feat(backend): Signal support for handshake nonce

.changeset/six-ears-wash.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': minor
+---
+
+When redirecting for a handshake to FAPI, send signal that current version of @clerk/backend supports the handshake nonce

packages/backend/src/constants.ts

@@ -35,6 +35,7 @@ const QueryParameters = {
   LegacyDevBrowser: '__dev_session',
   HandshakeReason: '__clerk_hs_reason',
   HandshakeNonce: Cookies.HandshakeNonce,
+  HandshakeFormat: 'format',
 } as const;
 
 const Headers = {

packages/backend/src/tokens/__tests__/handshake.test.ts

@@ -155,6 +155,7 @@ describe('HandshakeService', () => {
       expect(url.searchParams.get('redirect_url')).toBe('https://example.com/');
       expect(url.searchParams.get(constants.QueryParameters.SuffixedCookies)).toBe('true');
       expect(url.searchParams.get(constants.QueryParameters.HandshakeReason)).toBe('test-reason');
+      expect(url.searchParams.get(constants.QueryParameters.HandshakeFormat)).toBe('nonce');
     });
 
     it('should include dev browser token in development mode', () => {
@@ -168,6 +169,7 @@ describe('HandshakeService', () => {
       const url = new URL(location);
 
       expect(url.searchParams.get(constants.QueryParameters.DevBrowser)).toBe('dev-token');
+      expect(url.searchParams.get(constants.QueryParameters.HandshakeFormat)).toBe('nonce');
     });
 
     it('should throw error if clerkUrl is missing', () => {

packages/backend/src/tokens/handshake.ts

@@ -149,6 +149,7 @@ export class HandshakeService {
       this.authenticateContext.usesSuffixedCookies().toString(),
     );
     url.searchParams.append(constants.QueryParameters.HandshakeReason, reason);
+    url.searchParams.append(constants.QueryParameters.HandshakeFormat, 'nonce');
 
     if (this.authenticateContext.instanceType === 'development' && this.authenticateContext.devBrowserToken) {
       url.searchParams.append(constants.QueryParameters.DevBrowser, this.authenticateContext.devBrowserToken);