boocode/apps/server/src/services/system-prompt.ts

// v1.12: extracted from inference.ts to give the prompt-assembly logic its
// own home + test surface. Adds the container-guidance layer (BOOCHAT.md
// baked into the Docker image, injected between the base prompt and the
// agent block).
//
// Resolution order, last-wins on conflicts:
//   base prompt
//   + container guidance (this layer, NEW in v1.12)
//   + agent.system_prompt          (resolved from data/AGENTS.md by getAgentById)
//   + session.system_prompt OR project.default_system_prompt
//
// v1.13.8: byte-stability instrumentation. buildSystemPromptWithFingerprint
// returns the assembled string plus a SHA-256 fingerprint and a per-session
// drift signal. buildSystemPrompt stays a string→string shim for backward
// compat (tests use it). No cache added — recon proved input-layer mtime
// caches (this file + agents.ts) already deliver byte-stable inputs in
// steady state. v1.13.8 measures that claim against production traffic
// before any cache infrastructure earns its place.

import { createHash } from 'node:crypto';
import { readFile, stat } from 'node:fs/promises';
import type { Agent, Project, Session } from '../types/api.js';
import { getAgentsMtimes } from './agents.js';

const BASE_SYSTEM_PROMPT = (projectPath: string) =>
  `You are BooCode Chat, a code investigation assistant. The user is working on a project located at ${projectPath}. Use the file-read tools (view_file, list_dir, grep, find_files) to investigate code when needed. Be concise. Cite file paths and line numbers when discussing code. Do not hallucinate file contents — read the file first. Tool results may be truncated; if so, narrow your query rather than guessing.`;

// v1.12 mtime-watch cache. Mirrors the safeStat pattern in services/agents.ts.
// On every call we stat the file; if the mtime matches the cached entry we
// return the cached content without re-reading. If the file is missing we
// cache { mtime: 0, content: null } so the not-found case still benefits
// from caching (one stat per call, no readFile attempt on a known-missing
// path). Because BOOCHAT.md is bind-mounted from the host, edits land
// immediately on the next chat turn — no container restart needed.
let cachedGuidance: { mtime: number; content: string | null } | null = null;

function resolveGuidancePath(): string {
  return process.env['CONTAINER_GUIDANCE_FILE'] ?? '/app/BOOCHAT.md';
}

export async function loadContainerGuidance(): Promise<string | null> {
  const path = resolveGuidancePath();
  try {
    return await readFile(path, 'utf8');
  } catch {
    return null;
  }
}

export async function getContainerGuidance(): Promise<string | null> {
  const path = resolveGuidancePath();
  let mtimeMs: number;
  try {
    const s = await stat(path);
    mtimeMs = s.mtimeMs;
  } catch {
    cachedGuidance = { mtime: 0, content: null };
    return null;
  }
  if (cachedGuidance && cachedGuidance.mtime === mtimeMs) {
    return cachedGuidance.content;
  }
  const content = await loadContainerGuidance();
  cachedGuidance = { mtime: mtimeMs, content };
  return content;
}

// Test-only: clear the cache so consecutive tests don't share state.
export function _resetContainerGuidanceCacheForTests(): void {
  cachedGuidance = null;
}

// v1.13.8: expose the mtime currently held in the BOOCHAT cache so the
// fingerprint log can stamp it without re-statting (no I/O race against
// getContainerGuidance, which is the canonical mtime source).
function getCachedGuidanceMtime(): number | null {
  if (!cachedGuidance) return null;
  // mtime=0 is the sentinel for "file is missing" (set in the catch above).
  // Surface it as null so the log/diff doesn't treat absence as a number.
  return cachedGuidance.mtime > 0 ? cachedGuidance.mtime : null;
}

// v1.13.8: fingerprint emitted per turn, observer state keyed by session.
// Field set is intentionally small — we want the diff between two
// fingerprints to point at the exact input that drifted, not bury the
// signal in noise.
export interface PrefixFingerprint {
  msg: 'prefix-fingerprint';
  project_id: string;
  agent_id: string | null;
  agent_name: string | null;
  session_id: string;
  prefix_hash: string;
  prefix_length: number;
  mtime_boochat: number | null;
  mtime_agents_global: number | null;
  mtime_agents_project: number | null;
  has_agent_system_prompt: boolean;
  has_session_override: boolean;
  has_project_override: boolean;
}

export interface PrefixDrift {
  msg: 'prefix-drift';
  session_id: string;
  prev_hash: string;
  new_hash: string;
  prev_length: number;
  new_length: number;
  // Names of fields in PrefixFingerprint (excluding the hash + length pair
  // and the session_id key itself) whose values differ between the previous
  // observation and this one. The bug case is `changed_inputs: []` — hash
  // differs but no tracked input moved, which means assembly is
  // nondeterministic somewhere.
  changed_inputs: string[];
}

