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`Use Cache` Form Action Misses Cache After Hydrated Reload Due To `$ACTION_*` FormData Keys

When a form uses a cached server action, React includes internal `$ACTION_*` transport controls in the FormData. These controls differ between SSR reload and client navigation, causing the cache key to change and the cached function to execute again after hydration.

highConfidence 75%Next.js Affected V16.3.0

Origin Analysis

React's `$ACTION_*` transport controls become part of the FormData used for cache key derivation. Inline actions use `$ACTION_REF_*` with encrypted bound captures that change on reload, while file actions use stable `$ACTION_ID_*` but client navigation can add action controls, causing cache misses after hydration. Native no-JavaScript submissions pass through `decodeAction()`, which strips these controls, explaining why only JS submissions miss.

1. Load `/inline` directly and wait for hydration. 2. Select Reset cache. 3. Submit the cached form with `message=hello`. 4. Confirm the cached function executed once. 5. Reload the page and wait for hydration. 6. Submit the unchanged form again. 7. Observe the cached function executes a second time.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This patch assumes no user application fields legitimately start with `$ACTION_`; React reserves that prefix for internal controls. If an app uses such field names, they would be ignored in cache keys, leading to incorrect cache hits. The change should only affect cache key derivation, not the action invocation; ensure all call sites use the cleaned FormData only for caching. Rollback: revert this patch if unforeseen issues with server action routing occur. Add tests for direct SSR reload, client navigation reload, and no-JS scenarios.