test(engine): verify layer forwarding and cross-session cache invariant

packages/engine/src/__tests__/prompt-cache-integration.test.ts

@@ -0,0 +1,48 @@
+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 + MEMORY_INSTRUCTIONS,
+      agentInstructions: `Session ${sessionIndex}: custom instructions that vary per agent.`,
+      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);
+  });
+});

packages/engine/src/__tests__/session-helpers-layers.test.ts

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+import { describe, it, expect } from "vitest";
+import type { ResolvedSessionOptions } from "../agent-session-helpers.js";
+import type { SystemPromptLayers } from "../prompt-layers.js";
+
+describe("ResolvedSessionOptions layer forwarding", () => {
+  it("includes systemPromptLayers in the type", () => {
+    const layers: SystemPromptLayers = {
+      stable: "Stable prefix.",
+      dynamic: "Dynamic suffix.",
+    };
+
+    const options: Partial<ResolvedSessionOptions> = {
+      systemPrompt: "Stable prefix.\n\nDynamic suffix.",
+      systemPromptLayers: layers,
+    };
+
+    expect(options.systemPromptLayers).toBeDefined();
+    expect(options.systemPromptLayers!.stable).toBe("Stable prefix.");
+  });
+});