diff --git a/app/web/main.py b/app/web/main.py
index 21e5861..dee1952 100644
--- a/app/web/main.py
+++ b/app/web/main.py
@@ -611,6 +611,22 @@ def _run_view(run: Dict[str, Any]) -> Dict[str, Any]:
     }
 
 
+def _mark_stale_views(views: List[Dict[str, Any]]) -> None:
+    """Flag runs whose emb HTML was overwritten by a newer sibling with
+    identical params. Prefect returns runs sorted by START_TIME_DESC, so the
+    first occurrence of each stem is authoritative; later ones are stale.
+    Mutates views in place."""
+    seen: set = set()
+    for v in views:
+        stem = v["emb_file"][:-5] if v.get("emb_file") else None
+        if stem and stem in seen:
+            v["stale"] = True
+        else:
+            v["stale"] = False
+            if stem:
+                seen.add(stem)
+
+
 def _reducer_choices() -> List[Dict[str, str]]:
     return [
         {"key": k, "label": spec["label"], "blurb": spec["blurb"]}
@@ -632,6 +648,7 @@ async def index(request: Request) -> HTMLResponse:
         runs = await PREFECT.recent_runs(client, limit=10)
         dep_id = await PREFECT.deployment_id(client)
     views = [_run_view(r) for r in runs]
+    _mark_stale_views(views)
     return templates.TemplateResponse(
         request,
         "index.html",
@@ -668,6 +685,7 @@ async def runs_partial(request: Request) -> HTMLResponse:
     async with httpx.AsyncClient(timeout=5.0) as client:
         runs = await PREFECT.recent_runs(client, limit=10)
     views = [_run_view(r) for r in runs]
+    _mark_stale_views(views)
     return templates.TemplateResponse(
         request, "_runs.html", {"runs": views}
     )
@@ -770,6 +788,7 @@ async def submit(request: Request) -> HTMLResponse:
     async with httpx.AsyncClient(timeout=5.0) as client:
         runs = await PREFECT.recent_runs(client, limit=10)
     views = [_run_view(r) for r in runs]
+    _mark_stale_views(views)
     return templates.TemplateResponse(
         request,
         "_runs.html",
diff --git a/app/web/static/style.css b/app/web/static/style.css
index 21acdbc..f6072ef 100644
--- a/app/web/static/style.css
+++ b/app/web/static/style.css
@@ -518,6 +518,15 @@ button.submit:disabled { background: var(--faint); border-color: var(--faint); c
   padding-left: 0.55rem;
   margin-left: -0.55rem;
 }
+.runs li.run.stale {
+  opacity: 0.55;
+}
+.runs li.run.stale .stale-note {
+  color: var(--alarm);
+  font-style: italic;
+  font-size: 0.72rem;
+  margin-left: 0.3rem;
+}
 
 .run .stamp {
   font-family: var(--mono);
diff --git a/app/web/templates/_runs.html b/app/web/templates/_runs.html
index bf6d426..2254f6e 100644
--- a/app/web/templates/_runs.html
+++ b/app/web/templates/_runs.html
@@ -7,8 +7,8 @@
 {% else %}
   <ul class="runs">
     {% for r in runs %}
-      <li class="run {% if just_submitted is defined and r.id == just_submitted %}just-submitted{% endif %}">
-        {% if r.emb_exists %}
+      <li class="run {% if just_submitted is defined and r.id == just_submitted %}just-submitted{% endif %}{% if r.stale %} stale{% endif %}">
+        {% if r.emb_exists and not r.stale %}
           <input type="checkbox" class="compare-cb" data-stem="{{ r.emb_file[:-5] }}" aria-label="select run for comparison" />
         {% else %}
           <span class="compare-cb-slot" aria-hidden="true"></span>
@@ -47,6 +47,7 @@
 
           <div class="outputs">
             <span class="tag">fig</span>
+            {% if r.stale %}<span class="stale-note" title="a newer run with identical params overwrote this output">overwritten</span>{% endif %}
             {% if r.ref_file %}
               {% if r.ref_exists %}
                 <a href="/figs/{{ r.ref_file }}" target="_blank" rel="noopener">reference</a>
diff --git a/app/web/templates/compare.html b/app/web/templates/compare.html
index 3a8aa79..2df62da 100644
--- a/app/web/templates/compare.html
+++ b/app/web/templates/compare.html
@@ -4,7 +4,7 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>embedding notebook &middot; compare</title>
-  <link rel="stylesheet" href="/static/style.css?v=26" />
+  <link rel="stylesheet" href="/static/style.css?v=27" />
   <script type="importmap">
   {
     "imports": {
diff --git a/app/web/templates/index.html b/app/web/templates/index.html
index 967b0e9..a04e1db 100644
--- a/app/web/templates/index.html
+++ b/app/web/templates/index.html
@@ -4,7 +4,7 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>embedding notebook</title>
-  <link rel="stylesheet" href="/static/style.css?v=26" />
+  <link rel="stylesheet" href="/static/style.css?v=27" />
   <script src="https://unpkg.com/htmx.org@2.0.4"></script>
   <script type="importmap">
   {