# Releo > Releo is change verification for the API economy: it detects breaking API and SDK changes, opens migration pull requests across every affected repository, and proves with deterministic replay of real traffic that behavior did not change before anything merges. Releo is in private beta; access is by waitlist at https://tryreleo.com/#access. Key facts: - Category: API change management and verification (developer tools, SaaS). - Customers: API providers migrating their customers, SaaS teams that depend on third-party APIs, agencies and integrators, and teams running AI agents. - How it works: Detect (spec diff) → Scan (find call sites) → Generate (pull requests) → Verify (replay real traffic, diff status, headers, body and timing; block the PR on any divergence). - Verification is deterministic: the verdict comes from comparing real responses, never from a model grading its own output. - Status: coming soon, waitlist only. Contact: hello@tryreleo.com. ## Product - [The safety layer for API changes](https://tryreleo.com/): Releo automates the migration and verification of breaking API and SDK changes. Ship deprecations without breaking customers’ code. Join the waitlist. - [Documentation](https://tryreleo.com/docs): How Releo detects breaking API changes, migrates every affected call site with pull requests, and verifies behavior with deterministic traffic replay before merge. - [Security](https://tryreleo.com/security): What data Releo holds today, how the product is designed to handle your code and API traffic, and how to report a vulnerability. ## Guides - [API Breaking Changes: Examples and How to Detect Them](https://tryreleo.com/learn/api-breaking-changes): An API breaking change forces existing clients to change their code. See 12 common examples, why tests miss them, and how to detect and verify them before release. - [API Versioning Strategies: URL vs Header vs Date](https://tryreleo.com/learn/api-versioning): Compare URL, header, query and date-based API versioning, see how Stripe and GitHub version their APIs, and learn what versioning does not solve. - [How to Deprecate an API: Deprecation & Sunset Headers](https://tryreleo.com/learn/api-deprecation): API deprecation guide: Deprecation (RFC 9745) and Sunset (RFC 8594) headers, notice periods, brownouts, 410 Gone, and getting consumers migrated. - [OpenAPI Diff: How to Detect Breaking API Changes in CI](https://tryreleo.com/learn/openapi-diff): Use an OpenAPI diff in CI to catch breaking API changes before merge. Compare oasdiff and openapi-diff, with a GitHub Actions example. - [API Migration Guide: Upgrade API Versions Safely](https://tryreleo.com/learn/api-migration): Step-by-step API migration guide: inventory call sites, map breaking changes, verify behavior with real traffic, and roll out gradually. - [Traffic Replay Testing: Verify API Changes Safely](https://tryreleo.com/learn/traffic-replay-testing): Traffic replay testing re-sends real requests to old and new versions of a service and compares responses. How it works, handling noise, and safety. - [API Testing: Types, Tools and a Practical Strategy](https://tryreleo.com/learn/api-testing): API testing explained: unit, contract, integration, end-to-end, load, security and replay tests, the tool categories for each, and a layered strategy. - [REST vs GraphQL vs gRPC: Differences and When to Use](https://tryreleo.com/learn/rest-vs-graphql): REST vs GraphQL vs gRPC compared: data fetching, performance, tooling, caching, and how each API style handles change, versioning and deprecation. - [API Design Best Practices: A Practical Checklist](https://tryreleo.com/learn/api-design-best-practices): API design best practices for naming, resources, errors, pagination, idempotency, versioning and evolvability, with examples you can apply to REST APIs. - [API Security Best Practices and the OWASP Top 10](https://tryreleo.com/learn/api-security-best-practices): API security best practices: authentication, authorization, OAuth 2.0, API keys, rate limiting, input validation, logging, and the OWASP API Security Top 10. - [API Documentation: How to Write Docs Developers Love](https://tryreleo.com/learn/api-documentation): How to write API documentation: reference vs guides, OpenAPI, working examples, quickstarts, changelogs and deprecation notices developers can act on. ## Blog - [API Traffic Replay: What a Behavioral Diff Compares](https://tryreleo.com/blog/api-traffic-replay-behavioral-diff): A behavioral diff is only useful if it ignores noise and catches real changes. How to normalize IDs, timestamps, ordering, headers, latency and secrets. - [LLM-as-Judge vs Deterministic Verification for Code](https://tryreleo.com/blog/deterministic-verification-vs-llm-as-judge): A model grading its own patch is not evidence. Why code changes need deterministic, reproducible verification, and where LLMs