diff --git a/docs/engineering/ratelimit-observability.md b/docs/engineering/ratelimit-observability.md
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+# Rate-limit observability runbook
+
+> Companion to issue #64 ("RATE_LIMIT default re-tune analysis"). After
+> #60 deployed the per-tenant `keyFor` keying, the right RATE_LIMIT
+> default became data-dependent. This runbook documents the metrics +
+> queries an operator should run to confirm whether the current 600
+> req/min/key default is correct, too tight, or too loose.
+
+## What's already exposed
+
+The workspace-server's existing Prometheus middleware
+(`workspace-server/internal/metrics/metrics.go`) tracks every request
+on every path:
+
+```
+molecule_http_requests_total{method, path, status}      counter
+molecule_http_request_duration_seconds_total{method,path,status}  counter
+```
+
+Path is the matched route pattern (`/workspaces/:id/activity` etc), so
+high-cardinality workspace UUIDs do not explode the label space.
+
+The rate limiter middleware (#60, `workspace-server/internal/middleware/ratelimit.go`)
+also stamps every response with `X-RateLimit-Limit`, `X-RateLimit-Remaining`,
+and `X-RateLimit-Reset`. Operators with browser-side or proxy-side
+header capture can read per-request bucket state directly.
+
+No new instrumentation is needed for #64's acceptance criteria. The
+metric surface is sufficient — this runbook just collects the queries.
+
+## Queries to run after #60 deploys
+
+### 1. Is the bucket actually firing 429s?
+
+```promql
+sum(rate(molecule_http_requests_total{status="429"}[5m]))
+```
+
+If this is zero on a given tenant, the bucket isn't being hit. If it's
+sustained > 1/min, dig in.
+
+### 2. Which routes attract 429s?
+
+```promql
+topk(
+  10,
+  sum by (path) (
+    rate(molecule_http_requests_total{status="429"}[5m])
+  )
+)
+```
+
+Expected shape post-#60:
+- `/workspaces/:id/activity` should be near zero — the canvas no longer
+  polls it on a 30s/60s/5s cadence (PRs #69 / #71 / #76).
+- Probe / health / heartbeat paths should be ~0 (those routes have a
+  separate IP-fallback bucket).
+
+If `/workspaces/:id/activity` 429s persist post-PRs-69/71/76 deploy, the
+canvas isn't running the WS-subscriber path — investigate WS health
+on that tenant.
+
+### 3. Per-bucket-key inference (no direct exposure today)
+
+The bucket map itself is in-memory only; we deliberately do **not**
+expose `org:<uuid>` ↔ remaining-tokens because that map can include
+SHA-256 hashes of bearer tokens. A tenant that wants per-key visibility
+should rely on response headers (`X-RateLimit-Remaining` on every
+response from a given session is the bucket's view of that session).
+
+If you genuinely need server-side per-bucket counts for triage,
+file a follow-up — the proper shape is a `/internal/ratelimit-stats`
+endpoint that emits **counts per key prefix only** (e.g. `org:`, `tok:`,
+`ip:`), never the key payloads. Don't roll that ad-hoc; it's a security
+review surface.
+
+## Decision tree for the re-tune
+
+After 14 days of production traffic on a tenant, look at the queries
+above and walk this tree:
+
+```
+Q1: Is the 429 rate sustained > 0.1/sec on any tenant?
+  ├─ NO  → The 600 default has comfortable headroom. Either keep it,
+  │        or lower it carefully (300) ONLY if you have a documented
+  │        reason (e.g. a misbehaving client we want to throttle harder).
+  │        Default to "no change" — see #64 for the math.
+  └─ YES → Q2.
+
+Q2: Is the 429 rate concentrated on ONE tenant or spread across many?
+  ├─ ONE tenant → Operator override: set RATE_LIMIT=1200 or 1800 on that
+  │               tenant's box. Document in the tenant's ops note. The
+  │               default does not need to change.
+  └─ MANY tenants → Q3.
+
+Q3: Are the 429s on a route that polls (e.g. /activity / /peers)?
+  ├─ YES → Confirm PRs #69, #71, #76 have actually deployed to those
+  │         tenants. If they have and 429s persist, the canvas may have
+  │         a regression — do not raise RATE_LIMIT. File a canvas issue.
+  └─ NO  → 429s on mutating routes mean genuine load. Raise the default
+            to 1200 in `workspace-server/internal/router/router.go:54`.
+            Same PR should attach: the metric chart, the time window,
+            and a paragraph explaining what changed in our traffic shape.
+```
+
+## Alert rule template (drop-in for Prometheus)
+
+```yaml
+# Sustained 429s — file is the SLO trip-wire. If this fires, walk the
+# decision tree above. NB: the issue#64 acceptance criterion is "two
+# weeks of metrics"; this alert is the inverse — it tells you something
+# changed before the two weeks are up.
