diff --git a/packages/engine/src/__tests__/agent-runtime-layers.test.ts b/packages/engine/src/__tests__/agent-runtime-layers.test.ts
new file mode 100644
index 000000000..bad5e7ff1
--- /dev/null
+++ b/packages/engine/src/__tests__/agent-runtime-layers.test.ts
@@ -0,0 +1,31 @@
+import { describe, it, expect } from "vitest";
+import type { AgentRuntimeOptions } from "../agent-runtime.js";
+import type { SystemPromptLayers } from "../prompt-layers.js";
+
+describe("AgentRuntimeOptions.systemPromptLayers", () => {
+  it("accepts systemPromptLayers alongside systemPrompt", () => {
+    const layers: SystemPromptLayers = {
+      stable: "You are a reviewer.",
+      dynamic: "Check for bugs.",
+    };
+
+    const options: AgentRuntimeOptions = {
+      cwd: "/tmp/test",
+      systemPrompt: "You are a reviewer.\n\nCheck for bugs.",
+      systemPromptLayers: layers,
+    };
+
+    expect(options.systemPromptLayers).toBeDefined();
+    expect(options.systemPromptLayers!.stable).toBe("You are a reviewer.");
+    expect(options.systemPromptLayers!.dynamic).toBe("Check for bugs.");
+  });
+
+  it("works without systemPromptLayers (backward compatible)", () => {
+    const options: AgentRuntimeOptions = {
+      cwd: "/tmp/test",
+      systemPrompt: "You are a reviewer.",
+    };
+
+    expect(options.systemPromptLayers).toBeUndefined();
+  });
+});
diff --git a/packages/engine/src/__tests__/pi-layers-wiring.test.ts b/packages/engine/src/__tests__/pi-layers-wiring.test.ts
new file mode 100644
index 000000000..5eebfe2fb
--- /dev/null
+++ b/packages/engine/src/__tests__/pi-layers-wiring.test.ts
@@ -0,0 +1,318 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import type { PathLike } from "node:fs";
+
+/**
+ * Tests that createFnAgent correctly wires prompt layers to
+ * DefaultResourceLoader and that tools are sorted deterministically.
+ *
+ * These tests verify the actual subsystem wiring rather than testing
+ * the layer logic in isolation.
+ */
+
+const createAgentSessionMock = vi.fn();
+const createCodingToolsMock = vi.fn(() => []);
+const createReadOnlyToolsMock = vi.fn(() => []);
+const createExtensionRuntimeMock = vi.fn();
+const discoverAndLoadExtensionsMock = vi.fn().mockResolvedValue({
+  runtime: { pendingProviderRegistrations: [] },
+  errors: [],
+});
+const packageManagerResolveMock = vi.fn().mockResolvedValue({ extensions: [] });
+const findMock = vi.fn();
+const getAllMock = vi.fn(() => [] as any[]);
+const registerProviderMock = vi.fn();
+const refreshMock = vi.fn();
+const settingsManagerInMemoryMock = vi.fn(() => ({ kind: "settings-manager" }));
+const setFallbackResolverMock = vi.fn();
+const reloadMock = vi.fn(async () => {});
+const execSyncMock = vi.fn((_cmd?: any, _opts?: any) => "");
+const existsSyncMock = vi.fn((_path: PathLike) => false);
+const readFileSyncMock = vi.fn((_path?: any) => "{}");
+const readCustomProvidersMock = vi.fn(() => []);
+
+// Capture DefaultResourceLoader constructor args
+let capturedResourceLoaderOptions: any = null;
+
+vi.mock("node:child_process", () => {
+  const execSyncFn = execSyncMock;
+  const kPromisifyCustom = Symbol.for("nodejs.util.promisify.custom");
+
+  const execFn: any = vi.fn((cmd: string, opts: any, cb: any) => {
+    const callback = typeof opts === "function" ? opts : cb;
+    const options = typeof opts === "function" ? {} : (opts ?? {});
+    try {
+      const out = execSyncFn(cmd, { ...options, stdio: ["pipe", "pipe", "pipe"] });
+      const stdout = out === undefined ? "" : out.toString();
+      if (typeof callback === "function") callback(null, stdout, "");
+    } catch (err) {
+      if (typeof callback === "function") {
+        const error = err as { stdout?: string; stderr?: string };
+        callback(err, error?.stdout?.toString?.() ?? "", error?.stderr?.toString?.() ?? "");
+      }
+    }
+  });
+
+  execFn[kPromisifyCustom] = (cmd: string, opts?: any) =>
+    new Promise((resolve, reject) => {
+      execFn(cmd, opts, (err: any, stdout: string, stderr: string) => {
+        if (err) {
+          (err as Record<string, unknown>).stdout = stdout;
+          (err as Record<string, unknown>).stderr = stderr;
+          reject(err);
+        } else {
+          resolve({ stdout, stderr });
+        }
+      });
+    });
+  return { execSync: execSyncFn, exec: execFn };
+});
+
+vi.mock("node:fs", async () => {
+  const actual = await vi.importActual<typeof import("node:fs")>("node:fs");
+  return {
+    ...actual,
+    existsSync: existsSyncMock,
+    readFileSync: readFileSyncMock,
+  };
+});
+
+vi.mock("../custom-providers.js", () => ({
+  readCustomProviders: readCustomProvidersMock,
+}));
+
+vi.mock("@mariozechner/pi-coding-agent", () => ({
+  AuthStorage: {
+    create: () => ({
+      setFallbackResolver: setFallbackResolverMock,
+    }),
+  },
+  createAgentSession: createAgentSessionMock,
+  createBashTool: () => ({ name: "bash" }),
+  createCodingTools: createCodingToolsMock,
+  createEditTool: () => ({ name: "edit" }),
+  createExtensionRuntime: createExtensionRuntimeMock,
+  createFindTool: () => ({ name: "find" }),
+  createGrepTool: () => ({ name: "grep" }),
+  createLsTool: () => ({ name: "ls" }),
+  createReadOnlyTools: createReadOnlyToolsMock,
+  createReadTool: () => ({ name: "read" }),
+  createWriteTool: () => ({ name: "write" }),
+  DefaultResourceLoader: class {
+    constructor(options: any) {
+      capturedResourceLoaderOptions = options;
+    }
+    async reload() {
+      await reloadMock();
+    }
+  },
+  DefaultPackageManager: class {
+    async resolve() {
+      return packageManagerResolveMock();
+    }
+  },
+  discoverAndLoadExtensions: discoverAndLoadExtensionsMock,
+  getAgentDir: () => "/mock-agent-dir",
+  ModelRegistry: class {
+    static create(..._args: unknown[]) {
+      return new (this as unknown as new () => unknown)();
+    }
+    find(provider: string, modelId: string) {
+      return findMock(provider, modelId);
+    }
+    getAll() {
+      return getAllMock();
+    }
+    registerProvider(name: string, config: unknown) {
+      return registerProviderMock(name, config);
+    }
+    refresh() {
+      return refreshMock();
+    }
+  },
