diff --git a/gently/ui/web/static/js/operate.js b/gently/ui/web/static/js/operate.js
index 9c8775ea..362b7c69 100644
--- a/gently/ui/web/static/js/operate.js
+++ b/gently/ui/web/static/js/operate.js
@@ -1097,6 +1097,23 @@ const OperateManager = (function () {
         catch (e) { toastFail(`Roles failed (${why(e)})`); }
     }
 
+    // What `Start` will actually do, per mode. The button used to say "Start"
+    // for all four, while `startRun` branched into four different verbs — one
+    // of which (single) acquires a volume and finishes, and is not starting an
+    // experiment at all. The mode selector sits in a block above, so an
+    // operator who chose a mode and then looked away had nothing to read.
+    const RUN_VERB = {
+        single: 'Acquire one volume',
+        adaptive: 'Start timelapse',
+        library: 'Run tactic',
+        agent: 'Brief the agent',
+    };
+
+    function renderRunButton() {
+        const b = $('op-run-start');
+        if (b && !b.disabled) b.textContent = RUN_VERB[_mode] || 'Start';
+    }
+
     function setMode(m) {
         _mode = m;
         document.querySelectorAll('#op-modes [data-mode]').forEach(b =>
@@ -1106,6 +1123,7 @@ const OperateManager = (function () {
             if (p) p.hidden = k !== m;
         });
         if (m === 'library') loadLibrary();
+        renderRunButton();
         renderSingle();
     }
 
@@ -1136,15 +1154,40 @@ const OperateManager = (function () {
         } catch (_) { host.innerHTML = '<div class="op-empty">Library unavailable</div>'; }
     }
 
+    /**
+     * The embryos a run should image: everything not marked as a reference.
+     *
+     * There used to be a fallback — `subs.length ? subs : all` — meant to be
+     * kind to a roster with no roles assigned. It could never do that. An
+     * embryo with no role, or role 'test', or 'unassigned', already passes the
+     * filter, so `subs` is empty in exactly ONE case: every embryo is marked
+     * `calibration`. The fallback therefore fired only when the operator had
+     * said "these are all references", and answered by imaging all of them as
+     * subjects — the precise opposite of the instruction.
+     *
+     * Empty now, and the caller says so.
+     */
     function subjectIds() {
-        const subs = _embryos.filter(e => e.role !== 'calibration').map(e => e.id);
-        return subs.length ? subs : _embryos.map(e => e.id);
+        return _embryos.filter(e => e.role !== 'calibration').map(e => e.id);
+    }
+
+    // Every embryo marked as a reference means there is nothing to image. Say
+    // so once, here, rather than at each mode — and say which state it is in,
+    // because "no embryos" and "no subjects among your embryos" need
+    // different fixes.
+    function haveSubjects() {
+        if (!_embryos.length) { toastFail('No embryos registered yet'); return false; }
+        if (!subjectIds().length) {
+            toastFail('Every embryo is marked as a reference — assign at least one subject');
+            return false;
+        }
+        return true;
     }
 
     async function startRun() {
         const b = $('op-run-start');
-        const done = () => { if (b) { b.disabled = false; b.textContent = 'Start'; } };
-        if (b) { b.disabled = true; b.textContent = 'Starting…'; }
+        const done = () => { if (b) { b.disabled = false; renderRunButton(); } };
+        if (b) { b.disabled = true; b.textContent = 'Working…'; }
         try {
             if (_mode === 'single') {
                 if (!_selected) { toastFail('Select an embryo first'); return; }
@@ -1166,6 +1209,7 @@ const OperateManager = (function () {
                 return;
             }
             if (_mode === 'adaptive') {
+                if (!haveSubjects()) return;
                 const interval = Math.max(1, Number(($('op-tl-interval') || {}).value) || 120);
                 const sel = ($('op-tl-stop') || {}).value || 'manual';
                 const val = Math.max(1, Number(($('op-tl-condval') || {}).value) || 1);
@@ -1187,6 +1231,7 @@ const OperateManager = (function () {
             }
             if (_mode === 'library') {
                 if (!_selectedLib) { toastFail('Pick a saved tactic'); return; }
+                if (!haveSubjects()) return;
                 const d = await postJSON('/api/operate/run-tactic',
                     { library_id: _selectedLib, embryo_ids: subjectIds() });
                 if (d.success) { toast('Tactic started'); renderRun(); }
@@ -1560,6 +1605,7 @@ const OperateManager = (function () {
         // surface would never get its zoom.
         attachImageViews();
         mountPanels();
+        renderRunButton();
         if (_active) return;
         _active = true;
         showPaneInitial();
diff --git a/gently/ui/web/templates/index.html b/gently/ui/web/templates/index.html
index 96cf0693..7f9d04ef 100644
--- a/gently/ui/web/templates/index.html
+++ b/gently/ui/web/templates/index.html
@@ -770,7 +770,10 @@ <h2 class="devices-title"><span class="devices-title-script">Device</span> <em c
                           starts an Operation Plan for them.</p>
                       </div>
 
