diff --git a/docs/content-candidate-pool.md b/docs/content-candidate-pool.md
index bd4b57d..bb74a07 100644
--- a/docs/content-candidate-pool.md
+++ b/docs/content-candidate-pool.md
@@ -27,16 +27,27 @@ stays non‑commercial**.
 
 | id | clip | source | window (s) | license / credit |
 |----|------|--------|-----------|------------------|
-| `cosmos` | Orion Nebula flythrough | NASA/JPL‑Caltech — https://images.nasa.gov/details/JPL-20221122-SOLSYSf-0001-Orion%20Dust%20and%20Death | ~30–54 | PD (17 U.S.C. §105) |
+| `cosmos` | Cosmic Cliffs — Carina Nebula flythrough | NASA SVS 31348 — https://svs.gsfc.nasa.gov/31348/ (`Clifs-3d-STScI.mp4`, *Exploring the Cosmic Cliffs in 3D*, Webb NIRCam, **4K**) | 10–34 | CC‑BY‑class — credit **NASA, ESA, CSA, STScI** |
+
+> ℹ️ **Cosmos primary swapped to the Webb "Cosmic Cliffs" flythrough (2026-06-25, session 0018).**
+> History: the original primary was NASA/JPL's *"Orion: Dust and Death"* (captions
+> burned in → cropping lost framing); it was then swapped to STScI's text-free
+> *Flight Through the Orion Nebula* (SVS 30957, visible light). The operator found
+> the Orion clip merely good and asked for a more dramatic nebula, so the primary is
+> now STScI's **Exploring the Cosmic Cliffs in 3D** (Carina Nebula, Webb NIRCam,
+> **SVS 31348**) — a full-frame, text-free 3D flythrough sourced from its **4K**
+> master (`Clifs-3d-STScI.mp4`, 3840×2160) → supersampled 1080p base + 4K master.
+> Trim 10–34s clears the opening title card (gone by ~8s) and the end-credit card
+> (~102s). The earlier `decaption_cosmos` crop helper remains removed.
 | `cosmos_galaxies` | Flying Through Galaxies | NASA SVS — https://svs.gsfc.nasa.gov/vis/a010000/a014900/a014950/14950_Galaxies_FlyThrough_4k.mp4 | 8–32 | PD |
 | `cosmos_hudf` | Hubble Ultra Deep Field zoom | NASA SVS — https://svs.gsfc.nasa.gov/vis/a030000/a030600/a030687/hudf-b-1920x1080p30.mov | 20–44 | CC‑BY‑class — credit **NASA, ESA, STScI** |
 | `cosmos_xdf` | eXtreme Deep Field flythrough | NASA SVS — https://svs.gsfc.nasa.gov/vis/a030000/a030600/a030681/hxdf_fly-b-1920x1080p30.mov | 2–26 | CC‑BY‑class — credit **NASA, ESA, STScI** |
 
 > ⚠️ **DISCREPANCY to fix next session.** Operator selected cosmos **#1, #3
 > (Orion), #4, #5**. During processing, **#6 Galaxy Traverse was run by mistake
-> instead of #3 Orion**. The Orion clip (#3) is text‑free and already lives at
-> `simulator/sample_media/cosmos/base.mp4` (the prior real base), so the pool above
-> is correct. **`cosmos_traverse` is an UNSELECTED extra on disk — drop it** (or
+> instead of #3 Orion**. The cosmos primary now lives at
+> `simulator/sample_media/cosmos/base.mp4` as the Webb Cosmic Cliffs flythrough
+> (SVS 31348 — see the cosmos-primary note above). **`cosmos_traverse` is an UNSELECTED extra on disk — drop it** (or
 > confirm with operator). The current ring scale id is `cosmos`; when the pool is
 > wired, `cosmos` becomes one pool member.
 
