No: an AI agent should request a capability, not receive raw API credentials in its conversational context. Keep credentials with the client or server component that needs them, and make each token valid only for its intended resource. In particular, an MCP server must not forward the token it received from its MCP client to an upstream API; it needs a separate credential for that API. MCP authorization is optional overall, however, and its HTTP authorization profile does not describe STDIO credential handling.
What the security boundary actually protects
A credential carries authority: whoever can use it may be able to perform actions as the person or service it represents. The goal is therefore not merely to hide a string. It is to constrain where that authority can be stored, which resource can accept it, what operations it permits, and which components can expose it.
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In MCP’s HTTP authorization model, the client acts as the OAuth client, the protected MCP server is the resource server, and the authorization server issues a token for use at that MCP server on behalf of the resource owner. The model-facing agent and the components that handle OAuth tokens are not interchangeable roles. See the MCP Authorization specification, version 2025-11-25.
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That division is the practical answer to “Should an AI agent ever see my API credentials?” A system should let the agent ask for an authorized tool operation without inserting a reusable secret into model-visible prompts, messages, or tool descriptions. This is a security design principle; it is distinct from the protocol’s specific rule against passing an inbound MCP token to another resource.
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Can an MCP server pass its access token to another API?
No. Treat the MCP server and the upstream API as separate protected resources. The client requests a token intended for the MCP server. The server validates that token before processing a request. If the server then calls a third-party or upstream API, it must use a separate token issued for that API—not forward the client’s MCP token, even if the upstream call happens to work with it.
The MCP authorization security considerations, version 2026-07-28, state: “The MCP server MUST NOT pass through the token it received from the MCP client.” They also require servers to accept only tokens specifically intended for themselves and reject tokens that do not identify them as the audience or otherwise verify them as the intended recipient. Read the MCP Authorization Security Considerations.
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| Credential | Intended recipient | Who uses it |
|---|---|---|
| Client-to-MCP token | The MCP server | The client presents it; the MCP server validates it before handling the protected request. |
| Upstream API token | The upstream API | The MCP server uses a separate credential when it makes an upstream request. |
Audience restriction limits the damage from a token being misrouted: a credential issued for one resource should not automatically grant access to another. A server that accepts a token merely because it is syntactically valid—or because another service accepts it—has failed to preserve that boundary.
Is authorization required for every MCP connector?
No. MCP authorization is optional at the protocol level. The 2025-11-25 authorization specification describes the HTTP-based authorization profile; implementations using that profile should conform to it. The same specification says STDIO implementations should not follow that HTTP profile and should retrieve credentials from the environment. Other transports need security practices appropriate to their own transport and deployment.
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| Deployment case | What the cited guidance establishes |
|---|---|
| HTTP-based MCP authorization | The versioned authorization profile defines OAuth roles and behavior for HTTP-based transports. See the 2025-11-25 Authorization specification. |
| STDIO | Do not apply the HTTP authorization profile as though it describes STDIO; the specification says credentials should be retrieved from the environment. |
| Remote endpoint exposing non-public tools or data | OWASP recommends authentication, authorization validation on each protected request, and TLS for remote Streamable HTTP connections. See the OWASP MCP Security Cheat Sheet. |
Optional protocol authorization does not mean a deployment should leave private capabilities unprotected. Apply authentication and per-request authorization where tools or data are non-public; do not infer that a local STDIO connector uses the HTTP OAuth flow.
How to protect the authorization flow
Protecting a token means securing the process that obtains and uses it as well as the place it is stored. The MCP security considerations warn that stolen client-stored tokens and tokens cached or logged by servers can let an attacker make requests that appear legitimate.
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- Request a token for the right resource. The client should identify the intended MCP server as the resource, and the server must validate that the token was issued for it.
- Use PKCE and validate the authorization response. The client must verify that the authorization server supports PKCE before starting, use S256 when technically capable, and refuse to proceed if the
code_challenge_methods_supportedsignal is missing. It must validate OAuthstate. - Match redirects exactly. Clients need registered redirect URIs, and authorization servers must compare redirect values exactly against preregistered URIs.
- Protect secrets in storage and runtime. Use secure token storage, restrict access to credential stores and processes, and do not put tokens or authorization material in logs. A secrets manager can help control storage and access, but it does not by itself guarantee that secrets stay out of model context.
- Reduce the value of leakage. The security considerations identify short-lived access tokens as a mitigation; public clients must rotate refresh tokens.
These requirements and recommendations are described in the 2026-07-28 MCP Authorization Security Considerations. In the same release cycle, the MCP project said Client ID Metadata Documents are replacing Dynamic Client Registration as the standard, while DCR remains for backward compatibility and is slated for future removal. Check the applicable specification version before implementing client registration; see the 2026-07-28 specification announcement.
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An MCP server may act as a proxy between a client and a third-party API. Because it can make requests using upstream authority, it can become a confused deputy: an attacker may trick it into obtaining or using permissions without the user’s proper consent, or into acting in a context different from the one the user authorized.
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Preserve the user’s authorization context across the proxy flow and validate each inbound request. The 2026-07-28 security considerations require proxy servers using static client IDs to obtain user consent for each dynamically registered client before forwarding to third-party authorization servers. MCP’s Security Best Practices also warn that possession of a state handle is not authentication. An opaque handle may point to stored state; it does not prove who holds it or what that person is authorized to do.
Quick Recap
Implementation checks before deployment
- Can the agent request a tool operation without receiving a reusable upstream secret in its prompt, response, or other model-visible content?
- Does the client request an MCP token for the intended server, and does that server reject tokens not meant for it?
- Does every upstream API call use a credential intended for that API rather than the inbound MCP token?
- Are tokens protected from logs, unsafe caches, and overly broad access by processes or operators?
- For HTTP OAuth, are HTTPS authorization endpoints, PKCE, exact redirect matching, and state validation in place?
- Do non-public remote tools and data require authentication and per-request authorization, with TLS for remote Streamable HTTP?
- Does a proxy verify the request and preserve user consent and authorization context instead of treating a state handle as proof of identity?
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