diff --git a/.changeset/fix-signup-future-sso-transfer-stale-ref.md b/.changeset/fix-signup-future-sso-transfer-stale-ref.md
new file mode 100644
index 00000000000..1ac84a8a110
--- /dev/null
+++ b/.changeset/fix-signup-future-sso-transfer-stale-ref.md
@@ -0,0 +1,5 @@
+---
+"@clerk/clerk-js": patch
+---
+
+Fix `signUp.update()` sending a PATCH request to `/client/sign_ups` instead of `/client/sign_ups/{id}` after an SSO sign-in transitions to a sign-up flow (e.g. when legal acceptance is required). The `SignUpFuture` reference held by hooks is now preserved across the transition and reflects the correct sign-up id.
diff --git a/packages/clerk-js/src/core/resources/Client.ts b/packages/clerk-js/src/core/resources/Client.ts
index 6b690c7261c..b06ac0b80ae 100644
--- a/packages/clerk-js/src/core/resources/Client.ts
+++ b/packages/clerk-js/src/core/resources/Client.ts
@@ -143,7 +143,11 @@ export class Client extends BaseResource implements ClientResource {
       this.id = data.id;
       this.sessions = (data.sessions || []).map(s => new Session(s));
 
-      if (data.sign_up && this.signUp instanceof SignUp && this.signUp.id === data.sign_up.id) {
+      if (data.sign_up && this.signUp instanceof SignUp && (this.signUp.id === data.sign_up.id || !this.signUp.id)) {
+        // Update in-place when ids match OR when the existing signUp has no id yet
+        // (e.g. an SSO sign-in that transitions to a sign-up flow). Reusing the same
+        // SignUp instance preserves any SignUpFuture references held by hooks so they
+        // remain valid and reflect the correct id after the transition.
         this.signUp.__internal_updateFromJSON(data.sign_up);
       } else {
         this.signUp = new SignUp(data.sign_up);