@@ -108,5 +119,5 @@ Removed from local media (all were gitignored, so repo-invisible):
 - `forest` — the old Yosemite base, superseded by the `coast` pool. **dropped.**
 - `reef` — the original placeholder/early reef base, superseded by `reef_*`. **dropped.**
 - `orbit` / `abyss` — the Increment‑1 single real bases (NASA ISS Exp65 / ctenophore),
-  not in the new pools. **dropped** (the cosmos Orion base stays as the cosmos pool primary).
+  not in the new pools. **dropped** (the cosmos base — now the Webb Cosmic Cliffs flythrough, SVS 31348 — stays as the cosmos pool primary).
 - `review.html` gallery — now `.gitignore`d (local‑only, re‑buildable).
diff --git a/docs/superpowers/specs/2026-06-25-simulator-dev-mode-design.md b/docs/superpowers/specs/2026-06-25-simulator-dev-mode-design.md
new file mode 100644
index 0000000..3bafe46
--- /dev/null
+++ b/docs/superpowers/specs/2026-06-25-simulator-dev-mode-design.md
@@ -0,0 +1,64 @@
+# Simulator Dev Mode — design
+
+> 2026-06-25 · session 0017 · planning-and-executing
+> Anchor: leaf task (simulator dev-tooling — R2b, no upstream design needed)
+
+## Problem
+
+While previewing the experience, the operator can't choose *which* clip in an
+altitude's rotating pool is shown — the server picks a random pool member on each
+landing (content-pipeline §11.1). For by-eye review they want to step through every
+clip in a pool on demand, and to see the engine/clip data behind what's on screen.
+The RenderPlan JSON readout is also always visible, cluttering what should read as
+an audience-facing preview.
+
+## Goal
+
+A single **Dev Mode** toggle at the bottom of the control panel. Off by default
+(persisted in `localStorage`). When on, a panel appears directly below it with all
+dev controls and data; when off, the normal view is just the experience + the three
+control fieldsets.
+
+## Scope
+
+Frontend only — `simulator/static/{index.html,app.js,style.css}`. No API changes:
+every datum Dev Mode shows is already available client-side (`/api/clips`,
+`/api/ring`, the `/api/alteration` response, and the in-memory preload cache). The
+separate `/author.html` remains the place to *edit* labels; Dev Mode is
+inspect-and-select only.
+
+## Dev panel contents (top → bottom)
+
+1. **Pool picker** — a `<select>` of the current altitude's pool members
+   (`clip_id · title`), its value reflecting the clip currently playing. Choosing a
+   member sets `activeClipId` to it and reloads the base media, overriding the random
+   landing pick. A `🎲 Re-roll random` button re-runs the server-canonical random
+   pick for the current scale. The dropdown is rebuilt on every scale landing, so it
+   always lists the current pool; single-clip scales (cosmos/orbit) show one entry.
+2. **Active clip** — `id`, `title`, `source`, `license`, `base_file`.
+3. **Annotations & affect** — each annotation key with salience (or legacy
+   `min_level`), time window (`appear`–`disappear`), and tier count; each affect key
+   with `min_level` and tier count. Read-only.
+4. **RenderPlan** — the relocated `#readout` (the `/api/alteration` JSON).
+5. **Cache & playback** — preload status (`N/total cached`), whether the active
+   clip is served **from memory** (a preloaded blob) vs the network, and live video
+   resolution / duration / loop position.
+
+## Behavior
+
+- **Pool override is per-landing.** Changing altitude re-rolls randomly as today;
+  the operator then picks again from the new scale's pool. The pick is not persisted
+  across landings (YAGNI — "select any video while on an altitude").
+- **Refresh points.** Pool picker + clip metadata + annotations rebuild whenever the
+  scale lands (hooked into `renderScaleReadout()`) and after a manual pick/re-roll.
+  The RenderPlan readout updates on each `update()` (knob change) as today. The
+  cache/playback stats refresh on a light interval (~500 ms) while Dev Mode is on,
+  and the cache count also ticks up as the preloader fills.
+- **`pickRandomMember()`** is factored out of `landScale()` so the initial landing,
+  ring advances, and the Re-roll button all share the one server-canonical pick.
+
+## Testing
+
+No JS unit harness in the repo and no API change, so verification is: `node --check`
+on `app.js`, the Python suite stays green (unaffected), and by-eye review by the
+operator (no headless browser yet — §9 E2E policy noted, machinery pending).
diff --git a/simulator/app.py b/simulator/app.py
index 0266abb..7ebe72d 100644
--- a/simulator/app.py
+++ b/simulator/app.py
@@ -237,13 +237,15 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
     async def _cache_policy(request, call_next):
         # Dev preview server: never let a browser serve a stale app.js/style.css —
         # by-eye iteration needs a plain refresh to always get the latest assets.
-        # Media is the exception: the clips are static and large, so they must be
-        # browser-cacheable. The renderer also preloads them into in-memory blob
-        # URLs (static/app.js), but a cacheable header keeps the preload fetch and
-        # any fallback off the network on repeat loads — instant altitude swaps.
+        # Media uses `no-cache` (store, but REVALIDATE every load): instant altitude
+        # swaps already come from the in-memory blob preload (static/app.js), so the
+        # HTTP cache only matters on (re)load — where a conditional request returns a
+        # cheap 304 when unchanged, yet picks up a re-sourced/re-cropped clip
+        # immediately. `immutable` was wrong here: it pinned stale footage for a year
+        # so edited clips never showed without a manual cache clear.
         response = await call_next(request)
         if request.url.path.startswith("/media/"):
-            response.headers["Cache-Control"] = "public, max-age=31536000, immutable"
+            response.headers["Cache-Control"] = "no-cache"
         else:
             response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
         return response
diff --git a/simulator/build_pool_manifest.py b/simulator/build_pool_manifest.py
index d234f8c..f499dd0 100644
--- a/simulator/build_pool_manifest.py
+++ b/simulator/build_pool_manifest.py
@@ -51,12 +51,14 @@ RING_ORDER = ["cosmos", "orbit", "coast", "reef", "abyss"]
 PD = "public-domain (US Gov, 17 U.S.C. §105)"
 PD_NPS = "public-domain (NPS, no © — 17 U.S.C. §105)"
 CCBY_STSCI = "CC-BY-class — credit NASA, ESA, STScI"
+CCBY_WEBB = "CC-BY-class — credit NASA, ESA, CSA, STScI"
 