+  SessionManager: {
+    inMemory: () => ({ kind: "session-manager" }),
+  },
+  SettingsManager: {
+    create: vi.fn(),
+    inMemory: settingsManagerInMemoryMock,
+  },
+}));
+
+describe("createFnAgent prompt layer wiring", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    capturedResourceLoaderOptions = null;
+    execSyncMock.mockReturnValue("");
+    existsSyncMock.mockReturnValue(false);
+    readFileSyncMock.mockReturnValue("{}");
+    readCustomProvidersMock.mockReturnValue([]);
+    findMock.mockImplementation((provider: string, modelId: string) => ({ provider, id: modelId }));
+    createAgentSessionMock.mockResolvedValue({
+      session: {
+        prompt: vi.fn(),
+        subscribe: vi.fn(),
+        dispose: vi.fn(),
+        setThinkingLevel: vi.fn(),
+      },
+    });
+  });
+
+  it("passes stable layer as systemPromptOverride when layers provided", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Stable prefix.\n\nDynamic suffix.",
+      systemPromptLayers: {
+        stable: "Stable prefix.",
+        dynamic: "Dynamic suffix.",
+      },
+    });
+
+    expect(capturedResourceLoaderOptions).toBeDefined();
+    const override = capturedResourceLoaderOptions.systemPromptOverride();
+    expect(override).toBe("Stable prefix.");
+  });
+
+  it("passes dynamic layer via appendSystemPromptOverride when layers provided", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Stable.\n\nDynamic content.",
+      systemPromptLayers: {
+        stable: "Stable.",
+        dynamic: "Dynamic content.",
+      },
+    });
+
+    expect(capturedResourceLoaderOptions).toBeDefined();
+    const appended = capturedResourceLoaderOptions.appendSystemPromptOverride();
+    expect(appended).toEqual(["Dynamic content."]);
+  });
+
+  it("falls back to full systemPrompt when no layers provided", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Full system prompt.",
+    });
+
+    expect(capturedResourceLoaderOptions).toBeDefined();
+    const override = capturedResourceLoaderOptions.systemPromptOverride();
+    expect(override).toBe("Full system prompt.");
+  });
+
+  it("returns empty array from appendSystemPromptOverride when no layers", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Full prompt.",
+    });
+
+    const appended = capturedResourceLoaderOptions.appendSystemPromptOverride();
+    expect(appended).toEqual([]);
+  });
+
+  it("returns empty array from appendSystemPromptOverride when dynamic is empty", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Stable only.",
+      systemPromptLayers: {
+        stable: "Stable only.",
+        dynamic: "",
+      },
+    });
+
+    const appended = capturedResourceLoaderOptions.appendSystemPromptOverride();
+    expect(appended).toEqual([]);
+  });
+});
+
+describe("createFnAgent deterministic tool ordering", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    capturedResourceLoaderOptions = null;
+    execSyncMock.mockReturnValue("");
+    existsSyncMock.mockReturnValue(false);
+    readFileSyncMock.mockReturnValue("{}");
+    readCustomProvidersMock.mockReturnValue([]);
+    findMock.mockImplementation((provider: string, modelId: string) => ({ provider, id: modelId }));
+    createAgentSessionMock.mockResolvedValue({
+      session: {
+        prompt: vi.fn(),
+        subscribe: vi.fn(),
+        dispose: vi.fn(),
+        setThinkingLevel: vi.fn(),
+      },
+    });
+  });
+
+  it("passes tools to createAgentSession in alphabetical order", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Test.",
+      tools: "coding",
+    });
+
+    expect(createAgentSessionMock).toHaveBeenCalled();
+
+    const callArgs = createAgentSessionMock.mock.calls[0][0];
+    const toolNames = (callArgs.customTools ?? []).map((t: any) => t.name);
+
+    // Tools should be in alphabetical order
+    const sorted = [...toolNames].sort();
+    expect(toolNames).toEqual(sorted);
+  });
+
+  it("sorts custom tools mixed with built-in tools", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Test.",
+      tools: "coding",
+      customTools: [
+        { name: "zzz_custom", execute: vi.fn() } as any,
+        { name: "aaa_custom", execute: vi.fn() } as any,
+      ],
+    });
+
+    const callArgs = createAgentSessionMock.mock.calls[0][0];
+    const toolNames = (callArgs.customTools ?? []).map((t: any) => t.name);
+
+    const sorted = [...toolNames].sort();
+    expect(toolNames).toEqual(sorted);
+  });
+
+  it("sorts readonly tools with custom tools", async () => {
+    createReadOnlyToolsMock.mockReturnValueOnce([
+      { name: "read" },
+      { name: "grep" },
+      { name: "find" },
+    ] as any);
+
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/tmp/test-project",
+      systemPrompt: "Test.",
+      tools: "readonly",
+      customTools: [
+        { name: "fn_task_list", execute: vi.fn() } as any,
+      ],
+    });
+
+    const callArgs = createAgentSessionMock.mock.calls[0][0];
+    const toolNames = (callArgs.customTools ?? []).map((t: any) => t.name);
+
+    const sorted = [...toolNames].sort();
+    expect(toolNames).toEqual(sorted);
+  });
+});
diff --git a/packages/engine/src/__tests__/prompt-cache-integration.test.ts b/packages/engine/src/__tests__/prompt-cache-integration.test.ts
new file mode 100644
index 000000000..9e2b1e719
--- /dev/null
+++ b/packages/engine/src/__tests__/prompt-cache-integration.test.ts
@@ -0,0 +1,49 @@
+import { describe, it, expect } from "vitest";
+import { buildPromptLayers, collapsePromptLayers, type SystemPromptLayers } from "../prompt-layers.js";
+import { REVIEWER_SYSTEM_PROMPT } from "../reviewer.js";
+
+describe("cross-session prompt cache integration", () => {
+  const MEMORY_INSTRUCTIONS = "\n## Memory\n\nUse fn_memory_search to look up relevant context.";
+
+  function simulateReviewerSession(sessionIndex: number): SystemPromptLayers {
+    return buildPromptLayers({
+      basePrompt: REVIEWER_SYSTEM_PROMPT,
+      agentInstructions: `Session ${sessionIndex}: custom instructions that vary per agent.`,
+      memorySection: MEMORY_INSTRUCTIONS,
+      pluginContributions: sessionIndex % 2 === 0
+        ? "## Plugin: lint\n\nCheck lint rules."
