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Review an API change as a change to a contract, not merely as a code diff. A client can fail when a field is renamed, a previously optional value becomes required, validation tightens, an enum changes, an error status changes, or an operation keeps its shape but changes meaning. The safest review identifies every externally visible difference, tests it against real consumer behavior and generated SDK rules, and chooses a compatible release or an explicit versioned migration.
This guide gives you a repeatable workflow: establish the contract and consumer population, classify each difference, prove compatibility assumptions with executable tests, and select versioning and retirement steps when compatibility cannot be preserved.
What exactly is an API contract?
An API contract is the observable agreement between a service and its callers. It includes more than an OpenAPI file or endpoint list: request and response formats, requiredness, accepted values, status and error behavior, operation semantics, URL structure, authentication headers, and the rules used to select a version.
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Review the contract at the wire boundary. An internal refactor, database migration, or infrastructure change may be invisible to clients and therefore need no API migration. Conversely, a one-line schema edit can break a production caller even when the server implementation appears small.
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- Requests: paths, query parameters, headers, body fields, types, defaults, nullability, and accepted encodings.
- Responses: field names and types, required fields, unknown-field behavior, enum values, pagination, status codes, and error bodies.
- Behavior: validation rules, side effects, ordering, idempotency, limits, timing expectations, and the meaning of successful results.
- Routing: version identifiers, URL forms, media types, cache keys, and behavior when a version is missing or invalid.
Microsoft Azure Architecture Center summarizes the goal as “Make API changes backward compatible when possible.” That is a design objective, not permission to assume that every additive edit is safe.
How do I decide whether an API change is breaking?
Classify the proposed behavior from the perspective of each supported client. A change is breaking when an existing caller can no longer send a request, parse a response, route to the service, or obtain the same promised meaning without code or configuration changes.
| Change pattern | Typical risk | What to verify |
|---|---|---|
| Remove or rename a field, parameter, header, operation, or URL | Breaking | Search consumers, generated models, serializers, dashboards, and documentation for the old name. |
| Change a field or parameter type, format, nullability, or units | Often breaking | Check parsers, generated code, numeric limits, date handling, and whether the old representation remains accepted. |
| Add a required request field, parameter, or header | Breaking for old callers | Replay requests from every supported client with the new value omitted. |
| Tighten validation or remove previously accepted inputs | Breaking for affected callers | Use boundary, malformed, legacy, and undocumented-but-observed inputs from production telemetry. |
| Add an optional response field | Conditionally compatible | Prove that clients ignore unknown fields and that closed schemas, generated models, signatures, or security filters do not reject them. |
| Add an optional request field | Conditionally compatible | Keep omission valid and preserve the old default and meaning; test callers that do not know the field. |
| Add an enum value | Conditionally compatible to breaking | Inspect exhaustive switches, validation libraries, generated enums, and clients that reject unknown values. |
| Change status codes, error shapes, side effects, ordering, or meaning | Potentially breaking even with an unchanged schema | Compare documented and observed behavior, not only schema files. |
| Change version routing, URL, media type, or cache identity | Operational and client risk | Test omitted and invalid version identifiers, intermediaries, and version-aware cache keys. |
The classification is client-specific. A tolerant JSON parser may survive an additive response field while a generated client with a closed model fails. A server may continue accepting an old request syntactically but return a different business meaning, which is still a contract break.
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What should I collect before reviewing the diff?
Start by defining the review boundary so an apparently safe edit is not judged in isolation.
- Identify the contract. Record the specification revision, protocol and media types, affected operations, authentication rules, and deployment environments.
- List supported versions. Include default behavior when no version is supplied, the current support and deprecation policy, and any sunset commitments.
- Map consumers. Inventory first-party applications, partner integrations, public users, batch jobs, webhooks, generated SDKs, command-line tools, and data pipelines. Note each client language, parser mode, release cadence, and compatibility guarantee.
- Capture the baseline. Save representative requests, responses, status codes, error payloads, limits, and version-routing examples for every affected operation.
- Define the decision owner. Assign who can approve a compatible extension, authorize a new version, set migration dates, and accept residual unknown-client risk.
Do not treat a specification repository as the entire consumer population. Telemetry, gateway logs, SDK downloads, support records, and partner inventories can reveal callers that are absent from source control.
How do I inspect an API change for compatibility hazards?
Review the diff in categories, then connect each difference to a concrete consumer behavior. This prevents a schema-only review from missing semantic or routing failures.
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Request and response shape
- Mark every removal, rename, move, nesting change, and change in capitalization.
- Compare primitive types, precision, units, formats, nullability, defaults, and maximum lengths.
- Check requiredness in both directions: a newly required input and a response field that callers now must expect.
- Inspect arrays, pagination links, polymorphic discriminators, and unknown-property rules.
Validation and accepted inputs
- Compare regular expressions, ranges, allowed characters, enum sets, payload-size limits, and content-type requirements.
