diff --git a/apps/server/src/provider/AntigravityAuth.test.ts b/apps/server/src/provider/AntigravityAuth.test.ts
index 02e8b691a1334..21663e2b6f215 100644
--- a/apps/server/src/provider/AntigravityAuth.test.ts
+++ b/apps/server/src/provider/AntigravityAuth.test.ts
@@ -270,6 +270,56 @@ it.layer(NodeServices.layer)("AntigravityAuth", (it) => {
     }),
   );
 
+  it.effect("shows Antigravity's sanitized sign-in rejection only to the owner", () =>
+    Effect.gen(function* () {
+      const harness = yield* makeHarness();
+      yield* harness.auth.controller.start(owner);
+      yield* phase(harness.auth, "waiting");
+      yield* Deferred.fail(
+        harness.authenticated,
+        new AcpErrors.AcpRequestError({
+          code: -32603,
+          errorMessage:
+            "Onboarding failed: User is ineligible for free-tier.\n  Reason: Your current account is not eligible for Antigravity. Try signing in with another personal Google account.\u0007 ",
+          method: "authenticate",
+        }),
+      );
+      const failed = yield* phase(harness.auth, "failed");
+      assert.equal(
+        failed.message,
+        "Onboarding failed: User is ineligible for free-tier. Reason: Your current account is not eligible for Antigravity. Try signing in with another personal Google account.",
+      );
+      const other = yield* phase(harness.auth, "failed", otherOwner);
+      assert.equal(other.message, "Sign-in failed in another client.");
+      yield* Deferred.await(harness.closed);
+    }),
+  );
+
+  it.effect("keeps sensitive authenticate rejection details out of the sign-in status", () =>
+    Effect.gen(function* () {
+      for (const errorMessage of [
+        `Sign-in failed at ${callbackUrl}`,
+        "Redirect to http%3A%2F%2F127.0.0.1 failed",
+        "Rejected key sk-123456789012345678901234567890",
+        "Account person@example.com is not eligible",
+        "Onboarding failed. Visit support.google.com for help.",
+        "Onboarding failed: See https:example.com for help.",
+        "Authentication failed: token sk-live-abcd-efgh",
+      ]) {
+        const harness = yield* makeHarness();
+        yield* harness.auth.controller.start(owner);
+        yield* phase(harness.auth, "waiting");
+        yield* Deferred.fail(
+          harness.authenticated,
+          new AcpErrors.AcpRequestError({ code: -32603, errorMessage, method: "authenticate" }),
+        );
+        const failed = yield* phase(harness.auth, "failed");
+        assert.equal(failed.message, "Google sign-in failed. Start sign-in again.", errorMessage);
+        yield* Deferred.await(harness.closed);
+      }
+    }),
+  );
+
   it.effect("accepts direct local or cached completion without a callback RPC", () =>
     Effect.gen(function* () {
       const harness = yield* makeHarness({ interactive: false });
@@ -282,7 +332,7 @@ it.layer(NodeServices.layer)("AntigravityAuth", (it) => {
     }),
   );
 
-  it.effect("rejects mismatched callbacks without sending any HTTP request", () =>
+  it.effect("retains callback errors only for the owner until a valid callback is accepted", () =>
     Effect.gen(function* () {
       const harness = yield* makeHarness();
       const state = yield* harness.auth.controller.start(owner);
@@ -297,9 +347,26 @@ it.layer(NodeServices.layer)("AntigravityAuth", (it) => {
           .complete(owner, { flowId: state.flowId!, callbackUrl: invalidUrl })
           .pipe(Effect.exit);
         assert.isTrue(Exit.isFailure(result));
+        const waiting = yield* phase(harness.auth, "waiting");
+        assert.equal(
+          waiting.inputError,
+          "This redirect URL does not belong to the current sign-in.",
+        );
+        assert.equal(waiting.flowId, state.flowId);
+        assert.equal(waiting.authorizationUrl, authorizationUrl);
+        const other = yield* phase(harness.auth, "waiting", otherOwner);
+        assert.notExists(other.inputError);
+        assert.equal(other.message, "Sign-in is in progress in another client.");
       }
       assert.equal(harness.forwarded(), 0);
-      assert.equal((yield* phase(harness.auth, "waiting")).authorizationUrl, authorizationUrl);
+      const verifying = yield* harness.auth.controller.complete(owner, {
+        flowId: state.flowId!,
+        callbackUrl,
+      });
+      assert.equal(verifying.phase, "verifying");
+      assert.notExists(verifying.inputError);
+      assert.equal(harness.forwarded(), 1);
+      assert.notExists((yield* phase(harness.auth, "verifying")).inputError);
       yield* harness.auth.controller.cancel(owner, state.flowId!);
     }),
   );
@@ -343,9 +410,19 @@ it.layer(NodeServices.layer)("AntigravityAuth", (it) => {
         .cancel(otherOwner, state.flowId!)
         .pipe(Effect.exit);
       assert.isTrue(Exit.isFailure(wrongOwner));
+      const rejected = yield* harness.auth.controller
+        .complete(owner, { flowId: state.flowId!, callbackUrl: "not a URL" })
+        .pipe(Effect.exit);
+      assert.isTrue(Exit.isFailure(rejected));
+      assert.equal(
+        (yield* phase(harness.auth, "waiting")).inputError,
+        "Paste the complete redirect URL from the Google sign-in page.",
+      );
       const cancelled = yield* harness.auth.controller.cancel(owner, state.flowId!);
       assert.equal(cancelled.phase, "cancelled");
       assert.isNull(cancelled.authorizationUrl);
+      assert.notExists(cancelled.inputError);
+      assert.notExists((yield* phase(harness.auth, "cancelled")).inputError);
       assert.equal(harness.forwarded(), 0);
       yield* Deferred.await(harness.closed);
       assert.deepEqual(harness.catalog(), ["previous-account-model"]);
diff --git a/apps/server/src/provider/AntigravityAuth.ts b/apps/server/src/provider/AntigravityAuth.ts
index ea1c8f7448cc2..602944dbf3fb1 100644
--- a/apps/server/src/provider/AntigravityAuth.ts
+++ b/apps/server/src/provider/AntigravityAuth.ts
@@ -96,10 +96,29 @@ function visibleSnapshot(snapshot: AuthSnapshot, ownerSessionId: string): Provid
     authorizationUrl: null,
     interaction: null,
     expiresAt: null,
-    ...(busy ? { message: "Sign-in is in progress in another client." } : {}),
+    inputError: null,
+    ...(busy
+      ? { message: "Sign-in is in progress in another client." }
+      : snapshot.state.phase === "failed"
+        ? { message: "Sign-in failed in another client." }
+        : {}),
   };
 }
 
