v1.13.8: system-prompt prefix stability verify-and-measure

Recon during planning disproved the original v1.13.7 (DB-cache) premise:
buildSystemPrompt already runs over inputs mtime-cached at the file layer
(BOOCHAT.md in system-prompt.ts:25, AGENTS.md global+per-project in
agents.ts:245), and DB scalars are byte-stable until edited. The output
is microsecond pure-string concat with no I/O. Skills aren't in the
prefix; tools live in a separate request body field alpha-sorted by
v1.13.3.

This batch closes the verification gap with instrumentation, not
implementation:

- system-prompt.ts: buildSystemPromptWithFingerprint canonical impl
  computes SHA-256 over the assembled prefix, runs a per-session
  Map<sessionId, lastHash> observer, emits PrefixFingerprint per call
  and PrefixDrift (with field-level changed_inputs) on hash change.
  buildSystemPrompt is now a thin shim returning .prompt.
- agents.ts: getAgentsMtimes accessor — cache-read only, no I/O.
- payload.ts: buildMessagesPayload takes optional log argument; when
  passed, emits prefix-fingerprint (info) + prefix-drift (warn).
- turn.ts + sentinel-summaries.ts: pass ctx.log at 3 production call
  sites; sentinel summaries log too so any drift across cap-hit /
  doom-loop paths surfaces.
- system-prompt.test.ts: 4 new tests (byte-identical, no-drift-on-
  stable, drift-fires-with-changed-inputs, cross-session-no-drift).

194/194 tests pass (was 190).

Smoke: 5 messages in a fresh session produced 7 prefix-fingerprint
logs (extras from buildMessagesPayload being called from sentinel
summary paths), all with identical prefix_hash and prefix_length=2907,
zero prefix-drift. Prefix is byte-stable in steady-state.

Decision: original system_prompt_cache DB table from the roadmap is
permanently dropped. The v1.12.0 mtime caches at the input layer plus
alpha tool ordering at the request body (v1.13.3) already address the
load-bearing cache-stability surfaces. Instrumentation stays so the
claim can be re-verified at any time.

apps/server/src/services/__tests__/system-prompt.test.ts

@@ -6,7 +6,9 @@ import {
   loadContainerGuidance,
   getContainerGuidance,
   buildSystemPrompt,
+  buildSystemPromptWithFingerprint,
   _resetContainerGuidanceCacheForTests,
+  _resetPrefixObserverForTests,
 } from '../system-prompt.js';
 import type { Agent, Project, Session } from '../../types/api.js';
 
@@ -17,12 +19,14 @@ let tmpDir: string;
 beforeEach(async () => {
   tmpDir = await mkdtemp(join(tmpdir(), 'system-prompt-test-'));
   _resetContainerGuidanceCacheForTests();
+  _resetPrefixObserverForTests();
   delete process.env['CONTAINER_GUIDANCE_FILE'];
 });
 
 afterEach(async () => {
   delete process.env['CONTAINER_GUIDANCE_FILE'];
   _resetContainerGuidanceCacheForTests();
+  _resetPrefixObserverForTests();
   await rm(tmpDir, { recursive: true, force: true });
 });
 
@@ -176,3 +180,75 @@ describe('buildSystemPrompt', () => {
     expect(prompt).not.toContain('--- end container guidance ---');
   });
 });
+
+// v1.13.8: byte-stability instrumentation surface.
+describe('buildSystemPromptWithFingerprint (v1.13.8)', () => {
+  it('returns byte-identical prompts for two consecutive calls with the same inputs', async () => {
+    const path = join(tmpDir, 'BOOCHAT.md');
+    await writeFile(path, 'stable guidance', 'utf8');
+    process.env['CONTAINER_GUIDANCE_FILE'] = path;
+
+    const session = makeSession();
+    const project = makeProject({ path: '/tmp/stable-proj' });
+    const agent = makeAgent({ system_prompt: 'be terse' });
+
+    const first = await buildSystemPromptWithFingerprint(project, session, agent);
+    const second = await buildSystemPromptWithFingerprint(project, session, agent);
+
+    expect(first.prompt).toBe(second.prompt);
+    expect(first.fingerprint.prefix_hash).toBe(second.fingerprint.prefix_hash);
+    expect(first.fingerprint.prefix_length).toBe(second.fingerprint.prefix_length);
+  });
+
+  it('emits drift=null on the first call for a fresh session, then null again when nothing changes', async () => {
+    process.env['CONTAINER_GUIDANCE_FILE'] = join(tmpDir, 'absent.md');
+    const session = makeSession();
+    const project = makeProject({ path: '/tmp/stable-proj' });
+
+    const first = await buildSystemPromptWithFingerprint(project, session, null);
+    expect(first.drift).toBeNull();
+
+    const second = await buildSystemPromptWithFingerprint(project, session, null);
+    expect(second.drift).toBeNull();
+    expect(second.fingerprint.prefix_hash).toBe(first.fingerprint.prefix_hash);
+  });
+
+  it('emits drift with prev/new hashes and a changed_inputs entry when an input mutates', async () => {
+    // Two BOOCHAT.md contents with different mtimes → guidance cache picks
+    // up the change → fingerprint hash flips → drift fires.
+    const path = join(tmpDir, 'BOOCHAT.md');
+    await writeFile(path, 'first', 'utf8');
+    process.env['CONTAINER_GUIDANCE_FILE'] = path;
+
+    const session = makeSession();
+    const project = makeProject({ path: '/tmp/stable-proj' });
+
+    const first = await buildSystemPromptWithFingerprint(project, session, null);
+    expect(first.drift).toBeNull();
+
+    await writeFile(path, 'second — different content', 'utf8');
+    const later = new Date(Date.now() + 60_000);
+    await utimes(path, later, later);
+
+    const second = await buildSystemPromptWithFingerprint(project, session, null);
+    expect(second.drift).not.toBeNull();
+    expect(second.drift!.prev_hash).toBe(first.fingerprint.prefix_hash);
+    expect(second.drift!.new_hash).toBe(second.fingerprint.prefix_hash);
+    expect(second.drift!.prev_hash).not.toBe(second.drift!.new_hash);
+    expect(second.drift!.changed_inputs).toContain('mtime_boochat');
+  });
+
+  it('does not fire drift across distinct sessions even if their hashes differ', async () => {
+    process.env['CONTAINER_GUIDANCE_FILE'] = join(tmpDir, 'absent.md');
+    const sessionA = makeSession({ id: 'sess-A' });
+    const sessionB = makeSession({ id: 'sess-B', system_prompt: 'B-only override' });
+    const project = makeProject({ path: '/tmp/stable-proj' });
+
+    const a = await buildSystemPromptWithFingerprint(project, sessionA, null);
+    const b = await buildSystemPromptWithFingerprint(project, sessionB, null);
+
+    expect(a.drift).toBeNull();
+    expect(b.drift).toBeNull();
+    expect(a.fingerprint.prefix_hash).not.toBe(b.fingerprint.prefix_hash);
+  });
+});