diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
index fd7596e7e3bf..ca86c3ebe266 100644
--- a/agent/anthropic_adapter.py
+++ b/agent/anthropic_adapter.py
@@ -2427,7 +2427,10 @@ def _ensure_leading_user_turn(result: List[Dict[str, Any]]) -> None:
     (convert_messages_to_converse).
     """
     if result and result[0].get("role") != "user":
-        result.insert(0, {"role": "user", "content": [{"type": "text", "text": " "}]})
+        # ponytail: some Anthropic-compatible endpoints (e.g. zenmux) reject a
+        # whitespace-only text block with HTTP 400 "text content blocks must
+        # contain non-whitespace text"; use a minimal non-blank placeholder.
+        result.insert(0, {"role": "user", "content": [{"type": "text", "text": "."}]})
 
 
 def convert_messages_to_anthropic(
diff --git a/tests/agent/test_anthropic_adapter.py b/tests/agent/test_anthropic_adapter.py
index ae99f950f739..c2166522edf4 100644
--- a/tests/agent/test_anthropic_adapter.py
+++ b/tests/agent/test_anthropic_adapter.py
@@ -1394,7 +1394,9 @@ def test_leading_assistant_after_compaction_gets_user_turn_prepended(self):
 
         assert system == "You are helpful."
         assert result[0]["role"] == "user"
-        assert result[0]["content"] == [{"type": "text", "text": " "}]
+        # Placeholder text must be non-whitespace: strict Anthropic-compatible
+        # endpoints (zenmux) 400 on whitespace-only text blocks.
+        assert result[0]["content"][0]["text"].strip()
         assert result[1]["role"] == "assistant"
         assert any(
             m["role"] == "assistant" and "Context compaction summary" in str(m["content"])
@@ -1418,7 +1420,7 @@ def test_double_compaction_no_system_in_messages_leads_with_user(self):
 
         assert system is None
         assert result[0]["role"] == "user"
-        assert result[0]["content"] == [{"type": "text", "text": " "}]
+        assert result[0]["content"][0]["text"].strip()
         assert result[1]["role"] == "assistant"
         assert "Context compaction summary" in str(result[1]["content"])