pyfix-agent 1.0.2

An autonomous, multi-turn AI debugging agent built from scratch using AST surgery.

PyFix Agent: Autonomous ReAct Debugging Loop

An autonomous, multi-turn AI debugging agent built entirely from scratch in Python.

Unlike standard wrappers that simply ask an LLM to "fix this code," PyFix Agent implements a custom ReAct (Reasoning and Acting) state machine and utilizes Abstract Syntax Tree (AST) manipulation to surgically patch Python files in real-time. It evaluates its own fixes by executing the code inside a sandboxed subprocess, iterating dynamically until the script passes or it reaches the maximum iteration limit.

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Core Architecture

This project deliberately avoids high-level agentic abstractions (such as LangChain or LlamaIndex) to build the core agentic loop from first principles.

1. Execution Engine: Runs the target script via Python subprocesses, capturing standard outputs, standard errors, and stack traces with safety timeout thresholds.
2. Context Memory: Maintains a chronological conversation history array, allowing the LLM to learn from its previously failed patching attempts without losing the original code context.
3. AST Surgery: Parses the LLM's response and uses Python's native ast.NodeTransformer to swap out broken function nodes with the corrected logic, leaving the rest of the file entirely untouched.

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Design Choices & Trade-offs

Building an autonomous agent requires balancing safety, context window limits, and real-world unpredictability.

1. AST Function Surgery vs. Full File Overwrites

• The Problem: Asking an LLM to rewrite an entire 1,000-line script to fix a single typo is slow, expensive, and risks the model "truncating" or getting lazy with existing, working code.
• The Solution: The agent extracts the specific function_name from the traceback. It prompts the LLM only for the corrected function. The PythonSurgery class (inheriting from ast.NodeTransformer) then traverses the syntax tree, finds the broken ast.FunctionDef, and seamlessly swaps it with the new node.
• The Trade-off: While this guarantees perfect preservation of unrelated code, it requires specialized routing logic for errors that occur at the top-level <module> scope, which bypass the AST function surgery and require full-file patching.

2. Execution-Based Evaluation vs. Exact String Matching

• The Problem: How do we benchmark if the agent successfully fixed a bug? Traditional exact string matching fails because the LLM might use different variable names (e.g., x += 1 instead of x = x + 1), resulting in false negatives.
• The Solution: The evaluation suite uses Execution-Based Benchmarking. The benchmark dynamically runs automated unit tests or validation scripts containing assertion statements against the patched files. If the patched script exits with code 0, it is marked as a success.

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Evaluation Benchmark

The agent is evaluated against a curated dataset of scripts spanning 5 distinct error categories:

• NameError: Undefined variables, scope issues, and missing imports.
• IndexError: Off-by-one loop conditions and bounds checking.
• TypeError: Data type mismatches and unsupported operations.
• AttributeError: Typographical errors in object methods or calling methods on NoneType.
• Logic Bugs: Silent errors that require execution-based assertions to detect.

(Currently evaluated against an automated function-level testing benchmark suite inside eval_dataset/)

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Installation & Usage

Install via pip (Recommended)

You can install PyFix Agent directly from PyPI:

pip install pyfix-agent

This will automatically register the pyfix-agent executable in your terminal path.

Install from Source

If you want to run or modify the source code locally:

# Clone the repository
git clone https://github.com/LightY-01/pyfix-agent.git
cd pyfix-agent

# Install package in editable mode
pip install -e .

Configuration

Export your Hugging Face Hub token to your environment variables to ensure secure API access:

Bash (Linux/macOS):

export HF_TOKEN="your_huggingface_token_here"

PowerShell (Windows):

$env:HF_TOKEN="your_huggingface_token_here"

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CLI Usage Guide

Point the agent at any broken Python script. Use the --verbose flag to watch the ReAct state machine's internal thought process.

# Run the agent CLI
pyfix-agent --script my_broken_code.py --verbose --max_iter 5

CLI Command Options

Argument	Type	Default	Description
--script	str	Required	Path to the broken Python script to debug
--max_iter	int	5	Maximum number of debugging iterations
--verbose	flag	False	Enable logging of reasoning, tracebacks, and raw LLM responses