Change Impact Assistant

An MCP server that answers one question for a developer or an AI coding agent: "I just changed this — what else might I need to check?"

It combines two independent signals:

• Static code structure — reads the codebase with Python's built-in ast module to find which files/functions import or call each other right now.

• Git co-change history — reads the repo's commit history to find files that have changed together in the past, even when nothing in the code connects them.

Neither signal alone is enough: static analysis misses config files, docs, or tests that always change alongside a piece of code but never import it; history alone misses brand-new code with no track record. Combining both catches more than either does on its own.

This is a small, original Python implementation of both ideas, built from scratch and kept intentionally simple.

Example

Given a change to payment_service.py:

HIGH CONFIDENCE
- invoice_service.py
  Reason: calls refund (changed in payment_service.py)

MEDIUM CONFIDENCE
- checkout_api.py
  Reason: imports payment_service.py, but doesn't call refund directly
- test_refunds.py
  Reason: changed together with payment_service.py in 4 of its last 6 changes (67%)

REVIEW SUGGESTED
- payment_config.yaml
  Reason: changed together with payment_service.py in 1 of its last 6 changes (17%)

invoice_service.py is HIGH because it actually calls the function that changed. checkout_api.py imports the file but calls a different, unchanged function, so it's demoted to MEDIUM rather than being blanket-flagged. test_refunds.py has no code connection at all — the only reason it's here is git history. payment_config.yaml is the same idea, weaker signal.

Related MCP server: Scrooge

Install

Requires Python 3.10+ and Git.

python3 -m venv .venv
source .venv/bin/activate
pip install -e .

Usage

Command line:

python -m impact_assistant analyze /path/to/your/repo

This reads git diff in that repo and prints the tiered report. Useful flags:

• --against HEAD~1 — compare against a specific ref instead of the working tree vs HEAD

• --since "1 year ago" — limit how far back co-change history looks

• --file some/file.py — analyze a specific file instead of the live diff (repeatable)

As an MCP server (for Claude Code, Claude Desktop, or any MCP-compatible AI assistant), add to .mcp.json:

{
  "mcpServers": {
    "change-impact-assistant": {
      "command": "/path/to/change-impact-assistant/.venv/bin/mcp",
      "args": ["run", "/path/to/change-impact-assistant/src/impact_assistant/server.py"]
    }
  }
}

This exposes 5 tools: analyze_current_change, explain_affected_file, get_static_impact, get_historical_impact, get_changed_files.

Interactively, with the official MCP Inspector:

mcp dev --with-editable . src/impact_assistant/server.py

How it works

Known simplifications (deliberate, not oversights)

• File/function names, not full scope resolution. Calls and imports are matched by name across the whole codebase. Two files with a same-named function can be conflated. A real type checker would resolve this exactly; this trades some precision for staying simple.

• Change unit = one commit, not a PR/merge-aware grouping. A history with many small fixup commits per PR will look noisier than a tool that groups by PR.

• Top-level functions/classes only for symbol-level diffing — a changed method inside a class shows up as "the class changed," not the specific method.

• Python codebases only.

• No persistent cache — the code graph and git history are rebuilt on every call. Fine for small/medium repos (verified under 100ms even on a ~6,500-commit, 37-file real project); would need caching for much larger ones.

Testing

pip install -e ".[dev]"
pytest tests/

Tests build a small, deterministic fake project with a hand-crafted git history (see tests/conftest.py) so results are reproducible. The tool has also been run against a real, unrelated open-source project (psf/requests) during development, which surfaced and led to fixing two real bugs that a synthetic test alone didn't catch.

License

MIT — see LICENSE.