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feat: add TypeScript 6 support#2774

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benjamineckstein:feat/typescript-6-support
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feat: add TypeScript 6 support#2774
benjamineckstein wants to merge 2 commits intoopenapi-ts:mainfrom
benjamineckstein:feat/typescript-6-support

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@benjamineckstein benjamineckstein commented Apr 15, 2026 •
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Summary

Add full TypeScript 6 support — not just the peer dep range, but also a fix for a TS6 behavioral change that breaks Readable<T> and Writable<T>.

Changes

Peer dep & devDependency (commit 1):

  • packages/openapi-typescript/package.json: widen peer dep ^5.x → ^5.x || ^6.x
  • packages/openapi-typescript-helpers/package.json: upgrade devDependency 5.9.3 → 6.0.2

Fix Readable/Writable types (commit 2):
In TypeScript 6, Date extends object evaluates to true (changed from TS5). This caused Readable<T> and Writable<T> to structurally expand built-in objects like Date into { toString: {}; toDateString: {}; ... } instead of preserving the Date type.

Added a guard clause for Date | RegExp | ((...args: never[]) => unknown) before the T extends object branch in both type utilities, preserving built-in objects. This fix is backward compatible with TS5.

Changeset: patch release for both openapi-typescript and openapi-typescript-helpers

Validation

  • ✅ pnpm run build — all 4 packages build successfully
  • ✅ pnpm test — all 7 test suites pass
  • ✅ pnpm run lint — clean

Context

feat(openapi-typescript): add TypeScript 6 support
7bc63d0 
- Widen peer dep: `^5.x` → `^5.x || ^6.x`
- Upgrade devDependency in openapi-typescript-helpers: 5.9.3 → 6.0.2
- Build, tests, and lint all pass with TS6
- No code changes needed

Closes openapi-ts#2723
@benjamineckstein benjamineckstein requested a review from a team as a code owner April 15, 2026 08:44
@benjamineckstein benjamineckstein requested a review from drwpow April 15, 2026 08:44
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changeset-bot bot commented Apr 15, 2026 •
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🦋 Changeset detected

Latest commit: a749a52

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages
Name Type
openapi-typescript Patch
openapi-typescript-helpers Patch
openapi-fetch Patch
openapi-react-query Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

@benjamineckstein benjamineckstein mentioned this pull request Apr 15, 2026
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fix(openapi-typescript-helpers): preserve built-in objects in Readabl…
a749a52 
…e/Writable types for TS6

In TypeScript 6, `Date extends object` evaluates to `true` (changed from
TS5). This caused `Readable<T>` and `Writable<T>` to structurally expand
Date, RegExp, and function fields instead of preserving them.

Add a guard clause for built-in objects (Date, RegExp, functions) before
the `T extends object` branch in both type utilities.

Ref: openapi-ts#2723
@benjamineckstein benjamineckstein changed the title feat(openapi-typescript): add TypeScript 6 support feat: add TypeScript 6 support Apr 15, 2026
benjamineckstein
benjamineckstein commented Apr 15, 2026
View reviewed changes
Comment thread packages/openapi-typescript-helpers/src/index.ts
: T extends object
? { [K in keyof T as NonNullable<T[K]> extends $Write<any> ? never : K]: Readable<T[K]> }
: T;
: T extends Date | RegExp | ((...args: never[]) => unknown)
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@benjamineckstein benjamineckstein Apr 15, 2026

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In TypeScript 6, Date extends object now evaluates to true (it was false in TS5). Without this guard, Readable<T> would structurally expand Date fields into { toString: {}; toDateString: {}; ... } instead of preserving the Date type — making any schema containing Date fields incompatible with itself when accessed through FetchResponse.

This guard preserves built-in objects (Date, RegExp, functions) by matching them before the generic T extends object branch. It's backward compatible with TS5 since these types were never matched by T extends object there.

jeremy added a commit to basecamp/fizzy-sdk that referenced this pull request Apr 16, 2026
@jeremy
Keep typescript on 5.9.x until openapi-typescript supports TS 6
4b81f0a 
The Dependabot group update bumped typescript to ^6.0.2, but
openapi-typescript@7.13.0 still declares peer typescript ^5.x, so
npm ci fails to resolve. Revert just the typescript bump (keep the
other five: @types/node, @vitest/coverage-v8, msw, oxlint, vitest)
and regenerate the lockfile against 5.9.3.

