fix(agents): Implementer commits a dummy Python file instead of editing actual repo files

[archittmittal]

Summary

The implementer_node in backend/agents.py hardcodes the commit path to feature_issue_{issue_number}.py (line 607). This means the agent always creates a brand-new throwaway script in the target repository, regardless of what the Architect's directive or the issue description says. It never reads existing files, never applies a patch, and never makes a meaningful change to the actual codebase.

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Affected Code

# backend/agents.py:607  (current — broken behaviour)
path = f"feature_issue_{issue_number}.py"

And the LLM prompt reinforces this broken pattern:

# backend/agents.py:559-560
system_prompt = "You are the Implementer Agent. Write a tiny python script that implements
                 the core logic of the idea. Keep it very short. Output ONLY the Python script."
user_prompt = f"Write code for: {idea}"

The agent is instructed to write a generic script, not to modify any specific file in the repository.

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Steps to Reproduce

1. Run AutoMaintainer against any target repository with a real open issue.
2. Watch the Implementer commit its "code".
3. Check the resulting PR in the target repository — it will contain a single new file called feature_issue_N.py containing a short standalone Python script that has no relation to the actual codebase structure.

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Impact

Expectation	Actual Behaviour
Agent reads the target file identified by the Architect	❌ Agent never fetches any existing file
Agent patches the correct lines in the correct file	❌ Agent creates a new unrelated feature_issue_N.py
Merged PR improves the target repository	❌ Merged PR adds a useless throwaway script
Self-correcting loop fixes real bugs	❌ Maintainer reviews a script with no context

This is the single most critical functional bug in the system — it makes the entire agentic loop produce zero value.

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Root Cause

The Architect directive contains precise file and line references (e.g. "Modify agents.py:54-76 to add retry logic"), but the Implementer node never parses this directive. It ignores the file path entirely and writes free-form code to a hardcoded new file.

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Proposed Fix

The Implementer node needs to be rewritten to:

1. Parse the Architect directive to extract the target file path and line range.
2. Fetch the current file contents from GitHub using gh_repo.get_contents(file_path).
3. Prompt the LLM with the existing code as context, asking it to produce a targeted patch/replacement for the specified lines only.
4. Commit only the modified file (not a new feature_issue_N.py).

# backend/agents.py — proposed implementer_node logic sketch

# 1. Extract target file from directive
import re
match = re.search(r'([\w/.-]+\.\w+):(\d+)-(\d+)', directive)
if match:
    target_file = match.group(1)
    start_line, end_line = int(match.group(2)), int(match.group(3))
else:
    target_file = None

# 2. Fetch current file from GitHub
if target_file:
    try:
        file_obj = gh_repo.get_contents(target_file, ref=default_branch)
        original_content = file_obj.decoded_content.decode('utf-8')
    except Exception:
        original_content = ""

# 3. Ask LLM to patch specific lines
system_prompt = (
    "You are the Implementer Agent. You will receive the full contents of a source file "
    "and a directive specifying which lines to change and why. Output ONLY the complete "
    "modified file contents — no explanations, no markdown fences."
)
user_prompt = (
    f"Directive: {idea}\n\n"
    f"File: {target_file}\n"
    f"Lines to change: {start_line}–{end_line}\n\n"
    f"Current file contents:\n{original_content}"
)

# 4. Commit the modified target file (not a new file)
path = target_file  # ← use the actual file path

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Files to Change

• backend/agents.py:
  • Lines 556–560: Update implementer_node system/user prompts to include existing file context.
  • Line 607: Replace hardcoded path = f"feature_issue_{issue_number}.py" with dynamic path parsed from the Architect directive.
  • Lines 618–623: Update create_file to update_file when the target file already exists.

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Acceptance Criteria

• When the Architect directive references a specific file (e.g. agents.py:54-76), the Implementer fetches and modifies that exact file.
• The PR diff shows changes to an existing repository file, not a new feature_issue_N.py.
• The Implementer falls back to creating a new file only when no target file can be extracted from the directive.
• The Maintainer review prompt includes the original file content alongside the proposed changes for proper context.