTP status proves that the route reached an HTTP server; it does not by itself prove that the MCP handshake or authorization is valid. If the client is in a container, test inside that container because its DNS and network namespace can differ from the host.
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After the MCP endpoint responds, check the client’s tool list and permissions. Home Assistant can expose entities and actions through its APIs, but a local LLM client may be configured to use only selected tools. Calendar entities being visible in Home Assistant does not guarantee that the MCP client has been allowed to read them or invoke an event-creation action.
Repair Home Assistant OAuth and client compatibility
Home Assistant’s documented MCP OAuth flow has several details that commonly get mixed up.
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Use the right OAuth values
- The OAuth client ID identifies the LLM client application. It is not the Home Assistant URL.
- Home Assistant does not use a client secret for this flow. If a client insists on a required secret field, the Home Assistant documentation says any text can be entered.
- The URL entered in the client must exactly match Home Assistant’s configured Internal URL or External URL, including hostname, scheme, and (when used) port.
Changing from an internal name to a public domain, or from HTTP to HTTPS, changes the OAuth issuer and redirect context. Use one consistent URL during discovery, login, and token exchange. Repeated failed logins can trigger Home Assistant’s IP ban feature when IP bans are enabled; clear the underlying error before repeatedly retrying.
Check the registration method your client expects
Home Assistant uses the OAuth Client ID Metadata Document approach. It does not provide an RFC 7591 Dynamic Client Registration endpoint. A client that strictly requires automatic RFC 7591 registration may therefore be incompatible. If the client supports manual OAuth configuration, enter Home Assistant’s documented authorization and token endpoints rather than enabling an automatic-registration option that the server does not implement. The MCP Server documentation describes this compatibility boundary.
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- Delete the failed Home Assistant account or connection entry in the MCP client.
- Verify the URL, client ID, and redirect settings before starting a new authorization.
- Authorize once, then inspect the client log for the callback and token-exchange result.
- If the address or client identity changed, revoke the old client token in Home Assistant and create a fresh connection.
Do not copy a Google client secret into the Home Assistant MCP fields. Google credentials belong to the Google integrations described below; MCP authentication is a separate relationship between the LLM client and Home Assistant.
Fix reverse-proxy, tunnel, and remote-access mismatches
A local-only route, a reverse-proxy or tunnel route, and Home Assistant Cloud are different network designs. Diagnose the route the MCP client actually uses.
Match the public URL and discovery metadata
If the client connects through a custom domain, proxy, or tunnel, compare that public hostname and scheme with Home Assistant’s configured URL. The proxy must forward the original X-Forwarded-Host and X-Forwarded-Proto values, and Home Assistant must be configured to trust that proxy. A mismatch can cause OAuth discovery metadata to contain relative paths that a conforming client rejects.
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- Open the public Home Assistant URL from the same network location as the MCP client.
- Confirm that the browser remains on the intended hostname during login and callback.
- Inspect proxy configuration for forwarded host and protocol headers.
- Verify Home Assistant’s trusted-proxy and external-URL settings, then restart or reload the affected service as required by your deployment.
- Repeat discovery and authorization only after the URL and headers agree.
For readers who need remote access but do not want to maintain proxy or tunnel headers, Home Assistant documents Home Assistant Cloud as an alternative route. It changes the network path; it does not repair an invalid Google credential or an incorrectly configured MCP client.
Repair Google Calendar independently
Google Calendar is a separate Home Assistant integration with its own Google API, credentials, and authorization. Follow the Google Calendar integration documentation and the Application Credentials documentation.
Check the Google Cloud project and credential type
- Identify the Google Cloud project associated with the Home Assistant application credential.
- Enable the Google Calendar API in that same project.
- Use the currently recommended Web Auth credential type for the Home Assistant integration.
- Set the redirect URI exactly to
https://my.home-assistant.io/redirect/oauth. Do not add a trailing slash, change the scheme, or substitute the Home Assistant host. - Start the authorization flow from Home Assistant and verify the Home Assistant instance URL shown on the linking page before approving access.
If the authorization page reports that a code expired, restart the flow and complete it without leaving the page idle. If credentials are rejected, inspect Home Assistant logs for the failing credential identifier. Deleting and recreating the affected Application Credential can resolve a corrupted or mismatched entry, but recreate it only after confirming that the Google project and API are correct.
Confirm what a successful Calendar connection provides
Home Assistant represents each connected Google calendar as a calendar entity. The integration supports calendar actions, including creating events. That does not automatically make every calendar field available to every local LLM: the MCP client must be configured to use the Home Assistant API and tools that expose the relevant entity and action. Test the entity inside Home Assistant first, then test the same operation through the LLM client.
Repair Google Mail without confusing it with Calendar
Google Mail is a distinct Home Assistant integration, not an extension of the Calendar integration. It requires Gmail API enablement and account authorization. Use the Google Mail integration documentation for its current setup and actions.
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Validate Gmail authorization
- Open the Google Cloud project used by the Home Assistant mail credential.
- Enable the Gmail API in that project.
- Check the Home Assistant Application Credential selected for Google Mail.
- Reauthorize the account if the token was revoked, the account changed, or scopes were altered.
- Review Home Assistant logs for an invalid-client, permission, or token-refresh error before recreating credentials.
