A remote MCP server may reject what you call an API key because it expects a different credential—often an OAuth access token—or because the credential is missing, expired, meant for another resource, or lacks permission. Start with the HTTP status and the WWW-Authenticate response header; they help distinguish an authentication failure from a permission or proxy problem.
First, confirm which credential the server expects
“API key rejected” is a symptom, not a diagnosis. For HTTP-based MCP authorization, the specification describes an OAuth access token sent on each request as Authorization: Bearer <access-token>. A vendor may separately support an API key or a custom header, but those mechanisms are not interchangeable with OAuth. Check the documentation for the specific MCP server and client before changing credentials. The MCP authorization specification, version 2025-11-25, applies to HTTP transports; it says STDIO implementations should use environment-based credential handling instead of this HTTP authorization scheme.
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Also verify that the configured endpoint is the server’s remote HTTP MCP endpoint and that the client supports its transport. A local STDIO command is not a remote HTTP endpoint.
Use the status code and challenge to narrow the cause
| Response | What it usually indicates | What to inspect |
|---|---|---|
| 401 Unauthorized | Authorization is missing or the token is invalid, expired, revoked, malformed, or intended for a different resource. | Whether the expected credential was sent in the right header and format; token validity; the resource or audience; and the WWW-Authenticate challenge. |
| 403 Forbidden | The credential may be recognized, but its scopes, permissions, or an access policy do not allow the request. | Look for error="insufficient_scope" and a scope parameter in the challenge. If they are absent, check other server or proxy policies rather than assuming that broader OAuth scopes will solve it. |
| 400 Bad Request | The authorization request may be malformed. | Inspect the response body and the client’s authorization settings. |
These are the MCP specification’s usual authorization distinctions, not a guarantee that every product uses identical error messages. A response can originate from a proxy or identity layer rather than the MCP server, so check sanitized response details and the logs for the hop that returned it. The specification requires invalid or expired tokens to receive HTTP 401. The MCP TypeScript SDK v2 reference likewise maps invalid_token to 401 and insufficient_scope to 403.
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Fix a 401: check the header, token, and resource
- Confirm presence and format. For OAuth bearer authorization, send
Authorization: Bearer <access-token>. MCP requires the authorization header on every HTTP request, including requests within the same logical session. - Check token validity. Renew or reauthorize if the access token has expired or been revoked. If it is malformed or not recognized, confirm that the client is sending the credential issued by the expected identity provider and that the server’s verifier is configured correctly.
- Compare the resource or audience. A genuine token for one API or MCP endpoint is not automatically valid for another. Check which resource the client requested and which resource the server expects. In the TypeScript SDK’s JWT verifier example, a configured
expectedResourcecauses a 401 when the token reports another resource or no resource. That is an SDK example, not proof that every server validates or normalizes resource values in the same way. - Follow discovery when the challenge provides it. If
WWW-Authenticateincludesresource_metadata, use that Protected Resource Metadata URL to discover the authorization server. MCP clients can also use the specification’s well-known URI fallback. Verify the issuer and published endpoints instead of guessing URLs.
Fix a 403: identify the permission boundary
When the response includes error="insufficient_scope", examine its scope parameter for the permissions the server says are needed. Request the required scopes using the client’s supported authorization flow, then retry a limited number of times after authorization succeeds. A valid identity may still lack permission for a particular tool or operation.
A 403 without an insufficient-scope challenge may come from a different access rule, administrator policy, portal, or proxy. Identify which layer returned it before changing scopes or rotating credentials. MCP SDKs illustrate the general distinction: the Go SDK describes bearer-token middleware that can check expiry and configured scopes, while the Python SDK reference documents 401 for missing or invalid bearer credentials and 403 for a missing required scope. These implementations do not define how every vendor labels an API key.
Check OAuth discovery and redirect settings
If authorization fails during sign-in or callback rather than at the MCP request, inspect the OAuth client configuration as well as the token. Common configuration points include the registered redirect URI, client ID and secret, authorization and token endpoints, token endpoint authentication method, and requested scopes. Use the endpoints published through discovery or specified by the provider; do not invent them.
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If a portal or proxy is involved, test each hop
A portal can manage its own user authentication while also using separate upstream OAuth credentials to reach an MCP server. A successful login to the portal does not, by itself, prove that the upstream server accepts its token or proxy client.
Cloudflare’s remote MCP portal documentation describes non-browser clients receiving 401 responses with OAuth discovery information, upstream callback URLs that need to be allowlisted, and manual configuration of OAuth endpoints, client credentials, and scopes. It also documents cases where an upstream server rejects proxy-based clients with 403 and where an expired admin OAuth token requires upstream reauthentication. These are Cloudflare-specific behaviors, not a general explanation for every MCP 403.
- Check whether the portal authenticated you and whether its upstream credential is also valid.
- Find out which hop produced the status: client, portal or proxy, identity provider, or MCP resource server.
- Use the portal’s error diagnostics and the relevant server or proxy logs to distinguish an upstream failure from a portal-side one.
- If the upstream callback fails, confirm the exact callback URL is allowlisted and the OAuth client settings match the provider.
Retry safely, and keep credentials private
- Record the status, sanitized response body, and
WWW-Authenticateheader. Remove access tokens, API keys, authorization codes, and client secrets before sharing diagnostics. - Confirm the server’s supported credential type and the client’s configured HTTP endpoint and transport.
- Apply the correction indicated by the evidence: attach the expected credential, renew an expired token, correct its resource, request a challenged scope, or repair the relevant OAuth or proxy configuration.
- Retry only after the correction, and limit repeated authorization attempts. If the failure remains, ask the server or proxy administrator to check its verifier, expected resource, scopes, policies, and logs.
Never paste credentials into prompts, put access tokens in URL query strings, or expose secrets in public configuration or logs. The MCP authorization specification prohibits access tokens in URI query strings and requires servers to validate that tokens were issued specifically for the resource they protect. For a particular failure, the decisive evidence is the affected server and client, credential type and issuer, HTTP status, sanitized response and challenge, and whether a proxy or portal is in the path.
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