When an agent writes the pull request, the review is the only place a human decision happens. This page covers what is different about agent-written diffs (with the research behind each claim), a review method that survives a 3,000-line PR, and which tools help with which part of it. It applies whether the code came from Claude Code, Cursor, Codex, Copilot or Devin; the per-agent pages go into each one's habits.
updated September 4, 2026 · every fact links to where we checked it
None of these are bugs on a line. All of them are what makes the PR expensive later.
Agents don't get tired, so 1,500-line PRs become normal. Reviewer accuracy falls past roughly 400 lines per hour: in the SmartBear study of Cisco reviews, reviewers faster than about 450 lines an hour found fewer defects than average in 87% of cases.
Asked to “add the export and fix the flaky test”, an agent ships both, plus the refactor it noticed on the way. Three deliverables share one review, one revert and one line in history.
The PR description was written by the model that wrote the code, from its plan rather than from the diff. What it forgot to mention is usually what it forgot to think about. The description is a claim to verify, not a summary to trust.
Generated tests often mock the unit under test, assert on the implementation rather than the behavior, or get loosened until green. Coverage looks fine and nothing is checked.
GitClear's analysis of 211 million changed lines found copy/pasted lines up from 8.3% in 2020 to 12.3% in 2024, and “moved” (refactored) code down below 10%. Agents write the helper again rather than find yours.
Veracode's 2025 GenAI report found 45% of AI-generated samples introduced an OWASP-class flaw; models failed to defend against XSS in 86% of cases. In a Stanford study, participants with an AI assistant wrote less secure code and were more confident it was secure.
Decide what the PR is before you read what it says. The long version, with commands and time budgets, is the 5,000-line guide.
Make two lists: what was promised and delivered, and what was delivered and never mentioned. The second list is where the surprises live: the timeout that got bumped, the retry that got added, the default that changed.
where Firetrail does this for you
Intent drift (Pro and up) produces the second list automatically, with the evidence, and the PR comment warns when the diff quietly does something the description never said.
If the PR ships more than one thing that could have shipped alone, it should. A UI and the API behind it are still two. Tests and docs that accompany a change don't count against it.
where Firetrail does this for you
The Multi-Intent check scores this on every PR. On Business, Firetrail writes a Rework plan (one intent per PR, the findings each one fixes, acceptance criteria) and your agent opens the draft PRs when you say so.
Migrations, config, environment, CI and infrastructure files, dependency and lockfile changes, auth and tenancy code, public API contracts. Then the rest. Generated files last, or skipped.
where Firetrail does this for you
The Global Config Change and Package Update checks flag the wide-blast-radius files; the Review Brief lists changed endpoints with their contracts and the schema diff table.
What is mocked? Is the thing under test among the mocks? Does any assertion check behavior rather than structure? Did the PR change an existing test rather than add one?
where Firetrail does this for you
The Test Relevance check asks whether anything tests the changed behavior; patch coverage (Pro and up) estimates which added lines the PR's own tests reach and lists the gaps; Mike, the QA reviewer, names the missing cases when he is in the project's lineup.
Data fenced per customer, permissions checked on the server and not only in the UI, records not reachable by changing an ID, no secrets in code or config, no user input rendered as HTML. Details on the security review page.
where Firetrail does this for you
Rick (security) and Priya (privacy) review every PR on Pro and Business. The Security Audit, on every plan, runs the five checks across the whole codebase or one PR from your own coding agent.
Without looking. If you can't, you're approving code nobody understands, which is the failure mode agent-written code makes easy.
where Firetrail does this for you
The PR Quiz (Pro and up) turns the top risk drivers into two to four questions with three plausible readings each. Anyone on the team can take it; the answer is recorded on the PR.
Merge as is, split, or send back. Only after that decision is a line-by-line read worth the time, and by then it's a read of 500 lines, not 3,000. A line-comment bot does the mechanical part well here.
where Firetrail does this for you
Firetrail's comment gives you the score, the breakdown and the verdicts before you open a single file. Keep your bot for the lines.
