fix(server): alias Grok's product-name model slug to a valid ACP modelId

apps/server/scripts/acp-mock-agent.ts

@@ -278,15 +278,17 @@ function modeState(): AcpSchema.SessionModeState {
   };
 }
 
+// Mirrors the real Grok ACP: it only accepts "grok-4.6"/"grok-4.5" as
+// modelIds, never the CLI's own "grok-build" product name (see #7791).
 const grokAcpModels: ReadonlyArray<AcpSchema.ModelInfo> = [
-  { modelId: "grok-build", name: "Grok Build" },
+  { modelId: "grok-4.6", name: "Grok 4.6" },
   { modelId: "grok-mock-alt", name: "Grok Mock Alt" },
 ];
 
 function modelState(): AcpSchema.SessionModelState {
   const modelId = grokAcpModels.some((model) => model.modelId === currentModelId)
     ? currentModelId
-    : "grok-build";
+    : "grok-4.6";
   return {
     currentModelId: modelId,
     availableModels: grokAcpModels,

apps/server/src/provider/acp/GrokAcpSupport.test.ts

@@ -10,8 +10,11 @@ import {
 
 describe("resolveGrokAcpBaseModelId", () => {
   it("normalizes empty and custom Grok model ids", () => {
-    expect(resolveGrokAcpBaseModelId(undefined)).toBe("grok-build");
-    expect(resolveGrokAcpBaseModelId("   ")).toBe("grok-build");
+    // "grok-build" is the CLI's product name, not a modelId its ACP accepts —
+    // it must resolve to a real model alias, not pass through verbatim.
+    expect(resolveGrokAcpBaseModelId(undefined)).toBe("grok-4.6");
+    expect(resolveGrokAcpBaseModelId("   ")).toBe("grok-4.6");
+    expect(resolveGrokAcpBaseModelId("grok-build")).toBe("grok-4.6");
     expect(resolveGrokAcpBaseModelId("  grok-test-custom-model  ")).toBe("grok-test-custom-model");
   });
 });

packages/contracts/src/model.ts

@@ -211,6 +211,14 @@ export const MODEL_SLUG_ALIASES_BY_PROVIDER: Partial<
     "opus-4.5-thinking": "claude-opus-4-5",
     "opus-4.5": "claude-opus-4-5",
   },
+  [GROK_DRIVER_KIND]: {
+    // "grok-build" is the Grok Build CLI's own product name, not a model id
+    // its ACP accepts — sending it verbatim as `session/set_model`'s modelId
+    // fails with "unknown model id". Grok's ACP only knows "grok-4.6" and
+    // "grok-4.5"; alias the default/display slug to the current one so the
+    // wire-level id is always valid.
+    "grok-build": "grok-4.6",
+  },
   [OPENCODE_DRIVER_KIND]: {},
 };
 

packages/shared/src/model.test.ts

@@ -154,4 +154,10 @@ describe("model slug normalization", () => {
     expect(normalizeModelSlug("opus", claude)).toBe("claude-opus-5");
     expect(normalizeCustomModelSlug(" opus ")).toBe("opus");
   });
+
+  it("aliases Grok's product-name slug to a modelId its ACP accepts", () => {
+    const grok = ProviderDriverKind.make("grok");
+
+    expect(normalizeModelSlug("grok-build", grok)).toBe("grok-4.6");
+  });
 });