perf(ttft): cache skills prompt with shared skill_utils module (salvage #3366) (#3421)

Two-layer caching for build_skills_system_prompt():
  1. In-process LRU (OrderedDict, max 8) — same-process: 546ms → <1ms
  2. Disk snapshot (.skills_prompt_snapshot.json) — cold start: 297ms → 103ms

Key improvements over original PR #3366:
- Extract shared logic into agent/skill_utils.py (parse_frontmatter,
  skill_matches_platform, get_disabled_skill_names, extract_skill_conditions,
  extract_skill_description, iter_skill_index_files)
- tools/skills_tool.py delegates to shared module — zero code duplication
- Proper LRU eviction via OrderedDict.move_to_end + popitem(last=False)
- Cache invalidation on all skill mutation paths:
  - skill_manage tool (in-conversation writes)
  - hermes skills install (CLI hub)
  - hermes skills uninstall (CLI hub)
  - Automatic via mtime/size manifest on cold start

prompt_builder.py no longer imports tools.skills_tool (avoids pulling
in the entire tool registry chain at prompt build time).

6301 tests pass, 0 failures.

Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>

agent/prompt_builder.py

@@ -4,14 +4,27 @@ All functions are stateless. AIAgent._build_system_prompt() calls these to
 assemble pieces, then combines them with memory and ephemeral prompts.
 """
 
+import json
 import logging
 import os
 import re
+import threading
+from collections import OrderedDict
 from pathlib import Path
 
 from hermes_constants import get_hermes_home
 from typing import Optional
 
+from agent.skill_utils import (
+    extract_skill_conditions,
+    extract_skill_description,
+    get_disabled_skill_names,
+    iter_skill_index_files,
+    parse_frontmatter,
+    skill_matches_platform,
+)
+from utils import atomic_json_write
+
 logger = logging.getLogger(__name__)
 
 # ---------------------------------------------------------------------------
@@ -230,6 +243,111 @@ CONTEXT_TRUNCATE_HEAD_RATIO = 0.7
 CONTEXT_TRUNCATE_TAIL_RATIO = 0.2
 
 
+# =========================================================================
+# Skills prompt cache
+# =========================================================================
+
+_SKILLS_PROMPT_CACHE_MAX = 8
+_SKILLS_PROMPT_CACHE: OrderedDict[tuple, str] = OrderedDict()
+_SKILLS_PROMPT_CACHE_LOCK = threading.Lock()
+_SKILLS_SNAPSHOT_VERSION = 1
+
+
+def _skills_prompt_snapshot_path() -> Path:
+    return get_hermes_home() / ".skills_prompt_snapshot.json"
+
+
+def clear_skills_system_prompt_cache(*, clear_snapshot: bool = False) -> None:
+    """Drop the in-process skills prompt cache (and optionally the disk snapshot)."""
+    with _SKILLS_PROMPT_CACHE_LOCK:
+        _SKILLS_PROMPT_CACHE.clear()
+    if clear_snapshot:
+        try:
+            _skills_prompt_snapshot_path().unlink(missing_ok=True)
+        except OSError as e:
+            logger.debug("Could not remove skills prompt snapshot: %s", e)
+
+
+def _build_skills_manifest(skills_dir: Path) -> dict[str, list[int]]:
+    """Build an mtime/size manifest of all SKILL.md and DESCRIPTION.md files."""
+    manifest: dict[str, list[int]] = {}
+    for filename in ("SKILL.md", "DESCRIPTION.md"):
+        for path in iter_skill_index_files(skills_dir, filename):
+            try:
+                st = path.stat()
+            except OSError:
+                continue
+            manifest[str(path.relative_to(skills_dir))] = [st.st_mtime_ns, st.st_size]
+    return manifest
+
+
+def _load_skills_snapshot(skills_dir: Path) -> Optional[dict]:
+    """Load the disk snapshot if it exists and its manifest still matches."""
+    snapshot_path = _skills_prompt_snapshot_path()
+    if not snapshot_path.exists():
+        return None
+    try:
+        snapshot = json.loads(snapshot_path.read_text(encoding="utf-8"))
+    except Exception:
+        return None
+    if not isinstance(snapshot, dict):
+        return None
+    if snapshot.get("version") != _SKILLS_SNAPSHOT_VERSION:
+        return None
+    if snapshot.get("manifest") != _build_skills_manifest(skills_dir):
+        return None
+    return snapshot
+
+
+def _write_skills_snapshot(
+    skills_dir: Path,
+    manifest: dict[str, list[int]],
+    skill_entries: list[dict],
+    category_descriptions: dict[str, str],
+) -> None:
+    """Persist skill metadata to disk for fast cold-start reuse."""
+    payload = {
+        "version": _SKILLS_SNAPSHOT_VERSION,
+        "manifest": manifest,
+        "skills": skill_entries,
+        "category_descriptions": category_descriptions,
+    }
+    try:
+        atomic_json_write(_skills_prompt_snapshot_path(), payload)
+    except Exception as e:
+        logger.debug("Could not write skills prompt snapshot: %s", e)
+
+
+def _build_snapshot_entry(
+    skill_file: Path,
+    skills_dir: Path,
+    frontmatter: dict,
+    description: str,
+) -> dict:
+    """Build a serialisable metadata dict for one skill."""
+    rel_path = skill_file.relative_to(skills_dir)
+    parts = rel_path.parts
+    if len(parts) >= 2:
+        skill_name = parts[-2]
+        category = "/".join(parts[:-2]) if len(parts) > 2 else parts[0]
+    else:
+        category = "general"
+        skill_name = skill_file.parent.name
+
+    platforms = frontmatter.get("platforms") or []
+    if isinstance(platforms, str):
+        platforms = [platforms]
+
+    return {
+        "skill_name": skill_name,
+        "category": category,
+        "frontmatter_name": str(frontmatter.get("name", skill_name)),
+        "description": description,
+        "platforms": [str(p).strip() for p in platforms if str(p).strip()],
+        "conditions": extract_skill_conditions(frontmatter),
+    }
+
+
 # =========================================================================
 # Skills index
 # =========================================================================
@@ -241,22 +359,13 @@ def _parse_skill_file(skill_file: Path) -> tuple[bool, dict, str]:
     (True, {}, "") to err on the side of showing the skill.
     """
     try:
-        from tools.skills_tool import _parse_frontmatter, skill_matches_platform
-
         raw = skill_file.read_text(encoding="utf-8")[:2000]
-        frontmatter, _ = _parse_frontmatter(raw)
+        frontmatter, _ = parse_frontmatter(raw)
 