- Test values that were accepted historically, including boundary and legacy forms. Removing an accepted input is a breaking change for callers that use it, even if the input was not prominent in documentation.
- Review authentication and authorization headers for new required scopes, changed formats, or altered failure responses.
Behavior and errors
- Ask whether the operation still has the same side effects, idempotency, ordering, consistency, and retry meaning.
- Compare success and error status codes, error-code stability, retry headers, and machine-readable error fields.
- Check whether an unchanged field now means something different, uses another unit, or is calculated at a different point in time.
URLs and version selection
- Review path templates, query names, hostnames, redirects, media-type parameters, and headers used for version negotiation.
- Define behavior for an omitted, malformed, unsupported, or conflicting version identifier.
- Include proxies and caches: a cache key that ignores the version can serve one contract’s representation to another contract’s client.
Can I add a field without breaking existing clients?
Sometimes, but only after proving two separate conditions: old callers must continue sending valid requests, and old clients must tolerate the additional response data. “Additive” describes the shape of a diff; it does not establish compatibility.
Optional request fields
Keep omission valid on the old contract path. Preserve the old default, side effects, and interpretation when the field is absent. Test an old request captured before the change, not merely a newly generated request that happens to include the field.
Optional response fields
Verify the actual parser behavior. Lenient map-based JSON clients usually ignore unknown properties, but closed schemas, strict deserializers, signature verification, data-loss filters, and generated models can reject them. Check SDK generation settings and wire-format rules for every supported language.
Enums and discriminated data
An additional enum value can break exhaustive switch statements, validation annotations, database constraints, or generated code that models the enum as a closed type. Treat unknown values as an explicit test case and document the fallback behavior before release.
Defaults and semantics
A default is part of the contract. Changing it can alter results for every caller that omits the field, even though no schema requiredness changed. Record the old and new defaults, then test omitted, explicit-old, and explicit-new values separately.
Zalando’s RESTful API and Event Guidelines emphasize that an API “MUST not break backward compatibility.” Apply that principle to the wire contract and to the concrete parser and generator guarantees of your own consumers.
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What executable tests should a compatibility review require?
Put the contract and gateway configuration under version control and make compatibility checks part of the deployment pipeline. A passing unit-test suite for the new server is not evidence that old clients still work.
- Replay each supported version. Run a corpus of baseline requests against the candidate deployment and compare status, error, schema, and defined semantic fields.
- Exercise request validation. Test omitted, null, boundary, malformed, legacy, unsupported, and oversized values. Include old clients that do not know newly added options.
- Exercise response parsing. Deserialize additive fields, unknown enum values, nulls, empty collections, pagination changes, and error payloads with every supported SDK and strictness mode.
- Test version identifiers. Cover each supported form plus missing, malformed, unsupported, and conflicting identifiers. Assert the selected contract and status code.
- Test cache correctness. Send equivalent requests for different versions through the real cache or a faithful test configuration. Verify that the version participates in the cache key where required.
- Run consumer contracts. Have representative applications or consumer-driven contract tests make real calls, including retries, timeouts, authentication, and idempotent replays.
- Define rollback checks. Confirm that the previous server and schema can be restored without leaving incompatible data, cache entries, migrations, or queued messages behind.
Keep failure output actionable: identify the operation, version, request fixture, expected contract, observed response, and owning consumer. A generic “schema mismatch” is not enough to decide whether to fix the server, update a client, or create a new version.
When should I create a new API version?
Create a new contract version when the required behavior cannot be made compatible for the supported consumer population. Examples include removing or renaming a field, making an input mandatory, changing a type or meaning, rejecting previously valid requests, or altering semantics that callers depend on.
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Before versioning, check whether a compatible extension can solve the requirement: add an optional field, introduce a new operation, or preserve the old interpretation while exposing the new one separately. Zalando recommends compatible evolution where possible, while Azure describes URI, query, header, and media-type versioning as established implementation choices.
| Versioning approach | Routing location | Compatibility and operational considerations |
|---|---|---|
| URI | For example, a path segment or separate base path | Easy to see and document; requires route management, client updates, and cache separation by URL. |
| Query parameter | A version value in the request query | Convenient for testing and gradual adoption; ensure caches, signatures, analytics, and links preserve the parameter. |
| Header | A custom or standard request header | Keeps URLs stable; requires reliable propagation through gateways, observability, SDKs, and intermediary policies. |
| Media type | An `Accept` or content-type media-type parameter | Expresses representation negotiation; demands precise default, invalid-value, and cache `Vary` behavior. |
Choose one convention for the API family and document precedence when more than one mechanism is present. The review must specify what happens when the version is omitted or invalid; silently selecting an unexpected contract makes incidents difficult to diagnose.
How do I plan migration and deprecation for a breaking change?
Publish the new contract while keeping the previous supported contract available for the consumers that still need it. Migration is an operational program, not just a new route.
