SYSTEM = (
    f"You are a coding agent. "
    + open("docs/react-style.md").read()
    + open("docs/sql-style.md").read()
    + open("docs/api-design.md").read()
)

skills/
  agent-builder/SKILL.md
  code-review/SKILL.md
  mcp-builder/SKILL.md
  pdf/SKILL.md

class SkillLoader:
    def scan(self):
        self.skills.clear()
        for manifest in sorted(self.skills_dir.glob("*/SKILL.md")):
            content = manifest.read_text()
            metadata, body = self.parse_frontmatter(content)
            name = str(metadata.get("name") or manifest.parent.name).strip()
            description = metadata.get("description") or body.splitlines()[0]
            description = " ".join(str(description).lstrip("# ").split())
            self.skills[name] = {
                "name": name,
                "description": description,
                "content": content,
            }

- code-review: Perform thorough code reviews...
- pdf: Process PDF files...

def build_system_prompt() -> str:
    return (
        f"You are a coding agent at {WORKDIR}. Use tools to solve tasks. "
        "Act, don't explain.\n\n"
        f"Skills available:\n{SKILL_LOADER.catalog()}\n\n"
        "Use load_skill to read the full instructions when a skill applies."
    )

def load(self, name: str) -> str:
    skill = self.skills.get(name)
    if skill:
        return skill["content"]
    available = ", ".join(self.skills) or "none"
    return f"Error: Unknown skill '{name}'. Available: {available}"

cd learn-claude-code
python s07_skill_loading/code.py

#!/usr/bin/env python3
"""
s07_skill_loading.py - Skill Loading

The system prompt contains a catalog of skill names and descriptions.
The model loads the full SKILL.md only when it calls load_skill.

    skills/                    Startup
    +------------------+       +------------------+
    | code-review/     | ----> | SkillLoader      |
    |   SKILL.md       |       | name + summary   |
    | pdf/             |       +--------+---------+
    |   SKILL.md       |                |
    +------------------+                v
                                 system prompt catalog

    LLM -- load_skill(name) --> full SKILL.md
     ^                              |
     +--------- tool_result --------+
"""

import os
import subprocess
from pathlib import Path

import yaml

try:
    import readline
    readline.parse_and_bind('set bind-tty-special-chars off')
    readline.parse_and_bind('set input-meta on')
    readline.parse_and_bind('set output-meta on')
    readline.parse_and_bind('set convert-meta off')
except ImportError:
    pass

from anthropic import Anthropic
from dotenv import load_dotenv

load_dotenv(override=True)
if os.getenv("ANTHROPIC_BASE_URL"):
    os.environ.pop("ANTHROPIC_AUTH_TOKEN", None)

WORKDIR = Path.cwd()
SKILLS_DIR = WORKDIR / "skills"
client = Anthropic(base_url=os.getenv("ANTHROPIC_BASE_URL"))
MODEL = os.environ["MODEL_ID"]


# -- Skill catalog --

class SkillLoader:
    def __init__(self, skills_dir: Path):
        self.skills_dir = skills_dir
        self.skills: dict[str, dict[str, str]] = {}
        self.scan()

    @staticmethod
    def parse_frontmatter(text: str) -> tuple[dict, str]:
        if not text.startswith("---"):
            return {}, text
        parts = text.split("---", 2)
        if len(parts) < 3:
            return {}, text
        try:
            metadata = yaml.safe_load(parts[1]) or {}
        except yaml.YAMLError:
            metadata = {}
        if not isinstance(metadata, dict):
            metadata = {}
        return metadata, parts[2].lstrip()

    def scan(self):
        self.skills.clear()
        if not self.skills_dir.exists():
            return

        for manifest in sorted(self.skills_dir.glob("*/SKILL.md")):
            content = manifest.read_text()
            metadata, body = self.parse_frontmatter(content)
            name = str(metadata.get("name") or manifest.parent.name).strip()
            description = metadata.get("description") or body.splitlines()[0]
            description = " ".join(str(description).lstrip("# ").split())
            self.skills[name] = {
                "name": name,
                "description": description,
                "content": content,
            }

    def catalog(self) -> str:
        if not self.skills:
            return "(no skills found)"
        return "\n".join(
            f"- {skill['name']}: {skill['description']}"
            for skill in self.skills.values()
        )

    def load(self, name: str) -> str:
        skill = self.skills.get(name)
        if skill:
            return skill["content"]
        available = ", ".join(self.skills) or "none"
        return f"Error: Unknown skill '{name}'. Available: {available}"