Add a temporary dependabot.yml ignore on typescript semver-major
bumps so the same mismatch doesn't keep getting proposed. Remove
the ignore once openapi-typescript ships TS 6 support
(openapi-ts/openapi-typescript#2774).
@jeremy jeremy mentioned this pull request Apr 16, 2026
Merged
jeremy added a commit to basecamp/fizzy-sdk that referenced this pull request Apr 19, 2026
@dependabot @jeremy
Bump the npm-deps group across 1 directory with 6 updates (#50)
a83e268 
* Bump the npm-deps group across 1 directory with 6 updates

Bumps the npm-deps group with 5 updates in the /typescript directory:

| Package | From | To |
| --- | --- | --- |
| [@types/node](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/node) | `25.5.0` | `25.5.2` |
| [@vitest/coverage-v8](https://github.com/vitest-dev/vitest/tree/HEAD/packages/coverage-v8) | `4.1.0` | `4.1.2` |
| [msw](https://github.com/mswjs/msw) | `2.12.14` | `2.13.0` |
| [oxlint](https://github.com/oxc-project/oxc/tree/HEAD/npm/oxlint) | `1.56.0` | `1.58.0` |
| [typescript](https://github.com/microsoft/TypeScript) | `5.9.3` | `6.0.2` |



Updates `@types/node` from 25.5.0 to 25.5.2
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/node)

Updates `@vitest/coverage-v8` from 4.1.0 to 4.1.2
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.1.2/packages/coverage-v8)

Updates `msw` from 2.12.14 to 2.13.0
- [Release notes](https://github.com/mswjs/msw/releases)
- [Changelog](https://github.com/mswjs/msw/blob/main/CHANGELOG.md)
- [Commits](mswjs/msw@v2.12.14...v2.13.0)

Updates `oxlint` from 1.56.0 to 1.58.0
- [Release notes](https://github.com/oxc-project/oxc/releases)
- [Changelog](https://github.com/oxc-project/oxc/blob/main/npm/oxlint/CHANGELOG.md)
- [Commits](https://github.com/oxc-project/oxc/commits/oxlint_v1.58.0/npm/oxlint)

Updates `typescript` from 5.9.3 to 6.0.2
- [Release notes](https://github.com/microsoft/TypeScript/releases)
- [Commits](microsoft/TypeScript@v5.9.3...v6.0.2)

Updates `vitest` from 4.1.0 to 4.1.2
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.1.2/packages/vitest)

---
updated-dependencies:
- dependency-name: "@types/node"
  dependency-version: 25.5.2
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: npm-deps
- dependency-name: "@vitest/coverage-v8"
  dependency-version: 4.1.2
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: npm-deps
- dependency-name: msw
  dependency-version: 2.13.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
  dependency-group: npm-deps
- dependency-name: oxlint
  dependency-version: 1.58.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
  dependency-group: npm-deps
- dependency-name: typescript
  dependency-version: 6.0.2
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: npm-deps
- dependency-name: vitest
  dependency-version: 4.1.2
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: npm-deps
...

Signed-off-by: dependabot[bot] <support@github.com>

* Keep typescript on 5.9.x until openapi-typescript supports TS 6

The Dependabot group update bumped typescript to ^6.0.2, but
openapi-typescript@7.13.0 still declares peer typescript ^5.x, so
npm ci fails to resolve. Revert just the typescript bump (keep the
other five: @types/node, @vitest/coverage-v8, msw, oxlint, vitest)
and regenerate the lockfile against 5.9.3.

Add a temporary dependabot.yml ignore on typescript semver-major
bumps so the same mismatch doesn't keep getting proposed. Remove
the ignore once openapi-typescript ships TS 6 support
(openapi-ts/openapi-typescript#2774).

* Raise engines.node floor to >=20 (drop EOL Node 18)

Node 18 reached end-of-life on 2025-04-30. Currently supported Node
releases are 20 (Maintenance LTS until 2026-04-30), 22 (LTS), and 24.
Lift the consumer-facing engines floor accordingly.

Dev toolchain (oxlint, @inquirer/*, etc.) already requires Node 20.x
or newer at the transitive level — that's fine for contributors and
separate from the published SDK's runtime contract.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Jeremy Daer <jeremy@37signals.com>
This was referenced Apr 19, 2026
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Add support for TypeScript 6

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