         if not skill_matches_platform(frontmatter):
-            return False, {}, ""
+            return False, frontmatter, ""
 
-        desc = ""
-        raw_desc = frontmatter.get("description", "")
-        if raw_desc:
-            desc = str(raw_desc).strip().strip("'\"")
-            if len(desc) > 60:
-                desc = desc[:57] + "..."
-
-        return True, frontmatter, desc
+        return True, frontmatter, extract_skill_description(frontmatter)
     except Exception as e:
         logger.debug("Failed to parse skill file %s: %s", skill_file, e)
         return True, {}, ""
@@ -265,16 +374,9 @@ def _parse_skill_file(skill_file: Path) -> tuple[bool, dict, str]:
 def _read_skill_conditions(skill_file: Path) -> dict:
     """Extract conditional activation fields from SKILL.md frontmatter."""
     try:
-        from tools.skills_tool import _parse_frontmatter
         raw = skill_file.read_text(encoding="utf-8")[:2000]
-        frontmatter, _ = _parse_frontmatter(raw)
-        hermes = frontmatter.get("metadata", {}).get("hermes", {})
-        return {
-            "fallback_for_toolsets": hermes.get("fallback_for_toolsets", []),
-            "requires_toolsets": hermes.get("requires_toolsets", []),
-            "fallback_for_tools": hermes.get("fallback_for_tools", []),
-            "requires_tools": hermes.get("requires_tools", []),
-        }
+        frontmatter, _ = parse_frontmatter(raw)
+        return extract_skill_conditions(frontmatter)
     except Exception as e:
         logger.debug("Failed to read skill conditions from %s: %s", skill_file, e)
         return {}
@@ -317,10 +419,12 @@ def build_skills_system_prompt(
 ) -> str:
     """Build a compact skill index for the system prompt.
 
-    Scans ~/.hermes/skills/ for SKILL.md files grouped by category.
-    Includes per-skill descriptions from frontmatter so the model can
-    match skills by meaning, not just name.
-    Filters out skills incompatible with the current OS platform.
+    Two-layer cache:
+      1. In-process LRU dict keyed by (skills_dir, tools, toolsets)
+      2. Disk snapshot (``.skills_prompt_snapshot.json``) validated by
+         mtime/size manifest — survives process restarts
+
+    Falls back to a full filesystem scan when both layers miss.
     """
     hermes_home = get_hermes_home()
     skills_dir = hermes_home / "skills"
@@ -328,98 +432,140 @@ def build_skills_system_prompt(
     if not skills_dir.exists():
         return ""
 
