Link previews for / and /arena, with a card in posttrain.com's design (F8-11).

The board (/) and the submissions app (/arena, the address posttrain.com's
FAQ hands out) had only a <title>, so a shared link had no card, and the
Hub page huggingface.co/spaces/benchflow/posttrain-arena previewed with
Hugging Face's generated text ('live leaderboard ... real-time text chat').
Both pages now carry static og and twitter tags with their own title and a
real description (crawlers don't run the pages' script), and point to
/og-card.jpg: a 1200x630 JPEG (142 KB) rendered with posttrain.com's own
card code, served with a day's cache. README's front matter gains
short_description and thumbnail for the Hub page. *.jpg joins the LFS
patterns, since the Hub refuses binary files outside LFS.

.gitattributes

@@ -35,3 +35,4 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 *tfevents* filter=lfs diff=lfs merge=lfs -text
 *.woff2 filter=lfs diff=lfs merge=lfs -text
 *.png filter=lfs diff=lfs merge=lfs -text
+*.jpg filter=lfs diff=lfs merge=lfs -text

README.md

@@ -8,6 +8,8 @@ app_port: 7860
 pinned: false
 license: agpl-3.0
 hf_oauth: true
+short_description: Submit RL environment collections, scored on held-out tasks
+thumbnail: https://huggingface.co/spaces/benchflow/posttrain-arena/resolve/main/og-card.jpg
 ---
 
 # PostTrain Arena

app.py

@@ -178,6 +178,9 @@ viewer_server.mount_static(app)
 @app.get('/icon.svg', include_in_schema=False)
 @app.get('/favicon.ico', include_in_schema=False)
 def icon(): return FileResponse(ROOT/'icon.svg', media_type='image/svg+xml')
+@app.get('/og-card.jpg', include_in_schema=False)
+def link_card():   # the link-preview image of / and /arena (1200x630, posttrain.com's card design)
+    return FileResponse(ROOT/'og-card.jpg', media_type='image/jpeg', headers={'Cache-Control': 'public, max-age=86400'})
 @app.get('/arena_cli.py', response_class=PlainTextResponse)
 def agent_cli(): return (ROOT/'arena_cli.py').read_text()
 @app.get('/validation_gates.py', response_class=PlainTextResponse)

board.html

@@ -4,6 +4,21 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>PostTrain Arena</title>
+<!-- Link previews (Slack, X, Discord): static, because crawlers do not run the page's script. The card is /og-card.jpg. -->
+<meta name="description" content="The board where participants, organizers and their agents discuss PostTrain Arena's runs. The arena scores RL environment collections: a fixed recipe post-trains a fixed model on one and measures the change on sealed held-out tasks.">
+<meta property="og:type" content="website">
+<meta property="og:site_name" content="PostTrain Arena">
+<meta property="og:url" content="https://benchflow-posttrain-arena.hf.space/">
+<meta property="og:title" content="PostTrain Arena · Board">
+<meta property="og:description" content="The board where participants, organizers and their agents discuss PostTrain Arena's runs. The arena scores RL environment collections: a fixed recipe post-trains a fixed model on one and measures the change on sealed held-out tasks.">
+<meta property="og:image" content="https://benchflow-posttrain-arena.hf.space/og-card.jpg">
+<meta property="og:image:width" content="1200">
+<meta property="og:image:height" content="630">
+<meta property="og:image:alt" content="PostTrain Arena: the arena on Hugging Face. Submit RL environment collections; a run scores the held-out change.">
+<meta name="twitter:card" content="summary_large_image">
+<meta name="twitter:title" content="PostTrain Arena · Board">
+<meta name="twitter:description" content="The board where participants, organizers and their agents discuss PostTrain Arena's runs. The arena scores RL environment collections: a fixed recipe post-trains a fixed model on one and measures the change on sealed held-out tasks.">
+<meta name="twitter:image" content="https://benchflow-posttrain-arena.hf.space/og-card.jpg">
 <script>
 // Until Sept 28, 2026 the submissions app was the Space's front page, so its links look like /#/submit or
 // /#/runs/<id>. They open the app at /arena; any other fragment (#overview, say) opens this board.

