fix: reconcile memfs/standard prompt sections safely (#985)

src/tests/agent/memoryPrompt.test.ts

@@ -0,0 +1,70 @@
+import { describe, expect, test } from "bun:test";
+
+import {
+  detectMemoryPromptDrift,
+  reconcileMemoryPrompt,
+} from "../../agent/memoryPrompt";
+import {
+  SYSTEM_PROMPT_MEMFS_ADDON,
+  SYSTEM_PROMPT_MEMORY_ADDON,
+} from "../../agent/promptAssets";
+
+function countOccurrences(haystack: string, needle: string): number {
+  if (!needle) return 0;
+  return haystack.split(needle).length - 1;
+}
+
+describe("memoryPrompt reconciler", () => {
+  test("replaces existing standard memory section with memfs section", () => {
+    const base = "You are a test agent.";
+    const standard = `${base}\n\n${SYSTEM_PROMPT_MEMORY_ADDON.trimStart()}`;
+
+    const reconciled = reconcileMemoryPrompt(standard, "memfs");
+
+    expect(reconciled).toContain("## Memory Filesystem");
+    expect(reconciled).not.toContain(
+      "Your memory consists of core memory (composed of memory blocks)",
+    );
+    expect(countOccurrences(reconciled, "## Memory Filesystem")).toBe(1);
+  });
+
+  test("does not leave orphan memfs sync fragment when switching from memfs to standard", () => {
+    const base = "You are a test agent.";
+    const memfs = `${base}\n\n${SYSTEM_PROMPT_MEMFS_ADDON.trimStart()}`;
+
+    const reconciled = reconcileMemoryPrompt(memfs, "standard");
+
+    expect(reconciled).toContain(
+      "Your memory consists of core memory (composed of memory blocks)",
+    );
+    expect(reconciled).not.toContain("## Memory Filesystem");
+    expect(reconciled).not.toContain("# See what changed");
+    expect(reconciled).not.toContain('git commit -m "<type>: <what changed>"');
+  });
+
+  test("cleans orphan memfs tail fragment before rebuilding target mode", () => {
+    const tailStart = SYSTEM_PROMPT_MEMFS_ADDON.indexOf("# See what changed");
+    expect(tailStart).toBeGreaterThanOrEqual(0);
+    const orphanTail = SYSTEM_PROMPT_MEMFS_ADDON.slice(tailStart).trim();
+
+    const drifted = `Header text\n\n${orphanTail}`;
+    const drifts = detectMemoryPromptDrift(drifted, "standard");
+    expect(drifts.some((d) => d.code === "orphan_memfs_fragment")).toBe(true);
+
+    const reconciled = reconcileMemoryPrompt(drifted, "standard");
+    expect(reconciled).toContain(
+      "Your memory consists of core memory (composed of memory blocks)",
+    );
+    expect(reconciled).not.toContain("# See what changed");
+  });
+
+  test("memfs reconciliation is idempotent and keeps single syncing section", () => {
+    const base = "You are a test agent.";
+    const once = reconcileMemoryPrompt(base, "memfs");
+    const twice = reconcileMemoryPrompt(once, "memfs");
+
+    expect(twice).toBe(once);
+    expect(countOccurrences(twice, "## Syncing")).toBe(1);
+    expect(countOccurrences(twice, "# See what changed")).toBe(1);
+  });
+});