Summary

Thanks for adding the temporary JA4/TLS debug endpoint and route coverage. The implementation is straightforward and CI is green, but before this ships I think the endpoint should be explicitly gated so it cannot become a public same-origin fingerprint reflection API by default.

Additional finding folded into body

Document the debug endpoint and config gate

If this endpoint is retained behind a new trusted-server.toml debug flag, please also document the flag and endpoint behavior in the API/config docs. The docs should make clear that this is temporary/debug-only, disabled by default, what fields are returned (ja4, h2_fp, cipher, tls_version, user-agent, ch-mobile, ch-platform), the fallback values, and that the response uses Cache-Control: no-store, private.

Summary

Adds a GET /_ts/debug/ja4 endpoint to the Fastly adapter for inspecting JA4/H2/TLS fingerprint metadata and Client Hints. CI is green and the route-level test correctly exercises dispatch. Two blocking concerns remain before this can ship: the endpoint is reachable unauthenticated by default (carried over from the prior review round), and its Cache-Control: no-store, private header is silently overridable by operator [response_headers] because the response flows through finalize_response.

Blocking

🔧 wrench

• Public-by-default fingerprint reflection oracle: /_ts/debug/ja4 is not protected by basic-auth in the default config and reflects values JS cannot otherwise read directly (main.rs:217). Gate behind a debug flag, restrict to staging, or move under an admin path.
• Cache-Control overridable by operator [response_headers]: handler-set no-store, private is replaced by finalize_response if operators set Cache-Control in [response_headers] (main.rs:134 → main.rs:349-351). Handle the route alongside /health before route_request runs.

Non-blocking

♻️ refactor

• Unit test duplicates the route-level test: identical assertions; route-level test is strictly stronger (main.rs:359-411 vs route_tests.rs:253-317).

🤔 thinking

• Defensive Vary header: User-Agent, Client-Hints, and TLS metadata vary the response — useful backstop if the override on Cache-Control is fixed (main.rs:133-136).

🌱 seedling

• Track removal: PR description and code call this "temporary" but neither has a removal trigger or follow-up issue. Add a // TODO: remove after JA4 evaluation completes — see #645 and consider a scheduled cleanup PR.

📝 note

• Bypasses platform abstraction for fields that have one: tls_protocol and tls_cipher are available via runtime_services.client_info() (main.rs:115-116). JA4/H2 are Fastly-specific so direct calls are unavoidable for those; mixing in the abstraction for fields that have one would be more consistent.

⛏ nitpick

• Use header::CACHE_CONTROL in test assertions instead of "cache-control" (main.rs:376, route_tests.rs:282).
• Extract magic strings to consts: "unavailable", "not sent", "none" and label strings are duplicated across production and both tests (main.rs:113-130).

CI Status

• fmt: PASS
• clippy: PASS
• rust tests: PASS
• js tests: PASS
• wasm32-wasip1 release build: PASS

Summary

I reviewed the current branch again and am only leaving findings that do not appear to duplicate the existing pending review comments. These are non-blocking cleanup/documentation accuracy items.