Check the documented capability boundary
The documented Google Mail actions include setting a vacation response and a notify action that can draft or send plain-text email. The documentation does not establish Google Mail as a general-purpose Gmail search-and-read connector for arbitrary local LLM clients. If your goal is inbox search or message retrieval, verify that the specific Home Assistant API or tool you configured actually exposes that operation instead of assuming that account authorization supplies it.
If the setup uses Home Assistant’s IMAP integration rather than Google Mail, apply a different rule: the IMAP documentation states that OAuth2 is not supported. An OAuth-only IMAP account therefore needs another supported authentication arrangement or a different integration; repeatedly trying to force IMAP OAuth will not fix the connection.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Distinguish Google Calendar from a remote iCalendar feed
Home Assistant’s Remote Calendar integration fetches an iCalendar URL. It can optionally use HTTP Basic Authentication and retries after a failed initial update. This route does not use Google OAuth and is not the same as either direct Google Calendar access or the MCP connection.
Diagnose a remote feed
- Confirm the URL returns an iCalendar document from the Home Assistant host, including when that host has no browser session or interactive login.
- Check whether the provider requires HTTP Basic Authentication and enter the correct username and password in the Remote Calendar configuration.
- Inspect the integration after its retry interval rather than judging it solely from the first failed update.
- Verify that outbound DNS, HTTPS, and firewall rules allow Home Assistant to reach the feed host.
A feed that works in a desktop browser may still fail for Home Assistant if it depends on cookies, JavaScript, or an interactive OAuth page; the documented integration expects an iCalendar URL and optional Basic Authentication.
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| Route | Request origin | Typical authentication | Capabilities to verify | Common failure clue |
|---|---|---|---|---|
| MCP to Home Assistant | LLM/MCP client | Home Assistant token or documented OAuth flow | Home Assistant entities and actions exposed to that client | 404 for a wrong path/API ID, timeout for network route, or auth error for token/OAuth |
| Google Calendar integration | Home Assistant | Google Web Auth credential and Calendar API authorization | Calendar entities and event actions | Expired code, invalid credential, or missing Calendar API |
| Google Mail integration | Home Assistant | Google credential and Gmail API authorization | Documented vacation-response and plain-text draft/send actions | Gmail API disabled, revoked token, or wrong Application Credential |
| Remote Calendar | Home Assistant | iCalendar URL, optionally HTTP Basic Authentication | Reading the published feed | Feed unreachable or credentials rejected; integration retries |
| IMAP | Home Assistant | Supported IMAP authentication; OAuth2 unavailable | Mail retrieval through IMAP | OAuth-only server configuration cannot authenticate |
These routes are not interchangeable implementations. They differ in where the request originates, whether access is local or remote, which credential is presented, and whether the required operation is reading data, creating an event, sending mail, or controlling Home Assistant.
Follow an evidence-based failure checklist
If the error is “not found” or HTTP 404
- Check that the MCP Server integration is enabled.
- Compare the client path with
/api/mcp. - If an API-specific path is used, verify that the API ID exists.
- Confirm that a proxy is not rewriting or stripping the path.
If the error is timeout, refused connection, or DNS failure
- Test name resolution and the port from the MCP client’s host or container.
- Check firewall rules, container networks, VPN boundaries, and tunnel status.
- Verify whether the client is using an internal hostname that is unavailable from its network, or a public hostname that requires a proxy.
- For HTTPS, check certificate validity and hostname matching.
If the error is OAuth discovery, redirect, or client registration
- Use the exact configured Internal or External URL, including scheme and hostname.
- Check forwarded host and protocol headers and Home Assistant’s trusted-proxy configuration.
- Determine whether the client requires RFC 7591 Dynamic Client Registration; Home Assistant’s MCP flow does not provide that endpoint.
- Ensure the client ID identifies the LLM application, not Home Assistant’s URL.
If Google reports invalid credentials or an expired code
- Identify whether the failing integration is Calendar or Google Mail.
- Confirm the corresponding API is enabled in the correct Google Cloud project.
- Retry an expired authorization code immediately through Home Assistant.
- Inspect logs, then recreate only the affected Application Credential if it remains invalid.
If the LLM connects but cannot perform the requested task
- Test the entity or action directly in Home Assistant.
- Check that the MCP client has selected the relevant Home Assistant API and tool.
- Match the requested operation to documented capabilities: calendar entities and event actions differ from Google Mail’s documented vacation and plain-text notification actions.
- Do not infer Gmail search/read capability from successful account authorization.
Keep credentials and recovery changes controlled
Change one boundary at a time and save the original error before each change. A useful sequence is: prove network reachability, prove MCP authentication, prove the Home Assistant entity or action locally, then prove the Google integration separately. This prevents a proxy change from being mistaken for an OAuth fix.
- Do not paste tokens, client secrets, or authorization codes into public issue reports.
- Use separate credentials for MCP and Google integrations so revoking one does not unnecessarily break the other.
- After a hostname or proxy change, revoke stale OAuth sessions and authorize again with the final URL.
- Watch for IP bans after repeated failed Home Assistant logins and remove the cause before retrying.
- Record the exact working URL, redirect URI, API project, and enabled APIs for future recovery.
Once each boundary succeeds independently, run a minimal end-to-end test: ask the LLM to perform one harmless Home Assistant read, then one permitted calendar or mail action. A successful end-to-end result means the client route, Home Assistant authentication, Google authorization (if used), and tool exposure all align; it does not imply that unconfigured Gmail or Calendar operations are available.
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