 # --- Per-clip provenance: id -> (title, license, source) ---
 META: dict[str, tuple[str, str, str]] = {
     # cosmos
-    "cosmos": ("Orion Nebula flythrough (NASA/JPL-Caltech)", PD,
-               "NASA/JPL-Caltech — images.nasa.gov JPL-20221122-SOLSYSf-0001 (Orion); trim ~30–54s, crossfade-loop"),
+    "cosmos": ("Cosmic Cliffs — Carina Nebula flythrough (NASA/ESA/CSA/STScI)", CCBY_WEBB,
+               "NASA SVS 31348 Clifs-3d-STScI (Exploring the Cosmic Cliffs in 3D, Webb NIRCam); "
+               "4K source, trim 10–34s, crossfade-loop. Text-free — replaces the Orion SVS 30957 flythrough."),
     "cosmos_galaxies": ("Flying Through Galaxies (NASA SVS)", PD,
                         "NASA SVS a014950 14950_Galaxies_FlyThrough_4k; trim 8–32s, crossfade-loop"),
     "cosmos_hudf": ("Hubble Ultra Deep Field zoom (NASA/ESA/STScI)", CCBY_STSCI,
@@ -161,9 +163,9 @@ def measure(key, value, box, min_level):
 LABELS: dict[str, list[dict]] = {
     # ---------- cosmos ----------
     "cosmos": [
-        static_label("detected.nebula", 4, ["cloud", "nebula", "emission nebula", "emission nebula · ionized H II region"], [0.32, 0.28, 0.34, 0.36]),
-        static_label("detected.star", 2, ["star", "young star", "protostar", "protostar · <1 Myr old"], [0.60, 0.30, 0.10, 0.12]),
-        measure("measure.redshift", "z ≈ 0.0", [0.36, 0.70, 0.18, 0.08], 4),
+        static_label("detected.nebula", 4, ["cloud", "nebula", "emission nebula", "emission nebula · the Carina star-forming region"], [0.20, 0.42, 0.5, 0.42]),
+        static_label("detected.star", 2, ["star", "young star", "protostar", "protostar · a newborn star, <1 Myr old"], [0.54, 0.12, 0.12, 0.14]),
+        measure("measure.distance", "≈7,500 ly", [0.06, 0.06, 0.2, 0.08], 3),
     ],
     "cosmos_galaxies": [
         static_label("detected.galaxy", 4, ["galaxy", "spiral galaxy", "barred spiral", "barred spiral · ~10¹¹ stars"], [0.34, 0.30, 0.30, 0.34]),
@@ -355,7 +357,7 @@ def build_manifest() -> dict:
     return {"clips": clips, "ring": {"scales": scales, "transitions": transitions}}
 
 
-# --- Optional: generate the NEW transition placeholders (orbit-coast, coast-reef) ---
+# --- Generate the per-edge transition clips (zoom morph + reversed companion) ---
 
 def _ffmpeg() -> str:
     if shutil.which("ffmpeg"):
@@ -365,8 +367,10 @@ def _ffmpeg() -> str:
 
 
 def _make_transition(ff: str, scale_a: str, scale_b: str) -> Path:
-    """A placeholder zoom/warp morph between two scales, from their PRIMARY pool
-    members' bases (mirrors setup_scales_media.py but keyed by scale->primary)."""
+    """A zoom/warp morph between two scales, from their PRIMARY pool members'
+    bases (mirrors setup_scales_media.py but keyed by scale->primary). This is the
+    recipe behind the well-liked orbit-coast edge; generate_media() now applies it
+    uniformly to every edge so none carry stale footage from an earlier ring."""
     a = MEDIA / POOLS[scale_a][0] / "base.mp4"
     b = MEDIA / POOLS[scale_b][0] / "base.mp4"
     out = MEDIA / "transitions" / f"{scale_a}-{scale_b}.mp4"
@@ -381,13 +385,29 @@ def _make_transition(ff: str, scale_a: str, scale_b: str) -> Path:
     return out
 
