diff --git a/apps/server/src/provider/acp/AcpRuntimeModel.test.ts b/apps/server/src/provider/acp/AcpRuntimeModel.test.ts
index f2d999520e02..27cb3234b375 100644
--- a/apps/server/src/provider/acp/AcpRuntimeModel.test.ts
+++ b/apps/server/src/provider/acp/AcpRuntimeModel.test.ts
@@ -13,6 +13,7 @@ import {
   parsePermissionRequest,
   parseSessionModeState,
   parseSessionUpdateEvent,
+  sessionModelStateFromInitialize,
   sessionUpdateCountsAsLoadReplayActivity,
   sessionUpdateIsReplay,
   syntheticLoadSessionResponseFromInitialize,
@@ -216,6 +217,63 @@ describe("AcpRuntimeModel", () => {
     expect(response.modes?.availableModes).toHaveLength(2);
   });
 
+  it("keeps initialize mode state whose modes have null descriptions", () => {
+    const response = syntheticLoadSessionResponseFromInitialize({
+      protocolVersion: 1,
+      _meta: {
+        modeState: {
+          currentModeId: "code",
+          availableModes: [
+            { id: "ask", name: "Ask", description: null },
+            { id: "code", name: "Code", description: "Edit files", _meta: null },
+          ],
+          _meta: null,
+        },
+      },
+    } satisfies EffectAcpSchema.InitializeResponse);
+
+    expect(parseSessionModeState(response)).toEqual({
+      currentModeId: "code",
+      availableModes: [
+        { id: "ask", name: "Ask" },
+        { id: "code", name: "Code", description: "Edit files" },
+      ],
+    });
+  });
+
+  it("reads initialize model state and ignores malformed model state", () => {
+    const modelState = {
+      currentModelId: "grok-4.7",
+      availableModels: [
+        {
+          modelId: "grok-4.7",
+          name: "Grok 4.7",
+          description: null,
+          _meta: { reasoningEfforts: [{ id: "high", default: true }] },
+        },
+        { modelId: "grok-4.6", name: "Grok 4.6" },
+      ],
+    };
+
+    expect(sessionModelStateFromInitialize({ protocolVersion: 1, _meta: { modelState } })).toEqual(
+      modelState,
+    );
+    expect(
+      sessionModelStateFromInitialize({
+        protocolVersion: 1,
+        _meta: {
+          modelState: { currentModelId: "grok-4.7", availableModels: [{ modelId: "grok-4.7" }] },
+        },
+      }),
+    ).toBeUndefined();
+    expect(
+      sessionModelStateFromInitialize({
+        protocolVersion: 1,
+        _meta: { modelState: { ...modelState, _meta: "not an object" } },
+      }),
+    ).toBeUndefined();
+  });
+
   it("projects typed ACP tool call updates into runtime events", () => {
     const created = parseSessionUpdateEvent({
       sessionId: "session-1",
diff --git a/apps/server/src/provider/acp/AcpRuntimeModel.ts b/apps/server/src/provider/acp/AcpRuntimeModel.ts
index 522d8783dec3..694f761ff8f7 100644
--- a/apps/server/src/provider/acp/AcpRuntimeModel.ts
+++ b/apps/server/src/provider/acp/AcpRuntimeModel.ts
@@ -3,7 +3,9 @@ import * as Duration from "effect/Duration";
 import * as Effect from "effect/Effect";
 import * as Option from "effect/Option";
 import * as Ref from "effect/Ref";
-import type * as EffectAcpSchema from "effect-acp/compat";
+import * as Schema from "effect/Schema";
+import * as EffectAcpSchema from "effect-acp/compat";
+import * as EffectAcpSchemaV1 from "effect-acp/schema-v1";
 import {
   deriveToolActivityPresentation,
   mergeToolActivityData,
@@ -19,39 +21,10 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === "object" && value !== null && !Array.isArray(value);
 }
 
-function isSessionModelState(value: unknown): value is EffectAcpSchema.SessionModelState {
-  if (!isRecord(value) || typeof value.currentModelId !== "string") {
-    return false;
-  }
-  if (!Array.isArray(value.availableModels)) {
-    return false;
-  }
-  return value.availableModels.every(
-    (model) =>
-      isRecord(model) &&
-      typeof model.modelId === "string" &&
-      typeof model.name === "string" &&
-      (model.description === undefined ||
-        model.description === null ||
-        typeof model.description === "string"),
-  );
-}
-
-function isSessionModeState(value: unknown): value is EffectAcpSchema.SessionModeState {
-  if (!isRecord(value) || typeof value.currentModeId !== "string") {
-    return false;
-  }
-  if (!Array.isArray(value.availableModes)) {
-    return false;
-  }
-  return value.availableModes.every(
-    (mode) =>
-      isRecord(mode) &&
-      typeof mode.id === "string" &&
-      typeof mode.name === "string" &&
-      (mode.description === undefined || typeof mode.description === "string"),
-  );
-}
+// Guards for the untyped `initialize._meta` states some agents (Grok) advertise.
+// Modes were removed from ACP v2, so the v1 wire schema is the source of truth.
+const isSessionModelState = Schema.is(EffectAcpSchema.SessionModelState);
+const isSessionModeState = Schema.is(EffectAcpSchemaV1.SessionModeState);
 
 export interface AcpSessionMode {
   readonly id: string;