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Reviewed commit: d1fe67be4e

Pull request overview

Adds client/server support for the X-Vinext-Router-Skip optimization by propagating per-layout static/dynamic flags (__layoutFlags), sending cached static layout IDs from the client on RSC navigations, and filtering the outgoing RSC wire stream (and cached RSC bytes) to omit layouts the server agrees are static. Also introduces build-time classification manifest wiring + debug reason sidecar to reduce runtime probing overhead.

Changes:

• Introduce skip-header utilities (parseSkipHeader, buildSkipHeaderValue, header constant) and server-side skip decision logic (computeSkipDecision), plus __layoutFlags injection into the outgoing payload.
• Implement RSC wire-format byte filtering (createSkipFilterTransform / wrapRscBytesForResponse) and apply it to both live RSC responses and cached RSC reads.
• Wire build-time layout classifications into the generated RSC entry via a manifest + generateBundle patching of __VINEXT_CLASS / __VINEXT_CLASS_REASONS, with optional runtime debug logging.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 1 comment.

        const requestHeaders = createRscRequestHeaders(
          requestInterceptionContext,
          getBrowserRouterState().layoutFlags,
        );
        navResponse = await fetch(rscUrl, {
          headers: requestHeaders,
          credentials: "include",
        });

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Review: X-Vinext-Router-Skip header optimization

Overall this is well-architected. The key design decisions are sound:

• Canonical-first cache strategy: The tee in renderAppPageLifecycle happens BEFORE the skip filter, so isrRscDataPromise always captures full canonical bytes. The Codex P1 about ISR cache poisoning is a false positive — the architecture is correct. The skip filter is only applied to rscCapture.responseStream (the client-facing branch), while the capture branch gets unfiltered canonical bytes.
• Defense-in-depth: computeSkipDecision re-validates client-requested skips against server-side classification. The skip header is a hint, not a command.
• Byte-layer filtering: Keeping the skip filter in the RSC wire format (app-page-skip-filter.ts) rather than mutating the React element tree is the right call — it preserves the canonical payload for caching and avoids coupling to React internals.
• app-page-skip-filter.ts stream state machine: Clean architecture — buffer until row 0 arrives, compute live set, flush pending rows in stream order, then forward-pass subsequent rows. The fallback to passthrough on parse failure and the flush of pending rows if row 0 never arrives are good defensive choices.

Bug

1. Mounted-slots header dropped from navigation fetches (confirmed Copilot finding): The refactor of createRscRequestHeaders to accept layoutFlags removed the mountedSlotsHeader from the outgoing fetch. The header is still computed (line 919) and used for prefetch/visited-response cache lookups, but the actual fetch() no longer sends it to the server. This breaks server-side slot-aware payload tailoring.

Suggestions (non-blocking)

2. REFERENCE_PATTERN could use a source link: The regex [LBFQWK@] is correct for React 19.2.x but will silently miss new tags React adds. A comment linking to the React Flight source would help future maintainers audit coverage.

3. buildPageElements signature is now 9 positional parameters: Consider bundling the trailing params into an options object in a follow-up. It's easy to swap mountedSlotsHeader and skipLayoutIds at a call site without the type system catching it (generated code is untyped).

4. __VINEXT_CLASS_REASONS is always patched: The reasons dispatch table is always written into the bundle via generateBundle but only consulted when VINEXT_DEBUG_CLASSIFICATION is set. For apps with many routes this is dead code in production. Consider documenting the tradeoff or gating on a build-time flag.