feat(FN-742): add describeModel helper and log model in agent creation sites

- Add describeModel() helper in pi.ts to format provider/model info for logging
- Log resolved model details in executor, reviewer, and triage agent creation
- Update executor and reviewer to call describeModel before session start
- Add unit tests for describeModel covering all input combinations
- Fix test mocks to account for new describeModel dependency

packages/engine/src/executor.test.ts

@@ -4,6 +4,7 @@ import { AgentSemaphore } from "./concurrency.js";
 // Mock external dependencies
 vi.mock("./pi.js", () => ({
   createKbAgent: vi.fn(),
+  describeModel: vi.fn().mockReturnValue("mock-provider/mock-model"),
 }));
 vi.mock("./reviewer.js", () => ({
   reviewStep: vi.fn(),

packages/engine/src/executor.ts

@@ -5,7 +5,7 @@ import type { TaskStore, Task, TaskDetail, StepStatus, Settings, WorkflowStep, M
 import { findWorktreeUser } from "./merger.js";
 import { generateWorktreeName, slugify } from "./worktree-names.js";
 import { Type, type Static } from "@mariozechner/pi-ai";
-import { createKbAgent } from "./pi.js";
+import { createKbAgent, describeModel } from "./pi.js";
 import { reviewStep, type ReviewVerdict } from "./reviewer.js";
 import type { ToolDefinition, AgentSession, SessionManager } from "@mariozechner/pi-coding-agent";
 import { PRIORITY_EXECUTE, type AgentSemaphore } from "./concurrency.js";
@@ -593,6 +593,9 @@ export class TaskExecutor {
           defaultThinkingLevel: settings.defaultThinkingLevel,
         });
 
+        executorLog.log(`${task.id}: using model ${describeModel(session)}`);
+        await this.store.logEntry(task.id, `Executor using model: ${describeModel(session)}`);
+
         // Make session available to custom tools (task_update checkpoint capture, review_step rewind)
         sessionRef.current = session;
 
@@ -1369,6 +1372,9 @@ If issues are found that need attention, describe them clearly.`;
         defaultThinkingLevel: settings.defaultThinkingLevel,
       });
 
+      executorLog.log(`${task.id}: workflow step '${workflowStep.name}' using model ${describeModel(session)}`);
+      await this.store.logEntry(task.id, `Workflow step '${workflowStep.name}' using model: ${describeModel(session)}`);
+
       let output = "";
       session.subscribe((event) => {
         if (event.type === "message_update") {

packages/engine/src/pi.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from "vitest";
+import { describeModel } from "./pi.js";
+import type { AgentSession } from "@mariozechner/pi-coding-agent";
+
+describe("describeModel", () => {
+  it('returns "provider/modelId" when session has a model', () => {
+    const fakeSession = {
+      model: {
+        provider: "anthropic",
+        id: "claude-sonnet-4-5",
+        name: "Claude Sonnet",
+      },
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("anthropic/claude-sonnet-4-5");
+  });
+
+  it('returns "unknown model" when session model is undefined', () => {
+    const fakeSession = {
+      model: undefined,
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("unknown model");
+  });
+
+  it("handles different providers", () => {
+    const fakeSession = {
+      model: {
+        provider: "openai",
+        id: "gpt-4o",
+        name: "GPT-4o",
+      },
+    } as unknown as AgentSession;
+
+    expect(describeModel(fakeSession)).toBe("openai/gpt-4o");
+  });
+});

packages/engine/src/pi.ts

@@ -22,6 +22,17 @@ export interface AgentResult {
   session: AgentSession;
 }
 
+/**
+ * Extract a human-readable model description from an AgentSession.
+ * Returns `"<provider>/<modelId>"` (e.g. `"anthropic/claude-sonnet-4-5"`)
+ * or `"unknown model"` when the session has no model set.
+ */
+export function describeModel(session: AgentSession): string {
+  const model = session.model;
+  if (!model) return "unknown model";
+  return `${model.provider}/${model.id}`;
+}
+
 export interface AgentOptions {
   cwd: string;
   systemPrompt: string;

packages/engine/src/restart.integration.test.ts

@@ -16,6 +16,7 @@ import { AgentSemaphore } from "./concurrency.js";
 
 vi.mock("./pi.js", () => ({
   createKbAgent: vi.fn(),
+  describeModel: vi.fn().mockReturnValue("mock-provider/mock-model"),
 }));
 vi.mock("./reviewer.js", () => ({
   reviewStep: vi.fn(),

packages/engine/src/reviewer.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 
 vi.mock("./pi.js", () => ({
   createKbAgent: vi.fn(),
+  describeModel: vi.fn().mockReturnValue("mock-provider/mock-model"),
 }));
 
 import { reviewStep } from "./reviewer.js";

packages/engine/src/reviewer.ts

@@ -9,8 +9,9 @@
  */
 
 import type { TaskStore } from "@fusion/core";
-import { createKbAgent } from "./pi.js";
+import { createKbAgent, describeModel } from "./pi.js";
 import { AgentLogger } from "./agent-logger.js";
+import { reviewerLog } from "./logger.js";
 import { checkSessionError } from "./usage-limit-detector.js";
 
 const REVIEWER_SYSTEM_PROMPT = `You are an independent code and plan reviewer.
@@ -209,6 +210,11 @@ export async function reviewStep(
     defaultThinkingLevel: options.defaultThinkingLevel,
   });
 
+  reviewerLog.log(`${taskId}: reviewer using model ${describeModel(session)}`);
+  if (options.store && options.taskId) {
+    await options.store.logEntry(options.taskId, `Reviewer using model: ${describeModel(session)}`);
+  }
+
   let reviewText = "";
 
   // Capture the reviewer's full text output (still needed for verdict extraction)

packages/engine/src/triage.ts

@@ -11,7 +11,7 @@ import type {
   ToolDefinition,
   AgentSession,
 } from "@mariozechner/pi-coding-agent";
-import { createKbAgent } from "./pi.js";
+import { createKbAgent, describeModel } from "./pi.js";
 import { reviewStep, type ReviewVerdict } from "./reviewer.js";
 import { PRIORITY_SPECIFY, type AgentSemaphore } from "./concurrency.js";
 import { AgentLogger } from "./agent-logger.js";
@@ -477,6 +477,9 @@ export class TriageProcessor {
           defaultThinkingLevel: settings.defaultThinkingLevel,
         });
 
+        triageLog.log(`${task.id}: using model ${describeModel(session)}`);
+        await this.store.logEntry(task.id, `Triage using model: ${describeModel(session)}`);
+
         // Make session available to review_spec tool (for RETHINK rewind)
         sessionRef.current = session;