AI Coding Agents
API Development
Protecting API compatibility with coding agents
September 24, 2026 - 2 minute read
API compatibility checks turn a service change into a contract review. A handler can pass its unit tests while breaking a client through a removed field, narrower input, changed default, new authentication rule, or different error response. Coding agents need the old contract and consumer evidence before they can judge whether a change is safe.
The OpenAPI Specification defines a language-independent description for HTTP APIs. Keeping that description in version control gives an agent a concrete baseline to compare with the proposed schema. The specification describes the contract, while a compatibility policy decides which differences block a release.
Define compatibility before implementation
State which clients and versions must continue to work. Include the current API description, deprecation policy, generated SDKs, consumer tests, and any gateway rules. Mark whether the change may add optional fields, accept broader input, or introduce a new endpoint. Also state what requires a versioned replacement.
Compatibility depends on observed behavior as well as schema. A field may be optional in OpenAPI but required by a mobile client. An undocumented status code may drive retry logic. Ask the coding agent to search known consumers and contract tests, then report gaps rather than assume the specification is complete.
For delegated work, Factory's remote delegation guidance recommends a desired outcome, reproduction details, acceptance criteria, verification steps, and relevant links. An API task should name the baseline revision and require evidence for both existing and new consumers.
Compare contracts and trace effects
Generate the proposed API description and compare it with the baseline. The open source OpenAPI Diff project detects many breaking changes between OpenAPI documents and can run in CI. Treat its result as one input. A schema diff cannot discover a behavioral change hidden behind an unchanged response type.
The agent should trace validation, authorization, serialization, storage, events, SDK generation, and documentation from the changed endpoint. For a renamed field, check reads and writes during the transition. For a stricter validator, test values accepted by the previous release. For a changed error, confirm status, body shape, and retry behavior.
Prefer additive transitions. Add the new field or endpoint, migrate consumers, observe usage, and remove the old contract only after the deprecation window closes. Keep compatibility shims explicit and owned so temporary paths do not become permanent ambiguity.
Make API compatibility checks reviewable
The pull request should include the contract diff, affected consumers, migration order, and focused test results. Run provider tests against old requests and consumer tests against the new service. Exercise authentication and failure paths because compatibility failures often sit outside the success response.
Factory's Automated QA supports API flows alongside web and CLI testing, then produces structured pass, fail, or blocked evidence. Factory Custom Automations can trigger the same review pattern from GitHub events. Keep a human approval gate for accepted breaking changes and record the reason, owner, and migration deadline.
An agent should stop when it cannot locate the baseline contract, identify active consumers, or reproduce the old behavior. A narrower blocked report is safer than a confident compatibility claim based only on unit tests.
