@@ -28,15 +28,16 @@ Both are plausible, well-written, and wrong. Reading the code first catches both
 6. **Force every open question to an explicit decision**, even if the decision is "accept as-is for v1, revisit if feedback says otherwise." An open question left unresolved in a PRD gets silently decided by whoever implements it — usually differently than anyone intended.
 7. **Write the test plan as part of the PRD, not after.** Concrete test cases — naming real functions/queries, not "add tests for X" — force you to notice design gaps you'd otherwise miss; the moment you try to write "assert Y happens" and realize the current design can't produce Y is often the first time the gap becomes visible. Check the repo for an existing test pattern for this shape of change before inventing a new one (e.g. a prior presence-logic bug fixed via `test/db_test_helpers.py` fixtures is the template for the next one, not a reason to build new test infrastructure).
 8. **Ask explicitly whether validating this needs real end-to-end infrastructure** (a new or modified plugin, a UI click-through) or whether synthetic unit-level fixtures suffice — don't assume either way. Check whether the functions under test take a DB connection/dict/list as a parameter (testable in isolation, no real plugin needed) or require a real file on disk (harder to fake, may need one).
-9. **Check performance against the real schema and real scale, not assumptions.** For every new or changed query: does it use an existing index, or add an unindexed lookup, a new join, or a correlated subquery? Grep `CREATE INDEX` for *every* index on the tables involved, not just the first one you find — a column can have both a plain index and a separate expression index (e.g. `idx_eve_mac_date_type ON Events(eveMac, ...)` alongside `idx_eve_lower_mac_date_type ON Events(LOWER(eveMac), ...)`), and missing the second one produces a wrong verdict. Then weigh cost by how often the query runs (once is nothing; every few minutes forever is a standing cost) and by real scale — **production users run 10,000+ devices**, not a homelab handful. A `CurrentScan` with 2-5 rows per device (one per contributing plugin) is routinely 20,000-50,000+ rows in one cycle; reason about that number, not a smaller hopeful one. A correlated `EXISTS`/subquery re-evaluated per outer row is fine *if* the correlated column is indexed — e.g. `current_scan_presence_condition()` (`server/scan/presence.py`) is exactly this shape against `CurrentScan.scanMac`, covered by `idx_currentscan_scanmac`. The real risk is an unindexed correlated lookup: an accidental self-join scanning the full inner table per outer row, which looks fine and passes tests at small scale but isn't at production scale. Check with `EXPLAIN QUERY PLAN` at a realistic row count, built against the *complete* real index set (copy every `CREATE INDEX` for the table, or run it against an actual `app.db`) rather than a hand-picked subset — a partial index set produces a misleading plan in either direction, not just "looks worse than it is." If it comes back unindexed for real, a `GROUP BY` aggregate is the usual fix.
-10. **Do a dedicated final-check pass, out loud, before calling it done.** Re-read the whole document end to end and specifically check:
+9. **Ask explicitly whether this feature/mechanism should be exposed via the API, and give a recommendation, not just flag it as an open question.** This codebase already has three real API surfaces to consider extending rather than inventing a fourth: REST (`server/api_server/*_endpoint.py`), GraphQL (`graphql_endpoint.py`), and a Prometheus `/metrics` endpoint (`prometheus_endpoint.py`). Skipping this question doesn't mean "no API access needed"; it means the answer gets silently decided later by whoever first wants to query the new data externally, usually as its own separate feature request re-litigating a design this PRD already had full context to settle. Not everything needs exposure: purely internal/diagnostic state with no plausible external consumer doesn't, but say so explicitly, with the reason, rather than leaving it unaddressed.
+10. **Check performance against the real schema and real scale, not assumptions.** For every new or changed query: does it use an existing index, or add an unindexed lookup, a new join, or a correlated subquery? Grep `CREATE INDEX` for *every* index on the tables involved, not just the first one you find — a column can have both a plain index and a separate expression index (e.g. `idx_eve_mac_date_type ON Events(eveMac, ...)` alongside `idx_eve_lower_mac_date_type ON Events(LOWER(eveMac), ...)`), and missing the second one produces a wrong verdict. Then weigh cost by how often the query runs (once is nothing; every few minutes forever is a standing cost) and by real scale — **production users run 10,000+ devices**, not a homelab handful. A `CurrentScan` with 2-5 rows per device (one per contributing plugin) is routinely 20,000-50,000+ rows in one cycle; reason about that number, not a smaller hopeful one. A correlated `EXISTS`/subquery re-evaluated per outer row is fine *if* the correlated column is indexed — e.g. `current_scan_presence_condition()` (`server/scan/presence.py`) is exactly this shape against `CurrentScan.scanMac`, covered by `idx_currentscan_scanmac`. The real risk is an unindexed correlated lookup: an accidental self-join scanning the full inner table per outer row, which looks fine and passes tests at small scale but isn't at production scale. Check with `EXPLAIN QUERY PLAN` at a realistic row count, built against the *complete* real index set (copy every `CREATE INDEX` for the table, or run it against an actual `app.db`) rather than a hand-picked subset — a partial index set produces a misleading plan in either direction, not just "looks worse than it is." If it comes back unindexed for real, a `GROUP BY` aggregate is the usual fix.
