fix: use reference identity instead of length check for assembled view

- Compare assembled.messages !== sourceMessages instead of length, so engines that rewrite content without changing array count are respected
- Add test for same-count-different-reference assembly

src/agents/pi-embedded-runner/tool-result-context-guard.test.ts

@@ -392,7 +392,25 @@ describe("installContextEngineLoopHook", () => {
     expect(transformed).toBe(compactedView);
   });
 
-  it("returns the source messages when the assembled view has the same length", async () => {
+  it("returns the assembled view when the engine rewrites content without changing count", async () => {
+    const agent = makeGuardableAgent();
+    const rewrittenView = [makeUser("rewritten-1"), makeUser("rewritten-2")];
+    const engine = makeMockEngine({
+      assemble: async () => ({ messages: rewrittenView, estimatedTokens: 0 }),
+    });
+    installHook(agent, engine);
+
+    const initial = [makeUser("first"), makeToolResult("call_1", "r")];
+    await callTransform(agent, initial);
+
+    const withNew = [...initial, makeToolResult("call_2", "r2")];
+    const transformed = await callTransform(agent, withNew);
+
+    // Same count (2) but different array reference — engine's view should be used
+    expect(transformed).toBe(rewrittenView);
+  });
+
+  it("returns the source when the engine returns the same array reference", async () => {
     const agent = makeGuardableAgent();
     const engine = makeMockEngine();
     installHook(agent, engine);

src/agents/pi-embedded-runner/tool-result-context-guard.ts

@@ -239,11 +239,7 @@ export function installContextEngineLoopHook(params: {
         tokenBudget,
         model: modelId,
       });
-      if (
-        assembled &&
-        Array.isArray(assembled.messages) &&
-        assembled.messages.length !== sourceMessages.length
-      ) {
+      if (assembled && Array.isArray(assembled.messages) && assembled.messages !== sourceMessages) {
         lastAssembledView = assembled.messages;
         return assembled.messages;
       }