feat(ai): add the input option to fakeText

[AlemTuzlak]

fakeText() takes an input option again, for example fakeText({ input: ['text', 'image'] }). It sets the fake's inputModalities, so you can test code that checks what a model accepts with no network. #1682 left the option out, because inputModalities was not on main yet. #1685 has landed since then.

🎯 Changes

• FakeTextOptions and FakeTextAdapter get a TInput type parameter. Its default is ReadonlyArray<Modality>, so existing code does not change.
• input sets inputModalities at run time and '~types'.inputModalities at type level.
• inputModalities stays optional, as on TextAdapter, so the fake is still an adapter under exactOptionalPropertyTypes.
• Note: chat() accepts a plain ModelMessage too, on purpose, so that wire payloads can be forwarded. So input: ['text'] does not make an image a type error in chat(). It types the adapter's modality data for the helper types.
• Docs: an input row in the Options table of advanced/testing.md.
• Changeset: @tanstack/ai minor.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with pnpm run test:pr, or these tests do not apply to this pull request.
• I fully understand the code in this pull request, including any code generated with AI assistance.
• Docs: I updated docs/ for this change, or this change is not user-facing.
• Changeset: I added a changeset (pnpm changeset), or this PR does not change a published package.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Testing

Commands run

• vitest run in packages/ai: 2116 tests pass, including the new sets inputModalities from the input option.
• tsc for packages/ai, with type tests in fake-text.test-d.ts. They check two things:
  • input gives readonly ['text', 'image'].
  • inputModalities stays optional.
• test:oxlint: clean. pnpm test:docs: no broken links.
• Not run locally: pnpm test:pr and the E2E suite. CI runs them.

Manual test

1. Run pnpm --dir packages/ai exec vitest run tests/fake-text.test.ts.
2. In a scratch test, check that fakeText({ input: ['text', 'image'] }).inputModalities gives ['text', 'image'], and that fakeText().inputModalities is undefined.

How this PR makes testing easy: a unit test and a type test.

Risk / rollback

Low. This is a new optional option with a default type parameter. To undo, revert this PR.

Public API change

After

const fake = fakeText({ input: ['text', 'image'] })
fake.inputModalities // ['text', 'image']

Summary by CodeRabbit

• New Features
  • Fake text models can now declare the input modalities they support, such as text or images. This information is available for checking model input support during tests.
• Documentation
  • Updated the testing guide to explain how to specify and inspect a fake model’s input modalities.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: TanStack/ai/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: a5fd31be-4964-43af-97de-b940b2c3d60a

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📥 Commits

Reviewing files that changed from the base of the PR and between d84a49e and 32a38e3.

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📒 Files selected for processing (6)

• .changeset/fake-text-input.md
• docs/advanced/testing.md
• docs/config.json
• packages/ai/src/testing/fake-text.ts
• packages/ai/tests/fake-text.test-d.ts
• packages/ai/tests/fake-text.test.ts

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📝 Walkthrough

Walkthrough

fakeText accepts an optional input modality array and exposes it as inputModalities. Its generic types preserve the supplied modality tuple. Tests and documentation cover the option and its behavior.

Changes

Fake text input modalities

Layer / File(s)	Summary
Input option, adapter, and validation packages/ai/src/testing/fake-text.ts, packages/ai/tests/fake-text.test-d.ts, packages/ai/tests/fake-text.test.ts, docs/advanced/testing.md, docs/config.json, .changeset/fake-text-input.md	FakeTextOptions and fakeText carry a generic input modality type. FakeTextAdapter assigns the provided input array to inputModalities; the option remains unset when omitted. Type and runtime tests verify these behaviors. The testing documentation and changeset describe the option. The documentation entry’s updatedAt date is set to 2026-10-09.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

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Merge Risk: ⚪ Minimal · up to 32a38

The input option and its documented behavior appear ready to merge after normal checks.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely identifies the main change: adding the input option to fakeText().
Description check		The description includes the required Changes, Checklist, and Release Impact sections. It also documents implementation details, testing results, risks, and the public API change. The author clearly s…
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 3 files. (3 skipped: 3 …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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☁️ Nx Cloud last updated this comment at 2026-10-09 15:25:15 UTC

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commit: 32a38e3

[github-actions]

Coverage

✅ Coverage held across 66 compared package(s).

Each package is measured twice in this job — on this PR and on its merge-base with main — and compared. A metric fails only when it is at or above 60% on the merge-base and under 60% on this PR. Other drops are listed but do not fail. Packages your PR doesn't affect are not measured and not listed.

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