 
+def _make_reverse(ff: str, forward: Path) -> Path:
+    """The zoom-OUT companion of a forward (zoom-in) edge: the same clip played
+    backward, written alongside it as `<edge>.rev.mp4`. An ascending ring move
+    crosses its edge `reversed`; the renderer then plays this file, so zooming out
+    recedes from the current scale back to the higher one instead of zooming in."""
+    out = forward.with_suffix(".rev.mp4")
+    subprocess.run([
+        ff, "-y", "-i", str(forward), "-vf", "reverse", "-an", str(out),
+    ], check=True, capture_output=True)
+    return out
+
+
 def generate_media() -> None:
-    """Generate the new ring edges introduced by the coast scale; the cosmos-orbit,
-    reef-abyss and abyss-cosmos edges already exist from the prior 5-scale ring."""
+    """(Re)build EVERY ring-edge transition from the current real primary bases —
+    the orbit-coast recipe applied uniformly, overwriting any stale clips left from
+    an earlier ring — plus a reversed companion per edge for zoom-out moves."""
     ff = _ffmpeg()
-    for a, b in [("orbit", "coast"), ("coast", "reef")]:
-        _make_transition(ff, a, b)
-        print(f"generated transitions/{a}-{b}.mp4 (placeholder zoom)")
+    n = len(RING_ORDER)
+    for i in range(n):
+        a, b = RING_ORDER[i], RING_ORDER[(i + 1) % n]
+        fwd = _make_transition(ff, a, b)
+        _make_reverse(ff, fwd)
+        print(f"generated transitions/{a}-{b}.mp4 (+ .rev) from real bases")
 
 
 def main(argv: list[str]) -> None:
diff --git a/simulator/sample_media/manifest.json b/simulator/sample_media/manifest.json
index f660d0a..ca44349 100644
--- a/simulator/sample_media/manifest.json
+++ b/simulator/sample_media/manifest.json
@@ -2,41 +2,41 @@
   "clips": [
     {
       "id": "cosmos",
-      "title": "Orion Nebula flythrough (NASA/JPL-Caltech)",
+      "title": "Cosmic Cliffs — Carina Nebula flythrough (NASA/ESA/CSA/STScI)",
       "base_file": "cosmos/base.mp4",
-      "license": "public-domain (US Gov, 17 U.S.C. §105)",
-      "source": "NASA/JPL-Caltech — images.nasa.gov JPL-20221122-SOLSYSf-0001 (Orion); trim ~30–54s, crossfade-loop",
+      "license": "CC-BY-class — credit NASA, ESA, CSA, STScI",
+      "source": "NASA SVS 31348 Clifs-3d-STScI (Exploring the Cosmic Cliffs in 3D, Webb NIRCam); 4K source, trim 10–34s, crossfade-loop. Text-free — replaces the Orion SVS 30957 flythrough.",
       "right_variants": {},
       "annotations": [
         {
           "key": "detected.nebula",
           "salience": 4,
           "box": [
-            0.32,
-            0.28,
-            0.34,
-            0.36
+            0.2,
+            0.42,
+            0.5,
+            0.42
           ]
         },
         {
           "key": "detected.star",
           "salience": 2,
           "box": [
-            0.6,
-            0.3,
-            0.1,
-            0.12
+            0.54,
+            0.12,
+            0.12,
+            0.14
           ]
         },
         {
-          "key": "measure.redshift",
+          "key": "measure.distance",
           "box": [
-            0.36,
-            0.7,
-            0.18,
+            0.06,
+            0.06,
+            0.2,
             0.08
           ],
-          "min_level": 4
+          "min_level": 3
         }
       ],
       "affect": [
@@ -79,15 +79,15 @@
             "cloud",
             "nebula",
             "emission nebula",
-            "emission nebula · ionized H II region"
+            "emission nebula · the Carina star-forming region"
           ],
           "detected.star": [
             "star",
             "young star",
             "protostar",
-            "protostar · <1 Myr old"
+            "protostar · a newborn star, <1 Myr old"
           ],
-          "measure.redshift": "z ≈ 0.0",
+          "measure.distance": "≈7,500 ly",
           "feel.wonder": [
             "wow",
             "wonder",
diff --git a/simulator/static/app.js b/simulator/static/app.js
index 51a6082..86b0aff 100644
--- a/simulator/static/app.js
+++ b/simulator/static/app.js
@@ -52,22 +52,28 @@ async function loadData() {
   ringIndex = 0;
 }
 