+// Antigravity's authenticate rejection is the only text that says why Google turned the
+// account down, but the same channel can echo callback URLs, tokens, paths, or emails.
+function safeRejectionReason(message: string): string | undefined {
+  const text = message.replace(/[\p{Cc}\s]+/gu, " ").trim();
+  const plainProse =
+    text.length > 0 &&
+    text.length <= 300 &&
+    /\p{L}/u.test(text) &&
+    /^[\p{L}\p{N} .,;:!?'"()-]+$/u.test(text) &&
+    !/[\p{L}\p{N}][.:][\p{L}\p{N}]/u.test(text) &&
+    !/\S{20,}|\d{6,}|\bsk-[a-z0-9-]+/i.test(text);
+  return plainProse ? text : undefined;
+}
+
 function safeAuthFailure(cause: Cause.Cause<unknown>, usesBrowser: boolean): string {
   const error = Cause.findErrorOption(cause);
   if (Option.isSome(error)) {
@@ -119,6 +138,10 @@ function safeAuthFailure(cause: Cause.Cause<unknown>, usesBrowser: boolean): str
       if (!usesBrowser && error.value.code === -32602) {
         return "Antigravity rejected the configured credentials. Check the provider settings.";
       }
+      if (error.value.method === "authenticate") {
+        const reason = safeRejectionReason(error.value.errorMessage);
+        if (reason) return reason;
+      }
     }
   }
   return usesBrowser
@@ -143,6 +166,7 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
     authorizationUrl: null,
     expiresAt: null,
     message: null,
+    inputError: null,
   };
   const snapshot = yield* SubscriptionRef.make<AuthSnapshot>({
     ownerSessionId: null,
@@ -226,6 +250,7 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
           phase: Exit.isSuccess(result) ? "succeeded" : "failed",
           authorizationUrl: null,
           expiresAt: null,
+          inputError: null,
           message: Exit.isSuccess(result)
             ? usesBrowser
               ? "Signed in with Google."
@@ -276,6 +301,7 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
             ...flow.state,
             phase: "verifying",
             authorizationUrl: null,
+            inputError: null,
             message: "Checking Antigravity access and models.",
           });
         }),
@@ -306,6 +332,7 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
               phase,
               authorizationUrl: null,
               expiresAt: null,
+              inputError: null,
               message,
             });
             return true;
@@ -398,12 +425,17 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
             options.instanceId,
             flow.pending,
             input.callbackUrl,
+          ).pipe(
+            Effect.tapError((error) =>
+              publishFlow(flow, { ...flow.state, inputError: error.detail }),
+            ),
           );
           flow.callbackSent = true;
           yield* publishFlow(flow, {
             ...flow.state,
             phase: "verifying",
             authorizationUrl: null,
+            inputError: null,
             message: "Waiting for Google to finish sign-in.",
           });
           const forwarding = yield* (
@@ -452,6 +484,7 @@ export const makeAntigravityAuth = Effect.fn("makeAntigravityAuth")(function* <
                   phase: "cancelled",
                   authorizationUrl: null,
                   expiresAt: null,
+                  inputError: null,
                   message: "Google sign-in was cancelled by sign-out.",
                 });
               }
diff --git a/packages/contracts/src/providerSetup.ts b/packages/contracts/src/providerSetup.ts
index 69394bed3c146..821d34c4275c6 100644
--- a/packages/contracts/src/providerSetup.ts
+++ b/packages/contracts/src/providerSetup.ts
@@ -130,6 +130,8 @@ export const ProviderAuthState = Schema.Struct({
   authorizationUrl: Schema.NullOr(Schema.String),
   expiresAt: Schema.NullOr(IsoDateTime),
   message: Schema.NullOr(Schema.String),
+  // Callback validation feedback while the sign-in flow keeps waiting.
+  inputError: Schema.optionalKey(Schema.NullOr(Schema.String)),
   // Newer servers may add method types, interactions, or owners; older
   // clients drop what they cannot decode instead of rejecting the state.
   methods: Schema.optionalKey(