+        : "",
+    });
+  }
+
+  it("produces byte-identical stable prefixes across 10 reviewer sessions", () => {
+    const sessions = Array.from({ length: 10 }, (_, i) => simulateReviewerSession(i));
+
+    const stablePrefix = sessions[0].stable;
+    for (let i = 1; i < sessions.length; i++) {
+      expect(sessions[i].stable).toBe(stablePrefix);
+    }
+  });
+
+  it("dynamic layers vary across sessions as expected", () => {
+    const sessions = Array.from({ length: 5 }, (_, i) => simulateReviewerSession(i));
+
+    const uniqueDynamics = new Set(sessions.map((s) => s.dynamic));
+    expect(uniqueDynamics.size).toBeGreaterThan(1);
+  });
+
+  it("collapsed layers produce valid non-empty strings", () => {
+    const sessions = Array.from({ length: 5 }, (_, i) => simulateReviewerSession(i));
+
+    for (const session of sessions) {
+      const collapsed = collapsePromptLayers(session);
+      expect(collapsed.length).toBeGreaterThan(0);
+      expect(collapsed).toContain("independent code and plan reviewer");
+    }
+  });
+
+  it("stable prefix starts with REVIEWER_SYSTEM_PROMPT", () => {
+    const layers = simulateReviewerSession(0);
+    expect(layers.stable.startsWith(REVIEWER_SYSTEM_PROMPT)).toBe(true);
+  });
+});
diff --git a/packages/engine/src/__tests__/prompt-layers-backward-compat.test.ts b/packages/engine/src/__tests__/prompt-layers-backward-compat.test.ts
new file mode 100644
index 000000000..a323c2e69
--- /dev/null
+++ b/packages/engine/src/__tests__/prompt-layers-backward-compat.test.ts
@@ -0,0 +1,196 @@
+import { describe, it, expect } from "vitest";
+import { buildSystemPromptWithInstructions } from "../agent-instructions.js";
+import { buildPromptLayers, collapsePromptLayers } from "../prompt-layers.js";
+
+/**
+ * Backward compatibility tests: verify that the new layered prompt approach
+ * produces byte-identical output to the legacy buildSystemPromptWithInstructions
+ * + manual concatenation pattern used by each subsystem.
+ */
+describe("prompt layers backward compatibility", () => {
+  describe("collapsed layers match buildSystemPromptWithInstructions", () => {
+    it("matches when only base prompt is provided", () => {
+      const basePrompt = "You are a reviewer.";
+
+      const oldResult = buildSystemPromptWithInstructions(basePrompt, "");
+      const layers = buildPromptLayers({ basePrompt });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+
+    it("matches with base prompt and instructions", () => {
+      const basePrompt = "You are a reviewer.";
+      const instructions = "Check for SQL injection.";
+
+      const oldResult = buildSystemPromptWithInstructions(basePrompt, instructions);
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions: instructions,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+
+    it("matches with base prompt, instructions, and plugin contributions", () => {
+      const basePrompt = "You are an executor.";
+      const instructions = "Follow TDD.";
+      const plugins = "## Plugin: security\n\nScan for vulnerabilities.";
+
+      // Old approach: buildSystemPromptWithInstructions + manual concatenation
+      const oldSystemPrompt = buildSystemPromptWithInstructions(basePrompt, instructions);
+      const oldResult = `${oldSystemPrompt}\n\n${plugins}`;
+
+      // New approach: buildPromptLayers + collapsePromptLayers
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions: instructions,
+        pluginContributions: plugins,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+
+    it("matches with empty instructions and plugin contributions", () => {
+      const basePrompt = "You are a triage agent.";
+      const plugins = "## Plugin: research\n\nUse web search.";
+
+      // Old approach: buildSystemPromptWithInstructions returns base (empty instructions)
+      // then plugins appended
+      const oldSystemPrompt = buildSystemPromptWithInstructions(basePrompt, "");
+      const oldResult = `${oldSystemPrompt}\n\n${plugins}`;
+
+      const layers = buildPromptLayers({
+        basePrompt,
+        pluginContributions: plugins,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+
+    it("matches with empty instructions and no plugins", () => {
+      const basePrompt = "You are a heartbeat agent.";
+
+      const oldResult = buildSystemPromptWithInstructions(basePrompt, "");
+      const layers = buildPromptLayers({ basePrompt });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+  });
+
+  describe("reviewer assembly pattern", () => {
+    it("preserves memory-before-instructions ordering in dynamic layer", () => {
+      const basePrompt = "You are an independent code and plan reviewer.";
+      const memoryInstructions = "## Memory\n\nUse fn_memory_search.";
+      const agentInstructions = "Focus on security.";
+      const plugins = "## Plugin: lint\n\nCheck eslint.";
+
+      // The old reviewer pattern was:
+      //   buildSystemPromptWithInstructions(base + memory, instructions) + plugins
+      // which produced: base + memory → instructions → plugins
+      //
+      // The new pattern moves memory from stable to dynamic (so stable prefix
+      // is byte-identical even if memory changes mid-task), but preserves the
+      // relative ordering: memory → instructions → plugins in the dynamic layer.
