TanStack: Support both interpolatePath signatures

[BaconMan1168]

Closes #36330

What I did

@tanstack/router-core 1.171.30 changed interpolatePath from an object-argument signature (interpolatePath({ path, params }).interpolatedPath) to a positional one (interpolatePath(path, segments, params) returning a string directly), which throws when @storybook/tanstack-react calls it with the old signature.

Added interpolateStoryPath() in code/frameworks/tanstack-react/src/routing/path-utils.ts, a compatibility wrapper that detects the installed interpolatePath's calling convention by function arity and builds the minimal segment array the new signature needs. Both call sites now go through this helper: decorator.tsx (the one named in the issue) and duplicate-tree.ts's resolveStoryLeaf, which has the identical bug but wasn't mentioned in the issue.

The segment builder covers the route syntax this framework actually uses ($name dynamic params and $ splat params). TanStack's {-$optional} compound segment syntax isn't handled, so a route using that syntax would only resolve correctly against the old router-core.

This PR was drafted with AI assistance (Claude Code). I reviewed the change and stand behind it.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

1. In code/frameworks/tanstack-react, install @tanstack/router-core@^1.171.30 (or later) over the currently pinned version.
2. Run a sandbox using this framework, e.g. yarn task --task sandbox --start-from auto --template <a tanstack-react template>.
3. Open Storybook and navigate to a story that renders a route with a dynamic or splat path param.
4. Confirm the story renders without the previous path.endsWith is not a function crash, and that the interpolated path is correct.
5. Re-run the same steps against the previously pinned router-core version to confirm no regression on the old signature.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

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• code/frameworks/tanstack-react/src/routing/path-utils.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• code/frameworks/tanstack-react/src/routing/path-utils.ts

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Walkthrough

The TanStack React routing code now uses a local interpolation helper that supports both legacy and current TanStack Router APIs. Story router creation and story leaf resolution use this helper. Tests cover dynamic parameters, splats, and root paths.

Changes

TanStack Router interpolation compatibility

Layer / File(s)	Summary
Interpolation compatibility helper and tests code/frameworks/tanstack-react/src/routing/path-utils.ts, code/frameworks/tanstack-react/src/routing/path-utils.test.ts	Adds interpolateStoryPath, which selects the legacy object-argument API or the current positional API. Tests cover both signatures, dynamic parameters, splats, and root paths.
Routing call-site migration code/frameworks/tanstack-react/src/routing/decorator.tsx, code/frameworks/tanstack-react/src/routing/duplicate-tree.ts	Replaces direct legacy interpolatePath calls with interpolateStoryPath in story router creation and story leaf resolution.

Priority: ⬆️ High

Severity of issue fixed: High

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Actionable comments posted: 2

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Inline comments:
In `@code/frameworks/tanstack-react/src/routing/path-utils.test.ts`:
- Around line 11-15: Update the path-utils test setup to declare a top-level
vi.mock for `@tanstack/react-router` with spy: true, access interpolatePath via
vi.mocked(), and configure its implementation in beforeEach. Preserve coverage
for both interpolatePath.length branches in path-utils.ts by using a
Vitest-supported arity-preserving setup that remains observable after
vi.resetModules() and dynamic imports.

In `@code/frameworks/tanstack-react/src/routing/path-utils.ts`:
- Line 83: Update toPathInterpolationSegments() to recognize compound required
parameters in the form prefix{$name}suffix when using the positional API,
emitting the required interpolation tuple so values such as post-{$postId}
become post-42. Preserve existing handling for standalone and optional
parameters, and add coverage for this compound syntax.

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• code/frameworks/tanstack-react/src/routing/decorator.tsx
• code/frameworks/tanstack-react/src/routing/duplicate-tree.ts
• code/frameworks/tanstack-react/src/routing/path-utils.test.ts
• code/frameworks/tanstack-react/src/routing/path-utils.ts

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[obvious-autobuild]

Review from the parallel investigation side (@ghengeveld asked me to help get this ready-to-merge in favor of #36368).

The approach is sound and is the right one. Arity-based dual dispatch delegates interpolation semantics to the installed router-core in both generations — no behavior is reimplemented, and the ^1.168.9 dependency range keeps working without a floor bump. I verified locally against router-core 1.168.9 (legacy branch) and the routing suite passes (3 files, 29 tests).

One concrete finding — the hand-rolled segment builder diverges from the pre-1.171.30 behavior on braced segments and empty segments. I ran a parity check of the legacy object-form implementation (real 1.168.9) against the positional implementation (real 1.171.32) fed by this PR's toPathInterpolationSegments, over 19 cases. 14 match, 5 diverge — all in the positional branch, i.e. the branch that runs in the latest sandboxes where this break is live:

Path	Params	Legacy (<1.171.30)	This PR (positional)
/a//b	—	/a//b	/a/b
/files/v{$id}.json	id=3	/files/v3.json	/files/v{$id}.json
/users/{-$id}/edit	—	/users/edit	/users/{-$id}/edit
/users/{-$id}/edit	id=5	/users/5/edit	/users/{-$id}/edit
/files/{$}.json	—	/files/.json	/files/{$}.json

The legacy parser (and the 1.171.30+ segment compiler) handle TanStack's braced path syntax — {prefix$id}, {-$id} optionals, braced splats — and preserve empty segments; the builder here only recognizes bare $ and $-prefixed parts. Practical impact: a user whose TanStack routes use braced path syntax gets different interpolation depending on which router-core their install resolved — inside one Storybook release. Same input, different output across the version boundary this PR exists to bridge.

Suggested directions (either works, your call):

1. Scope it explicitly — if Storybook's inferred paths provably never contain braced segments or //, document that in a comment and add tests asserting the limited grammar, so the divergence is a known, deliberate boundary rather than an accident.
2. Port a faithful segmenter — note parsePathname is no longer exported from router-core 1.171.32 (I checked), so the library's own parser isn't reachable; porting the brace/optional/empty-segment rules from the legacy parser is the alternative.

I also added the ci:merged and qa:skip labels (Danger required exactly one ci:* and one qa:*), which should trigger the CircleCI tier on this PR. Once the divergence question is settled and CI is green, this should be in good shape for @huang-julien's final review.

[storybook-app-bot]

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^{Commit: fe41e25, ran on 29 September 2026 at 13:14:18 UTC}

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[obvious-autobuild]

Status update after the new head (6ebe67c, merge of next): I re-ran the routing suite locally against this head — path-utils.test.ts passes 4/4 (legacy object-signature and mocked positional-signature cases), and the full routing directory is green (3 files, 29 tests). The new dual-signature test coverage is a welcome addition and the arity-based dispatch is verified working against both real router-core implementations.

For completeness, the finding in my earlier review still applies at this head — toPathInterpolationSegments is unchanged, so the 5 divergent cases (braced params {$id} / {-$id} / braced splat, and empty-segment collapse on //) remain the one open design decision before this is fully behavior-preserving across the router-core boundary. Either of the two suggested directions (document the limited grammar with tests, or port the brace/optional rules) resolves it — happy to verify a follow-up commit against my parity suite either way.