diff --git a/blog/generate-openapi-from-traffic.md b/blog/generate-openapi-from-traffic.md new file mode 100644 index 00000000..41d4d79a --- /dev/null +++ b/blog/generate-openapi-from-traffic.md @@ -0,0 +1,225 @@ +--- +template: ../@theme/templates/BlogPost +title: No OpenAPI description yet? Generate one from real traffic +description: The new generate-spec command infers an OpenAPI description from recorded HTTP traffic - deterministically first, with optional AI refinement. +seo: + title: No OpenAPI description yet? Generate one from real traffic + description: The new generate-spec command infers an OpenAPI description from recorded HTTP traffic - deterministically first, with optional AI refinement. +author: adam-sobaniec +publishedDate: "2026-07-15" +categories: + - redocly:redocly-cli + - redocly:product-updates + - api-specifications:openapi +--- + +Plenty of production APIs have no OpenAPI description at all. +The endpoints live in code, the knowledge lives in people's heads, and the closest thing to documentation is a wiki page nobody fully trusts. + +For years that was survivable - human developers can read source code, ask a colleague, poke around with curl. +Your newest consumers can't. +AI agents don't attend onboarding calls or absorb tribal knowledge; they learn what your API can do from a machine-readable description of it. +An OpenAPI description is the foundation that everything an agent touches is built on: reference docs, SDKs, MCP tools, request validation. +Without one, your API's capabilities are simply invisible to them. + +So if your API doesn't have an OpenAPI description yet, this is a good moment to start. +The obstacle was never conviction - it's the blank page. +Writing a description by hand for an API with dozens of existing endpoints is exactly the kind of backfill work that never makes it into a sprint. + +The new `generate-spec` command in Redocly CLI removes the blank page. +Your API already produces the raw material every time someone uses it: traffic. +`generate-spec` reads recorded HTTP traffic and infers an OpenAPI 3.1 description from it. + +## From traffic to description + +The command accepts HAR files, Kong logs, Nginx/Apache JSON logs, and NDJSON - a file or a whole folder of them. +Traffic parsing is shared with the [`drift` command](./catch-api-drift.md), so any log that works with `drift` works here too. + +From the recorded exchanges it builds a baseline deterministically - no AI involved, same traffic in, same description out: + +- Identifier-like path segments (numeric IDs, UUIDs, prefixed tokens like `prd_…`) become named path parameters, so a hundred URLs collapse into one templated path. +- Request and response schemas are merged across all observations; a property becomes optional as soon as one sample omits it. +- Alternative body shapes for the same operation are preserved as `oneOf` variants, and object shapes that repeat across the document are extracted into `components/schemas`. +- String values are analyzed conservatively: consistent well-known patterns get a `format` (`uuid`, `date-time`, `email`, `uri`), and strings that only ever take a small set of repeated values become an `enum`. + +## Try it on the Cafe API + +Like last time, you don't need your own API to see it work - Redocly Cafe, our public demo API, will do. +Pretend for a moment that its OpenAPI description doesn't exist, and let's reconstruct it from traffic. + +First, record some. +The `proxy` command starts a local reverse proxy that captures everything passing through into a HAR file: + +```bash +redocly proxy --target https://api.cafe.redocly.com --har ./cafe.har +``` + +``` +Proxy listening on http://127.0.0.1:4040 → forwarding to https://api.cafe.redocly.com/ +Recording traffic to ./cafe.har +Press Ctrl+C to stop. +``` + +Send a few requests through it - browse the menu, filter it, download some menu item photos: + +```bash +curl http://127.0.0.1:4040/menu +curl "http://127.0.0.1:4040/menu?category=dessert" +curl "http://127.0.0.1:4040/menu?category=beverage" +curl -o tiramisu.png http://127.0.0.1:4040/menu-item-images/prd_0000000000seedtrams0000000 +curl -o tea.png http://127.0.0.1:4040/menu-item-images/prd_0000000000seedteabv0000000 +curl -o cheesecake.png http://127.0.0.1:4040/menu-item-images/prd_0000000000seedchesc0000000 +``` + +Press `Ctrl` + `C` to stop the proxy and write the HAR file: + +``` +Captured 6 exchange(s) to ./cafe.har +``` + +Now ask for a description: + +```bash +redocly generate-spec ./cafe.har --title "Cafe API" -o cafe-openapi.yaml +``` + +``` +Inferred a baseline OpenAPI description from traffic: 2 operation(s). +Written to: cafe-openapi.yaml +Done in 0s. +``` + +The result is a valid OpenAPI 3.1 description - about a hundred lines of it, from six requests. +The frame: a server URL inferred from the capture, and one path per discovered endpoint: + +```yaml +openapi: 3.1.0 +info: + title: Cafe API + version: 1.0.0 +servers: + - url: https://api.cafe.redocly.com +paths: + /menu: + # … + /menu-item-images/{menu-item-imageId}: + # … +``` + +Rather than paste the whole file, let's zoom in on what the inference did. + +Start with the paths: the three photo URLs collapsed into one templated path, because the `prd_…` identifiers were recognized as IDs and became a required path parameter: + +```yaml +/menu-item-images/{menu-item-imageId}: + get: + operationId: get-menu-item-images-menu-item-imageId + responses: + '200': + description: OK + parameters: + - name: menu-item-imageId + in: path + required: true + schema: + type: string +``` + +Inside the `/menu` response schema, every menu item property got a type, and the observed values were mined for more: + +```yaml +properties: + # … + price: + type: integer + category: + type: string + enum: + - beverage + - dessert + createdAt: + type: string + format: date-time + photoUrl: + type: string + format: uri +``` + +`category` became an enum because all eighteen observed values were one of the two, and `createdAt` and `photoUrl` matched well-known patterns in every sample. +The `category` query parameter on the same operation stayed a plain string - two observations aren't enough evidence, so the inference stays conservative rather than guessing: + +```yaml +parameters: + - name: category + in: query + required: false + schema: + type: string +``` + +The merge across samples also noticed which properties come and go. +Beverages have volume, desserts have calories, so `volume`, `containsCaffeine`, and `calories` are typed but absent from the `required` list: + +```yaml +properties: + # … + volume: + type: integer + containsCaffeine: + type: boolean + calories: + type: integer +required: + - id + - name + - price + - photoTextDescription + - category + - createdAt + - updatedAt + - object + - photoUrl +``` + +It's still a hypothesis, though - the description only knows what the traffic showed. +`price` is an integer because every observed price happened to be whole. +The photo response has no schema because its body is a PNG, not JSON. +Endpoints nobody called don't exist, nothing has a human-readable description, and names like `{menu-item-imageId}` are derived mechanically - rename them when you review. +More traffic tightens the hypothesis, and your e2e test suite is a free source of it: route the tests through `proxy` once, and the whole run becomes input. + +## Refine it with AI + +For everything determinism can't reach, add `--with-ai`: + +```bash +redocly generate-spec ./cafe.har --title "Cafe API" --with-ai --ai-provider claude -o cafe-openapi.yaml +``` + +The command refines the baseline one operation at a time, sending each operation together with a shape-diverse sample of its real recorded exchanges to an AI provider. +The AI narrows types, adds formats, enums, descriptions, and examples, and models alternative payloads explicitly with `oneOf` and `allOf`. + +Three providers are supported - `claude` (Claude Code), `codex` (Codex CLI), and `cursor` (Cursor CLI). +Each runs the corresponding locally installed CLI in non-interactive mode, so the provider you already use and pay for is the one doing the work; pick a model with `--ai-model` or let the provider use its default. + +The AI's answer is never trusted blindly. +A refined operation is only accepted when it keeps the operation's path and method, keeps every response status code observed in the traffic, and passes validation with the `spec` ruleset. +A rejected refinement keeps its deterministic baseline, and if refinement fails entirely, the command falls back to the baseline description - you never end up with less than the deterministic run would have given you. + +{% admonition type="warning" name="Traffic leaves your machine" %} +`--with-ai` sends samples of the recorded traffic - URLs, query strings, request and response bodies - to the selected AI provider. +Capture in a sandboxed environment with synthetic data, and make sure the traffic contains no secrets or personal data you are not allowed to share. +{% /admonition %} + +{% admonition type="warning" name="Experimental" %} +The `generate-spec` command is experimental. +Flags, output, and behavior may change - including breaking changes - in upcoming releases while we shape it with your feedback. +{% /admonition %} + +## Get started + +The `generate-spec` command is available now in the latest [Redocly CLI](https://redocly.com/docs/cli) - see the [command reference](https://redocly.com/docs/cli/commands/generate-spec) for all options. +Run it on your traffic, review what comes out, lint it with your own ruleset, and tell us what you think on the [Redocly CLI GitHub repository](https://github.com/Redocly/redocly-cli/issues). + +And once you have a description, don't let it rot. +Point the [`drift` command](./catch-api-drift.md) at next week's traffic and it tells you the moment reality moves. +Generate, then guard.