-// Land on the current scale: pick a random pool member (content-pipeline §11.1).
-// A real server ring picks via a delta=0 advance (Python-canonical pick); the
-// synthesized fallback ring has a pool of one, so it just takes that member.
-async function landScale() {
+// Server-canonical random pick of a pool member for the current scale: a delta=0
+// advance (content-pipeline §11.1, Python owns the randomness). The synthesized
+// fallback ring has a pool of one, so it just returns that member. Returns a
+// clip_id or null. Shared by the initial landing AND the Dev Mode re-roll button.
+async function pickRandomMember() {
   const scale = ring && ring.scales[ringIndex];
-  if (!scale) { activeClipId = null; return; }
+  if (!scale) return null;
   if (serverRing) {
     try {
       const resp = await fetch("/api/ring/advance", {
         method: "POST", headers: { "content-type": "application/json" },
         body: JSON.stringify({ from_index: ringIndex, delta: 0 }),
       });
-      if (resp.ok) { activeClipId = (await resp.json()).target_clip_id; currentClipId = null; return; }
+      if (resp.ok) return (await resp.json()).target_clip_id;
     } catch (_) { /* fall through to the primary member */ }
   }
-  activeClipId = scale.clip_id;
+  return scale.clip_id;
+}
+
+// Land on the current scale: pick a random pool member and force its media to load.
+async function landScale() {
+  activeClipId = await pickRandomMember();
   currentClipId = null;
 }
 
@@ -87,7 +93,11 @@ function mediaUrl(file) { return mediaBlobs[file] || ("/media/" + file); }
 function preloadList() {
   const files = new Set();
   for (const c of Object.values(clipsById)) if (c && c.base_file) files.add(c.base_file);
-  if (ring && ring.transitions) for (const t of ring.transitions) if (t && t.file) files.add(t.file);
+  // Each ring edge ships both directions: the forward (zoom-in) clip and its baked
+  // reverse (zoom-out) companion — cache both so up and down moves are instant.
+  if (ring && ring.transitions) for (const t of ring.transitions) {
+    if (t && t.file) { files.add(t.file); files.add(reverseFile(t.file)); }
+  }
   return [...files];
 }
 
@@ -104,7 +114,7 @@ function preloadOrder() {
       else if (s) add((clipsById[s.clip_id] || {}).base_file);
     }
   }
-  if (ring.transitions) for (const t of ring.transitions) add(t.file);
+  if (ring.transitions) for (const t of ring.transitions) { add(t.file); add(reverseFile(t.file)); }
   for (const f of preloadList()) add(f);   // sweep up anything not on the ring
   return ordered;
 }
@@ -462,6 +472,7 @@ function renderScaleReadout() {
   const poolTag = poolN > 1 ? ` · pool ${poolN}` : "";
   $("scale-name").textContent = `${s.id} · ${member}  (${ringIndex + 1}/${ring.scales.length}${poolTag})`;
   renderDial();
+  refreshDevClip();      // keep the Dev Mode pool picker + clip data in sync with the landing
 }
 
 function controls() {
@@ -508,11 +519,17 @@ function debounced() { clearTimeout(timer); timer = setTimeout(update, 80); }
 
 const FAST_BLEND_RATE = 2.5;   // playback speed for a collapsed fast-spin pass
 
-function playTransition(file, blended) {
+// The zoom-OUT companion of an edge clip: the baked `<edge>.rev.mp4` reverse. A
+// `reversed` step (an ascending / zoom-out ring move) plays this instead of the
+// forward zoom-in clip, so going up recedes from the current scale.
+function reverseFile(file) { return file.replace(/\.mp4$/, ".rev.mp4"); }
+
+function playTransition(file, blended, reversed) {
   // Play one zoom/warp transition clip start-to-finish, with the alteration
   // layers hidden. A `blended` (fast-spin) pass runs at FAST_BLEND_RATE so a
   // quick encoder spin lands fast instead of grinding through every morph.
-  // Resolves on 'ended' (or a safety timeout that scales with playback rate).
+  // Zoom-out moves (`reversed`) play the baked reverse clip. Resolves on 'ended'
+  // (or a safety timeout that scales with playback rate).
   return new Promise((resolve) => {
     overlay.style.opacity = "0";
     affectLayer.style.opacity = "0";
@@ -520,7 +537,7 @@ function playTransition(file, blended) {
     vid.style.filter = "none";
     vid.loop = false;
     vid.style.opacity = "1";
-    vid.src = mediaUrl(file);
+    vid.src = mediaUrl(reversed ? reverseFile(file) : file);
     vid.playbackRate = blended ? FAST_BLEND_RATE : 1;
     let done = false;
     const finish = () => {
@@ -549,7 +566,7 @@ async function advance(delta) {
     // A fast spin comes back collapsed to a single blended step (move.fast);
     // a slow spin chains one full transition per scale crossed.
     for (const step of move.steps) {
-      await playTransition(step.file, step.blended);
+      await playTransition(step.file, step.blended, step.reversed);
     }
     ringIndex = move.to_index;
     activeClipId = move.target_clip_id;   // the randomly-picked pool member to load
@@ -685,6 +702,136 @@ function jumpToScale(idx) {
   if (d) advance(d);
 }
 
