diff --git a/assets/brand/README.md b/assets/brand/README.md
index ada5f1318..0ef5f1f73 100644
--- a/assets/brand/README.md
+++ b/assets/brand/README.md
@@ -11,6 +11,7 @@ assets/brand/
 ├── master/      tight-cropped, alpha-correct, native-res masters (checked in)
 ├── mask/        grayscale luminance masks for dynamic UI tinting (checked in)
 ├── derived/     sized/format variants for app/web/installer (gitignored; built)
+├── social/      the one checked-in derived image: the GitHub social preview
 └── build.py     orchestrates master + mask + derived generation
 ```
 
@@ -31,6 +32,11 @@ and `derived/`. Idempotent.
 - **`mask/`** — grayscale masks for dynamically tinting the logo at runtime
   in the UI. See `mask/README.md` for the three flavors of dynamic-tint and
   Leptos usage.
+- **`social/`** — `hypercolor-social.png`, the 1280x640 repository social
+  preview (the luminary field with the mark over the wordmark). GitHub has
+  no API for it, so it is uploaded by hand under Settings, General, Social
+  preview; regenerate with `uv run assets/brand/build.py` and copy
+  `derived/social/github-social.png` here when the masters change.
 - **`derived/`** — final sized/format outputs: Tauri app icon set, Windows ICO,
   favicons, OG images, WiX/NSIS installer BMPs, social avatars. Regenerated on
   release, not edited by hand. The Windows installer assets are also mirrored
diff --git a/assets/brand/build.py b/assets/brand/build.py
index 26258fe88..7d5637b3b 100644
--- a/assets/brand/build.py
+++ b/assets/brand/build.py
@@ -824,14 +824,23 @@ def build_social() -> None:
     bg.paste(h_scaled, ((1500 - target_w) // 2, (500 - target_h) // 2), h_scaled)
     bg.convert("RGB").save(out / "twitter-banner.png")
 
-    # GitHub org banner 1280x640
+    # GitHub org banner 1280x640. Fit by width first: the horizontal lockup
+    # is wider than 4:1, so a height-only fit ran past both edges.
     bg = radial_gradient((1280, 640), inner=(35, 15, 70), outer=VOID_BLACK).convert("RGBA")
-    target_h = 360
-    target_w = int(hw * target_h / hh)
-    h_scaled = h_lockup.resize((target_w, target_h), Image.LANCZOS)
-    bg.paste(h_scaled, ((1280 - target_w) // 2, (640 - target_h) // 2), h_scaled)
+    h_scaled = _fit_width(h_lockup, int(1280 * 0.84))
+    if h_scaled.height > 360:
+        h_scaled = _fit_height(h_lockup, 360)
+    bg.paste(h_scaled, ((1280 - h_scaled.width) // 2, (640 - h_scaled.height) // 2), h_scaled)
     bg.convert("RGB").save(out / "github-banner.png")
 
+    # GitHub repository social preview 1280x640: the luminary nebula field
+    # with the mark over the glowing wordmark. This is the image the repo
+    # Settings page takes under Social preview; the checked-in copy at
+    # social/hypercolor-social.png is regenerated from here.
+    _luminary_vertical(
+        1280, 640, mark_wf=0.28, mark_y=0.11, wm_wf=0.58, wm_y=0.57, sparkles=160
+    ).save(out / "github-social.png")
+
     # Discord server icon 512
     bg = radial_gradient((512, 512), inner=(35, 15, 70), outer=VOID_BLACK).convert("RGBA")
     mark_scaled = sq.resize((420, 420), Image.LANCZOS)
diff --git a/assets/brand/social/hypercolor-social.png b/assets/brand/social/hypercolor-social.png
new file mode 100644
index 000000000..94b41b09f
Binary files /dev/null and b/assets/brand/social/hypercolor-social.png differ