SKILL_LOADER = SkillLoader(SKILLS_DIR)


def build_system_prompt() -> str:
    return (
        f"You are a coding agent at {WORKDIR}. Use tools to solve tasks. "
        "Act, don't explain.\n\n"
        f"Skills available:\n{SKILL_LOADER.catalog()}\n\n"
        "Use load_skill to read the full instructions when a skill applies."
    )


SYSTEM = build_system_prompt()


# -- Tools --

def run_bash(command: str) -> str:
    try:
        result = subprocess.run(
            command, shell=True, cwd=WORKDIR,
            capture_output=True, text=True, timeout=120,
        )
        output = (result.stdout + result.stderr).strip()
        return output[:50000] if output else "(no output)"
    except subprocess.TimeoutExpired:
        return "Error: Timeout (120s)"


def run_read(path: str, limit: int | None = None) -> str:
    try:
        lines = (WORKDIR / path).resolve().read_text().splitlines()
        if limit and limit < len(lines):
            lines = lines[:limit] + [f"... ({len(lines) - limit} more lines)"]
        return "\n".join(lines)
    except Exception as e:
        return f"Error: {e}"


def run_write(path: str, content: str) -> str:
    try:
        file_path = (WORKDIR / path).resolve()
        file_path.parent.mkdir(parents=True, exist_ok=True)
        file_path.write_text(content)
        return f"Wrote {len(content)} bytes to {path}"
    except Exception as e:
        return f"Error: {e}"


def run_edit(path: str, old_text: str, new_text: str) -> str:
    try:
        file_path = (WORKDIR / path).resolve()
        text = file_path.read_text()
        if old_text not in text:
            return f"Error: text not found in {path}"
        file_path.write_text(text.replace(old_text, new_text, 1))
        return f"Edited {path}"
    except Exception as e:
        return f"Error: {e}"


def run_glob(pattern: str) -> str:
    import glob
    try:
        matches = []
        for match in glob.glob(pattern, root_dir=WORKDIR):
            if (WORKDIR / match).resolve().is_relative_to(WORKDIR):
                matches.append(match)
        return "\n".join(matches) if matches else "(no matches)"
    except Exception as e:
        return f"Error: {e}"


TOOLS = [
    {"name": "bash", "description": "Run a shell command.",
     "input_schema": {"type": "object", "properties": {"command": {"type": "string"}}, "required": ["command"]}},
    {"name": "read_file", "description": "Read file contents.",
     "input_schema": {"type": "object", "properties": {"path": {"type": "string"}, "limit": {"type": "integer"}}, "required": ["path"]}},
    {"name": "write_file", "description": "Write content to a file.",
     "input_schema": {"type": "object", "properties": {"path": {"type": "string"}, "content": {"type": "string"}}, "required": ["path", "content"]}},
    {"name": "edit_file", "description": "Replace exact text in a file once.",
     "input_schema": {"type": "object", "properties": {"path": {"type": "string"}, "old_text": {"type": "string"}, "new_text": {"type": "string"}}, "required": ["path", "old_text", "new_text"]}},
    {"name": "glob", "description": "Find files matching a glob pattern.",
     "input_schema": {"type": "object", "properties": {"pattern": {"type": "string"}}, "required": ["pattern"]}},
    {"name": "load_skill", "description": "Load the full SKILL.md content by skill name.",
     "input_schema": {"type": "object", "properties": {"name": {"type": "string"}}, "required": ["name"]}},
]

TOOL_HANDLERS = {
    "bash": run_bash,
    "read_file": run_read,
    "write_file": run_write,
    "edit_file": run_edit,
    "glob": run_glob,
    "load_skill": SKILL_LOADER.load,
}


# -- Hooks --

HOOKS = {"UserPromptSubmit": [], "PreToolUse": [], "PostToolUse": [], "Stop": []}


def register_hook(event: str, callback):
    HOOKS[event].append(callback)


def trigger_hooks(event: str, *args):
    for callback in HOOKS[event]:
        result = callback(*args)
        if result is not None:
            return result
    return None