+// --- Dev Mode: pool picker + live analysis, all under one toggle ---
+// Off by default, state persisted in localStorage. Everything here is read from
+// data the renderer already has (clips, ring, the alteration response, the preload
+// cache) — no server endpoints. Editing labels stays in /author.html.
+const DEV_KEY = "hef.devMode";
+let devMode = false;
+let devStatsTimer = null;
+
+const escapeHtml = (s) => String(s).replace(/[&<>"]/g,
+  (c) => ({ "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;" }[c]));
+
+function applyDevVisibility() {
+  const panel = $("dev-panel");
+  if (panel) panel.classList.toggle("hidden", !devMode);
+  if (devMode) { refreshDevClip(); renderDevStats(); }
+}
+
+function initDev() {
+  const toggle = $("dev-mode");
+  if (!toggle) return;
+  try { devMode = localStorage.getItem(DEV_KEY) === "1"; } catch (_) { devMode = false; }
+  toggle.checked = devMode;
+  toggle.addEventListener("change", () => {
+    devMode = toggle.checked;
+    try { localStorage.setItem(DEV_KEY, devMode ? "1" : "0"); } catch (_) {}
+    applyDevVisibility();
+  });
+  $("pool-select").addEventListener("change", onPoolSelect);
+  $("pool-reroll").addEventListener("click", reroll);
+  // Cache + playback stats are live-ish; cheap to repaint twice a second when shown.
+  devStatsTimer = setInterval(() => { if (devMode) renderDevStats(); }, 500);
+  applyDevVisibility();
+}
+
+// Rebuild the pool picker for the current scale and repaint the clip data. Cheap
+// (pools are <= a handful), so just rebuild on every landing/pick.
+function refreshDevClip() {
+  if (!devMode) return;
+  const sel = $("pool-select");
+  const scale = ring && ring.scales[ringIndex];
+  if (sel && scale) {
+    const pool = (scale.pool && scale.pool.length) ? scale.pool : [{ clip_id: scale.clip_id, title: scale.title }];
+    sel.innerHTML = "";
+    for (const m of pool) {
+      const o = document.createElement("option");
+      o.value = m.clip_id;
+      o.textContent = `${m.clip_id} · ${m.title || (clipsById[m.clip_id] || {}).title || ""}`;
+      sel.appendChild(o);
+    }
+    if (activeClipId) sel.value = activeClipId;
+    sel.disabled = pool.length < 2;
+  }
+  renderDevMeta();
+  renderDevAnno();
+}
+
+function renderDevMeta() {
+  const dl = $("clip-meta");
+  const c = activeClip();
+  if (!dl) return;
+  if (!c) { dl.innerHTML = "<dt>—</dt><dd>no clip</dd>"; return; }
+  const rows = [["id", c.id], ["title", c.title], ["source", c.source],
+               ["license", c.license], ["base_file", c.base_file]];
+  dl.innerHTML = rows.map(([k, v]) =>
+    `<dt>${escapeHtml(k)}</dt><dd>${escapeHtml(v != null ? v : "—")}</dd>`).join("");
+}
+
+function renderDevAnno() {
+  const box = $("clip-anno");
+  const c = activeClip();
+  if (!box) return;
+  const strings = (c && c.strings && c.strings.en) || {};
+  const win = (a) => (typeof a.appear === "number" || typeof a.disappear === "number")
+    ? ` win ${(a.appear ?? 0).toFixed(2)}–${(a.disappear ?? 1).toFixed(2)}` : "";
+  const rows = [];
+  for (const a of (c && c.annotations) || []) {
+    const measure = a.key.startsWith("measure.");
+    const sal = typeof a.salience === "number" ? `sal ${a.salience}`
+              : typeof a.min_level === "number" ? `min L${a.min_level}` : "sal 1";
