Contract testing checks whether a service honors the messages another service relies on at their integration boundary. In a consumer-driven workflow, the consumer tests the interactions it needs against a mock and produces a contract; the provider verifies those interactions against its implementation. This can check compatibility without deploying both services together for every test, but it does not prove that the complete deployed system works.
What a service contract test checks
A contract is the agreed shape and behavior of messages exchanged at an integration point. It is not a legal document or a complete description of either service.
For HTTP, the consumer initiates a request and the provider responds. An HTTP interaction in Pact describes an expected request and the response the consumer needs. For asynchronous messaging, the consumer is the service that reads a message; the provider, also called the producer in this context, writes it. A message interaction describes the minimal message the consumer needs.
In a consumer-driven contract, the expectations come from the consumer’s actual usage. The contract focuses on interactions that consumer depends on, rather than every possible state in a broad resource schema. A passing test therefore means that the selected expectations were met—not that every conceivable request, response, or message is valid.
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How the consumer-driven workflow works
Pact documents one implementation of this model. Its workflow separates recording consumer expectations from checking the provider against them:
- Write a consumer test against a Pact mock. Exercise the consumer code using the interaction it needs, such as a request and the minimal response it relies on. For a message integration, test the consumer against the message it expects to read.
- Generate the contract. Pact records the interactions in a JSON contract. It represents the expectations exercised by the consumer tests, not every behavior the provider might support.
- Share the contract with the provider team or verification job. The consumer publishes or otherwise shares the generated contract so it can be used to check provider compatibility.
- Run provider verification. Pact retrieves the contract and replays the recorded requests against a locally running provider. For message interactions, verification checks the provider’s message against the consumer’s expectations.
- Include both sides in CI. Run consumer tests when consumer behavior changes and provider verification when provider behavior changes. Treat verification as a compatibility check for the contract being tested, not as a replacement for the rest of either service’s test suite.
The Pact Go provider verification guide recommends stubbing the provider’s dependencies during verification. That keeps the check focused on the provider’s integration boundary and helps make it fast and deterministic. This is a common strategy, not a requirement that every team use identical infrastructure or workflow.
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Make provider states explicit
A provider state sets up the condition needed before a particular interaction can be verified. For example, a hypothetical interaction might require that an account exists. The provider verification setup should create or arrange that condition before replaying the request.
Keep interactions independently verifiable. Do not rely on an earlier interaction in the same test run to create data needed by a later one. Instead, give each interaction its own explicit precondition. That makes failures easier to diagnose and avoids tests whose result depends on execution order or hidden shared state.
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What a passing contract test does—and does not—establish
A passing result establishes that the provider met the selected message expectations under the conditions used for verification. It can expose a compatibility break at the seam between a particular consumer and provider without requiring both services to be deployed together for every check.
It does not establish that the full application works end to end. A contract test does not, by itself, validate all business behavior, deployment configuration, infrastructure, network conditions, production data, or interactions that the contract does not describe. Keep broader tests for those concerns:
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- Consumer and provider unit tests check local logic and edge cases that are outside the shared interaction.
- Integration tests check behavior involving real dependencies or components where their interaction matters.
- End-to-end and deployment checks exercise the assembled system and its deployed configuration.
Choose each layer for the question it can answer. Contract verification is useful for compatibility at an integration boundary; it is not a substitute for evidence about the whole running system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Consumer-driven contracts and schema checks answer different questions
A consumer-driven contract and a provider-authored schema or API specification can complement one another. They differ in where expectations come from, what they check, and what confidence they provide.
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| Approach | Where expectations come from | What is checked | Confidence provided |
|---|---|---|---|
| Consumer-driven contract | The interactions a consumer actually uses and needs. | Concrete request/response or message interactions recorded by consumer tests. | Whether the tested consumer expectations match provider behavior. |
| Provider schema or specification check | A provider-authored API description or shared specification. | Whether implementation behavior conforms to the declared description. | Whether the provider matches its published schema or specification. |
A schema check can help keep implementation and API documentation aligned; an interaction contract adds consumer-specific assurance. Neither is a universal winner, and some teams need both. The choice depends on whether the gap to address is undocumented consumer needs, implementation drift from a published specification, or both.
Where ScreenshotNeo fits—and where it does not
ScreenshotNeo is a website screenshot API and MCP server, not a contract-testing framework. It does not verify service messages or replace Pact or other contract checks. It may be relevant as a separate tool if a service team also needs website screenshots; its API can return a PNG, JPEG, WebP, or PDF, and its MCP server provides tools for AI agents.
For website captures, ScreenshotNeo says it removes cookie and consent banners, newsletter popups, and chat widgets before capture. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. The free plan includes 1,000 shots per month with no card required; paid plans start at $5 for 3,000 shots. These capture features are separate from testing whether services honor an integration contract.
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