-    # Collect skills with descriptions, grouped by category.
-    # Each entry: (skill_name, description)
-    # Supports sub-categories: skills/mlops/training/axolotl/SKILL.md
-    # -> category "mlops/training", skill "axolotl"
-    # Load disabled skill names once for the entire scan
-    try:
-        from tools.skills_tool import _get_disabled_skill_names
-        disabled = _get_disabled_skill_names()
-    except Exception:
-        disabled = set()
+    # ── Layer 1: in-process LRU cache ─────────────────────────────────
+    cache_key = (
+        str(skills_dir.resolve()),
+        tuple(sorted(str(t) for t in (available_tools or set()))),
+        tuple(sorted(str(ts) for ts in (available_toolsets or set()))),
+    )
+    with _SKILLS_PROMPT_CACHE_LOCK:
+        cached = _SKILLS_PROMPT_CACHE.get(cache_key)
+        if cached is not None:
+            _SKILLS_PROMPT_CACHE.move_to_end(cache_key)
+            return cached
+
+    disabled = get_disabled_skill_names()
+
+    # ── Layer 2: disk snapshot ────────────────────────────────────────
+    snapshot = _load_skills_snapshot(skills_dir)
 
     skills_by_category: dict[str, list[tuple[str, str]]] = {}
-    for skill_file in skills_dir.rglob("SKILL.md"):
-        is_compatible, frontmatter, desc = _parse_skill_file(skill_file)
-        if not is_compatible:
-            continue
-        rel_path = skill_file.relative_to(skills_dir)
-        parts = rel_path.parts
-        if len(parts) >= 2:
-            skill_name = parts[-2]
-            category = "/".join(parts[:-2]) if len(parts) > 2 else parts[0]
-        else:
-            category = "general"
-            skill_name = skill_file.parent.name
-        # Respect user's disabled skills config
-        fm_name = frontmatter.get("name", skill_name)
-        if fm_name in disabled or skill_name in disabled:
-            continue
-        # Extract conditions inline from already-parsed frontmatter
-        # (avoids redundant file re-read that _read_skill_conditions would do)
-        hermes_meta = (frontmatter.get("metadata") or {}).get("hermes") or {}
-        conditions = {
-            "fallback_for_toolsets": hermes_meta.get("fallback_for_toolsets", []),
-            "requires_toolsets": hermes_meta.get("requires_toolsets", []),
-            "fallback_for_tools": hermes_meta.get("fallback_for_tools", []),
-            "requires_tools": hermes_meta.get("requires_tools", []),
+    category_descriptions: dict[str, str] = {}
+
+    if snapshot is not None:
+        # Fast path: use pre-parsed metadata from disk
+        for entry in snapshot.get("skills", []):
+            if not isinstance(entry, dict):
+                continue
+            skill_name = entry.get("skill_name") or ""
+            category = entry.get("category") or "general"
+            frontmatter_name = entry.get("frontmatter_name") or skill_name
+            platforms = entry.get("platforms") or []
+            if not skill_matches_platform({"platforms": platforms}):
+                continue
+            if frontmatter_name in disabled or skill_name in disabled:
+                continue
+            if not _skill_should_show(
+                entry.get("conditions") or {},
+                available_tools,
+                available_toolsets,
+            ):
+                continue
+            skills_by_category.setdefault(category, []).append(
+                (skill_name, entry.get("description", ""))
+            )
+        category_descriptions = {
+            str(k): str(v)
+            for k, v in (snapshot.get("category_descriptions") or {}).items()
         }
-        if not _skill_should_show(conditions, available_tools, available_toolsets):
-            continue
-        skills_by_category.setdefault(category, []).append((skill_name, desc))
+    else:
+        # Cold path: full filesystem scan + write snapshot for next time
+        skill_entries: list[dict] = []
+        for skill_file in iter_skill_index_files(skills_dir, "SKILL.md"):
+            is_compatible, frontmatter, desc = _parse_skill_file(skill_file)
+            entry = _build_snapshot_entry(skill_file, skills_dir, frontmatter, desc)
+            skill_entries.append(entry)
+            if not is_compatible:
+                continue
+            skill_name = entry["skill_name"]
+            if entry["frontmatter_name"] in disabled or skill_name in disabled:
+                continue
+            if not _skill_should_show(
+                extract_skill_conditions(frontmatter),
+                available_tools,
+                available_toolsets,
+            ):
+                continue
+            skills_by_category.setdefault(entry["category"], []).append(
+                (skill_name, entry["description"])
+            )
 
