fix(engine): preserve memory-before-instructions ordering in dynamic layer

packages/engine/src/__tests__/prompt-layers-backward-compat.test.ts

@@ -83,15 +83,19 @@ describe("prompt layers backward compatibility", () => {
   });
 
   describe("reviewer assembly pattern", () => {
-    it("produces the expected reviewer prompt with memory in dynamic layer", () => {
+    it("preserves memory-before-instructions ordering in dynamic layer", () => {
       const basePrompt = "You are an independent code and plan reviewer.";
-      const memoryInstructions = "\n## Memory\n\nUse fn_memory_search.";
+      const memoryInstructions = "## Memory\n\nUse fn_memory_search.";
       const agentInstructions = "Focus on security.";
       const plugins = "## Plugin: lint\n\nCheck eslint.";
 
-      // New reviewer pattern: memory goes in dynamic layer (not stable) so
-      // the stable prefix is byte-identical across sessions even when memory
-      // changes mid-task.
+      // 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,
@@ -100,12 +104,16 @@ describe("prompt layers backward compatibility", () => {
       });
       const result = collapsePromptLayers(layers);
 
-      // Base prompt is the stable layer
+      // Base prompt is the stable layer (no memory)
       expect(layers.stable).toBe(basePrompt);
-      // Dynamic layer contains instructions, memory, and plugins
-      expect(result).toContain(agentInstructions);
-      expect(result).toContain("Memory");
-      expect(result).toContain(plugins);
+
+      // 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);
     });
   });
 

packages/engine/src/__tests__/prompt-layers.test.ts

@@ -64,7 +64,7 @@ describe("buildPromptLayers", () => {
     });
 
     expect(layers.dynamic).toBe(
-      "## Custom Instructions\n\nInstructions.\n\nMemory.\n\nPlugins."
+      "Memory.\n\n## Custom Instructions\n\nInstructions.\n\nPlugins."
     );
   });
 

packages/engine/src/prompt-layers.ts

@@ -42,16 +42,20 @@ export function buildPromptLayers(input: PromptLayerInput): SystemPromptLayers {
 
   const dynamicParts: string[] = [];
 
-  const trimmedInstructions = agentInstructions?.trim() ?? "";
-  if (trimmedInstructions) {
-    dynamicParts.push(`## Custom Instructions\n\n${trimmedInstructions}`);
-  }
-
+  // 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);