diff --git a/cauldron/forge.py b/cauldron/forge.py
index d2e1afa..68d1292 100644
--- a/cauldron/forge.py
+++ b/cauldron/forge.py
@@ -1204,11 +1204,28 @@ def _extract_plan_payload(forge_result: dict) -> tuple[list, str]:
 
 
 def _parse_json_blob(s: str):
+    """Parse the FIRST balanced JSON value out of a string. Tolerates Sonnet
+    appending extra prose/notes after the JSON object (which violates the
+    'no prose' rule but happens occasionally). Also strips ```json fences."""
     s = s.strip()
     # Strip code fences if Sonnet wrapped its output
     s = re.sub(r"^```(?:json)?\s*", "", s)
     s = re.sub(r"\s*```$", "", s)
     try:
+        # Plain decode first — fastest path when output is clean
         return json.loads(s)
+    except Exception:
+        pass
+    # Fall back to raw_decode which extracts the first JSON value and
+    # tells us where it ended. Anything after gets ignored. Handles the
+    # "Extra data: line 54" failure mode where Sonnet appended notes.
+    try:
+        decoder = json.JSONDecoder()
+        # Skip any leading whitespace before scanning
+        idx = 0
+        while idx < len(s) and s[idx] in " \t\n\r":
+            idx += 1
+        obj, _end = decoder.raw_decode(s[idx:])
+        return obj
     except Exception as e:
         raise ForgeError(f"could not parse model JSON: {e}; head={s[:200]!r}") from e