diff --git a/src/client/auth.test.ts b/src/client/auth.test.ts
index 48be870b..82d55909 100644
--- a/src/client/auth.test.ts
+++ b/src/client/auth.test.ts
@@ -414,6 +414,22 @@ describe("OAuth Authorization", () => {
   });
 
   describe("exchangeAuthorization", () => {
+    const mockProvider: OAuthClientProvider = {
+      get redirectUrl() { return "http://localhost:3000/callback"; },
+      get clientMetadata() {
+        return {
+          redirect_uris: ["http://localhost:3000/callback"],
+          client_name: "Test Client",
+        };
+      },
+      clientInformation: jest.fn(),
+      tokens: jest.fn(),
+      saveTokens: jest.fn(),
+      redirectToAuthorization: jest.fn(),
+      saveCodeVerifier: jest.fn(),
+      codeVerifier: jest.fn(),
+    };
+    
     const validTokens = {
       access_token: "access123",
       token_type: "Bearer",
@@ -449,12 +465,11 @@ describe("OAuth Authorization", () => {
         }),
         expect.objectContaining({
           method: "POST",
-          headers: {
-            "Content-Type": "application/x-www-form-urlencoded",
-          },
         })
       );
 
+      const headers = mockFetch.mock.calls[0][1].headers as Headers;
+      expect(headers.get("Content-Type")).toBe("application/x-www-form-urlencoded");
       const body = mockFetch.mock.calls[0][1].body as URLSearchParams;
       expect(body.get("grant_type")).toBe("authorization_code");
       expect(body.get("code")).toBe("code123");
@@ -464,6 +479,48 @@ describe("OAuth Authorization", () => {
       expect(body.get("redirect_uri")).toBe("http://localhost:3000/callback");
     });
 
+    it("exchanges code for tokens with auth", async () => {
+      mockProvider.authToTokenEndpoint = function(headers: Headers, params: URLSearchParams) {
+        headers.set("Authorization", "Basic " + btoa(validClientInfo.client_id + ":" + validClientInfo.client_secret));
+        params.set("example_param", "example_value")
+      };
+
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => validTokens,
+      });
+
+      const tokens = await exchangeAuthorization("https://auth.example.com", {
+        clientInformation: validClientInfo,
+        authorizationCode: "code123",
+        codeVerifier: "verifier123",
+        redirectUri: "http://localhost:3000/callback",
+      }, mockProvider);
+
+      expect(tokens).toEqual(validTokens);
+      expect(mockFetch).toHaveBeenCalledWith(
+        expect.objectContaining({
+          href: "https://auth.example.com/token",
+        }),
+        expect.objectContaining({
+          method: "POST",
+        })
+      );
+
+      const headers = mockFetch.mock.calls[0][1].headers as Headers;
+      expect(headers.get("Content-Type")).toBe("application/x-www-form-urlencoded");
+      expect(headers.get("Authorization")).toBe("Basic Y2xpZW50MTIzOnNlY3JldDEyMw==");
+      const body = mockFetch.mock.calls[0][1].body as URLSearchParams;
+      expect(body.get("grant_type")).toBe("authorization_code");
+      expect(body.get("code")).toBe("code123");
+      expect(body.get("code_verifier")).toBe("verifier123");
+      expect(body.get("client_id")).toBe("client123");
+      expect(body.get("redirect_uri")).toBe("http://localhost:3000/callback");
+      expect(body.get("example_param")).toBe("example_value");
+      expect(body.get("client_secret")).toBeUndefined;
+    });
+
     it("validates token response schema", async () => {
       mockFetch.mockResolvedValueOnce({
         ok: true,
@@ -502,6 +559,22 @@ describe("OAuth Authorization", () => {
   });
 
   describe("refreshAuthorization", () => {
+    const mockProvider: OAuthClientProvider = {
+      get redirectUrl() { return "http://localhost:3000/callback"; },
+      get clientMetadata() {
+        return {
+          redirect_uris: ["http://localhost:3000/callback"],
+          client_name: "Test Client",
+        };
+      },
+      clientInformation: jest.fn(),
+      tokens: jest.fn(),
+      saveTokens: jest.fn(),
+      redirectToAuthorization: jest.fn(),
+      saveCodeVerifier: jest.fn(),
+      codeVerifier: jest.fn(),
+    };
+	  
     const validTokens = {
       access_token: "newaccess123",
       token_type: "Bearer",
@@ -538,12 +611,11 @@ describe("OAuth Authorization", () => {
         }),
         expect.objectContaining({
           method: "POST",
-          headers: {
-            "Content-Type": "application/x-www-form-urlencoded",
-          },
         })
       );
 
+      const headers = mockFetch.mock.calls[0][1].headers as Headers;
+      expect(headers.get("Content-Type")).toBe("application/x-www-form-urlencoded");
       const body = mockFetch.mock.calls[0][1].body as URLSearchParams;
       expect(body.get("grant_type")).toBe("refresh_token");
       expect(body.get("refresh_token")).toBe("refresh123");
@@ -551,6 +623,44 @@ describe("OAuth Authorization", () => {
       expect(body.get("client_secret")).toBe("secret123");
     });
 