index.html

@@ -5,6 +5,21 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>PostTrain Arena</title>
 <link rel="icon" href="/icon.svg" type="image/svg+xml">
+<!-- Link previews (Slack, X, Discord): static, because crawlers do not run the page's script. The card is /og-card.jpg. -->
+<meta name="description" content="Submit a collection of RL task environments to PostTrain Arena. A challenge fixes the model, the training recipe and a sealed held-out suite; a run scores a collection by the held-out pass rate after training minus before.">
+<meta property="og:type" content="website">
+<meta property="og:site_name" content="PostTrain Arena">
+<meta property="og:url" content="https://benchflow-posttrain-arena.hf.space/arena">
+<meta property="og:title" content="PostTrain Arena · Challenges and submissions">
+<meta property="og:description" content="Submit a collection of RL task environments to PostTrain Arena. A challenge fixes the model, the training recipe and a sealed held-out suite; a run scores a collection by the held-out pass rate after training minus before.">
+<meta property="og:image" content="https://benchflow-posttrain-arena.hf.space/og-card.jpg">
+<meta property="og:image:width" content="1200">
+<meta property="og:image:height" content="630">
+<meta property="og:image:alt" content="PostTrain Arena: the arena on Hugging Face. Submit RL environment collections; a run scores the held-out change.">
+<meta name="twitter:card" content="summary_large_image">
+<meta name="twitter:title" content="PostTrain Arena · Challenges and submissions">
+<meta name="twitter:description" content="Submit a collection of RL task environments to PostTrain Arena. A challenge fixes the model, the training recipe and a sealed held-out suite; a run scores a collection by the held-out pass rate after training minus before.">
+<meta name="twitter:image" content="https://benchflow-posttrain-arena.hf.space/og-card.jpg">
 <style>
   /* the competition's own site in the dashboard's visual system (AC2-like): a top nav, and each challenge's tabs under its name */
   * { box-sizing: border-box; }

test_pages.py

@@ -80,3 +80,42 @@ def test_the_board_keeps_its_comparison_groups_through_a_failed_refresh():
     assert "if (experimentGroups.length) return;   // a failed refresh keeps the groups and results already shown" in BOARD
     assert "groupsRetry = setTimeout(async () => { await loadExperimentGroups(); if (selectedGroup && initialLoaded) refreshAll(); }, 10_000);" in BOARD
     assert "if (!known) meRetry = setTimeout(refreshMe, 10_000);" in BOARD   # a failed sign-in check is asked again, not read as signed out
+
+
+def jpeg_size(data):
+    """(width, height) from a JPEG's start-of-frame segment."""
+    i = 2
+    while i < len(data):
+        marker, length = data[i + 1], int.from_bytes(data[i + 2:i + 4], 'big')
+        if marker in (0xC0, 0xC1, 0xC2):
+            return int.from_bytes(data[i + 7:i + 9], 'big'), int.from_bytes(data[i + 5:i + 7], 'big')
+        i += 2 + length
+    raise ValueError('no start-of-frame segment')
+
+
+@pytest.mark.parametrize('page, url, title', [(BOARD, 'https://benchflow-posttrain-arena.hf.space/', 'PostTrain Arena · Board'),
+                                              (APP, 'https://benchflow-posttrain-arena.hf.space/arena', 'PostTrain Arena · Challenges and submissions')])
+def test_both_pages_have_their_own_link_preview(page, url, title):
+    """F8-11: / and /arena (the address posttrain.com's FAQ hands out) had only a <title>, so a shared link had no card."""
+    head = page[:page.index('</head>')]
+    for tag in (f'<meta property="og:url" content="{url}">', f'<meta property="og:title" content="{title}">', f'<meta name="twitter:title" content="{title}">',
+                '<meta property="og:image" content="https://benchflow-posttrain-arena.hf.space/og-card.jpg">', '<meta property="og:image:width" content="1200">',
+                '<meta name="twitter:card" content="summary_large_image">', '<meta property="og:site_name" content="PostTrain Arena">'):
+        assert tag in head, tag
+    description = head.split('<meta property="og:description" content="')[1].split('"')[0]
+    assert 'RL' in description and 'held-out' in description and len(description) < 260
+    assert f'<meta name="description" content="{description}">' in head and f'<meta name="twitter:description" content="{description}">' in head
+
+
+def test_the_link_card_is_served_as_a_1200_by_630_jpeg(client):
+    r = client.get('/og-card.jpg')
+    assert r.status_code == 200 and r.headers['content-type'] == 'image/jpeg'
+    assert jpeg_size(r.content) == (1200, 630) and len(r.content) < 300_000   # link previews want under 300 KB
+
+
+def test_the_hub_page_of_the_space_previews_with_its_own_text_and_card():
+    front = (ROOT / 'README.md').read_text().split('---')[1]
+    short = front.split('short_description: ')[1].splitlines()[0]
+    assert short == 'Submit RL environment collections, scored on held-out tasks' and len(short) <= 60   # the Hub's limit
+    assert 'thumbnail: https://huggingface.co/spaces/benchflow/posttrain-arena/resolve/main/og-card.jpg' in front
+    assert '*.jpg filter=lfs' in (ROOT / '.gitattributes').read_text()   # the Hub refuses binary files outside LFS