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions,
+        memorySection: memoryInstructions,
+        pluginContributions: plugins,
+      });
+      const result = collapsePromptLayers(layers);
+
+      // Base prompt is the stable layer (no memory)
+      expect(layers.stable).toBe(basePrompt);
+
+      // Dynamic layer preserves: memory → instructions → plugins ordering
+      const memoryIdx = result.indexOf("## Memory");
+      const instructionsIdx = result.indexOf("## Custom Instructions");
+      const pluginsIdx = result.indexOf("## Plugin:");
+      expect(memoryIdx).toBeGreaterThan(0);
+      expect(instructionsIdx).toBeGreaterThan(memoryIdx);
+      expect(pluginsIdx).toBeGreaterThan(instructionsIdx);
+    });
+  });
+
+  describe("executor assembly pattern", () => {
+    it("reproduces the executor prompt assembly", () => {
+      const basePrompt = "You are a task execution agent.";
+      const agentInstructions = "Follow the spec precisely.";
+      const plugins = "## Plugin: deploy\n\nCheck CI status.";
+
+      // Old executor pattern:
+      // 1. buildSystemPromptWithInstructions(base, instructions)
+      // 2. if plugins: concatenate
+      const oldSystemPrompt = buildSystemPromptWithInstructions(
+        basePrompt,
+        agentInstructions,
+      );
+      const oldResult = `${oldSystemPrompt}\n\n${plugins}`;
+
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions,
+        pluginContributions: plugins,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+  });
+
+  describe("heartbeat assembly pattern", () => {
+    it("reproduces the heartbeat prompt assembly with multi-part instructions", () => {
+      const basePrompt = "You are a heartbeat agent.";
+      const identitySection = "## Identity\n\nYou are Agent-1.";
+      const memoryInstructions = "## Memory\n\nUse memory tools.";
+      const selfImprovePrompt = "## Self-Improvement\n\nReview your performance.";
+
+      // Old heartbeat pattern:
+      // 1. Join identity + memory + selfImprove with \n\n
+      // 2. buildSystemPromptWithInstructions(base, joined)
+      // 3. if plugins: concatenate
+      const joinedInstructions = [identitySection, memoryInstructions, selfImprovePrompt]
+        .filter((part) => part.trim())
+        .join("\n\n");
+      const oldResult = buildSystemPromptWithInstructions(basePrompt, joinedInstructions);
+
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions: joinedInstructions,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+  });
+
+  describe("triage assembly pattern", () => {
+    it("reproduces the triage prompt assembly with multi-part instructions", () => {
+      const basePrompt = "You are a task specification agent.";
+      const identitySection = "## Identity\n\nYou are TriageBot.";
+      const triageInstructions = "Be thorough.";
+      const researchGuidance = "## Research\n\nUse web search when needed.";
+
+      // Old triage pattern:
+      // 1. Join identity + instructions + research with \n\n
+      // 2. buildSystemPromptWithInstructions(base, joined)
+      const joinedInstructions = [identitySection, triageInstructions, researchGuidance]
+        .filter((section) => section.trim())
+        .join("\n\n");
+      const oldResult = buildSystemPromptWithInstructions(basePrompt, joinedInstructions);
+
+      const layers = buildPromptLayers({
+        basePrompt,
+        agentInstructions: joinedInstructions,
+      });
+      const newResult = collapsePromptLayers(layers);
+
+      expect(newResult).toBe(oldResult);
+    });
+  });
+});
diff --git a/packages/engine/src/__tests__/prompt-layers.test.ts b/packages/engine/src/__tests__/prompt-layers.test.ts
new file mode 100644
index 000000000..dd4eaf5b3
--- /dev/null
+++ b/packages/engine/src/__tests__/prompt-layers.test.ts
@@ -0,0 +1,125 @@
+import { describe, it, expect } from "vitest";
+import {
+  type SystemPromptLayers,
+  buildPromptLayers,
+  collapsePromptLayers,
+} from "../prompt-layers.js";
+
+describe("buildPromptLayers", () => {
+  it("separates base prompt into stable layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+    });
+
+    expect(layers.stable).toBe("You are a reviewer.");
+    expect(layers.dynamic).toBe("");
+  });
+
+  it("puts agent instructions into dynamic layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+      agentInstructions: "Always check for SQL injection.",
+    });
+
+    expect(layers.stable).toBe("You are a reviewer.");
+    expect(layers.dynamic).toContain("Always check for SQL injection.");
+  });
+
+  it("puts memory section into dynamic layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+      memorySection: "## Agent Memory\n\nRemember to check tests.",
+    });
+
+    expect(layers.stable).toBe("You are a reviewer.");
+    expect(layers.dynamic).toContain("Agent Memory");
+  });
+
+  it("puts plugin contributions into dynamic layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+      pluginContributions: "## Plugin: security\n\nScan for CVEs.",
+    });
+
+    expect(layers.stable).toBe("You are a reviewer.");
+    expect(layers.dynamic).toContain("security");
+  });
+
+  it("puts performance feedback into dynamic layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+      performanceFeedback: "## Performance Feedback\n\n- Average score: 8.5",
+    });
+
+    expect(layers.stable).toBe("You are a reviewer.");
+    expect(layers.dynamic).toContain("Performance Feedback");
+  });
+
+  it("combines multiple dynamic sections with double newlines", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "Base.",
+      agentInstructions: "Instructions.",
+      memorySection: "Memory.",
+      pluginContributions: "Plugins.",
+    });
+
+    expect(layers.dynamic).toBe(
+      "Memory.\n\n## Custom Instructions\n\nInstructions.\n\nPlugins."
+    );
+  });
+
+  it("omits empty dynamic sections", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "Base.",
+      agentInstructions: "",
+      memorySection: "",
+      pluginContributions: "Plugins.",
+    });
+
+    expect(layers.dynamic).toBe("Plugins.");
+    expect(layers.dynamic).not.toContain("Custom Instructions");
+  });
+
+  it("produces deterministic output for identical inputs", () => {
+    const input = {
+      basePrompt: "Base.",
+      agentInstructions: "Inst.",
+      memorySection: "Mem.",
+      pluginContributions: "Plug.",
+      performanceFeedback: "Perf.",
+    };
+
+    const layers1 = buildPromptLayers(input);
+    const layers2 = buildPromptLayers(input);
+
+    expect(layers1.stable).toBe(layers2.stable);
+    expect(layers1.dynamic).toBe(layers2.dynamic);
+  });
+});
+
+describe("collapsePromptLayers", () => {
+  it("returns stable when dynamic is empty", () => {
+    const result = collapsePromptLayers({ stable: "Base.", dynamic: "" });
+    expect(result).toBe("Base.");
+  });
+
+  it("joins stable and dynamic with double newline", () => {
+    const result = collapsePromptLayers({
+      stable: "Base.",
+      dynamic: "Dynamic.",
+    });
+    expect(result).toBe("Base.\n\nDynamic.");
+  });
+
+  it("matches legacy buildSystemPromptWithInstructions output", () => {
+    const layers = buildPromptLayers({
+      basePrompt: "You are a reviewer.",
+      agentInstructions: "Check for bugs.",
+    });
+    const collapsed = collapsePromptLayers(layers);
+
+    expect(collapsed).toBe(
+      "You are a reviewer.\n\n## Custom Instructions\n\nCheck for bugs."