-    if not skills_by_category:
-        return ""
-
-    # Read category-level descriptions from DESCRIPTION.md
-    # Checks both the exact category path and parent directories
-    category_descriptions = {}
-    for category in skills_by_category:
-        cat_path = Path(category)
-        desc_file = skills_dir / cat_path / "DESCRIPTION.md"
-        if desc_file.exists():
+        # Read category-level DESCRIPTION.md files
+        for desc_file in iter_skill_index_files(skills_dir, "DESCRIPTION.md"):
             try:
                 content = desc_file.read_text(encoding="utf-8")
-                match = re.search(r"^---\s*\n.*?description:\s*(.+?)\s*\n.*?^---", content, re.MULTILINE | re.DOTALL)
-                if match:
-                    category_descriptions[category] = match.group(1).strip()
+                fm, _ = parse_frontmatter(content)
+                cat_desc = fm.get("description")
+                if not cat_desc:
+                    continue
+                rel = desc_file.relative_to(skills_dir)
+                cat = "/".join(rel.parts[:-1]) if len(rel.parts) > 1 else "general"
+                category_descriptions[cat] = str(cat_desc).strip().strip("'\"")
             except Exception as e:
                 logger.debug("Could not read skill description %s: %s", desc_file, e)
 
-    index_lines = []
-    for category in sorted(skills_by_category.keys()):
-        cat_desc = category_descriptions.get(category, "")
-        if cat_desc:
-            index_lines.append(f"  {category}: {cat_desc}")
-        else:
-            index_lines.append(f"  {category}:")
-        # Deduplicate and sort skills within each category
-        seen = set()
-        for name, desc in sorted(skills_by_category[category], key=lambda x: x[0]):
-            if name in seen:
-                continue
-            seen.add(name)
-            if desc:
-                index_lines.append(f"    - {name}: {desc}")
-            else:
-                index_lines.append(f"    - {name}")
+        _write_skills_snapshot(
+            skills_dir,
+            _build_skills_manifest(skills_dir),
+            skill_entries,
+            category_descriptions,
+        )
 
-    return (
-        "## Skills (mandatory)\n"
-        "Before replying, scan the skills below. If one clearly matches your task, "
-        "load it with skill_view(name) and follow its instructions. "
-        "If a skill has issues, fix it with skill_manage(action='patch').\n"
-        "After difficult/iterative tasks, offer to save as a skill. "
-        "If a skill you loaded was missing steps, had wrong commands, or needed "
-        "pitfalls you discovered, update it before finishing.\n"
-        "\n"
-        "<available_skills>\n"
-        + "\n".join(index_lines) + "\n"
-        "</available_skills>\n"
-        "\n"
-        "If none match, proceed normally without loading a skill."
-    )
+    if not skills_by_category:
+        result = ""
+    else:
+        index_lines = []
+        for category in sorted(skills_by_category.keys()):
+            cat_desc = category_descriptions.get(category, "")
+            if cat_desc:
+                index_lines.append(f"  {category}: {cat_desc}")
+            else:
+                index_lines.append(f"  {category}:")
+            # Deduplicate and sort skills within each category
+            seen = set()
+            for name, desc in sorted(skills_by_category[category], key=lambda x: x[0]):
+                if name in seen:
+                    continue
+                seen.add(name)
+                if desc:
+                    index_lines.append(f"    - {name}: {desc}")
+                else:
+                    index_lines.append(f"    - {name}")
+
+        result = (
+            "## Skills (mandatory)\n"
+            "Before replying, scan the skills below. If one clearly matches your task, "
+            "load it with skill_view(name) and follow its instructions. "
+            "If a skill has issues, fix it with skill_manage(action='patch').\n"
+            "After difficult/iterative tasks, offer to save as a skill. "
+            "If a skill you loaded was missing steps, had wrong commands, or needed "
+            "pitfalls you discovered, update it before finishing.\n"
+            "\n"
+            "<available_skills>\n"
+            + "\n".join(index_lines) + "\n"
+            "</available_skills>\n"
+            "\n"
+            "If none match, proceed normally without loading a skill."
+        )
+
+    # ── Store in LRU cache ────────────────────────────────────────────
+    with _SKILLS_PROMPT_CACHE_LOCK:
+        _SKILLS_PROMPT_CACHE[cache_key] = result
+        _SKILLS_PROMPT_CACHE.move_to_end(cache_key)
+        while len(_SKILLS_PROMPT_CACHE) > _SKILLS_PROMPT_CACHE_MAX:
+            _SKILLS_PROMPT_CACHE.popitem(last=False)
+
+    return result
 
 
 # =========================================================================