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- Publish the delta. Describe removed and changed fields, validation differences, semantic changes, error behavior, version-selection rules, and concrete before-and-after examples.
- Provide client support. Release updated SDKs, migration guides, test fixtures, and dual-read or dual-write techniques where data transitions are involved.
- Measure adoption. Track requests by version, consumer identity, operation, and error class. Set an evidence-based retirement threshold rather than assuming every caller has upgraded.
- Communicate dates and policy. Announce deprecation, support boundaries, escalation contacts, and the final retirement date through the channels each consumer actually monitors.
- Operate both paths safely. Apply security fixes, capacity planning, observability, and incident response to every supported version. Test routing and caches for each path.
- Retire deliberately. Recheck traffic, obtain confirmation from critical consumers, remove documentation and credentials tied to the old contract, and retain a rollback plan for the shutdown window.
Do not copy another provider’s timeline as a universal standard. Microsoft Graph publishes its own version-specific deprecation and support policy; that policy is an example of one product’s governance, not a generally applicable number of months.
How should I compare versioning and migration options?
Use a decision record that makes trade-offs explicit rather than choosing a mechanism by habit.
| Decision axis | Questions to answer |
|---|---|
| Compatibility guarantee | Can old requests and responses continue unchanged, and what parser or generator assumptions must hold? |
| Consumer migration effort | How many clients need code, configuration, credential, data, or release changes, and how quickly can they ship? |
| Routing and cache behavior | Will gateways, proxies, CDNs, signatures, logs, and cache keys preserve the selected version? |
| Support cost | How many versions will run simultaneously, and what security, capacity, documentation, and on-call burden follows? |
| Testability | Can CI exercise every version, invalid selector, schema rule, and rollback path with representative consumers? |
| Observability | Can you attribute traffic and failures to a version and consumer without relying on fragile inference? |
| Reversibility | Can the change be rolled back without corrupting persisted data or leaving mixed-version messages in flight? |
The least disruptive option is usually the one that preserves the old contract and adds a separate capability. If a new version is unavoidable, select the routing style that your gateways, SDK tooling, cache infrastructure, and consumer teams can test consistently.
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“The schema diff is additive, so we approved it.”
Inspect strict deserializers, generated classes, enum handling, signatures, and validation filters. Add consumer fixtures that include the new output and unknown values, then require each client to demonstrate its behavior.
“No one uses the old field.”
Verify with traffic data and an owned consumer inventory. Undiscovered partners, dormant mobile releases, scheduled jobs, and webhooks can call an operation long after its maintainers stop seeing it in local code.
“The new version works, but users receive the wrong representation.”
Check omitted and conflicting version selectors, proxy forwarding, `Vary` headers, and cache keys. A correct handler behind incorrect routing is still a compatibility incident.
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“The old client gets a 400 after deployment.”
Compare its exact request with the new validation rules, including headers, content type, omitted fields, and legacy formats. Restore acceptance on the old contract or route that client to the previous version while its migration is prepared.
“Rollback restored the code but not the behavior.”
Inspect persisted schema changes, queued messages, cache entries, feature flags, and data written in the new format. A rollback procedure must cover state and traffic, not only the application binary.
What should be on the final approval checklist?
Require a recorded answer to every item before releasing a contract-affecting change.
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- Every removal, rename, type or meaning change, requiredness change, validation change, accepted-input change, status change, URL change, and routing change is classified.
- Compatibility assumptions for optional requests, additive responses, enums, defaults, generated SDKs, and closed schemas are proven by tests.
- Each supported version is tested with valid, omitted, malformed, unsupported, and conflicting version identifiers.
- Gateway, cache, authentication, observability, and rollback behavior are covered.
- The release path states whether the old contract remains, how migration is measured, and who owns deprecation and retirement decisions.
- Documentation, examples, SDKs, changelogs, and support communications match the deployed behavior.
This record becomes the audit trail for why a change was considered compatible or why a new version and migration were required. It also gives incident responders the exact assumptions to verify when a client fails after release.
Frequently Asked Questions
Does semantic versioning by itself prove that an API change is safe?
No. A semantic version label is a communication convention, not a test of wire compatibility. The API’s documented support policy, consumer parser behavior, routing rules, and executable contract tests determine whether callers can continue unchanged.
How can I review an API when some consumers are unknown?
Use gateway logs, authentication identities, user-agent and SDK metadata, partner records, and support tickets to build an observed consumer list. Mark unidentifiable traffic as residual risk and keep the old contract available until that traffic is attributed or retired.
Should an API version be selected by the client or by the server?
Client-selected versioning makes the requested contract explicit and supports parallel migrations. If the server supplies a default, document that default and its change policy; otherwise clients that omit a selector can change behavior unexpectedly.
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Keep the reviewed contract diff, consumer compatibility results, baseline fixtures, version-routing tests, cache tests, rollout metrics, and rollback procedure with the release record. Preserve the exact API policy and SDK-generation settings used for the decision so it can be reproduced later.
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