+    );
+  });
+});
diff --git a/packages/engine/src/__tests__/reviewer-prompt-layers.test.ts b/packages/engine/src/__tests__/reviewer-prompt-layers.test.ts
new file mode 100644
index 000000000..09694e15e
--- /dev/null
+++ b/packages/engine/src/__tests__/reviewer-prompt-layers.test.ts
@@ -0,0 +1,49 @@
+import { describe, it, expect } from "vitest";
+import { buildPromptLayers, collapsePromptLayers } from "../prompt-layers.js";
+
+describe("reviewer prompt layering", () => {
+  const REVIEWER_BASE = "You are an independent code and plan reviewer.";
+  const MEMORY_INSTRUCTIONS = "\n## Memory\n\nUse fn_memory_search to look up context.";
+
+  it("puts base prompt in stable layer and memory in dynamic layer", () => {
+    const layers = buildPromptLayers({
+      basePrompt: REVIEWER_BASE,
+      agentInstructions: "Custom reviewer guidance.",
+      memorySection: MEMORY_INSTRUCTIONS,
+      pluginContributions: "## Plugin: lint\n\nCheck lint.",
+    });
+
+    expect(layers.stable).toBe(REVIEWER_BASE);
+    expect(layers.stable).not.toContain("Memory");
+    expect(layers.dynamic).toContain("Memory");
+    expect(layers.dynamic).toContain("Custom reviewer guidance");
+  });
+
+  it("produces identical stable layer across simulated sessions", () => {
+    const layers1 = buildPromptLayers({
+      basePrompt: REVIEWER_BASE,
+      agentInstructions: "Session 1 instructions.",
+      memorySection: MEMORY_INSTRUCTIONS,
+    });
+    const layers2 = buildPromptLayers({
+      basePrompt: REVIEWER_BASE,
+      agentInstructions: "Session 2 instructions.",
+      memorySection: MEMORY_INSTRUCTIONS,
+    });
+
+    expect(layers1.stable).toBe(layers2.stable);
+  });
+
+  it("collapsed layers match legacy concatenation", () => {
+    const layers = buildPromptLayers({
+      basePrompt: REVIEWER_BASE,
+      agentInstructions: "Check for bugs.",
+      pluginContributions: "## Plugin: sec\n\nScan.",
+    });
+    const collapsed = collapsePromptLayers(layers);
+
+    expect(collapsed).toBe(
+      `${REVIEWER_BASE}\n\n## Custom Instructions\n\nCheck for bugs.\n\n## Plugin: sec\n\nScan.`
+    );
+  });
+});
diff --git a/packages/engine/src/__tests__/token-usage-cache-ratio.test.ts b/packages/engine/src/__tests__/token-usage-cache-ratio.test.ts
new file mode 100644
index 000000000..5e0f1ec47
--- /dev/null
+++ b/packages/engine/src/__tests__/token-usage-cache-ratio.test.ts
@@ -0,0 +1,20 @@
+import { describe, it, expect } from "vitest";
+import { computeCacheHitRatio } from "../session-token-usage.js";
+
+describe("computeCacheHitRatio", () => {
+  it("returns 0 when no tokens used", () => {
+    expect(computeCacheHitRatio(0, 0)).toBe(0);
+  });
+
+  it("returns 0 when no cached tokens", () => {
+    expect(computeCacheHitRatio(1000, 0)).toBe(0);
+  });
+
+  it("returns ratio of cached to total input", () => {
+    expect(computeCacheHitRatio(500, 500)).toBeCloseTo(0.5);
+  });
+
+  it("returns close to 1 when mostly cached", () => {
+    expect(computeCacheHitRatio(100, 9900)).toBeCloseTo(0.99);
+  });
+});
diff --git a/packages/engine/src/agent-heartbeat.ts b/packages/engine/src/agent-heartbeat.ts
index 6ea0ec775..5f01f7594 100644
--- a/packages/engine/src/agent-heartbeat.ts
+++ b/packages/engine/src/agent-heartbeat.ts
@@ -26,10 +26,10 @@ import { createTaskCreateTool, createTaskLogToolWithContext, createTaskDocumentW
 import { AgentLogger } from "./agent-logger.js";
 import {
   resolveAgentInstructionsWithRatings,
-  buildSystemPromptWithInstructions,
   buildPluginPromptSection,
   resolveAgentHeartbeatProcedure,
 } from "./agent-instructions.js";
+import { buildPromptLayers, collapsePromptLayers } from "./prompt-layers.js";
 import { heartbeatLog, formatError } from "./logger.js";
 import { createRunAuditor, type EngineRunContext } from "./run-audit.js";
 import { promptWithFallback } from "./pi.js";
@@ -1838,23 +1838,22 @@ export class HeartbeatMonitor {
           }
         }
 
-        const systemPrompt = buildSystemPromptWithInstructions(
-          baseHeartbeatSystemPrompt,
-          [resolvedInstructionsForIdentity, memoryInstructions, selfImprovePrompt].filter((part) => part.trim()).join("\n\n"),
-        );
-        const heartbeatContributions = this.pluginRunner
-          ?.getPromptContributionsForSurface("heartbeat")
-          ?? [];
-        if (heartbeatContributions.length > 0) {
-          heartbeatLog.log(`applied ${heartbeatContributions.length} plugin prompt contributions for heartbeat surface`);
-        }
+        // Build structured layers for cross-session prompt caching.
         const heartbeatPluginContributions = buildPluginPromptSection(
           "heartbeat",
           this.pluginRunner,
         );
-        const systemPromptFinal = heartbeatPluginContributions
-          ? `${systemPrompt}\n\n${heartbeatPluginContributions}`
-          : systemPrompt;
+        if (heartbeatPluginContributions) {
+          heartbeatLog.log(`applied plugin prompt contributions for heartbeat surface`);
+        }
+
+        const heartbeatLayers = buildPromptLayers({
+          basePrompt: baseHeartbeatSystemPrompt,
+          agentInstructions: [resolvedInstructionsForIdentity, memoryInstructions, selfImprovePrompt].filter((part) => part.trim()).join("\n\n"),
+          pluginContributions: heartbeatPluginContributions,
+        });
+
+        const systemPromptFinal = collapsePromptLayers(heartbeatLayers);
 
         // fn_heartbeat_done must be the last tool in the array (stable terminal signal)
         heartbeatTools.push(heartbeatDoneTool);
@@ -1948,6 +1947,7 @@ export class HeartbeatMonitor {
           pluginRunner: this.pluginRunner,
           cwd: rootDir,
           systemPrompt: systemPromptFinal,
+          systemPromptLayers: heartbeatLayers,
           tools: "coding",
           customTools: heartbeatTools,
           defaultProvider: heartbeatSessionModels.defaultProvider,
diff --git a/packages/engine/src/agent-runtime.ts b/packages/engine/src/agent-runtime.ts
index af8c2a4ab..9b378aba0 100644
--- a/packages/engine/src/agent-runtime.ts
+++ b/packages/engine/src/agent-runtime.ts
@@ -19,6 +19,7 @@ import type { PermanentAgentGatingContext } from "@fusion/core";
 import type { SkillSelectionContext } from "./skill-resolver.js";
 import type { FallbackModelUsedPayload } from "./pi.js";
 import type { AgentActionGateContext } from "./agent-action-gate.js";
+import type { SystemPromptLayers } from "./prompt-layers.js";
 
 /**
  * Options for creating an agent session.
