Changes

When no formId is configured, createForm provides no fallback, so FormApi generates a random uuid() — seeded from Math.random(), and therefore different between the server render and the client render. Binding that generated id produces a hydration mismatch under SolidStart:

const form = createForm(() => ({ defaultValues: { firstName: '' } }))

<form id={form.formId}>   {/* server: "d8efe189-…"   client: "4b1c07a2-…" */}

An explicitly provided formId was already forwarded and is unchanged by this PR.

This is the same bug as #2254, which #2259 fixed for Vue. Solid is the remaining adapter with an SSR story that hadn't had the treatment the others already have:

• react-form/src/useFormId.ts — React.useId, falling back to useUUID on React 17
• preact-form/src/useFormId.ts — useId from preact/hooks
• vue-form/src/useFormId.ts — Vue's useId (fix(vue-form): generate an SSR-safe default formId #2259)

Solid's counterpart is createUniqueId(), which derives the id from the hydration context when one is present, so both renders agree. packages/solid-form/src/createForm.tsx now uses it for the default.

Test plan

packages/solid-form/tests/createFormId.test.tsx covers both branches — an explicit formId is returned unchanged, and the default comes from Solid's id sequence rather than a random uuid. Before the change the second test fails with:

expected 'd8efe189-c90f-4667-ac96-e9685217a6a4' to match /^cl-\d+$/

The first test passes both before and after, which is the evidence that explicit ids were never affected.

Full @tanstack/solid-form suite passes (46 tests, 5 files), tsc --noEmit is clean, and eslint ./src ./tests reports no new problems.

One thing worth flagging

I wasn't able to assert the server/client round-trip directly the way vue-form/tests/useFormId.test.tsx does. Under solid-form's current vitest config, solid-js/web resolves to the client build — isServer is false and renderToString() returns undefined — so an SSR render can't be exercised without a second vitest project configured with server conditions and solid({ ssr: true }).

Rather than grow this PR into a test-infrastructure change, the test pins the default to Solid's id sequence, which is the property that makes hydration agree. Happy to add the SSR project setup here or in a follow-up if you'd prefer the stronger assertion — just say which.

Summary by CodeRabbit

• Bug Fixes

  • Improved server-side rendering and hydration reliability for Solid forms with stable default form IDs.
  • Explicitly configured form IDs continue to work unchanged.

• Tests

  • Added coverage for custom form IDs and hydration-safe default IDs.