-                      <button class="op-btn op-btn-primary op-btn-wide" id="op-run-start" type="button">Start</button>
+                      <!-- Label is set per mode by renderRunButton(); this is single mode's verb
+                           so there is no flash of the wrong one on load. -->
+                      <button class="op-btn op-btn-primary op-btn-wide" id="op-run-start"
+                              type="button">Acquire one volume</button>
                     </div>
 
                     <div class="op-col">
diff --git a/tests/test_run_button_names_its_verb.py b/tests/test_run_button_names_its_verb.py
new file mode 100644
index 00000000..fe241524
--- /dev/null
+++ b/tests/test_run_button_names_its_verb.py
@@ -0,0 +1,88 @@
+"""`Start` must say which of four things it will do.
+
+`startRun` branches on `_mode` into four different verbs — `single` acquires
+one volume and finishes, `adaptive` starts a timelapse, `library` runs a saved
+tactic, `agent` hands over a prompt. The button said "Start" for all four, and
+the mode selector lives in a block above it, so an operator who chose a mode
+and then looked away had nothing on the button to read.
+
+Two invariants worth pinning, because both rot silently:
+
+1. Every mode has a verb. A mode added to `setMode`'s list but not to
+   `RUN_VERB` falls back to "Start" and quietly reintroduces the ambiguity.
+2. `subjectIds()` keeps no fallback. The old `subs.length ? subs : all` could
+   only ever fire when EVERY embryo was marked `calibration` — an embryo with
+   no role, or 'test', or 'unassigned' already passes the filter. So it fired
+   exactly when the operator had said "these are all references" and answered
+   by imaging all of them as subjects.
+
+ponytail: source assertions, because operate.js is an IIFE with no export
+surface and CI runs no JavaScript. They check the shape, not the behaviour.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+OPERATE = (
+    Path(__file__).resolve().parents[1] / "gently" / "ui" / "web" / "static" / "js" / "operate.js"
+)
+
+
+def _src() -> str:
+    return OPERATE.read_text(encoding="utf-8")
+
+
+def test_every_run_mode_has_a_verb() -> None:
+    src = _src()
+
+    block = src[src.index("const RUN_VERB = {") : src.index("function renderRunButton")]
+    verbs = set(re.findall(r"(\w+):\s*'", block))
+
+    assert re.search(r"\['single', 'adaptive', 'library', 'agent'\]", src), (
+        "setMode's mode list changed shape — update this test alongside it"
+    )
+    modes = {"single", "adaptive", "library", "agent"}
+
+    assert modes <= verbs, (
+        f"modes with no verb in RUN_VERB: {sorted(modes - verbs)} — the button "
+        "falls back to 'Start' for those and the ambiguity is back"
+    )
+
+
+def test_single_mode_does_not_claim_to_start_anything() -> None:
+    """It acquires one volume and finishes. The label must not imply a run."""
+    src = _src()
+    block = src[src.index("const RUN_VERB = {") : src.index("function renderRunButton")]
+    single = re.search(r"single:\s*'([^']+)'", block)
+    assert single, "single mode lost its verb"
+    label = single.group(1).lower()
+    assert "start" not in label, (
+        f"single mode's label is {label!r} — it acquires one volume and stops, "
+        "so it must not read as starting an experiment"
+    )
+
+
+def test_subject_ids_keeps_no_fallback() -> None:
+    src = _src()
+    body = src[src.index("function subjectIds()") :]
+    body = body[: body.index("}")]
+    assert "_embryos.map" not in body, (
+        "subjectIds() has a fallback again — it can only fire when every embryo "
+        "is a reference, and it answers by imaging all of them as subjects"
+    )
+
+
+def test_a_reference_only_roster_is_refused_rather_than_imaged() -> None:
+    src = _src()
+    assert "function haveSubjects()" in src
+    assert src.count("haveSubjects()") >= 3, (
+        "haveSubjects is defined but not guarding both roster-driven run modes"
+    )
+    # "no embryos" and "no subjects among your embryos" need different fixes,
+    # so the guard must distinguish them rather than emitting one message.
+    guard = src[src.index("function haveSubjects()") :]
+    guard = guard[: guard.index("\n    }")]
+    assert "No embryos registered" in guard
+    assert "marked as a reference" in guard