mcp: add client-side OAuth flow

[samthanawalla]

This is a preliminary implementation of OAuth 2.1 for the client.

When a StreamableClientTransport encounters a 401 Unauthorized response
from the server, it initiates the OAuth flow described in thec
authorization section of the MCP spec
(https://modelcontextprotocol.io/specification/2025-06-18/basic/authorization).
On success, the transport obtains an access token which it passes
to all subsequent requests.

Much remains to be done here:

• Resource Indicators, as described in section 2.5.1 of the MCP spec.

• All of this is unexported, so it is available only to our own
  StreamingClientTransport. We should add API so people can use it
  with their own transports.

• And, of course, tests. We should test against fake implementations
  but also, if we can find any, real reference implementations.

For #19
Co-authored-by: Jonathan Amsterdam jba@google.com

[jba]

Let's not submit until after the release.

…

On Tue, Sep 30, 2025, 10:34 AM Sam Thanawalla ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In auth/client.go <#544 (comment)> : > + return resp, nil + } + if _, ok := base.(*oauth2.Transport); ok { + // We failed to authorize even with a token source; give up. + return resp, nil + } + + resp.Body.Close() + // Try to authorize. + t.mu.Lock() + defer t.mu.Unlock() + // If we don't have a token source, get one by following the OAuth flow. + // (We may have obtained one while t.mu was not held above.) + if _, ok := t.opts.Base.(*oauth2.Transport); !ok { + authHeaders := resp.Header[http.CanonicalHeaderKey("WWW-Authenticate")] + ts, err := t.handler(req.Context(), OAuthHandlerArgs{ Done ------------------------------ In go.mod <#544 (comment)> : > @@ -1,6 +1,8 @@ module github.com/modelcontextprotocol/go-sdk -go 1.23.0 +go 1.24.0 Done — Reply to this email directly, view it on GitHub <#544 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AENAOZKQNJW6RLLWIA22MDT3VKIG7AVCNFSM6AAAAACH2NKQZ6VHI2DSMVQWIX3LMV43YUDVNRWFEZLROVSXG5CSMV3GSZLXHMZTEOBVGI4TSNJXHA> . You are receiving this because your review was requested.Message ID: ***@***.***>

[rumpl]

In cagent we use the same trick for OAuth. I just spent the morning debugging and fixing our code for streamable mcp servers and thought I'd tell you what was wrong, might be useful for you.

This second request that comes after the oauth flow has a half-broken request, the body of the request was already read by the first call on line 70, so you would get a 400 Bad Request back from the server. You need to hold on the original body and put it back here before sending a second RoundTrip.

docker/cagent#547