SP3 generative song cards

arc.py

@@ -350,6 +350,31 @@ def build_fragment_prompt() -> str:
     return "\n\n".join([_FRAGMENT_PERSONA, _output_contract_block()])
 
 
+# ---------------------------------------------------------------------------
+# Generative song card (SP3): the model writes only the creative title+artist
+# into the JSON text field; the server assigns the (valid) musical params.
+# ---------------------------------------------------------------------------
+_SONG_CARD_PERSONA = (
+    "You are the music director of a 1970s late-night radio station, inventing FICTIONAL "
+    "records. Titles are evocative and a little melancholy; artists are fictional bands or "
+    "singers. Never use a real song or a real artist."
+)
+
+
+def build_song_card_prompt() -> str:
+    """Card-specific contract: the `text` field carries '<title> by <artist>'."""
+    moods = ", ".join(MOODS)
+    return (
+        _SONG_CARD_PERSONA + "\n\n"
+        "Respond with ONE JSON object and nothing else, with exactly these keys:\n"
+        '  "text": the record as "<title> by <artist>" -- nothing else, no quotes, no labels.\n'
+        '  "mood": one of [' + moods + "].\n"
+        '  "arc_cue": "none".\n'
+        'Example: {"text": "Tail Lights in the Rain by The Sleeping Overpass", '
+        '"mood": "nostalgic", "arc_cue": "none"}'
+    )
+
+
 # ---------------------------------------------------------------------------
 # Trigger detection (server-side, no model). Advances the meter.
 # ---------------------------------------------------------------------------