// Fields tracked per-session for the drift diff. Stored alongside the hash
// so we can recompute changed_inputs without re-running buildSystemPrompt.
interface ObservedInputs {
  agent_id: string | null;
  mtime_boochat: number | null;
  mtime_agents_global: number | null;
  mtime_agents_project: number | null;
  has_agent_system_prompt: boolean;
  has_session_override: boolean;
  has_project_override: boolean;
}

interface ObserverEntry {
  hash: string;
  length: number;
  inputs: ObservedInputs;
}

// Unbounded by design for v1.13.8 (instrumentation, short-lived sessions in
// the smoke test). TODO(v1.13.x follow-up if v1.13.8 surfaces stable):
// LRU-bound this Map at 1000 sessions when the in-process surface lives long
// enough to matter.
const prefixObserver = new Map<string, ObserverEntry>();

// Test-only: clear the observer so consecutive tests don't share state.
export function _resetPrefixObserverForTests(): void {
  prefixObserver.clear();
}

function computeChangedInputs(prev: ObservedInputs, curr: ObservedInputs): string[] {
  const out: string[] = [];
  const keys = Object.keys(curr) as (keyof ObservedInputs)[];
  for (const k of keys) {
    if (prev[k] !== curr[k]) out.push(k);
  }
  return out;
}

export async function buildSystemPromptWithFingerprint(
  project: Project,
  session: Session,
  agent: Agent | null,
): Promise<{ prompt: string; fingerprint: PrefixFingerprint; drift: PrefixDrift | null }> {
  let out = BASE_SYSTEM_PROMPT(project.path);
  const guidance = await getContainerGuidance();
  if (guidance) {
    out += `\n\n--- Container guidance ---\n${guidance}\n--- end container guidance ---\n`;
  }
  if (agent && agent.system_prompt.trim().length > 0) {
    out += '\n\n' + agent.system_prompt.trim();
  }
  const sessionPrompt = session.system_prompt?.trim() ?? '';
  const projectPrompt = project.default_system_prompt?.trim() ?? '';
  const userPrompt = sessionPrompt || projectPrompt;
  if (userPrompt.length > 0) {
    out += '\n\n' + userPrompt;
  }

  const hash = createHash('sha256').update(out, 'utf8').digest('hex');
  const agentsMtimes = getAgentsMtimes(project.path);
  const inputs: ObservedInputs = {
    agent_id: agent?.id ?? null,
    mtime_boochat: getCachedGuidanceMtime(),
    mtime_agents_global: agentsMtimes.global,
    mtime_agents_project: agentsMtimes.project,
    has_agent_system_prompt: !!(agent && agent.system_prompt.trim().length > 0),
    has_session_override: sessionPrompt.length > 0,
    has_project_override: projectPrompt.length > 0,
  };

  const fingerprint: PrefixFingerprint = {
    msg: 'prefix-fingerprint',
    project_id: project.id,
    agent_id: agent?.id ?? null,
    agent_name: agent?.name ?? null,
    session_id: session.id,
    prefix_hash: hash,
    prefix_length: out.length,
    mtime_boochat: inputs.mtime_boochat,
    mtime_agents_global: inputs.mtime_agents_global,
    mtime_agents_project: inputs.mtime_agents_project,
    has_agent_system_prompt: inputs.has_agent_system_prompt,
    has_session_override: inputs.has_session_override,
    has_project_override: inputs.has_project_override,
  };

  let drift: PrefixDrift | null = null;
  const prev = prefixObserver.get(session.id);
  if (prev && prev.hash !== hash) {
    drift = {
      msg: 'prefix-drift',
      session_id: session.id,
      prev_hash: prev.hash,
      new_hash: hash,
      prev_length: prev.length,
      new_length: out.length,
      changed_inputs: computeChangedInputs(prev.inputs, inputs),
    };
  }
  prefixObserver.set(session.id, { hash, length: out.length, inputs });

  return { prompt: out, fingerprint, drift };
}

// Backward-compatible string-returning shim. Kept so existing callers
// (tests, future code paths that don't want to log) work unchanged.
export async function buildSystemPrompt(
  project: Project,
  session: Session,
  agent: Agent | null,
): Promise<string> {
  const { prompt } = await buildSystemPromptWithFingerprint(project, session, agent);
  return prompt;
}