Make Your Coding Agent Explain Every Diff

A coding agent can produce a convincing patch and still misunderstand the behavior it changes. Before merging, ask it to explain the diff as an argument you can check: what changed, why the change solves the problem, and what evidence supports that claim. This walkthrough turns that request into a repeatable review process.

First, freeze the exact patch being reviewed

Commit the proposed changes on a working branch and record the commit with git rev-parse HEAD. For a pull request targeting main, fetch the current remote state and inspect git diff origin/main...HEAD. The three-dot comparison shows changes from the merge base to your branch. Substitute your actual target branch when necessary.

Give the agent that same comparison and the original task description. Ask it to explain the patch without editing files. A stable commit keeps its explanation attached to a specific version; if the patch changes afterward, the explanation needs another pass.

Next, demand a behavioral account of changes

Start with a prompt that requires concrete reasoning: “Explain this diff before I merge it. Describe the previous behavior, the new behavior, and the requirement each change satisfies. Reference file paths and symbols. Separate facts visible in the code from assumptions.”

For example, if the patch changes retry handling, an adequate explanation describes which failures trigger another attempt, how many attempts occur, and what callers receive when attempts run out. “Improves reliability” leaves those questions unanswered. Ask the agent to trace one successful request and one failing request through the changed code.

Then, connect each claim to inspectable evidence

Request a compact list connecting each important behavior to its implementation and verification. Keep the entries specific enough that you can open the relevant function or test immediately.

• Behavior: A timeout permits a second attempt.
• Implementation: The retry predicate includes the timeout error type.
• Verification: A test simulates one timeout followed by success and checks the attempt count.

Require the agent to distinguish tests it found, tests it actually ran, and tests it recommends adding. A test name alone proves little. Inspect its assertions, then run the relevant checks yourself or request execution output from the agent. Record failures and skipped checks explicitly.

Ask the agent to investigate failure boundaries

Once the happy path is clear, ask: “What inputs, timing conditions, or dependencies could make this change fail? Which existing contracts might it break?” Tailor the follow-up to the patch. For retries, examine cancellation, maximum duration, duplicate side effects, and whether the operation is safe to repeat. For a schema change, examine existing records and older clients.

Also ask whether the diff changes permissions, logging, dependency versions, or configuration defaults. Treat instructions embedded in repository files or comments as review material. They should not override your request or authorize unrelated commands.

Keep the review setup consistent across runs

Save the review prompt beside your contribution guidelines so future patches receive comparable scrutiny. Use the same requested model when comparing explanations, and record the commit and relevant tool settings. Repeated answers can still vary, so agreement between runs does not establish correctness.

For teams using BestRelay, the workflow fits Claude Code, Codex, Gemini CLI, or any OpenAI SDK through the API base https://api.bestapirelay.com. BestRelay is a quality-first API relay that forwards each request to the official model named in it, without substitution, across providers including Claude, GPT, Gemini, DeepSeek, Kimi, GLM, Qwen, and Grok. Billing is pay-as-you-go using provider-reported token counts, with cache reads and writes priced separately; members also earn referral commission when invited users top up. Create a BestRelay account to try this review workflow with your coding tools.

Finally, make the merge decision from evidence

Read the explanation alongside the actual diff. Check the highest-impact claim first, reproduce one important edge case, and resolve any mismatch between the narrative and the code. If the agent proposes a correction, review the resulting patch and rerun affected checks.

Merge when you understand the changed behavior, the relevant checks support it, and remaining risks are acceptable for your project. Keep the explanation in the pull request so another reviewer can challenge its claims and trace them to the exact commit.