fix(svelte-form): use stable module-level counter for default formId by okxint · Pull Request #2430 · TanStack/form

okxint · 2026-10-13T05:12:10Z

Follows the same approach as #2415 (angular-form) — scoping this to svelte-form as that PR explicitly called out.

Problem

When formId is not provided, FormApi falls back to uuid(). In SvelteKit SSR a new UUID is generated server-side, and a different UUID is generated again on the client during hydration, causing a mismatch when form.formId is bound to a DOM attribute (id, aria-labelledby, etc.).

Fix

Add a module-level counter (nextFormId()) that increments in component-tree order. Since SvelteKit evaluates modules in the same order on server and client, the counter produces identical IDs on both sides. Explicit opts.formId is always used as-is — no change for users who supply their own ID.

Summary by CodeRabbit

Improvements
- Forms now receive an ID automatically when one isn’t provided. IDs increment within the Svelte context, with a randomly generated ID used when that context is unavailable. Explicitly supplied form IDs are preserved, so existing forms that set their own IDs continue to use them.

When formId is not provided, FormApi falls back to uuid(), generating a different ID on the server and client, causing SSR hydration mismatches when form.formId is bound to a DOM attribute in SvelteKit. Add a module-level counter (nextFormId) that increments in component-tree order, matching both SSR and hydration passes deterministically. Mirrors the same pattern used by angular-form (APP_ID + counter, PR TanStack#2415). Explicit opts.formId is always respected unchanged. Fixes TanStack#2388

changeset-bot · 2026-10-13T05:12:14Z

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coderabbitai · 2026-10-13T05:12:27Z

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packages/svelte-form/src/createForm.svelte.ts

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📝 Walkthrough

The Svelte adapter now generates a default form ID when options.formId is nullish. It preserves an explicitly supplied ID. ID generation uses a Svelte context counter, with a random ID fallback if context access fails.

Changes: Svelte form ID assignment

Layer / File(s)	Summary
Generate default form IDs `packages/svelte-form/src/createForm.svelte.ts`	`nextFormId` generates incrementing IDs using a Svelte context counter and returns a random ID if context access fails. `createForm` uses this generated ID when `options.formId` is nullish and preserves a supplied ID.

Priority: ➖ Normal

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

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: dummdidumm

Merge Risk: 🔵 Low · up to 6dec7

Sibling forms can share a generated ID, causing devtools to combine their state or apply a targeted action to both. This is a bounded tooling risk; the PR remains low risk to merge with owner awareness.

Pre-merge checks: 2 · 3

Failed checks: 3 warnings

Check name	Explanation	Resolution
Title check	The title identifies the correct package and default formId problem, but it states that the fix uses a module-level counter. The reported implementation uses a Svelte component-context-scoped counter …	Update the title to describe the final implementation, for example: "fix(svelte-form): use context-scoped counter for default formId".
Description check	The description clearly explains the SSR hydration problem and the intended fix, but it omits the required Checklist and Release Impact sections. It also provides no testing status or changeset status…	Add the template sections and complete each checklist item. State whether tests were run or do not apply, and state whether a changeset is required and was generated.
Docstring Coverage	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 2 passed

Check name	Status	Explanation
Linked Issues check		Issue `#2388` requires an SSR-safe default `formId` for `svelte-form`. The reviewed code assigns `svelte-form-${++ctx.n}` from a Svelte context counter when no ID is supplied. The context is created per…
Out of Scope Changes check		The diff changes only `packages/svelte-form/src/createForm.svelte.ts`. The added context counter and fallback implement the requested Svelte default-ID behavior. The change does not modify Angular, Li…

Full details: Linked Issues check

Explanation

Issue #2388 requires an SSR-safe default formId for svelte-form. The reviewed code assigns svelte-form-${++ctx.n} from a Svelte context counter when no ID is supplied. The context is created per component tree, so the default does not use a module-global request-shared counter. The code retains an explicit options?.formId value. The random fallback applies only when createForm runs outside Svelte component context, such as unit tests. The change addresses the Svelte scope identified in #2388. No linked issue requires a specific automated test.

Full details: Out of Scope Changes check

Explanation

The diff changes only packages/svelte-form/src/createForm.svelte.ts. The added context counter and fallback implement the requested Svelte default-ID behavior. The change does not modify Angular, Lit, form-core, or unrelated functionality. Leaving those adapters unchanged matches the stated Svelte-only scope.

Full details: Title check

Explanation

The title identifies the correct package and default formId problem, but it states that the fix uses a module-level counter. The reported implementation uses a Svelte component-context-scoped counter with a random fallback outside component context.

Resolution

Update the title to describe the final implementation, for example: "fix(svelte-form): use context-scoped counter for default formId".

Full details: Description check

Explanation

The description clearly explains the SSR hydration problem and the intended fix, but it omits the required Checklist and Release Impact sections. It also provides no testing status or changeset status.

Resolution

Add the template sections and complete each checklist item. State whether tests were run or do not apply, and state whether a changeset is required and was generated.

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Inline comments:
Review comments at @packages/svelte-form/src/createForm.svelte.ts:
- Line 5: Replace the module-level _formCounter with an ID source scoped to each
render, so forms without explicit IDs receive the same formId sequence during
SSR and client hydration.

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packages/svelte-form/src/createForm.svelte.ts

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A module-level counter persists across SvelteKit SSR requests, so the server counter drifts ahead of the client (which always starts at zero), causing hydration mismatches on forms without explicit ids. Using Svelte getContext/setContext scopes the counter to each component tree, which resets naturally per SSR request.

okxint · 2026-10-18T09:17:10Z

Replaced the module-level counter with a Svelte context-scoped one using getContext/setContext. Each SvelteKit SSR request creates a fresh component tree so the context resets naturally, keeping the server and client counters in sync. Falls back to a random suffix outside component context (tests, etc).

coderabbitai Bot reviewed Oct 13, 2026

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fix(svelte-form): use stable module-level counter for default formId - #2430

fix(svelte-form): use stable module-level counter for default formId#2430
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okxint:fix/svelte-form-ssr-stable-formid

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