+groups:
+  - name: workspace-server-ratelimit
+    rules:
+      - alert: WorkspaceServerRateLimit429Sustained
+        expr: |
+          sum by (instance) (
+            rate(molecule_http_requests_total{status="429"}[10m])
+          ) > 0.1
+        for: 30m
+        labels:
+          severity: warning
+          owner: workspace-server
+        annotations:
+          summary: "{{ $labels.instance }} sustained 429s — see ratelimit-observability runbook"
+          runbook: "https://git.moleculesai.app/molecule-ai/molecule-core/blob/main/docs/engineering/ratelimit-observability.md"
+```
+
+Threshold rationale: 0.1 req/s = 6/min sustained over 10min. Below
+that, a 429 is almost certainly a transient burst that the canvas's
+retry-once handler at `canvas/src/lib/api.ts:55` already absorbs. The
+30m `for:` keeps the alert from chattering on a brief blip.
+
+## Companion probe script
+
+For one-off triage when an operator can reproduce the problem in their
+own browser, `scripts/edge-429-probe.sh` (#62) reproduces a canvas-
+sized burst against a tenant subdomain and dumps each 429's response
+shape so the operator can distinguish workspace-server bucket overflow
+from CF/Vercel edge rate-limiting without dashboard access.
+
+```sh
+./scripts/edge-429-probe.sh hongming.moleculesai.app --burst 80 --out /tmp/edge.txt
+```
+
+The script's report header explains how to read the output.
diff --git a/scripts/edge-429-probe.sh b/scripts/edge-429-probe.sh
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+#!/usr/bin/env bash
+# edge-429-probe.sh — capture 429 origin (workspace-server vs CF/Vercel edge)
+# during a simulated canvas-burst against a tenant subdomain.
+#
+# Issue molecule-core#62. The post-#60 verification step asks an
+# operator with CF/Vercel dashboard access to confirm whether the
+# layout-chunk 429s observed in DevTools were:
+#   (a) workspace-server bucket overflow (closes once #60 deploys), or
+#   (b) actual edge-layer rate-limiting (CF or Vercel).
+#
+# This script doesn't need dashboard access. It reproduces the burst
+# pattern locally and dumps every 429's response shape so the operator
+# can distinguish (a) from (b) by inspection: workspace-server emits a
+# JSON body, CF emits HTML, Vercel emits a different HTML. Headers tell
+# the same story (cf-ray vs x-vercel-*).
+#
+# Usage:
+#   ./scripts/edge-429-probe.sh <tenant-host> [--burst N] [--waves N] [--pause SECS] [--out file]
+#
+# Example:
+#   ./scripts/edge-429-probe.sh hongming.moleculesai.app --burst 80 --out /tmp/edge.txt
+#
+# The script is read-only against the target — it only issues GETs to
+# public-by-design endpoints. No mutating requests, no credential use.
+
+set -euo pipefail
+
+# ── Help / usage handling first, before positional capture ────────────────────
+case "${1:-}" in
+  -h|--help|"")
+    sed -n '/^# edge-429-probe.sh/,/^$/p' "$0" | sed 's/^# \{0,1\}//'
+    exit 0
+    ;;
+esac
+
+HOST="$1"; shift
+BURST=80
+WAVES=3
+WAVE_PAUSE=2
+OUT=""
+
+while [ "${1:-}" != "" ]; do
+  case "$1" in
+    --burst) BURST="$2"; shift 2 ;;
+    --waves) WAVES="$2"; shift 2 ;;
+    --pause) WAVE_PAUSE="$2"; shift 2 ;;
+    --out)   OUT="$2";   shift 2 ;;
+    -h|--help)
+      sed -n '/^# edge-429-probe.sh/,/^$/p' "$0" | sed 's/^# \{0,1\}//'
+      exit 0
+      ;;
+    *) echo "unknown arg: $1" >&2; exit 2 ;;
+  esac
+done
+
+# ── Endpoint discovery ────────────────────────────────────────────────────────
+echo "→ Discovering a layout-chunk URL from canvas root..." >&2
+ROOT_BODY=$(curl -fsSL --max-time 10 "https://${HOST}/" 2>/dev/null || true)
+LAYOUT_PATH=$(echo "$ROOT_BODY" \
+  | grep -oE '/_next/static/chunks/layout-[A-Za-z0-9_-]+\.js' \
+  | head -1 || true)
+if [ -z "$LAYOUT_PATH" ]; then
+  LAYOUT_PATH="/_next/static/chunks/layout-probe-not-found.js"
+  echo "  (no layout chunk discovered — using sentinel path; 404 on this is expected)" >&2
+else
+  echo "  layout chunk: $LAYOUT_PATH" >&2
+fi
+
+# Probe URL: a generic activity endpoint. The rate-limiter middleware
+# runs BEFORE workspace-id validation, so unauth/invalid-id requests
+# still hit the bucket.