+11. **Do a dedicated final-check pass, out loud, before calling it done.** Re-read the whole document end to end and specifically check:
     - Did a correction made mid-document propagate everywhere it needed to (the Design subsection *and* Affected Files *and* Tests *and* any execution-plan summary)? A correction landing in one place and not its siblings is worse than never catching it, because now the document silently contradicts itself.
     - Does every "this is the cleanest/simplest real case" claim still hold up if you actually re-read that specific piece of code right now, or was it asserted by pattern-matching a name/category? Re-verify, don't re-assert.
     - Does anything render incorrectly as markdown — an unfenced ASCII diagram or code block will collapse into one line under lazy-paragraph-continuation, the same class of bug as a list missing its preceding blank line.
     - Do any internal anchor links' slugs match their headings?
     - Does the design still cleanly separate its axes, or did a later addition quietly re-conflate two concerns inside what's supposed to be a single-purpose mechanism (the same mistake step 3 exists to catch at the top level can reappear one level down inside an individual mechanism's own value set — e.g. a 3-value enum where two of the values are secretly independent booleans in a trenchcoat).
-11. **Leave a visible trail of corrections instead of silently rewriting.** When a review pass — yours or someone else's — finds something wrong, write "**Correction (caught in review):** ..." inline rather than quietly fixing the earlier text and moving on. This is what makes a PRD trustworthy to a second reader: they can see what was checked and what changed, not just receive a polished final answer with no visible seams.
-12. **When marking a PRD Implemented, ask once whether anything found along the way generalizes beyond this one document** — a tooling trick, a wrong assumption that took real digging to disprove, a review catch that would recur on the next PRD. The PRD's own "Implementation notes" section is the right place for what happened *in this PRD*; a skill or memory update is the right place for anything that would otherwise have to be rediscovered next time. One question, not a mandatory new section — skip it when nothing generalizes, don't force it on every PRD regardless of size.
+12. **Leave a visible trail of corrections instead of silently rewriting.** When a review pass — yours or someone else's — finds something wrong, write "**Correction (caught in review):** ..." inline rather than quietly fixing the earlier text and moving on. This is what makes a PRD trustworthy to a second reader: they can see what was checked and what changed, not just receive a polished final answer with no visible seams.
+13. **When marking a PRD Implemented, ask once whether anything found along the way generalizes beyond this one document** — a tooling trick, a wrong assumption that took real digging to disprove, a review catch that would recur on the next PRD. The PRD's own "Implementation notes" section is the right place for what happened *in this PRD*; a skill or memory update is the right place for anything that would otherwise have to be rediscovered next time. One question, not a mandatory new section — skip it when nothing generalizes, don't force it on every PRD regardless of size.
 
 ## Structure to follow
 
@@ -46,7 +47,7 @@ Both are plausible, well-written, and wrong. Reading the code first catches both
 - **Open issues** — each with an explicit recorded decision (step 6), not left dangling.
 - **Affected files** — concrete `file:function:line` references, not bare filenames.
 - **Backward compatibility** — explicit default values and why they preserve current behavior for existing consumers.
-- **Performance impact** — the baseline (what's unindexed/slow *today*, independent of this change), what the change adds that's negligible, what's new and worth mitigating, and concrete mitigations rather than a vague "should be fine" (step 9).
+- **Performance impact** — the baseline (what's unindexed/slow *today*, independent of this change), what the change adds that's negligible, what's new and worth mitigating, and concrete mitigations rather than a vague "should be fine" (step 10).
 - **Docs/skills to update** — anywhere this needs to be reflected outside the code itself (external docs, paired skill files, template files new authors copy from).
 - **Tests** — organized by mechanism, each case naming the real function/query it exercises and the concrete assertion (step 7), plus a manual verification checklist for anything that can't be unit-tested (including an `EXPLAIN QUERY PLAN` check at realistic scale if the Performance impact section found a genuine risk).
 - **(Optional) Execution plan** — phased, referencing the same file/function names used above rather than restating the design in vaguer terms.