diff --git a/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.test.ts
index 63286a4bcea79..ebcde1741e14b 100644
--- a/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.test.ts
@@ -35,6 +35,7 @@ import { normalizeAntigravitySessionUpdate } from "../../provider/acp/Antigravit
 import { ANTIGRAVITY_SIGN_IN_REQUIRED_MESSAGE } from "../../provider/antigravityAuthSupport.ts";
 import * as IdAllocator from "@t3tools/provider-core/server/IdAllocator";
 import * as ProviderAdapter from "@t3tools/provider-core/server/ProviderAdapter";
+import { makeProviderFailure } from "@t3tools/provider-core/server/failure";
 import {
   makeAntigravityAcpAdapterFlavor,
   makeAntigravityAdapterV2,
@@ -142,6 +143,58 @@ const layerSession = Layer.mergeAll(
   TestProviderHost.layer().pipe(Layer.provide(NodeServices.layer)),
 );
 
+describe("AntigravityAdapterV2 model availability", () => {
+  it.live("explains ACP access restrictions when the session does not offer the saved model", () =>
+    Effect.gen(function* () {
+      const childProcessSpawner = yield* ChildProcessSpawner.ChildProcessSpawner;
+      const path = yield* Path.Path;
+      const crypto = yield* Crypto.Crypto;
+      const mockAgentPath = yield* path.fromFileUrl(
+        new URL("../../../scripts/acp-mock-agent.ts", import.meta.url),
+      );
+      const instanceId = ProviderInstanceId.make("antigravity-model-access-test");
+      const adapter = yield* makeAntigravityAdapterV2({
+        instanceId,
+        selfInvocation: yield* resolveSelfInvocation(),
+        makeRuntime: (input) =>
+          makeAntigravityAcpRuntime({
+            ...input,
+            childProcessSpawner,
+            spawn: {
+              command: process.execPath,
+              args: [mockAgentPath],
+              cwd: input.cwd,
+              env: { T3_ACP_ANTIGRAVITY: "1" },
+            },
+          }).pipe(Effect.provideService(Crypto.Crypto, crypto)),
+        withProcess: (_stop, task) => task,
+        // @effect-diagnostics-next-line effectSucceedWithVoid:off - This optional result requires undefined, not the wider void type.
+        defaultModel: Effect.succeed(undefined),
+      });
+      const threadId = ThreadId.make("thread-antigravity-model-access");
+      const runtimePolicy = ProviderAdapter.ProviderAdapterV2RuntimePolicy.make({
+        runtimeMode: "full-access",
+        interactionMode: "default",
+        cwd: process.cwd(),
+      });
+      const open = (model: string) =>
+        adapter.openSession({
+          threadId,
+          providerSessionId: ProviderSessionId.make(`provider-session-${model}`),
+          modelSelection: { instanceId, model },
+          runtimePolicy,
+        });
+      const error = yield* open("claude-test").pipe(Effect.flip);
+      const failure = makeProviderFailure({ cause: error });
+      assert.include(failure.message, '"claude-test"');
+      assert.include(failure.message, "sign-in method and ACP client");
+      assert.include(failure.message, "Antigravity app");
+      assert.include(failure.message, "refresh Antigravity models");
+      yield* open("gemini-test-low");
+    }).pipe(Effect.provide(layerSession), Effect.scoped),
+  );
+});
+
 describe("AntigravityAdapterV2 compaction", () => {
   it.effect("does not expose unsupported manual compaction on the provider session", () =>
     Effect.gen(function* () {
diff --git a/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.ts b/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.ts
index 8c5ccb48b149d..91695195fed20 100644
--- a/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.ts
@@ -234,6 +234,8 @@ export function makeAntigravityAcpAdapterFlavor(
     // Loading history replays tool calls; resume avoids replay.
     subagentsIdleOnTurnCompletion: true,
     ...(options.onSessionEvent === undefined ? {} : { onSessionEvent: options.onSessionEvent }),
+    modelSelectionGuidance:
+      "Antigravity model access can depend on the sign-in method and ACP client, and can differ from the Antigravity app. Update T3 Code and refresh Antigravity models, then choose an available model and retry.",
     applyModelSelection: Effect.fn("AntigravityAdapterV2.applyModelSelection")(function* ({
       runtime,
       modelSelection,
diff --git a/apps/server/src/provider/acp/AntigravityAcpSupport.ts b/apps/server/src/provider/acp/AntigravityAcpSupport.ts
index a2be5ccaa3f6a..f162d34f68f1b 100644
--- a/apps/server/src/provider/acp/AntigravityAcpSupport.ts
+++ b/apps/server/src/provider/acp/AntigravityAcpSupport.ts
@@ -256,7 +256,7 @@ export const applyAntigravityAcpModelSelection = Effect.fn("applyAntigravityAcpM
       return yield* Effect.fail(
         input.mapError(
           EffectAcpErrors.AcpRequestError.invalidParams(
-            `Antigravity model '${resolved}' is unavailable for this Google account. Select an available model.`,
+            `Antigravity model '${resolved}' is unavailable in this ACP session. Select an available model.`,
           ),
         ),
       );
diff --git a/packages/provider-acp/src/server/adapter.ts b/packages/provider-acp/src/server/adapter.ts
index 2110f9616de54..fcb7ca836ed27 100644
--- a/packages/provider-acp/src/server/adapter.ts
+++ b/packages/provider-acp/src/server/adapter.ts
@@ -219,11 +219,12 @@ class AcpModelSelectionError extends Schema.TaggedError<AcpModelSelectionError>(
   "AcpModelSelectionError",
   {
     model: Schema.String,
+    guidance: Schema.optional(Schema.String),
     cause: Schema.Defect(),
   },
 ) {
   override get message(): string {
-    return `The agent did not accept the model "${this.model}". Choose another model for this provider, then retry.`;
+    return `The agent did not accept the model "${this.model}". ${this.guidance ?? "Choose another model for this provider, then retry."}`;
   }
 }
 
@@ -364,6 +365,7 @@ export interface AcpAdapterV2Flavor {
     readonly startResult: AcpSessionRuntime.AcpSessionRuntimeStartResult;
     readonly modelSelection: ModelSelection;
   }) => Effect.Effect<string | undefined, EffectAcpErrors.AcpError>;
+  readonly modelSelectionGuidance?: string;
   /**
    * Model-selection option ids `applyModelSelection` sets itself under a
    * native id, which the generic option loop then skips (Kiro's
@@ -7102,7 +7104,11 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
           // own error.
           const rejectedModel = (error: EffectAcpErrors.AcpError) =>
             error._tag === "AcpRequestError" && error.code === -32602
-              ? new AcpModelSelectionError({ model: modelSelection.model, cause: error })
+              ? new AcpModelSelectionError({
+                  model: modelSelection.model,
+                  guidance: flavor.modelSelectionGuidance,
+                  cause: error,
+                })
               : error;
           let appliedModel: string | undefined;
           if (flavor.applyModelSelection !== undefined) {