+    const tiers = tierCount(strings[a.key]);
+    rows.push(`<div class="anno-row"><span class="anno-key ${measure ? "measure" : ""}">${escapeHtml(a.key)}</span>` +
+      `<span class="anno-meta"> · ${sal} · ${tiers} tier${tiers > 1 ? "s" : ""}${win(a)}</span></div>`);
+  }
+  for (const f of (c && c.affect) || []) {
+    const tiers = tierCount(strings[f.key]);
+    rows.push(`<div class="anno-row"><span class="anno-key affect">${escapeHtml(f.key)}</span>` +
+      `<span class="anno-meta"> · min L${f.min_level ?? 1} · ${tiers} tier${tiers > 1 ? "s" : ""}</span></div>`);
+  }
+  box.innerHTML = rows.length ? rows.join("") : `<div class="anno-empty">no annotations or affect</div>`;
+}
+
+function renderDevStats() {
+  const dl = $("dev-stats");
+  if (!dl) return;
+  const c = activeClip();
+  const total = preloadList().length;
+  const cached = Object.keys(mediaBlobs).length;
+  const fromMem = !!(c && mediaBlobs[c.base_file]);
+  const res = vid.videoWidth ? `${vid.videoWidth}×${vid.videoHeight}` : "—";
+  const dur = vid.duration ? `${vid.duration.toFixed(1)}s` : "—";
+  const loop = vid.duration ? `${Math.round(loopT() * 100)}%` : "—";
+  dl.innerHTML =
+    `<dt>cache</dt><dd>${cached}/${total} cached</dd>` +
+    `<dt>active source</dt><dd class="${fromMem ? "mem" : "net"}">${fromMem ? "memory (blob)" : "network"}</dd>` +
+    `<dt>resolution</dt><dd>${res}</dd>` +
+    `<dt>duration</dt><dd>${dur}</dd>` +
+    `<dt>loop pos</dt><dd>${loop}</dd>`;
+}
+
+// Pick a specific pool member (Dev Mode override of the random landing pick).
+function onPoolSelect() {
+  const id = $("pool-select").value;
+  if (!id || id === activeClipId) return;
+  activeClipId = id;
+  currentClipId = null;          // force the base media to reload
+  renderScaleReadout();          // refresh scale name + dev clip data
+  update();                      // reload media + plan
+}
+
+// Re-roll: a fresh server-canonical random pick for the current scale.
+async function reroll() {
+  if (busy) return;
+  const id = await pickRandomMember();
+  if (!id) return;
+  activeClipId = id;
+  currentClipId = null;
+  renderScaleReadout();
+  update();
+}
+
 // Dev live-reload: poll the asset version and reload when it changes, so an open
 // tab never keeps running a stale renderer while we iterate (the readout updates
 // from the live API and masks it otherwise). Reloads on my edits AND on a server
@@ -707,6 +854,7 @@ async function main() {
   await landScale();          // pick the initial scale's pool member before first render
   preloadAllMedia();          // background: cache every clip in memory for instant altitude swaps
   buildDial();                // draw the altitude knob from the ring's scales
+  initDev();                  // wire the Dev Mode toggle + pool picker (reads persisted state)
   renderScaleReadout();
   for (const id of ["content", "left", "right", "mood"]) {
     $(id).addEventListener("input", debounced);
diff --git a/simulator/static/index.html b/simulator/static/index.html
index 68e91ab..1417144 100644
--- a/simulator/static/index.html
+++ b/simulator/static/index.html
@@ -50,11 +50,40 @@
         <label>Mood — dark ◀ 0 ▶ light <input type="range" id="mood" min="-4" max="4" value="0" /></label>
       </fieldset>
 