✓ does it · ◐ partly · — doesn't. “Bots” means CodeRabbit, Greptile, Copilot code review, Cursor Bugbot, Claude Code Review; “your agent” means asking Claude Code or Cursor to review its own work.
| Review job | Line-comment bots | Firetrail | Your own coding agent |
|---|---|---|---|
| Find bugs on the changed lines | ✓ | — | ◐same model, same blind spots |
| Say whether the PR should be split, and how | ◐some group a large diff into layers | ✓Multi-Intent; Rework plan on Business | ◐if you ask, without evidence |
| Find what the description never mentioned | — | ✓intent drift | — |
| Estimate whether the new code is tested | — | ✓patch coverage, Pro and up | ◐ |
| Verdicts from several specialties | ◐category-tagged comments | ✓ten reviewers | — |
| Explain a huge PR on one page | ◐walkthrough comments | ✓Review Brief, every plan | ◐ |
| Split the PR into small ones | — | ✓Rework, executed by your agent; Business | ◐by hand, from your instructions |
| Security pass across the application | ◐SAST integrations on some | ✓Security Audit via the MCP, every plan | ◐ |
| Learn whether the warnings predicted your bugs | — | ✓origin tracing, Business | — |
| Reads | Diff, usually plus repository context | The diff only | Your whole checkout |
Firetrail analyzes every PR the moment it opens on GitHub. Ten checks read the diff and build a Future Debug Cost score with an itemized breakdown; a team of reviewers gives verdicts on the whole change; the description is reconciled against what the analysis found; the added code's test coverage is estimated. It posts one comment and a neutral check, and the full result lives in the app. It doesn't care who wrote the code: an agent's PR and a person's PR are scored the same way, which is the point.
Through the Firetrail MCP, your own coding agent can ask for a Review Brief (one page: endpoints, schema diff, change mix, intent drift, findings, coverage, key changes per file, wireframes), a Security Audit (the five checks across the codebase, evidence only), or on Business a Rework plan that it executes as a chain of small draft PRs. Firetrail never writes to your repository; the agent does, on your account.
What it doesn't do: inline comments, line-level bug finding, reading code outside the diff, anything on GitLab. Every number it produces is an AI estimate and is labeled as one; a PR too large to read in full is marked partial rather than quietly sampled. None of this is a replacement for a human reading the risky part.
Thorough, well-described, larger than needed, and it adds a co-author trailer you can grep for. Firetrail runs inside Claude Code through MCP.
Read →Mixed human and generated lines under one name, wide agent-mode edits, Background Agent PRs. Firetrail in Cursor's MCP config.
Read →The five failures to check first, the evidence that agents make them, and how the Security Audit covers the whole codebase.
Read →Two tools, doing different jobs: a line-comment bot (CodeRabbit, Greptile, Copilot code review, Cursor Bugbot or Claude Code Review) for defects on the lines, and Firetrail for the change itself: size, bundled intents, undisclosed changes, untested logic, and a plan to split it. Agent-written PRs fail on the second set far more than on the first.
Partly. Reviewing with the same model that wrote the code shares its blind spots, which is why a different reviewer helps and why Anthropic's own review product adds a verification step. Firetrail uses several independent checks and reviewers with different specialties, shows the evidence for every finding, and labels estimates as estimates. Keep a human on the merge decision.
Yes. Start from the shape (how many deliverables, what the description doesn't say, which files have blast radius, whether the tests prove anything) and only then read lines. Human PRs usually arrive already shaped by the author; agent PRs don't.
Often you can't from the diff. Claude Code adds a co-author trailer to its commits by default; Cursor edits land under the developer's name. Firetrail doesn't try to detect authorship: it scores the change the same way regardless, which is what you want when every PR is partly generated.
Yes, twice over. On GitHub, Firetrail analyzes every PR regardless of what wrote it. In the terminal, the Firetrail MCP connects to Claude Code, Cursor, Codex CLI, VS Code or any MCP client, so you can ask for a Review Brief, a Security Audit or a Rework plan without leaving the tool that wrote the code.
Independent studies we cite above, plus the Firetrail docs the tool facts come from.
Free for one repository and 50 PRs a month. The comment arrives a minute or two after the PR does.