mcp: add client-side OAuth flow (preliminary) #176
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| // Copyright 2025 The Go MCP SDK Authors. All rights reserved. | ||
| // Use of this source code is governed by an MIT-style | ||
| // license that can be found in the LICENSE file. | ||
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| //go:build mcp_go_client_oauth | ||
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| package auth | ||
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| import ( | ||
| "context" | ||
| "log" | ||
| "net/http" | ||
| "sync" | ||
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| "github.com/modelcontextprotocol/go-sdk/internal/oauthex" | ||
| "golang.org/x/oauth2" | ||
| ) | ||
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| // An OAuthHandler conducts an OAuth flow and returns a [oauth2.TokenSource] if the authorization | ||
| // is approved, or an error if not. | ||
| type OAuthHandler func(context.Context, OAuthHandlerArgs) (oauth2.TokenSource, error) | ||
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| // OAuthHandlerArgs are arguments to an [OAuthHandler]. | ||
| type OAuthHandlerArgs struct { | ||
| // The URL to fetch protected resource metadata, extracted from the WWW-Authenticate header. | ||
| // Empty if not present or there was an error obtaining it. | ||
| ResourceMetadataURL string | ||
| } | ||
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| // HTTPTransport is an [http.RoundTripper] that follows the MCP | ||
| // OAuth protocol when it encounters a 401 Unauthorized response. | ||
| type HTTPTransport struct { | ||
| handler OAuthHandler | ||
| mu sync.Mutex // protects opts.Base | ||
| opts HTTPTransportOptions | ||
| } | ||
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| // NewHTTPTransport returns a new [*HTTPTransport]. | ||
| // The handler is invoked when an HTTP request results in a 401 Unauthorized status. | ||
| // It is called only once per transport. Once a TokenSource is obtained, it is used | ||
| // for the lifetime of the transport; subsequent 401s are not processed. | ||
| func NewHTTPTransport(handler OAuthHandler, opts *HTTPTransportOptions) (*HTTPTransport, error) { | ||
| t := &HTTPTransport{} | ||
| if opts != nil { | ||
| t.opts = *opts | ||
| } | ||
| if t.opts.Base == nil { | ||
| t.opts.Base = http.DefaultTransport | ||
| } | ||
| return t, nil | ||
| } | ||
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| // HTTPTransportOptions are options to [NewHTTPTransport]. | ||
| type HTTPTransportOptions struct { | ||
| // Base is the [http.RoundTripper] to use. | ||
| // If nil, [http.DefaultTransport] is used. | ||
| Base http.RoundTripper | ||
| } | ||
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| func (t *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) { | ||
| t.mu.Lock() | ||
| base := t.opts.Base | ||
| _, haveTokenSource := base.(*oauth2.Transport) | ||
| t.mu.Unlock() | ||
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| resp, err := base.RoundTrip(req) | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
| if resp.StatusCode != http.StatusUnauthorized { | ||
| return resp, nil | ||
| } | ||
| if haveTokenSource { | ||
| // We failed to authorize even with a token source; give up. | ||
| return resp, nil | ||
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Collaborator
There was a problem hiding this comment. Should we return an error here explaining that we tried to authorize and it failed, or is that going to be handled higher in the stack?
Contributor
Author
There was a problem hiding this comment. Higher up. The caller will see the unauthorized status and should perform the OAuth dance again. |
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| } | ||
| // Try to authorize. | ||
| t.mu.Lock() | ||
| // 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{ | ||
| ResourceMetadataURL: extractResourceMetadataURL(authHeaders), | ||
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| }) | ||
| if err != nil { | ||
| t.mu.Unlock() | ||
| return nil, err | ||
| } | ||
| t.opts.Base = &oauth2.Transport{Base: t.opts.Base, Source: ts} | ||
| } | ||
| t.mu.Unlock() | ||
| // Only one level of recursion, because we now have a token source. | ||
| return t.RoundTrip(req) | ||
| } | ||
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| func extractResourceMetadataURL(authHeaders []string) string { | ||
| cs, err := oauthex.ParseWWWAuthenticate(authHeaders) | ||
| if err != nil { | ||
| log.Printf("parsing auth headers %q: %v", authHeaders, err) | ||
| return "" | ||
| } | ||
| return oauthex.ResourceMetadataURL(cs) | ||
| } | ||
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Check if handler is nil, otherwise we will panic when trying to initialize the token source in RoundTrip.