fixing ProxyOAuthServerProvider: redirect_uri missing in token request (#519)

src/server/auth/handlers/token.test.ts

@@ -322,7 +322,8 @@ describe('Token Handler', () => {
             client_secret: 'valid-secret',
             grant_type: 'authorization_code',
             code: 'valid_code',
-            code_verifier: 'any_verifier'
+            code_verifier: 'any_verifier',
+            redirect_uri: 'https://example.com/callback'
           });
 
         expect(response.status).toBe(200);
@@ -342,6 +343,69 @@ describe('Token Handler', () => {
         global.fetch = originalFetch;
       }
     });
+
+    it('passes through redirect_uri when using proxy provider', async () => {
+      const originalFetch = global.fetch;
+
+      try {
+        global.fetch = jest.fn().mockResolvedValue({
+          ok: true,
+          json: () => Promise.resolve({
+            access_token: 'mock_access_token',
+            token_type: 'bearer',
+            expires_in: 3600,
+            refresh_token: 'mock_refresh_token'
+          })
+        });
+
+        const proxyProvider = new ProxyOAuthServerProvider({
+          endpoints: {
+            authorizationUrl: 'https://example.com/authorize',
+            tokenUrl: 'https://example.com/token'
+          },
+          verifyAccessToken: async (token) => ({
+            token,
+            clientId: 'valid-client',
+            scopes: ['read', 'write'],
+            expiresAt: Date.now() / 1000 + 3600
+          }),
+          getClient: async (clientId) => clientId === 'valid-client' ? validClient : undefined
+        });
+
+        const proxyApp = express();
+        const options: TokenHandlerOptions = { provider: proxyProvider };
+        proxyApp.use('/token', tokenHandler(options));
+
+        const redirectUri = 'https://example.com/callback';
+        const response = await supertest(proxyApp)
+          .post('/token')
+          .type('form')
+          .send({
+            client_id: 'valid-client',
+            client_secret: 'valid-secret',
+            grant_type: 'authorization_code',
+            code: 'valid_code',
+            code_verifier: 'any_verifier',
+            redirect_uri: redirectUri
+          });
+
+        expect(response.status).toBe(200);
+        expect(response.body.access_token).toBe('mock_access_token');
+
+        expect(global.fetch).toHaveBeenCalledWith(
+          'https://example.com/token',
+          expect.objectContaining({
+            method: 'POST',
+            headers: {
+              'Content-Type': 'application/x-www-form-urlencoded'
+            },
+            body: expect.stringContaining(`redirect_uri=${encodeURIComponent(redirectUri)}`)
+          })
+        );
+      } finally {
+        global.fetch = originalFetch;
+      }
+    });
   });
 
   describe('Refresh token grant', () => {

src/server/auth/handlers/token.ts

@@ -31,6 +31,7 @@ const TokenRequestSchema = z.object({
 const AuthorizationCodeGrantSchema = z.object({
   code: z.string(),
   code_verifier: z.string(),
+  redirect_uri: z.string().optional(),
 });
 
 const RefreshTokenGrantSchema = z.object({
@@ -88,7 +89,7 @@ export function tokenHandler({ provider, rateLimit: rateLimitConfig }: TokenHand
             throw new InvalidRequestError(parseResult.error.message);
           }
 
-          const { code, code_verifier } = parseResult.data;
+          const { code, code_verifier, redirect_uri } = parseResult.data;
 
           const skipLocalPkceValidation = provider.skipLocalPkceValidation;
 
@@ -102,7 +103,12 @@ export function tokenHandler({ provider, rateLimit: rateLimitConfig }: TokenHand
           }
 
           // Passes the code_verifier to the provider if PKCE validation didn't occur locally
-          const tokens = await provider.exchangeAuthorizationCode(client, code, skipLocalPkceValidation ? code_verifier : undefined);
+          const tokens = await provider.exchangeAuthorizationCode(
+            client, 
+            code, 
+            skipLocalPkceValidation ? code_verifier : undefined,
+            redirect_uri
+          );
           res.status(200).json(tokens);
           break;
         }

src/server/auth/provider.ts

@@ -36,7 +36,12 @@ export interface OAuthServerProvider {
   /**
    * Exchanges an authorization code for an access token.
    */
-  exchangeAuthorizationCode(client: OAuthClientInformationFull, authorizationCode: string, codeVerifier?: string): Promise<OAuthTokens>;
+  exchangeAuthorizationCode(
+    client: OAuthClientInformationFull, 
+    authorizationCode: string, 
+    codeVerifier?: string,
+    redirectUri?: string
+  ): Promise<OAuthTokens>;
 
   /**
    * Exchanges a refresh token for an access token.

src/server/auth/providers/proxyProvider.test.ts

@@ -142,6 +142,28 @@ describe("Proxy OAuth Server Provider", () => {
       expect(tokens).toEqual(mockTokenResponse);
     });
 
+    it("includes redirect_uri in token request when provided", async () => {
+      const redirectUri = "https://example.com/callback";
+      const tokens = await provider.exchangeAuthorizationCode(
+        validClient,
+        "test-code",
+        "test-verifier",
+        redirectUri
+      );
+
+      expect(global.fetch).toHaveBeenCalledWith(
+        "https://auth.example.com/token",
+        expect.objectContaining({
+          method: "POST",
+          headers: {
+            "Content-Type": "application/x-www-form-urlencoded",
+          },
+          body: expect.stringContaining(`redirect_uri=${encodeURIComponent(redirectUri)}`)
+        })
+      );
+      expect(tokens).toEqual(mockTokenResponse);
+    });
+
     it("exchanges refresh token for new tokens", async () => {
       const tokens = await provider.exchangeRefreshToken(
         validClient,

src/server/auth/providers/proxyProvider.ts

@@ -151,7 +151,8 @@ export class ProxyOAuthServerProvider implements OAuthServerProvider {
   async exchangeAuthorizationCode(
     client: OAuthClientInformationFull,
     authorizationCode: string,
-    codeVerifier?: string
+    codeVerifier?: string,
+    redirectUri?: string
   ): Promise<OAuthTokens> {
     const params = new URLSearchParams({
       grant_type: "authorization_code",
@@ -167,6 +168,10 @@ export class ProxyOAuthServerProvider implements OAuthServerProvider {
       params.append("code_verifier", codeVerifier);
     }
 
+    if (redirectUri) {
+      params.append("redirect_uri", redirectUri);
+    }
+
     const response = await fetch(this._endpoints.tokenUrl, {
       method: "POST",
       headers: {