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API client generation with coding agents
September 25, 2026 - 2 minute read
Suppose a generator upgrade produces a large API client diff and renames one public method that an application calls. Generation succeeds, but the consumer no longer compiles. A coding agent must separate the mechanical regeneration from the compatibility decision that determines whether the client can be released.
Keep the service contract as the source of truth. Pin the generator and configuration, define which outputs belong in version control, and fix generator inputs rather than hand-editing emitted code. OpenAPI Generator's usage guide documents the invocation options that a repository can reduce to one repeatable command.
Bound the API client generation task
Start with the reviewed OpenAPI document, target language, generator name, generator version, configuration file, and exact command. State which generated directory may change and whether generated files are committed or published as a package.
Separate contract changes from generator upgrades when possible. If both move in one pull request, reviewers cannot easily tell whether a diff came from an API decision or a tooling change. When an endpoint changes, classify it as additive, deprecated, or breaking before regeneration.
Factory’s AGENTS.md guidance supports repository-scoped commands and completion checks. Put the canonical generation command and the rule against hand edits near the generated directory so a coding agent encounters them before making changes.
Regenerate and test the artifact
Run generation from the pinned inputs in a clean workspace, then run it again and require no additional diff. Investigate changing timestamps, template versions, or local configuration when output keeps moving. A stable second run establishes repeatability for those inputs, not compatibility with an existing consumer.
Compile the client and run contract-focused tests against a controlled server or mock. Test representative success and error responses, optional fields, pagination, and serialization boundaries. A successful generator command proves only that files were emitted.
For the renamed-method example, compile a representative consumer against the regenerated client. A client can compile internally while its public interface breaks callers. Add a check for the method signature and request serialization that the consumer depends on.
Review API client generation as a release
Compare both the service contract and the generated public API with the previous release. Method names, required fields, nullability, and enum handling can change the consumer contract even when the server remains compatible. Decide whether to preserve the old interface through approved generator configuration or release a breaking client update with migration guidance.
Generated volume can hide a small consequential change. Ask the agent to summarize the contract delta, generator delta, dependency delta, and validation results separately. Keep the raw generated diff available, but give reviewers a short map to the files that define public behavior.
Use CI to regenerate and fail when committed output is stale. Factory’s Droid Exec can run the repository command non-interactively, while deterministic compile and contract checks remain required gates. An independent review should inspect handwritten templates or configuration with the same care as application source.
Choose the client package version from the compatibility impact, not from the size of the generated diff. Publish only after the contract and generated artifact are both approved. If multiple language clients share one contract, update them in coordinated, separately testable changes rather than assuming one successful target proves the rest.