@@ -36,6 +37,17 @@ export interface AgentRuntimeOptions {
   cwd: string;
   /** System prompt for the agent */
   systemPrompt: string;
+  /**
+   * Optional structured prompt layers for cross-session caching.
+   * When present, runtimes that support prompt caching use the `stable`
+   * layer as a cacheable prefix and the `dynamic` layer as the per-session
+   * suffix. Runtimes that don't support caching ignore this and use
+   * `systemPrompt` (the collapsed string) instead.
+   *
+   * Callers MUST also provide `systemPrompt` as the collapsed equivalent
+   * for backward compatibility.
+   */
+  systemPromptLayers?: SystemPromptLayers;
   /** Tool set to use: "coding" for full tools, "readonly" for read-only access */
   tools?: "coding" | "readonly";
   /** Additional custom tools to merge with the base toolset */
diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index 68b06989a..278f0c954 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -53,6 +53,7 @@ import {
   buildSystemPromptWithInstructions,
   buildPluginPromptSection,
 } from "./agent-instructions.js";
+import { buildPromptLayers, collapsePromptLayers } from "./prompt-layers.js";
 import type { AgentReflectionService } from "./agent-reflection.js";
 import { createRunAuditor, generateSyntheticRunId, type EngineRunContext } from "./run-audit.js";
 import { evaluateSpecStaleness, getPromptPath } from "./spec-staleness.js";
@@ -3077,21 +3078,23 @@ export class TaskExecutor {
 
         // Resolve per-agent custom instructions for the executor role
         const executorInstructions = await this.resolveInstructionsForRole("executor");
-        const executorSystemPrompt = buildSystemPromptWithInstructions(
-          getExecutorSystemPrompt(settings),
-          executorInstructions,
-        );
-        const executorSystemContributions = this.options.pluginRunner?.getPromptContributionsForSurface("executor-system") ?? [];
-        if (executorSystemContributions.length > 0) {
-          executorLog.log(`${task.id}: applied ${executorSystemContributions.length} plugin prompt contributions for executor-system surface`);
-        }
+
+        // Build structured layers for cross-session prompt caching.
         const executorPluginContributions = buildPluginPromptSection(
           "executor-system",
           this.options.pluginRunner,
         );
-        const executorSystemPromptFinal = executorPluginContributions
-          ? `${executorSystemPrompt}\n\n${executorPluginContributions}`
-          : executorSystemPrompt;
+        if (executorPluginContributions) {
+          executorLog.log(`${task.id}: applied plugin prompt contributions for executor-system surface`);
+        }
+
+        const executorLayers = buildPromptLayers({
+          basePrompt: getExecutorSystemPrompt(settings),
+          agentInstructions: executorInstructions,
+          pluginContributions: executorPluginContributions,
+        });
+
+        const executorSystemPromptFinal = collapsePromptLayers(executorLayers);
 
         // sessionFile must be let because it's destructured alongside session which is reassigned
         // eslint-disable-next-line prefer-const
@@ -3101,6 +3104,7 @@ export class TaskExecutor {
           pluginRunner: this.options.pluginRunner,
           cwd: worktreePath,
           systemPrompt: executorSystemPromptFinal,
+          systemPromptLayers: executorLayers,
           tools: "coding",
           customTools,
           onText: agentLogger.onText,
@@ -3430,6 +3434,7 @@ export class TaskExecutor {
                 pluginRunner: this.options.pluginRunner,
                 cwd: worktreePath,
                 systemPrompt: executorSystemPromptFinal,
+                systemPromptLayers: executorLayers,
                 tools: "coding",
                 customTools,
                 onText: agentLogger.onText,
diff --git a/packages/engine/src/pi.ts b/packages/engine/src/pi.ts
index 6dbb89184..e9d16cb46 100644
--- a/packages/engine/src/pi.ts
+++ b/packages/engine/src/pi.ts
@@ -56,6 +56,7 @@ import {
   type AgentActionGateContext,
 } from "./agent-action-gate.js";
 import { resolvePermanentAgentToolDecision } from "./permanent-agent-gating.js";
+import type { SystemPromptLayers } from "./prompt-layers.js";
 
 export interface AgentResult {
   session: AgentSession;
@@ -711,6 +712,11 @@ export type BuiltinWebToolName = "WebSearch" | "WebFetch";
 export interface AgentOptions {
   cwd: string;
   systemPrompt: string;
+  /** Structured prompt layers for cross-session caching. When provided,
+   *  the stable layer is used as systemPromptOverride and the dynamic
+   *  layer as appendSystemPromptOverride. Falls back to systemPrompt
+   *  when not provided. */
+  systemPromptLayers?: SystemPromptLayers;
   tools?: "coding" | "readonly";
   customTools?: ToolDefinition[];
   /** Optional allowlist of builtin runtime web tools to keep enabled. */
@@ -1715,8 +1721,11 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
     cwd: resolvedProjectRoot,
     agentDir: getFusionAgentDir(),
     settingsManager,
-    systemPromptOverride: () => options.systemPrompt,
-    appendSystemPromptOverride: () => [],
+    systemPromptOverride: () => options.systemPromptLayers?.stable ?? options.systemPrompt,
+    appendSystemPromptOverride: () =>
+      options.systemPromptLayers?.dynamic
+        ? [options.systemPromptLayers.dynamic]
+        : [],
     ...(effectiveExtensionPaths.length > 0 ? { additionalExtensionPaths: [...effectiveExtensionPaths] } : {}),
     ...(skillsOverrideFn ? { skillsOverride: skillsOverrideFn } : {}),
   });
@@ -1748,6 +1757,17 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
       boundaryContext.worktreePath,
       boundaryContext.worktreeProjectRoot,
     );
+    // Sort tools alphabetically by name for deterministic ordering.
+    // Prompt caching requires the tool list to be byte-identical across
+    // sessions — reordering breaks cache prefix matching.
+    // Exception: fn_heartbeat_done must remain last (stable terminal signal
+    // required by the heartbeat executor — see agent-heartbeat.ts).
