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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"OpenAPI 3.1: schemas with const but no type lose type information","articleBody":"A schema containing only `const`, without an explicit `type`, generates an alias to `any`. This affects all constants: scalars, arrays and objects.\n\nThe reproduction below compares scalar `const` schemas with their explicitly typed equivalents.\n\nReproduced against `main` at commit `833a968f84b4b0ec7c2f3fb5bb119e3c449f7a4f` (`v2.8.1-0.20260924063949-833a968f84b4`). The scalar reproduction below also fails on the latest release, `v2.8.0`.\n\nSave as `spec.yaml`:\n\n```yaml\nopenapi: 3.1.0\ninfo:\n  title: Scalar const inference reproduction\n  version: \"1\"\npaths: {}\ncomponents:\n  schemas:\n    StringConst:\n      const: active\n    TypedStringConst:\n      type: string\n      const: active\n\n    IntegerConst:\n      const: 0\n    TypedIntegerConst:\n      type: integer\n      const: 0\n\n    BooleanConst:\n      const: false\n    TypedBooleanConst:\n      type: boolean\n      const: false\n\n    NumberConst:\n      const: 1.5\n    TypedNumberConst:\n      type: number\n      const: 1.5\n```\n\nSave as `config.yaml`:\n\n```yaml\npackage: repro\noutput: types.gen.go\ngenerate:\n  models: true\noutput-options:\n  skip-prune: true\n```\n\n`skip-prune: true` retains the unreferenced schemas. Other settings use their defaults.\n\nRun in a directory containing those two files:\n\n```sh\ngo run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@833a968f84b4b0ec7c2f3fb5bb119e3c449f7a4f \\\n  --config=config.yaml spec.yaml\n```\n\nGeneration succeeds, but produces different representations for each equivalent pair:\n\n| Constant | Without explicit `type` | With explicit `type` |\n| --- | --- | --- |\n| `active` | `type StringConst = any` | `type TypedStringConst string` |\n| `0` | `type IntegerConst = any` | `type TypedIntegerConst int` |\n| `false` | `type BooleanConst = any` | `type TypedBooleanConst bool` |\n| `1.5` | `type NumberConst = any` | `type TypedNumberConst float32` |\n\nOnly the explicitly typed schemas receive enum constants and `Valid()` methods.\n\nOn `v2.8.0`, the untyped schemas generate `interface{}` instead of `any`; the typed controls behave as above.\n\nAdditional checks with the scalar reproduction on the pinned `main` commit:\n\n- Explicit `compatibility.schema-merging-behavior: v1`, `v2`, and experimental `v3`: same result as the default configuration.\n- `output-options.skip-enum-via-oneof: true`: same result.\n- `output-options.skip-enum-validate: true`: untyped schemas still generate `any`; the typed controls retain their scalar types and constants, with `Valid()` methods disabled as expected.\n- Setting `output-options.type-mapping.number.default.type: float64` changes `TypedNumberConst` to `float64`, while `NumberConst` remains `any`.\n- Changing the document version to `openapi: 3.1.1`: same result.\n\nExpected: scalar `const` schemas should receive the same typing and singleton-enum support as their explicitly typed counterparts, respecting configured Go type mappings.\n\nNon-scalar constraints may not be expressible through Go types alone, but scalars already have supported representations.\n\nUnder OpenAPI 3.1's JSON Schema semantics, `const` requires equality with the specified value. Adding `type` to the scalar examples above does not change which values they accept. [OpenAPI Schema Object](https://spec.openapis.org/oas/v3.1.1.html#schema-object), [JSON Schema `const`](https://json-schema.org/draft/2020-12/json-schema-validation#section-6.1.3).\n\nThe same behavior occurs inside object properties: a property containing only `const: active` becomes an `any` field.\n\nIn `generateGoSchema`, a missing `type` reaches the generic-schema branch, which selects `any` and returns before the scalar `const`/enum handler. [Generic-schema branch](https://github.com/oapi-codegen/oapi-codegen/blob/833a968f84b4b0ec7c2f3fb5bb119e3c449f7a4f/pkg/codegen/schema.go#L1272-L1293), [`const`/enum handler](https://github.com/oapi-codegen/oapi-codegen/blob/833a968f84b4b0ec7c2f3fb5bb119e3c449f7a4f/pkg/codegen/schema.go#L1476-L1506).\n\nRelated: [#2520](https://github.com/oapi-codegen/oapi-codegen/pull/2520) added inference for the `oneOf` + `title` + `const` pattern. This report concerns standalone `const` schemas without `type` and reproduces after that change.\n\nThe reproduction is provided under Apache-2.0.\n","author":{"url":"https://github.com/g-logunov","@type":"Person","name":"g-logunov"},"datePublished":"2026-09-24T14:30:59Z","url":"https://github.com/oapi-codegen/oapi-codegen/issues/2567"}