diff --git a/aborist/qa/client.py b/aborist/qa/client.py
index 26d0a97..dc7b4b9 100644
--- a/aborist/qa/client.py
+++ b/aborist/qa/client.py
@@ -114,6 +114,22 @@ class OpenAICompatibleClient:
         self.close()
         return False
 
+    @staticmethod
+    def _scrub_surrogates(s: str) -> str:
+        """Replace lone UTF-16 surrogates with U+FFFD.
+
+        Wikipedia chunks (and other ingested text) occasionally
+        contain unpaired surrogates from the ingest of invalid-UTF-8
+        source. httpx's json= path does ``.encode('utf-8')`` on the
+        serialized request body, which raises UnicodeEncodeError on
+        any lone surrogate. Sanitize incoming message content here
+        so the outbound HTTP request always serializes cleanly. We
+        round-trip through WTF-8 (surrogatepass) bytes, then decode
+        as standard UTF-8 with replacement — invalid sequences
+        become U+FFFD (REPLACEMENT CHARACTER).
+        """
+        return s.encode("utf-8", errors="surrogatepass").decode("utf-8", errors="replace")
+
     def chat_completion(
         self,
         messages: list[dict],
@@ -130,6 +146,18 @@ class OpenAICompatibleClient:
 
         client = self._http  # persistent connection pool from __init__
         headers = {"Content-Type": "application/json"}
+        # Sanitize message content for httpx's json-encode path.
+        messages = [
+            {
+                **m,
+                "content": (
+                    self._scrub_surrogates(m["content"])
+                    if isinstance(m.get("content"), str)
+                    else m.get("content")
+                ),
+            }
+            for m in messages
+        ]
         if self.api_key:
             headers["Authorization"] = f"Bearer {self.api_key}"
         payload = {