A desktop MCP client can use a different local callback port each time it starts. An authorization server that demands an exact match on that port may reject the client even when its loopback redirect follows the native-app standard. The fix is to validate the registered redirect’s scheme, loopback IP host, and path while allowing the port to vary for loopback IP redirects, as RFC 8252 requires. Confirm that the specific authorization server implements this exception.
Why a desktop redirect URI mismatch happens
A native desktop client needs a way to receive the authorization response. With a loopback redirect, it starts a temporary HTTP listener on the local machine and supplies a callback URI such as http://127.0.0.1:49152/callback in its authorization request. The operating system can choose an available port when the client starts, so the port may change between runs.
MCP authorization guidance calls for exact validation against preregistered redirect URIs. Applied without accounting for native loopback redirects, a server may compare the request with a registered URI including its port and reject the request when the runtime port differs. That is a standards-based explanation for this class of redirect URI mismatch, not evidence of a defect in any particular MCP client or provider.
What the standards require
The MCP authorization specification requires redirect URI validation against preregistered values to help prevent redirection attacks. For native-app loopback IP redirects, RFC 8252 defines a specific exception: the authorization server must allow the client to specify its listening port at request time.
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“The authorization server MUST allow any port to be specified at the time of the request for loopback IP redirect URIs, to accommodate clients that obtain an available ephemeral port from the operating system at the time of the request.”
That requirement appears in RFC 8252, Section 7.3 (OAuth 2.0 for Native Apps), published October 2017. It changes how the port is validated; it does not permit arbitrary redirect hosts or paths.
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Match the URI except for the permitted port
For a loopback IP redirect, keep validating the scheme, IP host, and complete registered path. The exception is that the request’s port can be the runtime port on which the client is listening. Do not treat the rule as permission to accept a different host or an unregistered callback path.
Prefer a loopback IP literal over localhost
RFC 8252 describes loopback redirects using HTTP and an IP literal: IPv4 127.0.0.1 or IPv6 [::1]. Prefer one of these over localhost so the redirect uses the form covered by the loopback IP port rule. Ensure the client registration and authorization request agree on the chosen host and path.
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How to fix the callback flow
- Choose a loopback IP callback. Use a URI such as
http://127.0.0.1:{port}/callback, where the client substitutes its listening port. An IPv6 loopback URI uses the bracketed address form, such ashttp://[::1]:{port}/callback. - Register the complete redirect URI. Include the scheme, loopback IP, and callback path in the client’s registration. Configure the authorization server to permit a variable port for loopback IP redirects rather than requiring the registered port to match the request port.
- Start a short-lived listener on loopback only. The client should bind its callback server only to the loopback interface, use the same port in the authorization request and listener, and keep the listener active during the authorization window.
- Receive the response and close the listener. Once the authorization response arrives, stop listening. A temporary callback endpoint should not remain open after it has served its purpose.
- Verify the target server’s behavior. Check the server’s current documentation or configuration to determine whether it supports the RFC 8252 exception. The standards requirement does not establish that every product implements it.
Loopback versus app-claimed HTTPS redirects
RFC 8252 describes both loopback redirects and app-claimed HTTPS redirects. An app-claimed HTTPS URI can provide stronger assurance about the destination app through operating-system URI dispatch, and is preferred where supported. A loopback redirect is suited to desktop systems where the client can open a local port without special permissions.
| Option | How the response reaches the app | Key implementation consideration |
|---|---|---|
| Loopback HTTP | The app listens on a local loopback address and receives the response on its chosen port. | The server must apply the RFC 8252 variable-port rule to loopback IP redirects while validating the rest of the URI. |
| App-claimed HTTPS | The operating system dispatches the URI to the app that claims it. | Use where supported; assess operating-system support, destination identity assurance, client requirements, and server acceptance. |
Troubleshoot an MCP OAuth callback rejection
- Inspect the actual request. Read the
redirect_urisent to the authorization endpoint. Compare its scheme, host, and path with the client registration. - Check whether it is a loopback IP URI. If it uses
127.0.0.1or[::1], check whether the server is rejecting it solely because the runtime port differs from the registered port. - Check the local listener. Confirm it binds only to loopback and that its port matches the port in the authorization request.
- Check its lifetime. Confirm the listener is active during authorization and closes after receiving the callback.
- If the URI uses localhost, try the IP literal form. Update both registration and request consistently, including the path, and check the server’s acceptance of that form.
- If Dynamic Client Registration is used, inspect its result. Confirm the registration records the redirect URI. Dynamic registration does not remove the authorization endpoint’s responsibility to validate redirects safely.
- Check product-specific support. Consult current official documentation or reproduce the behavior against the specific server before concluding that it supports, or fails to support, the native loopback exception.
Do not treat a desktop app as a confidential client by default
Native desktop applications are public clients unless registration provisions per-instance secrets. A secret embedded in an app distributed to users can be extracted, so it should not be relied on as a confidential-client secret. Redirect validation remains important regardless of whether Dynamic Client Registration is involved.
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