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What a contract test can—and cannot—guarantee
A contract test checks the assumptions your application makes at an integration boundary. It can catch a changed model identifier, missing request option, renamed response field, or unhandled stream event before that change reaches production. It cannot prove that a model will answer a prompt consistently.
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Keep these two risks separate:
- Interface risk: request, response, SDK, API revision, and event-schema changes can break serialization, parsing, or control flow.
- Behavior risk: a model may produce different wording, decisions, or tool choices while the API calls still succeed.
OpenAI says it seeks to avoid breaking changes in major API versions where reasonably possible, but separately cautions that prompting behavior can change between model snapshots. It recommends pinning model versions and running application evaluations for consistency: OpenAI API overview. A passing schema test therefore is not a substitute for an evaluation of your product’s acceptance criteria.
Put a narrow adapter between your app and each provider
Test the boundary your application actually depends on, rather than writing tests around a broad, generic “LLM” interface that hides meaningful differences. Define a small application-facing adapter for each operation—such as generating a response, streaming, or invoking a tool—and retain provider-specific options where the product needs them.
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interface TextGenerationAdapter {
generate(input: { prompt: string }): Promise<{ text: string }>;
}
This is only an illustrative interface, not a provider SDK contract. Your actual adapter should reflect the operations and data your application uses. If one provider has a capability another lacks, expose or handle that distinction deliberately instead of silently normalizing it away.
Build deterministic tests around the integration boundary
Assert the outbound request
Exercise the production request builder and check the fields that define the call. Include the exact model identifier, required and optional settings, tools, structured-output configuration, and API revision or headers where applicable. A request that still returns a successful HTTP response can nevertheless be wrong for your application if it silently drops a setting or targets a different model.
Keep intentional changes explicit: if you mean to move from a pinned model snapshot to another snapshot or rolling alias, update the expected contract deliberately and review the behavior change separately. Record the provider, requested model, SDK package and version, API revision, and test date in test output so a changed result has useful context.
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- TypeScript implements a superset of syntax for strictly typed development, facilitating deep static analysis and enhanced development environment integration. The compiler translates source into standard script formats, ensuring parity across any runtime.
- TypeScript is ideal for front-end developers, full-stack engineers, and software architects who build large-scale web applications. It serves those looking to improve code excellence, reduce bugs through static checking, and maintain complex projects more.
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Parse fixtures through production code
Keep representative response fixtures for each schema shape your parser handles, then pass them through the same parsing code used by the application. Assert required fields and their types, plus the application-level result that depends on them. Make failures explain which required field or shape was missing rather than allowing an undefined value to travel deeper into the system.
Do not treat a successful request as evidence that parsing is safe. Google’s Interactions API migration illustrates why: its documented response content moved from outputs to steps. A client expecting the old field could fail after the request itself succeeds. See the Gemini Interactions migration guide.
Test streams as protocols
Streaming code depends on event order and type, not just the final text. Use fixtures to check partial content handling, event names, terminal events, and tool-call events. Verify that the parser accumulates or dispatches data correctly and does not mistake a terminal event for another content delta.
Google’s migration guide documented a change from content.delta to step.delta, alongside the shift from outputs to steps. Those names make good examples of assertions: test the event types your parser accepts and fail clearly when a required event is absent or unknown. The same guide also calls out user_input, model_output, function-call steps, and server-side-tool steps; include the cases your integration relies on.
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Test tool-call parsing and dispatch independently from ordinary text generation. Check the structure your application needs to select a tool, validate its arguments, execute it, and process the result. Also test provider-specific features separately from any shared compatibility path; a common request shape does not establish common semantics.
Treat SDKs, API revisions, and model identifiers as contract inputs
Pin and record the SDK version used by the integration tests. An SDK upgrade may change which API schema your code receives, even if application code still compiles. Google’s migration guide documented that JavaScript SDK version 2.0.0 and later opted into the new Interactions schema, while 1.x temporarily returned legacy responses. It also documented REST clients using an Api-Revision header during the transition. The guide’s stated legacy-removal date, June 8, 2026, has passed; it is a dated example of SDK/API coupling, not a current deadline. Check the current provider documentation before relying on any transition detail.
Version labels also mean different things across providers. Google describes v1 as its stable API version and v1beta as a changing surface for early capabilities; it says breaking changes to stable APIs result in a new major API version, with the existing version deprecated after a reasonable period. Non-breaking additions may still arrive within a major version. See Gemini API versions. Even a stable API version is not a promise of identical model output.
For an OpenAI-compatible endpoint, test the exact features you use rather than inferring full parity from compatibility. Google says its OpenAI SDK compatibility path is most suitable when a unified Chat Completions schema matters more than provider-specific functionality; features may be limited and translation overhead exists because the schemas do not map one-to-one. See Gemini OpenAI compatibility.
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Use deterministic fixture tests for request serialization, response parsing, and event handling. Add a separate live smoke check to verify that the configured credentials, endpoint, model identifier, and deployed integration still work together. Keep that check distinct from unit-level contract tests so external availability does not make every test run nondeterministic.
Best Value
For changes that may alter answers, run evaluations against your own acceptance criteria: for example, whether the result includes required information, chooses an allowed action, or follows a product-specific format. Pin model versions where consistent behavior matters, and treat a move to a new snapshot or alias as an intentional behavior change to evaluate—not merely a dependency bump.
Use deprecation notices to plan migrations before failures
A deprecation notice is an operational signal, not just documentation housekeeping. OpenAI says its notice periods are intended to give customers time to evaluate replacements, test application behavior, and complete migrations. Its deprecations page is the place to verify current model and product schedules; dates can change, so check it when planning work: OpenAI deprecations.
Quick Recap
- Identify which production integrations and pinned identifiers are affected by the notice.
- Choose a replacement candidate and run the same contract checks against its request, response, stream, and tool behavior.
- Run application evaluations to assess behavior that schema assertions cannot cover.
- Stage the change with monitoring and a rollback path, then remove the old integration when the migration is verified.
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