+ACTIVITY_PATH="/workspaces/00000000-0000-0000-0000-000000000000/activity?probe=edge-429"
+
+# ── Fire one curl, write a single-line JSON-ish status record to stdout ──────
+# Inlined into xargs as a heredoc-style command rather than a function so
+# the function-export pitfalls (some shells lose `export -f` across xargs)
+# don't apply. Each output line is a parseable record; failed curls emit
+# a curl_err record so request volume is preserved.
+TMP_RESULTS="$(mktemp -t edge-429-probe.XXXXXX)"
+trap 'rm -f "$TMP_RESULTS"' EXIT
+
+run_burst() {
+  # $1 = path; $2 = label; $3 = wave_id
+  local path="$1" label="$2" wave="$3"
+  local i
+  for i in $(seq 1 "$BURST"); do
+    {
+      out=$(curl -sS --max-time 10 -o /dev/null \
+        -w 'status=%{http_code} size=%{size_download} time=%{time_total} server=%{header.server} cf_ray=%{header.cf-ray} x_vercel=%{header.x-vercel-id} retry_after=%{header.retry-after} content_type=%{header.content-type} x_ratelimit_limit=%{header.x-ratelimit-limit} x_ratelimit_remaining=%{header.x-ratelimit-remaining} x_ratelimit_reset=%{header.x-ratelimit-reset}\n' \
+        "https://${HOST}${path}" 2>/dev/null) || out="status=curl_err"
+      printf 'label=%s-%s-%s %s\n' "$label" "$wave" "$i" "$out" >> "$TMP_RESULTS"
+    } &
+  done
+  wait
+}
+
+emit() {
+  if [ -n "$OUT" ]; then
+    printf '%s\n' "$*" >> "$OUT"
+  else
+    printf '%s\n' "$*"
+  fi
+}
+
+if [ -n "$OUT" ]; then : > "$OUT"; fi
+
+emit "# edge-429-probe report"
+emit "# host=$HOST burst=$BURST waves=$WAVES pause=${WAVE_PAUSE}s"
+emit "# layout_path=$LAYOUT_PATH"
+emit "# activity_path=$ACTIVITY_PATH"
+emit "# generated=$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+emit ""
+
+for wave in $(seq 1 "$WAVES"); do
+  emit "## wave $wave"
+  : > "$TMP_RESULTS"
+  run_burst "$LAYOUT_PATH" "layout" "$wave"
+  run_burst "$ACTIVITY_PATH" "activity" "$wave"
+  while read -r line; do
+    emit "  $line"
+  done < "$TMP_RESULTS"
+  if [ "$wave" -lt "$WAVES" ]; then
+    sleep "$WAVE_PAUSE"
+  fi
+done
+
+emit ""
+emit "## summary — how to read the report"
+emit "#   status=429 + content_type starts with application/json + x_ratelimit_limit set"
+emit "#     => workspace-server bucket overflow. Closes when #60 deploys."
+emit "#   status=429 + cf_ray set + content_type=text/html"
+emit "#     => Cloudflare WAF / rate-limit. Audit dashboard rules per #62."
+emit "#   status=429 + x_vercel set + content_type=text/html"
+emit "#     => Vercel edge / Bot Fight Mode. Audit Vercel project per #62."
+emit "#   status=429 with no server/cf_ray/x_vercel"
+emit "#     => corporate proxy or VPN. Not actionable in this repo."
+
+if [ -n "$OUT" ]; then
+  echo "→ Report written to $OUT" >&2
+  # Match only data lines (begin with two-space indent + "label="),
+  # not the summary's reference text which also mentions "status=429".
+  # grep -c outputs "0" + exits 1 when zero matches; `|| true` masks
+  # the exit status so set -e doesn't trip without losing the count.
+  total=$(grep -c '^  label=' "$OUT" 2>/dev/null || true)
+  total429=$(grep -c '^  label=.*status=429' "$OUT" 2>/dev/null || true)
+  total=${total:-0}
+  total429=${total429:-0}
+  echo "→ Totals: ${total429} of ${total} requests returned 429" >&2
+  if [ "${total429}" -gt 0 ]; then
+    echo "→ Per-label 429 counts:" >&2
+    grep '^  label=.*status=429' "$OUT" \
+      | sed -E 's/^  label=([^-]+).*/  \1/' \
+      | sort | uniq -c >&2
+  fi
+fi