diff --git a/README.md b/README.md
index ac3f69e..ef42233 100644
--- a/README.md
+++ b/README.md
@@ -70,7 +70,7 @@ Future work:
 
 ## Header cards
 
-Generate a 1200×628 PNG card:
+Generate a 1200×630 PNG card:
 
 ```bash
 bun run thread-cards/generate-card.ts \
diff --git a/thread-cards/SKILL.md b/thread-cards/SKILL.md
index c5b0936..ed8d5ca 100644
--- a/thread-cards/SKILL.md
+++ b/thread-cards/SKILL.md
@@ -61,11 +61,13 @@ dispatch:
 
 ## Output
 
-- Dimensions: 1200x628 (Open Graph standard)
+- Dimensions: 1200x630 (Open Graph / social-card standard)
 - Format: PNG
 - Size: ~50-60 KB
 - Bluesky embeds include `aspectRatio` automatically (reads from PNG IHDR)
 
+Jun 18 2026 correction: do not use 1200x628. Cameron flagged the header size again on the Jun 18 daily-brief announce. The generator default is 1200x630; if a generated card reports any other dimensions, treat it as a bug before posting.
+
 ## Files
 
 - `generate-card.ts` — the generator (~280 lines)
diff --git a/thread-cards/generate-card.ts b/thread-cards/generate-card.ts
index 9fc38f5..dd1bed8 100644
--- a/thread-cards/generate-card.ts
+++ b/thread-cards/generate-card.ts
@@ -18,8 +18,10 @@ import { dirname, resolve } from "node:path"
 import { fileURLToPath } from "node:url"
 
 // ── Config ──────────────────────────────────────────────────────────
+// Open Graph / social-card standard. Keep this exact 1.91:1-ish size so
+// Bluesky, X, and link previews read the card as a proper header image.
 const WIDTH = 1200
-const HEIGHT = 628
+const HEIGHT = 630
 const BG = "#000000"
 const FG = "#ffffff"
 const MUTED = "#aaaaaa"