still earn their place. - [How API Providers Can Cut Customer Migration Time](https://tryreleo.com/blog/reduce-api-migration-time-for-customers): Every breaking change costs your customers engineering time. Machine-readable changelogs, OpenAPI diffs, codemods and automated PRs shrink that cost. - [AI Agents and API Changes: Why Schema Drift Hurts](https://tryreleo.com/blog/ai-agents-api-changes-schema-drift): AI agents call tools backed by real APIs. When those APIs drift, agents fail quietly or act confidently wrong. Here is how to version, check and replay. - [How to Ship a Breaking API Change: A Checklist](https://tryreleo.com/blog/how-to-ship-a-breaking-api-change-checklist): A provider checklist for breaking API changes: announcing, Deprecation and Sunset headers, precise changelogs, migration tooling, dual running and removal. - [API Migration Testing: Why Passing Tests Lie](https://tryreleo.com/blog/api-migration-tests-passed-production-broke): Mocks encode the old API, so migration tests pass while production breaks. A worked billing example shows behavioral vs contract breaks and how to catch both. - [The Hidden Cost of API Breaking Changes](https://tryreleo.com/blog/hidden-cost-of-api-breaking-changes): An API breaking change looks like a one-line fix. For consumers the hours go to discovery, finding call sites, patching, verifying, rollout and incidents. - [Why API Change Verification Needs to Exist](https://tryreleo.com/blog/why-api-change-verification-needs-to-exist): Detecting API breaking changes is largely solved and patches are getting easier to write. Proving a migration did not change behavior is the missing step. ## API glossary - [What is an API?](https://tryreleo.com/glossary/api): An API (application programming interface) is a defined contract that lets one piece of software request data or actions from another without knowing how it… - [What is a REST API?](https://tryreleo.com/glossary/rest-api): A REST API is an HTTP API organised around resources identified by URLs and manipulated with standard methods such as GET, POST, PUT, PATCH and DELETE. - [What is the OpenAPI Specification?](https://tryreleo.com/glossary/openapi): The OpenAPI Specification is a standard, language-agnostic format (YAML or JSON) for describing HTTP APIs: their paths, operations, parameters, schemas and… - [What is a breaking change in an API?](https://tryreleo.com/glossary/breaking-change): A breaking change is a change to an API that causes a correctly written client, which worked before, to fail or behave differently without modifying its code. - [What is backward compatibility in APIs?](https://tryreleo.com/glossary/backward-compatibility): An API change is backward compatible when every client built against the previous version keeps working, with the same behavior, after the change ships. - [What is API versioning?](https://tryreleo.com/glossary/api-versioning): API versioning is the practice of exposing multiple versions of an API at once so providers can ship breaking changes while existing clients keep using the… - [What is semantic versioning?](https://tryreleo.com/glossary/semantic-versioning): Semantic versioning is a MAJOR.MINOR.PATCH numbering scheme where a major bump signals a breaking change, a minor bump adds backward-compatible features, and… - [What is API deprecation?](https://tryreleo.com/glossary/api-deprecation): API deprecation is the formal announcement that an endpoint, field or version is going away, with a timeline for clients to migrate before it is removed. - [What is the Sunset HTTP header?](https://tryreleo.com/glossary/sunset-header): The Sunset header (RFC 8594) is an HTTP response header that gives the date after which a resource is expected to stop responding. - [What is contract testing?](https://tryreleo.com/glossary/contract-testing): Contract testing verifies that an API provider and its consumers agree on the requests and responses they exchange, so either side can change without… - [What is an API gateway?](https://tryreleo.com/glossary/api-gateway): An API gateway is a server that sits in front of one or more APIs and handles cross-cutting concerns such as routing, authentication, rate limiting, logging… - [What is API rate limiting?](https://tryreleo.com/glossary/rate-limiting): Rate limiting caps how many requests a client can make to an API in a given time window, protecting the service from overload and abuse. - [What is idempotency in APIs?](https://tryreleo.com/glossary/idempotency): An API operation is idempotent when performing it multiple times has the same effect as performing it once, which makes retries safe. - [What is API pagination?](https://tryreleo.com/glossary/pagination): API pagination splits large result sets into pages that clients fetch one at a time, using offsets, page numbers or opaque cursors. - [What is a