+    customToolList.sort((a, b) => a.name.localeCompare(b.name));
+    const heartbeatDoneIdx = customToolList.findIndex((t) => t.name === "fn_heartbeat_done");
+    if (heartbeatDoneIdx >= 0 && heartbeatDoneIdx < customToolList.length - 1) {
+      const [doneTool] = customToolList.splice(heartbeatDoneIdx, 1);
+      customToolList.push(doneTool);
+    }
     // Last-chance abort hook. Fires *here* — after every awaited setup step
     // in createFnAgent (provider registration, worktree validation, resource
     // loader reload) and immediately before the actual LLM session spawn.
@@ -1774,7 +1794,7 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
           ...customToolList.map((tool) => tool.name),
           ...options.builtinToolsAllowlist,
         ]),
-      ];
+      ].sort();
     }
 
     return createAgentSession(createSessionOptions);
diff --git a/packages/engine/src/prompt-layers.ts b/packages/engine/src/prompt-layers.ts
new file mode 100644
index 000000000..4dd4ceddb
--- /dev/null
+++ b/packages/engine/src/prompt-layers.ts
@@ -0,0 +1,85 @@
+/**
+ * Structured system prompt layers for cross-session caching.
+ *
+ * The `stable` layer contains content that is identical across sessions of
+ * the same role (base role prompt). The `dynamic` layer holds per-session
+ * content (agent instructions, memory, performance feedback, plugins).
+ *
+ * When the stable layer is byte-identical across consecutive API calls,
+ * Anthropic's prompt cache gives a 90% read discount. OpenAI caches
+ * matching prefixes automatically at 50% discount.
+ */
+export interface SystemPromptLayers {
+  /** Role-specific base prompt — identical across all sessions of this role. */
+  stable: string;
+  /** Per-session content: agent instructions, memory, feedback, plugins. */
+  dynamic: string;
+}
+
+export interface PromptLayerInput {
+  /** The base role system prompt (e.g. REVIEWER_SYSTEM_PROMPT). */
+  basePrompt: string;
+  /** Resolved agent instructions (instructionsText + instructionsPath + soul). */
+  agentInstructions?: string;
+  /** Formatted memory section (agent memory + workspace memory). */
+  memorySection?: string;
+  /** Formatted plugin prompt contributions. */
+  pluginContributions?: string;
+  /** Formatted performance feedback section. */
+  performanceFeedback?: string;
+}
+
+/**
+ * Build structured prompt layers from the components that currently get
+ * concatenated into a single system prompt string.
+ *
+ * The stable layer is ONLY the base role prompt. Everything else goes into
+ * the dynamic layer so that the stable prefix is byte-identical across
+ * sessions of the same role, enabling cross-session prompt caching.
+ */
+export function buildPromptLayers(input: PromptLayerInput): SystemPromptLayers {
+  const { basePrompt, agentInstructions, memorySection, pluginContributions, performanceFeedback } = input;
+
+  const dynamicParts: string[] = [];
+
+  // Memory section comes before instructions to preserve the relative
+  // ordering from the legacy buildSystemPromptWithInstructions approach,
+  // where memory was concatenated onto basePrompt before instructions
+  // were appended.
+  const trimmedMemory = memorySection?.trim() ?? "";
+  if (trimmedMemory) {
+    dynamicParts.push(trimmedMemory);
+  }
+
+  const trimmedInstructions = agentInstructions?.trim() ?? "";
+  if (trimmedInstructions) {
+    dynamicParts.push(`## Custom Instructions\n\n${trimmedInstructions}`);
+  }
+
+  const trimmedPlugins = pluginContributions?.trim() ?? "";
+  if (trimmedPlugins) {
+    dynamicParts.push(trimmedPlugins);
+  }
+
+  const trimmedFeedback = performanceFeedback?.trim() ?? "";
+  if (trimmedFeedback) {
+    dynamicParts.push(trimmedFeedback);
+  }
+
+  return {
+    stable: basePrompt,
+    dynamic: dynamicParts.join("\n\n"),
+  };
+}
+
+/**
+ * Collapse layers back into a single string for backward compatibility.
+ * Runtimes that don't support structured caching use this to get the same
+ * concatenated prompt as before.
+ */
+export function collapsePromptLayers(layers: SystemPromptLayers): string {
+  if (!layers.dynamic) {
+    return layers.stable;
+  }
+  return `${layers.stable}\n\n${layers.dynamic}`;
+}
diff --git a/packages/engine/src/reviewer.ts b/packages/engine/src/reviewer.ts
index 415a1f91b..d399e2539 100644
--- a/packages/engine/src/reviewer.ts
+++ b/packages/engine/src/reviewer.ts
@@ -18,9 +18,9 @@ import { reviewerLog } from "./logger.js";
 import { checkSessionError } from "./usage-limit-detector.js";
 import {
   resolveAgentInstructions,
-  buildSystemPromptWithInstructions,
   buildPluginPromptSection,
 } from "./agent-instructions.js";
+import { buildPromptLayers, collapsePromptLayers } from "./prompt-layers.js";
 import { createFallbackModelObserver } from "./fallback-model-observer.js";
 import { createMemoryGetTool, createMemorySearchTool, createWebFetchTool } from "./agent-tools.js";
 
@@ -405,26 +405,36 @@ export async function reviewStep(
     }
   }
   const reviewerBasePrompt = resolveAgentPrompt("reviewer", options.agentPrompts) || REVIEWER_SYSTEM_PROMPT;
+  // Memory goes in the dynamic layer (not concatenated onto basePrompt) so the
+  // stable prefix is byte-identical across sessions even if memory changes.
+  // The leading "\n" separator is no longer needed — buildPromptLayers handles
+  // section joining with "\n\n".
   const memorySection = options.rootDir && options.settings?.memoryEnabled !== false
-    ? "\n" + buildReviewerMemoryInstructions(options.rootDir, options.settings)
+    ? buildReviewerMemoryInstructions(options.rootDir, options.settings)
     : "";
-  const reviewerSystemPrompt = buildSystemPromptWithInstructions(
-    reviewerBasePrompt + memorySection,
-    reviewerInstructions,
-  );
-  const reviewerContributions = options.pluginRunner
-    ?.getPromptContributionsForSurface("reviewer")
-    ?? [];
-  if (reviewerContributions.length > 0) {
-    reviewerLog.log(`applied ${reviewerContributions.length} plugin prompt contributions for reviewer surface`);
-  }
+
+  // Build structured layers for cross-session prompt caching.
