Cross-repo PR compatibility

Last updated

Cross-repo dependencies are a known GitHub gap — there's no native way to say "my frontend PR depends on this backend PR in another repo". Argus closes this gap by detecting linked PRs and running a compatibility check.

How to trigger

Paste a GitHub PR URL into your PR body, or use the shorthand owner/repo#N. Anywhere in the description is fine. No manifest, no YAML.

## Context
This PR updates our frontend to consume the new API
from backend-team/api#142.

It also depends on infra-team/config#88 for the new
feature flag.

What Argus does

  1. Finds all GitHub PR URLs and owner/repo#N references in the PR body (up to 5 by default).
  2. After the primary review completes, runs two async stages in parallel:
    • Combination-risk judge — hydrates each linked PR's diff + prior findings from their Argus review (if any), asks the LLM to probe 9 categories: schema/migration race, serialization contract drift, type/interface drift, config contradiction, deployment ordering, security posture, enum exhaustiveness, locale/temporal, and propagated findings.
    • Joint issue coverage — when 2+ linked PRs share a referenced issue, judges whether the combined change addresses each acceptance criterion with per-criterion evidence (file:line).
  3. Edits the sticky review comment in place, adding Cross-Repo PR Coverage and, if applicable, Joint Issue Coverage sections.
  4. When a linked PR's review completes later, re-runs only the cross-PR stage on already-reviewed PRs so late-arriving siblings refresh earlier PRs' sections.

Inaccessible repos

If a linked repo doesn't have Argus installed, the compatibility check skips it and notes "not reviewed by Argus — diff context only". The review still completes. Install Argus on the linked repo to enable full finding propagation.

Concurrent reviews

When linked PRs are reviewed at the same time, each initial cross-PR pass may run with partial data (siblings still reviewing). The event-driven refresh catches this: as each sibling completes, Argus re-runs the cross-PR stage on every already-reviewed PR that links to it, so the final state converges to full coverage across the family.

Default on

Cross-repo PR checks are enabled by default for new installations. Cost is 1–5 LLM calls per review depending on how many linked PRs and shared issues are involved; bounded by a per-install rate limit (30/hour) and a per-PR refresh cap (2 per 10 minutes). Disable in Settings → Features. Existing installations keep whatever toggle value was stored before the default flip.

Severity policy

Combination risks and joint-coverage gaps are reported informationallyin the sticky comment. They don't bump finding severity or block a merge — the reviewer has full context to decide.