webhook?](https://tryreleo.com/glossary/webhook): A webhook is an HTTP callback: when an event happens, a provider sends an HTTP request with the event data to a URL the consumer registered. - [What is an API SDK?](https://tryreleo.com/glossary/api-sdk): An API SDK (software development kit) is a client library that wraps an API in idiomatic functions and types for a specific programming language. - [What is GraphQL?](https://tryreleo.com/glossary/graphql): GraphQL is a query language and runtime for APIs where clients request exactly the fields they need from a typed schema, usually through a single endpoint. - [What is gRPC?](https://tryreleo.com/glossary/grpc): gRPC is an open-source remote procedure call framework that uses HTTP/2 and Protocol Buffers to let services call typed methods on each other efficiently. - [What is an API key?](https://tryreleo.com/glossary/api-key): An API key is a secret string a client sends with each API request to identify its application or account for authentication, quotas and billing. - [What is OAuth 2.0?](https://tryreleo.com/glossary/oauth): OAuth 2.0 (RFC 6749) is an authorization framework that lets an app get limited, revocable access to an API on a user's behalf without their password. - [What is a JSON Web Token (JWT)?](https://tryreleo.com/glossary/jwt): A JSON Web Token (JWT, RFC 7519) is a compact, URL-safe token of signed JSON claims, widely used as an access or ID token to authenticate API calls. - [What is CORS?](https://tryreleo.com/glossary/cors): CORS (Cross-Origin Resource Sharing) is a browser mechanism where HTTP headers let a server permit pages from other origins to read its responses. - [What are HTTP status codes?](https://tryreleo.com/glossary/http-status-codes): HTTP status codes are three-digit numbers a server returns with each response to signal its outcome, grouped in five classes from 1xx to 5xx. - [What is an API endpoint?](https://tryreleo.com/glossary/api-endpoint): An API endpoint is a specific address, usually a URL path combined with an HTTP method, where a client sends requests to perform one operation on an API. - [What is JSON Schema?](https://tryreleo.com/glossary/json-schema): JSON Schema is a declarative vocabulary for describing the structure of JSON data (types, required fields, formats, limits) so documents can be validated. - [What is API mocking?](https://tryreleo.com/glossary/api-mocking): API mocking replaces a real API with a simulated one that returns predefined responses, so teams can build and test clients without the real service. - [What is API monitoring?](https://tryreleo.com/glossary/api-monitoring): API monitoring continuously checks that an API is available, fast and returning correct responses in production, and alerts teams when it stops doing so. - [What is the API lifecycle?](https://tryreleo.com/glossary/api-lifecycle): The API lifecycle is the sequence of stages an API passes through, from design and build to release, versioning, deprecation and eventual retirement. - [What is an API-first approach?](https://tryreleo.com/glossary/api-first): API-first is a development approach where the API contract is designed and agreed before implementation, and treated as a product in its own right. - [What is API security?](https://tryreleo.com/glossary/api-security): API security is the practice of protecting APIs from misuse and attack with authentication, authorization, input validation, rate limits and monitoring. - [What is API documentation?](https://tryreleo.com/glossary/api-documentation): API documentation is the reference and guide material that explains how to use an API: its endpoints, parameters, authentication, errors and examples. - [What is API testing?](https://tryreleo.com/glossary/api-testing): API testing verifies that an API returns correct responses, status codes, data and performance for given requests, directly at the interface without a UI. ## Optional - [Privacy policy](https://tryreleo.com/privacy): What Releo collects through its waitlist, why, where it is stored, and how to have it deleted. - [Terms of use](https://tryreleo.com/terms): Terms for using tryreleo.com and joining the Releo waitlist, covering site content, acceptable use and liability. - [Guides to API Change Management](https://tryreleo.com/learn): Practical guides to API breaking changes, versioning, deprecation, OpenAPI diffs, migrations and traffic replay testing, for teams that publish and consume APIs. - [Blog: Shipping API Changes Safely](https://tryreleo.com/blog): Engineering essays and checklists on API breaking changes, migrations, deprecation, AI agents and deterministic verification, from the team building Releo. - [API Glossary: Definitions of Common API Terms](https://tryreleo.com/glossary): Clear definitions of API terms: breaking changes, versioning, deprecation, the Sunset header, idempotency, rate limiting, webhooks and more.