feat: surface resolved model name in default model option description (#982)

## Summary

- When the SDK's `"default"` model entry carries a `resolvedModel` (e.g.
`"claude-sonnet-5"`), `buildConfigOptions` now sets the option's
`description` to the human-readable `displayName` of the matching named
model (e.g. `"Claude Sonnet 5"`)
- Falls back to the raw `resolvedModel` string if no named entry shares
that resolved ID
- No behaviour change when `resolvedModel` is absent

This lets ACP clients show the actual model behind "Default" — e.g.
"Default (Claude Sonnet 5)" — instead of just "Default (recommended)".

## Test plan

- [ ] `buildConfigOptions model option resolved description > sets
description to the named model's displayName when resolvedModel matches`
- [ ] `buildConfigOptions model option resolved description > falls back
to resolvedModel itself when no named model shares it`
- [ ] `buildConfigOptions model option resolved description > leaves
description undefined when default model has no resolvedModel`
- [ ] All 753 existing tests still pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/acp-agent.ts

@@ -6946,11 +6946,23 @@ export function buildConfigOptions(
       category: "model",
       type: "select",
       currentValue: models.currentModelId,
-      options: models.availableModels.map((m) => ({
-        value: m.modelId,
-        name: m.name,
-        description: m.description ?? undefined,
-      })),
+      options: models.availableModels.map((m) => {
+        if (m.modelId === "default") {
+          const defaultInfo = modelInfos.find((mi) => mi.value === "default");
+          const resolvedModel = defaultInfo?.resolvedModel;
+          if (resolvedModel) {
+            const namedMatch = modelInfos.find(
+              (mi) => mi.value !== "default" && mi.resolvedModel === resolvedModel,
+            );
+            return {
+              value: m.modelId,
+              name: m.name,
+              description: namedMatch?.displayName ?? resolvedModel,
+            };
+          }
+        }
+        return { value: m.modelId, name: m.name, description: m.description ?? undefined };
+      }),
     },
   ];
 

src/tests/acp-agent.test.ts

@@ -13462,6 +13462,83 @@ describe("agent selection config option", () => {
     });
   });
 
+  describe("buildConfigOptions model option resolved description", () => {
+    const modes = { currentModeId: "default", availableModes: [] };
+    const models = {
+      currentModelId: "default",
+      availableModels: [{ modelId: "default", name: "Default", description: "" }],
+    };
+
+    it("sets description to the named model's displayName when resolvedModel matches", () => {
+      const modelInfos = [
+        {
+          value: "default",
+          displayName: "Default",
+          description: "",
+          resolvedModel: "claude-sonnet-5",
+        },
+        {
+          value: "sonnet",
+          displayName: "Claude Sonnet 5",
+          description: "Balanced",
+          resolvedModel: "claude-sonnet-5",
+        },
+      ];
+      const options = buildConfigOptions(
+        modes,
+        models,
+        modelInfos as any,
+        undefined,
+        [],
+        "default",
+      );
+      const modelOption = options.find((o) => o.id === "model");
+      const defaultEntry = (modelOption as any).options.find((o: any) => o.value === "default");
+      expect(defaultEntry.description).toBe("Claude Sonnet 5");
+    });
+
+    it("falls back to resolvedModel itself when no named model shares it", () => {
+      const modelInfos = [
+        {
+          value: "default",
+          displayName: "Default",
+          description: "",
+          resolvedModel: "claude-opus-5-20251201",
+        },
+      ];
+      const options = buildConfigOptions(
+        modes,
+        models,
+        modelInfos as any,
+        undefined,
+        [],
+        "default",
+      );
+      const modelOption = options.find((o) => o.id === "model");
+      const defaultEntry = (modelOption as any).options.find((o: any) => o.value === "default");
+      expect(defaultEntry.description).toBe("claude-opus-5-20251201");
+    });
+
+    it("leaves description undefined when default model has no resolvedModel", () => {
+      const modelsNoDesc = {
+        currentModelId: "default",
+        availableModels: [{ modelId: "default", name: "Default" }],
+      };
+      const modelInfos = [{ value: "default", displayName: "Default", description: "" }];
+      const options = buildConfigOptions(
+        modes,
+        modelsNoDesc,
+        modelInfos as any,
+        undefined,
+        [],
+        "default",
+      );
+      const modelOption = options.find((o) => o.id === "model");
+      const defaultEntry = (modelOption as any).options.find((o: any) => o.value === "default");
+      expect(defaultEntry.description).toBeUndefined();
+    });
+  });
+
   describe("switching the agent", () => {
     function createMockAgent() {
       const mockClient = { sessionUpdate: async () => {} } as unknown as AcpClient;