+  // The stable layer (base prompt only) is byte-identical across all
+  // reviewer sessions in this task, enabling cache hits. Memory goes
+  // into the dynamic layer because it can change between sessions.
   const reviewerPluginContributions = buildPluginPromptSection(
     "reviewer",
     options.pluginRunner,
   );
-  const reviewerSystemPromptFinal = reviewerPluginContributions
-    ? `${reviewerSystemPrompt}\n\n${reviewerPluginContributions}`
-    : reviewerSystemPrompt;
+  if (reviewerPluginContributions) {
+    reviewerLog.log(`applied plugin prompt contributions for reviewer surface`);
+  }
+
+  const layers = buildPromptLayers({
+    basePrompt: reviewerBasePrompt,
+    agentInstructions: reviewerInstructions,
+    memorySection,
+    pluginContributions: reviewerPluginContributions,
+  });
+
+  // Collapsed string for backward compatibility with runtimes that don't
+  // support layers (plugin runtimes, older pi versions).
+  const reviewerSystemPromptFinal = collapsePromptLayers(layers);
 
   // Build skill selection context (assigned agent skills take precedence over role fallback)
   let skillContext = undefined;
@@ -495,6 +505,7 @@ export async function reviewStep(
       pluginRunner: options.pluginRunner,
       cwd,
       systemPrompt: reviewerSystemPromptFinal,
+      systemPromptLayers: layers,
       tools: "readonly",
       customTools: [createWebFetchTool(), ...(memoryTools ?? [])],
       onText: agentLogger ? agentLogger.onText : (delta) => options.onText?.(delta),
diff --git a/packages/engine/src/session-token-usage.ts b/packages/engine/src/session-token-usage.ts
index ba7641172..9d93f63ab 100644
--- a/packages/engine/src/session-token-usage.ts
+++ b/packages/engine/src/session-token-usage.ts
@@ -90,3 +90,21 @@ export async function accumulateSessionTokenUsage(
     log.warn(`${taskId}: session token usage accumulate failed: ${message}`);
   }
 }
+
+/**
+ * Compute the cache hit ratio: `cachedTokens / (inputTokens + cachedTokens)`.
+ * Returns a number in [0, 1], or 0 when both arguments are 0.
+ *
+ * Compatible with stored `task.tokenUsage` fields: pass `inputTokens` (which
+ * includes cache-write tokens per `accumulateSessionTokenUsage`) and
+ * `cachedTokens` (cache-read tokens). Note this differs slightly from the
+ * Anthropic console metric, which excludes cache-write from the denominator.
+ */
+export function computeCacheHitRatio(
+  inputTokens: number,
+  cachedTokens: number,
+): number {
+  const total = inputTokens + cachedTokens;
+  if (total === 0) return 0;
+  return cachedTokens / total;
+}
diff --git a/packages/engine/src/skill-resolver.ts b/packages/engine/src/skill-resolver.ts
index 265d302ec..cf106886c 100644
--- a/packages/engine/src/skill-resolver.ts
+++ b/packages/engine/src/skill-resolver.ts
@@ -484,3 +484,4 @@ export function createSkillsOverrideFromSelection(
     };
   };
 }
+
diff --git a/packages/engine/src/triage.ts b/packages/engine/src/triage.ts
index 1b544cc5d..2cc62e27f 100644
--- a/packages/engine/src/triage.ts
+++ b/packages/engine/src/triage.ts
@@ -30,9 +30,9 @@ import { AgentLogger } from "./agent-logger.js";
 import {
   resolveAgentInstructions,
   resolveAgentInstructionsWithRatings,
-  buildSystemPromptWithInstructions,
   buildPluginPromptSection,
 } from "./agent-instructions.js";
+import { buildPromptLayers, collapsePromptLayers } from "./prompt-layers.js";
 import { createFallbackModelObserver } from "./fallback-model-observer.js";
 import { planLog, reviewerLog, formatError } from "./logger.js";
 import {
@@ -1014,30 +1014,29 @@ export class TriageProcessor {
         const triageIdentitySection = assignedAgent
           ? `## Identity\n\nYou are ${assignedAgent.name}${assignedAgent.title?.trim() ? `, ${assignedAgent.title.trim()}` : ""} (agent ID: ${assignedAgent.id}, role: ${assignedAgent.role}).`
           : "";
-        const triageSystemPrompt = buildSystemPromptWithInstructions(
-          resolveAgentPrompt("triage", settings.agentPrompts)
+        // Build structured layers for cross-session prompt caching.
+        const triagePluginContributions = buildPluginPromptSection(
+          "triage",
+          this.options.pluginRunner,
+        );
+        if (triagePluginContributions) {
+          planLog.log(`${task.id}: applied plugin prompt contributions for triage surface`);
+        }
+
+        const triageLayers = buildPromptLayers({
+          basePrompt: resolveAgentPrompt("triage", settings.agentPrompts)
             || (isFast ? FAST_TRIAGE_SYSTEM_PROMPT : TRIAGE_SYSTEM_PROMPT),
-          [
+          agentInstructions: [
             triageIdentitySection,
             triageInstructions,
             isResearchToolSurfaceEnabled(settings)
               ? getResearchGuidanceForSurface("triage")
               : "",
           ].filter((section) => section.trim()).join("\n\n"),
-        );
-        const triageContributions = this.options.pluginRunner
-          ?.getPromptContributionsForSurface("triage")
-          ?? [];
-        if (triageContributions.length > 0) {
-          planLog.log(`${task.id}: applied ${triageContributions.length} plugin prompt contributions for triage surface`);
-        }
-        const triagePluginContributions = buildPluginPromptSection(
-          "triage",
-          this.options.pluginRunner,
-        );
-        const triageSystemPromptFinal = triagePluginContributions
-          ? `${triageSystemPrompt}\n\n${triagePluginContributions}`
-          : triageSystemPrompt;
+          pluginContributions: triagePluginContributions,
+        });
+
+        const triageSystemPromptFinal = collapsePromptLayers(triageLayers);
 
         // Build skill selection context (assigned agent skills take precedence over role fallback)
         const skillContext = await buildSessionSkillContext({
@@ -1054,6 +1053,7 @@ export class TriageProcessor {
           pluginRunner: this.options.pluginRunner,
           cwd: this.rootDir,
           systemPrompt: triageSystemPromptFinal,
+          systemPromptLayers: triageLayers,
           tools: "coding",
           customTools,
           onText: agentLogger.onText,
@@ -1294,6 +1294,7 @@ export class TriageProcessor {
               pluginRunner: this.options.pluginRunner,
               cwd: this.rootDir,
               systemPrompt: triageSystemPromptFinal,
+              systemPromptLayers: triageLayers,
               tools: "coding",
               customTools,
               onText: agentLogger.onText,