diff --git a/apps/server/src/provider/acp/AntigravityAcpSupport.test.ts b/apps/server/src/provider/acp/AntigravityAcpSupport.test.ts
index dc755d1e7a5a4..4145a8bdec0cf 100644
--- a/apps/server/src/provider/acp/AntigravityAcpSupport.test.ts
+++ b/apps/server/src/provider/acp/AntigravityAcpSupport.test.ts
@@ -50,6 +50,7 @@ function makeModelRuntime(
     selections.push(
       `${configId}=${String(value)}${options?.allowUnlistedValue === true ? " (unlisted)" : ""}`,
     );
+    if (failure) return yield* failure;
     return { configOptions };
   });
   return {
@@ -155,24 +156,27 @@ describe("applyAntigravityAcpModelSelection", () => {
     }),
   );
 
-  it.effect.each(["gemini-removed", "Gemini saved", "gemini-saved[reasoning=high]"])(
-    "rejects unavailable or non-native model ID %s without selecting a fallback",
-    (model) =>
-      Effect.gen(function* () {
-        const { runtime, selections } = makeModelRuntime();
-        const error = yield* applyAntigravityAcpModelSelection({
-          runtime,
-          model,
-          mapError: (cause) => cause,
-        }).pipe(Effect.flip);
-
-        expect(error).toMatchObject({
-          _tag: "AcpRequestError",
-          code: -32602,
-          errorMessage: expect.stringContaining(`'${model}' is unavailable`),
-        });
-        expect(selections).toEqual([]);
-      }),
+  it.effect("sends an explicit model absent from cached choices to the provider", () =>
+    Effect.gen(function* () {
+      const { runtime, selections } = makeModelRuntime();
+      const model = yield* applyAntigravityAcpModelSelection({
+        runtime,
+        model: "gemini-new",
+        mapError: (cause) => cause,
+      });
+      expect(model).toBe("gemini-new");
+      expect(selections).toEqual(["model=gemini-new (unlisted)"]);
+
+      const failure = EffectAcpErrors.AcpRequestError.invalidParams("Provider rejected it.");
+      const rejecting = makeModelRuntime([modelConfig], failure);
+      const error = yield* applyAntigravityAcpModelSelection({
+        runtime: rejecting.runtime,
+        model: "gemini-removed",
+        mapError: (cause) => ({ cause }),
+      }).pipe(Effect.flip);
+      expect(error).toEqual({ cause: failure });
+      expect(rejecting.selections).toEqual(["model=gemini-removed (unlisted)"]);
+    }),
   );
 
   it.effect("accepts exact model IDs from grouped native options", () =>
diff --git a/apps/server/src/provider/acp/AntigravityAcpSupport.ts b/apps/server/src/provider/acp/AntigravityAcpSupport.ts
index 7fd64e2534fa7..7d893a5816815 100644
--- a/apps/server/src/provider/acp/AntigravityAcpSupport.ts
+++ b/apps/server/src/provider/acp/AntigravityAcpSupport.ts
@@ -273,17 +273,18 @@ export const applyAntigravityAcpModelSelection = Effect.fn("applyAntigravityAcpM
     });
     const explicit = Boolean(input.model) && input.model !== ANTIGRAVITY_DEFAULT_MODEL;
     if (resolved === undefined || (!explicit && resolved === current)) return current;
-    const options = antigravityModelOptions(configOptions);
-    const legacyFamily = options.some((option) => option.value === resolved)
-      ? undefined
-      : legacyAntigravityFamily(configOptions, resolved);
-    if (legacyFamily !== undefined) {
+    const listed = antigravityModelOptions(configOptions).some(
+      (option) => option.value === resolved,
+    );
+    const legacyFamily = listed ? undefined : legacyAntigravityFamily(configOptions, resolved);
+    // A live session's choices go stale when the account gains models; let the agent decide.
+    if (legacyFamily !== undefined || (explicit && !listed)) {
       yield* input.runtime
         .setConfigOption("model", resolved, { allowUnlistedValue: true })
         .pipe(Effect.mapError(input.mapError));
-      return legacyFamily;
+      return legacyFamily ?? resolved;
     }
-    if (!options.some((option) => option.value === resolved)) {
+    if (!listed) {
       return yield* Effect.fail(
         input.mapError(
           EffectAcpErrors.AcpRequestError.invalidParams(