+      <!-- Dev Mode: a single toggle; everything dev (pool picker + analysis) lives below it. -->
+      <label class="dev-switch" for="dev-mode">
+        <input type="checkbox" id="dev-mode" />
+        <span class="dev-switch-track"><span class="dev-switch-thumb"></span></span>
+        <span class="dev-switch-label">Dev Mode</span>
+      </label>
 
-      <fieldset>
-        <legend>RenderPlan readout</legend>
-        <pre id="readout">—</pre>
-      </fieldset>
+      <div id="dev-panel" class="dev-panel hidden">
+        <fieldset>
+          <legend>Pool — pick a clip</legend>
+          <select id="pool-select" aria-label="Choose a clip from this altitude's pool"></select>
+          <button type="button" id="pool-reroll" class="dev-btn">🎲 Re-roll random</button>
+        </fieldset>
+
+        <fieldset>
+          <legend>Active clip</legend>
+          <dl id="clip-meta" class="dev-dl"></dl>
+        </fieldset>
+
+        <fieldset>
+          <legend>Annotations &amp; affect</legend>
+          <div id="clip-anno" class="dev-anno"></div>
+        </fieldset>
+
+        <fieldset>
+          <legend>RenderPlan</legend>
+          <pre id="readout">—</pre>
+        </fieldset>
+
+        <fieldset>
+          <legend>Cache &amp; playback</legend>
+          <dl id="dev-stats" class="dev-dl"></dl>
+        </fieldset>
+      </div>
     </section>
   </main>
   <script src="/app.js"></script>
diff --git a/simulator/static/style.css b/simulator/static/style.css
index 4e41e39..f64d528 100644
--- a/simulator/static/style.css
+++ b/simulator/static/style.css
@@ -66,3 +66,37 @@ input[type=range], select { width: 100%; }
 .dial-hub { fill: #2c3c5c; stroke: #9cf; stroke-width: 0.6; }
 .scale-name { display: block; text-align: center; font-size: 12px; color: #cde; }
 .hint { margin: 0.3rem 0 0; font-size: 11px; color: #789; }
+
+/* --- Dev Mode --- a single switch; all dev controls/data live in #dev-panel below it. */
+.dev-switch { display: flex; align-items: center; gap: 0.5rem; cursor: pointer;
+              padding: 0.5rem 0.2rem 0.2rem; border-top: 1px solid #222; user-select: none; }
+.dev-switch input { position: absolute; opacity: 0; width: 0; height: 0; }
+.dev-switch-track { position: relative; width: 34px; height: 18px; border-radius: 9px;
+                    background: #2a2a2a; border: 1px solid #444; transition: background 0.15s; }
+.dev-switch-thumb { position: absolute; top: 1px; left: 1px; width: 14px; height: 14px;
+                    border-radius: 50%; background: #888; transition: transform 0.15s, background 0.15s; }
+.dev-switch input:checked + .dev-switch-track { background: #1d3a2a; border-color: #2e6; }
+.dev-switch input:checked + .dev-switch-track .dev-switch-thumb { transform: translateX(16px); background: #4e9; }
+.dev-switch input:focus-visible + .dev-switch-track { outline: 2px solid #9af; outline-offset: 1px; }
+.dev-switch-label { color: #9af; font-weight: 600; letter-spacing: 0.3px; }
+.dev-panel { display: flex; flex-direction: column; gap: 0.8rem; }
+/* Beat the generic `.hidden` (defined earlier, equal specificity): two classes
+   win, so the panel truly collapses when Dev Mode is off. */
+.dev-panel.hidden { display: none; }
+.dev-panel legend { color: #4e9; }
+.dev-btn { margin-top: 0.5rem; width: 100%; padding: 0.4rem; cursor: pointer;
+           background: #182028; color: #cde; border: 1px solid #2c3c5c; border-radius: 4px; }
+.dev-btn:hover { background: #20303f; }
+.dev-dl { margin: 0; display: grid; grid-template-columns: auto 1fr; gap: 0.15rem 0.6rem; font-size: 12px; }
+.dev-dl dt { color: #789; }
+.dev-dl dd { margin: 0; color: #dde; word-break: break-word; }
+.dev-dl dd.mem { color: #4e9; }
+.dev-dl dd.net { color: #fc6; }
+.dev-anno { font-size: 11px; max-height: 220px; overflow: auto;
+            font-family: ui-monospace, monospace; }
+.dev-anno .anno-row { padding: 0.2rem 0; border-bottom: 1px solid #1d1d1d; }
+.dev-anno .anno-key { color: #aee6ff; }
+.dev-anno .anno-key.affect { color: #e3cfff; }
+.dev-anno .anno-key.measure { color: #ffd79a; }
+.dev-anno .anno-meta { color: #678; }
+.dev-anno .anno-empty { color: #567; font-style: italic; }
diff --git a/tests/test_simulator_api.py b/tests/test_simulator_api.py
index 0fcc4d8..b3c5669 100644
--- a/tests/test_simulator_api.py
+++ b/tests/test_simulator_api.py
@@ -113,8 +113,11 @@ def test_media_is_cacheable(tmp_path, monkeypatch):
     resp = client.get("/media/cosmos/base.mp4")
     assert resp.status_code == 200
     cc = resp.headers.get("cache-control", "")
+    # Stored but revalidated: not no-store (so the blob preload can cache it), and
+    # not pinned immutable (so a re-sourced/re-cropped clip shows on a plain reload).
     assert "no-store" not in cc
-    assert "max-age" in cc
+    assert "immutable" not in cc
+    assert "no-cache" in cc
 
 
 @pytest.fixture