How to stop AI coding agents from breaking code they didn't see

An agent edits what it has read. If it changes a function's signature or behaviour, every caller it didn't open is a guess. Type checks catch some of it in typed code, but not a changed meaning, a new side effect, or callers in Python and JavaScript. Tests catch the rest only where tests exist.

With or without any tool:

• Before changing a shared function, ask the agent to list every caller first, and to read them.
• Prefer adding a new parameter with a default over changing an existing one.
• Ask which tests cover the function, and run those before you push.
• Keep each change small enough that a reviewer can see every caller in the diff.
• Grep is fine for unique names; for common names (save, update, get) it misses and over-matches.

Synapse keeps a graph of who calls what in your repository. Before Claude Code or Cursor edits, it gets a brief over MCP: the exact lines to change, the callers that could break, which of them have no tests, and the pattern the team already uses. Before the agent says done, it can check the change for broken callers and duplicated code. On pull requests, the Synapse PR bot posts the same blast radius for reviewers.

In our benchmark (Claude Code with Sonnet, 7 real tasks on a repository Synapse had never seen, 2 runs each), Synapse cut tokens by 58% and cost by 48%, and every edit still compiled. Planning tasks saved the most. Results vary by repository and task.

Synapse is free to start: connect a GitHub repository and add it to Claude Code, Cursor or Claude in about two minutes. Create an account or read the docs.