+    it("exchanges refresh token for new tokens with auth", async () => {
+      mockProvider.authToTokenEndpoint = function(headers: Headers, params: URLSearchParams) {
+        headers.set("Authorization", "Basic " + btoa(validClientInfo.client_id + ":" + validClientInfo.client_secret));
+        params.set("example_param", "example_value")
+      };
+
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => validTokensWithNewRefreshToken,
+      });
+
+      const tokens = await refreshAuthorization("https://auth.example.com", {
+        clientInformation: validClientInfo,
+        refreshToken: "refresh123",
+      }, mockProvider);
+
+      expect(tokens).toEqual(validTokensWithNewRefreshToken);
+      expect(mockFetch).toHaveBeenCalledWith(
+        expect.objectContaining({
+          href: "https://auth.example.com/token",
+        }),
+        expect.objectContaining({
+          method: "POST",
+        })
+      );
+
+      const headers = mockFetch.mock.calls[0][1].headers as Headers;
+      expect(headers.get("Content-Type")).toBe("application/x-www-form-urlencoded");
+      expect(headers.get("Authorization")).toBe("Basic Y2xpZW50MTIzOnNlY3JldDEyMw==");
+      const body = mockFetch.mock.calls[0][1].body as URLSearchParams;
+      expect(body.get("grant_type")).toBe("refresh_token");
+      expect(body.get("refresh_token")).toBe("refresh123");
+      expect(body.get("client_id")).toBe("client123");
+      expect(body.get("example_param")).toBe("example_value");
+      expect(body.get("client_secret")).toBeUndefined;
+    });
+
     it("exchanges refresh token for new tokens and keep existing refresh token if none is returned", async () => {
       mockFetch.mockResolvedValueOnce({
         ok: true,
diff --git a/src/client/auth.ts b/src/client/auth.ts
index 16f0a550..827f23f7 100644
--- a/src/client/auth.ts
+++ b/src/client/auth.ts
@@ -66,6 +66,8 @@ export interface OAuthClientProvider {
    * the authorization result.
    */
   codeVerifier(): string | Promise<string>;
+  
+  authToTokenEndpoint?(headers: Headers, params: URLSearchParams): void | Promise<void>;
 }
 
 export type AuthResult = "AUTHORIZED" | "REDIRECT";
@@ -137,7 +139,7 @@ export async function auth(
       authorizationCode,
       codeVerifier,
       redirectUri: provider.redirectUrl,
-    });
+    }, provider);
 
     await provider.saveTokens(tokens);
     return "AUTHORIZED";
@@ -153,7 +155,7 @@ export async function auth(
         metadata,
         clientInformation,
         refreshToken: tokens.refresh_token,
-      });
+      }, provider);
 
       await provider.saveTokens(newTokens);
       return "AUTHORIZED";
@@ -372,6 +374,7 @@ export async function exchangeAuthorization(
     codeVerifier: string;
     redirectUri: string | URL;
   },
+  provider?: OAuthClientProvider
 ): Promise<OAuthTokens> {
   const grantType = "authorization_code";
 
@@ -392,6 +395,9 @@ export async function exchangeAuthorization(
   }
 
   // Exchange code for tokens
+  const headers = new Headers({
+    "Content-Type": "application/x-www-form-urlencoded",
+  });
   const params = new URLSearchParams({
     grant_type: grantType,
     client_id: clientInformation.client_id,
@@ -400,15 +406,15 @@ export async function exchangeAuthorization(
     redirect_uri: String(redirectUri),
   });
 
-  if (clientInformation.client_secret) {
+  if (provider?.authToTokenEndpoint) {
+    provider.authToTokenEndpoint(headers, params);
+  } else if (clientInformation.client_secret) {
     params.set("client_secret", clientInformation.client_secret);
   }
 
   const response = await fetch(tokenUrl, {
     method: "POST",
-    headers: {
-      "Content-Type": "application/x-www-form-urlencoded",
-    },
+    headers: headers,
     body: params,
   });
 
@@ -433,6 +439,7 @@ export async function refreshAuthorization(
     clientInformation: OAuthClientInformation;
     refreshToken: string;
   },
+  provider?: OAuthClientProvider,
 ): Promise<OAuthTokens> {
   const grantType = "refresh_token";
 
@@ -453,21 +460,24 @@ export async function refreshAuthorization(
   }
 
   // Exchange refresh token
+  const headers = new Headers({
+    "Content-Type": "application/x-www-form-urlencoded",
+  });
   const params = new URLSearchParams({
     grant_type: grantType,
     client_id: clientInformation.client_id,
     refresh_token: refreshToken,
   });
 
-  if (clientInformation.client_secret) {
+  if (provider?.authToTokenEndpoint) {
+    provider.authToTokenEndpoint(headers, params);
+  } else if (clientInformation.client_secret) {
     params.set("client_secret", clientInformation.client_secret);
   }
 
   const response = await fetch(tokenUrl, {
     method: "POST",
-    headers: {
-      "Content-Type": "application/x-www-form-urlencoded",
-    },
+    headers: headers,
     body: params,
   });
   if (!response.ok) {