""" Lecture 5.3 — Code Agent v1: Full Agent Loop with Real Tools Agent Engineering A working coding agent. Real implementations of read_file, list_files, and edit_file backed by the local filesystem. The agent loop runs until the user quits — tool calls are handled automatically, results are fed back to the model, and the loop continues until the model produces a text reply. Usage: python code-agent-v1.py Type a request at the prompt. Press Enter twice (empty line) to quit. """ import os from dotenv import load_dotenv import anthropic load_dotenv() client = anthropic.Anthropic() # ───────────────────────────────────────────── # System prompt # ───────────────────────────────────────────── SYSTEM_PROMPT = """You are a coding assistant. Your job is to help users read, understand, and modify code files in their project directory. ## Available Tools **list_files(path)**: List the files and directories at a given path. Use this to understand the project structure before looking for specific files. Returns file names and types. **read_file(filename)**: Read the complete contents of a file. Use this to understand existing code before making changes. Always read a file before editing it. **edit_file(path, old_str, new_str)**: Create or edit a file. - To create: set old_str to empty string, new_str to file contents. - To edit: set old_str to exact text to replace, new_str to replacement. You must read the file first to know what text to use for old_str. ## How to Work - Read before editing. Never modify a file you haven't read in this session. - Be concise. Respond with what the user needs, not explanations of your process unless the user asks. - When in doubt, ask. If a request is ambiguous, ask one clarifying question before acting. - Report what you did. After completing a task, briefly confirm what was done. Example: "Added the multiply function to math_utils.py on line 12." ## Constraints - Only access files within the current project directory. Do not attempt to read or modify files outside the project. - Never delete files. If the user asks you to delete something, explain that you cannot and suggest alternatives. - Do not execute code. You can read and write files, but not run them.""" # ───────────────────────────────────────────── # Tool schemas # ───────────────────────────────────────────── TOOLS = [ { "name": "list_files", "description": ( "List the files and directories at a given path. " "Use this to understand the project structure before looking for " "specific files. Returns file names and types." ), "input_schema": { "type": "object", "properties": { "path": { "type": "string", "description": "Directory path to list. Use '.' for the current directory." } }, "required": ["path"] } }, { "name": "read_file", "description": ( "Read the complete contents of a file. " "Use this to understand existing code before making changes. " "Always read a file before editing it." ), "input_schema": { "type": "object", "properties": { "filename": { "type": "string", "description": "Path to the file to read." } }, "required": ["filename"] } }, { "name": "edit_file", "description": ( "Create or edit a file. " "To create: set old_str to empty string, new_str to file contents. " "To edit: set old_str to the exact text to replace, new_str to the replacement. " "You must read the file first to know what text to use for old_str." ), "input_schema": { "type": "object", "properties": { "path": { "type": "string", "description": "Path to the file to create or edit." }, "old_str": { "type": "string", "description": "Text to replace. Empty string to create a new file." }, "new_str": { "type": "string", "description": "Replacement text, or full file contents when creating." } }, "required": ["path", "old_str", "new_str"] } } ] # ───────────────────────────────────────────── # Real tool implementations # ───────────────────────────────────────────── def list_files(path): """Return a directory listing with file names and types.""" try: entries = os.listdir(path) lines = [] for entry in sorted(entries): full = os.path.join(path, entry) kind = "dir" if os.path.isdir(full) else "file" lines.append(f"{entry} [{kind}]") return "\n".join(lines) if lines else "(empty directory)" except FileNotFoundError: return f"Error: directory not found: {path}" except PermissionError: return f"Error: permission denied: {path}" def read_file(filename): """Return the complete contents of a file.""" try: with open(filename, "r", encoding="utf-8") as f: return f.read() except FileNotFoundError: return f"Error: file not found: {filename}" except PermissionError: return f"Error: permission denied: {filename}" except UnicodeDecodeError: return f"Error: file is not readable as text: {filename}" def edit_file(path, old_str, new_str): """Create a new file or replace the first occurrence of old_str in an existing file.""" if old_str == "": # Create (or overwrite) the file try: os.makedirs(os.path.dirname(path), exist_ok=True) if os.path.dirname(path) else None with open(path, "w", encoding="utf-8") as f: f.write(new_str) return f"Created {path}" except PermissionError: return f"Error: permission denied: {path}" else: # Edit: replace first occurrence try: with open(path, "r", encoding="utf-8") as f: contents = f.read() except FileNotFoundError: return f"Error: file not found: {path}. Read the file before editing it." if old_str not in contents: return f"Error: text not found in {path}:\n{old_str!r}" updated = contents.replace(old_str, new_str, 1) with open(path, "w", encoding="utf-8") as f: f.write(updated) return f"Edited {path}" def dispatch_tool(name, inputs): """Call the named tool and return its result as a string.""" print(f" [tool call] {name}({', '.join(f'{k}={v!r}' for k, v in inputs.items())})") if name == "list_files": result = list_files(**inputs) elif name == "read_file": result = read_file(**inputs) elif name == "edit_file": result = edit_file(**inputs) else: result = f"Error: unknown tool: {name}" print(f" [tool result] {result[:120]}{'...' if len(result) > 120 else ''}") return result # ───────────────────────────────────────────── # Agent loop # ───────────────────────────────────────────── def run_agent(): messages = [] print("Coding Agent — type your request. Empty line to quit.\n") while True: user_input = input("You: ").strip() if not user_input: print("Goodbye.") break messages.append({"role": "user", "content": user_input}) # Inner loop: keep calling the API until the model produces a text reply while True: response = client.messages.create( model="claude-sonnet-4-6", max_tokens=4096, system=SYSTEM_PROMPT, tools=TOOLS, messages=messages ) # Append the full assistant response to history messages.append({"role": "assistant", "content": response.content}) if response.stop_reason == "tool_use": # Execute every tool the model requested, collect results tool_results = [] for block in response.content: if block.type == "tool_use": result = dispatch_tool(block.name, block.input) tool_results.append({ "type": "tool_result", "tool_use_id": block.id, "content": result }) # Feed all results back in a single user turn messages.append({"role": "user", "content": tool_results}) else: print(response.stop_reason) # Model is done — print the text reply for block in response.content: if block.type == "text": print(f"\nAssistant: {block.text}\n") break # Back to the outer loop for the next user message if __name__ == "__main__": run_agent()