DENY_LIST = ["rm -rf /", "sudo", "shutdown", "reboot", "mkfs", "dd if="]
DESTRUCTIVE = ["rm ", "> /etc/", "chmod 777"]


def permission_hook(block):
    """PreToolUse: block denied operations and ask about risky ones."""
    if block.name == "bash":
        command = block.input.get("command", "")
        for pattern in DENY_LIST:
            if pattern in command:
                print(f"\n\033[31m[blocked] '{pattern}'\033[0m")
                return "Permission denied by deny list"
        for keyword in DESTRUCTIVE:
            if keyword in command:
                print("\n\033[33m[permission] Potentially destructive command\033[0m")
                print(f"   Tool: {block.name}({block.input})")
                choice = input("   Allow? [y/N] ").strip().lower()
                if choice not in ("y", "yes"):
                    return "Permission denied by user"

    if block.name in ("read_file", "write_file", "edit_file"):
        path = block.input.get("path", "")
        if not (WORKDIR / path).resolve().is_relative_to(WORKDIR):
            print("\n\033[33m[permission] Access outside workspace\033[0m")
            print(f"   Tool: {block.name}({block.input})")
            choice = input("   Allow? [y/N] ").strip().lower()
            if choice not in ("y", "yes"):
                return "Permission denied by user"
    return None


def log_hook(block):
    """PreToolUse: log every tool call."""
    args_preview = str(list(block.input.values())[:2])[:60]
    print(f"\033[90m[HOOK] {block.name}({args_preview})\033[0m")
    return None


def large_output_hook(block, output):
    """PostToolUse: warn on large output."""
    if len(str(output)) > 100000:
        print(f"\033[33m[HOOK] Large output from {block.name}: {len(str(output))} chars\033[0m")
    return None


def context_inject_hook(query: str):
    """UserPromptSubmit: log the working directory."""
    print(f"\033[90m[HOOK] UserPromptSubmit: working in {WORKDIR}\033[0m")
    return None


def summary_hook(messages: list):
    """Stop: print the number of tool results in this message list."""
    tool_count = sum(
        1
        for message in messages
        for block in (
            message.get("content")
            if isinstance(message.get("content"), list)
            else []
        )
        if isinstance(block, dict) and block.get("type") == "tool_result"
    )
    print(f"\033[90m[HOOK] Stop: session used {tool_count} tool calls\033[0m")
    return None


register_hook("UserPromptSubmit", context_inject_hook)
register_hook("PreToolUse", permission_hook)
register_hook("PreToolUse", log_hook)
register_hook("PostToolUse", large_output_hook)
register_hook("Stop", summary_hook)


def execute_tool(block) -> str:
    blocked = trigger_hooks("PreToolUse", block)
    if blocked:
        return str(blocked)

    handler = TOOL_HANDLERS.get(block.name)
    try:
        output = handler(**block.input) if handler else f"Unknown: {block.name}"
    except Exception as e:
        output = f"Error: {e}"

    trigger_hooks("PostToolUse", block, output)
    return str(output)


def agent_loop(messages: list):
    while True:
        response = client.messages.create(
            model=MODEL,
            system=SYSTEM,
            messages=messages,
            tools=TOOLS,
            max_tokens=8000,
        )
        messages.append({"role": "assistant", "content": response.content})

        if response.stop_reason != "tool_use":
            force = trigger_hooks("Stop", messages)
            if force:
                messages.append({"role": "user", "content": force})
                continue
            return

        results = []
        for block in response.content:
            if block.type != "tool_use":
                continue
            output = execute_tool(block)
            results.append({
                "type": "tool_result",
                "tool_use_id": block.id,
                "content": output,
            })
        messages.append({"role": "user", "content": results})


if __name__ == "__main__":
    print("s07: Skill Loading - catalog first, full content on demand")
    print("Enter a question, press Enter to send. Type q to quit.\n")

    history = []
    while True:
        try:
            query = input("\033[36ms07 >> \033[0m")
        except (EOFError, KeyboardInterrupt):
            break
        if query.strip().lower() in ("q", "exit", ""):
            break
        trigger_hooks("UserPromptSubmit", query)
        history.append({"role": "user", "content": query})
        agent_loop(history)
        for block in history[-1]["content"]:
            if getattr(block, "type", None) == "text":
                print(block.text)
        print()