ContractGuard: Zero-Downtime API Schema Repair

The Problem When a backend API changes — a field is renamed, a type changes, or an endpoint is removed — the frontend breaks silently. There's no compiler error, no warning; the failure surfaces only when a user hits a runtime error or a build fails in CI. A developer then has to manually diff two API versions, grep the frontend for every affected call site, and patch each one by hand. This is slow, error-prone, and disproportionately costly on fast-moving teams without strict API governance — exactly the release/deployment and maintenance friction this challenge targets. The Solution ContractGuard is an IBM Bob 2.0 agent pipeline that goes further than detection: it closes the entire loop. A Diff Agent compares a FastAPI backend's OpenAPI spec before and after a change, classifying each difference (field renamed, field type changed, field added, endpoint removed) with severity. For every breaking change, three Bob 2.0 subagents take over: an Impact Agent scans the frontend codebase to find every affected call site; a Repair Agent generates and applies the actual code fix — updated types, corrected call sites; a Verify Agent runs the frontend's build and test suite to confirm the patch genuinely works, retrying the Repair Agent on failure rather than claiming false success. Every result is compiled into a plain-English Impact Report — readable by a PM or junior developer, not just whoever wrote the original diff tool. Target Users & Interaction ContractGuard is built for small, fast-moving engineering teams — startups, hackathon teams, or squads without a dedicated API governance process — where a single developer often owns both frontend and backend changes and has no automated safety net between them. A developer works exactly as before: they change a Pydantic schema or remove a route. ContractGuard runs automatically in the background, surfacing a report the moment a breaking change is detected, with the fix already applied and verified by the time they see it. What Makes It Unique Existing tools — OpenAPI diff linters, Pact-style contract tests, TypeScript compiler errors — stop at detection. They raise a flag and leave a human to do the actual repair work. ContractGuard is the only piece in this space that completes the cycle: detect, explain, repair, and verify, fully automated, using genuine multi-agent orchestration rather than a single monolithic prompt. Each subagent has one clear responsibility, mirroring how a careful human engineer would actually approach the problem — understand the change, find what it touches, fix it, then prove the fix works. Effectiveness We validated ContractGuard against three real, independently isolated breaking-change scenarios on a FastAPI + React reference application: a field rename, a field type change, and an endpoint removal. In every scenario, the Diff Agent correctly classified the break, and the full pipeline produced a verified, working fix without any manual frontend edits — turning what is normally minutes-to-hours of manual debugging into an automated, auditable process.