diff --git a/run_agent.py b/run_agent.py
index 5bc644e45c8..04e52c37241 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -3529,6 +3529,19 @@ class AIAgent:
         # instead of returning structured reasoning fields.  Only fall back
         # to inline extraction when no structured reasoning was found.
         content = getattr(assistant_message, "content", None)
+        if not reasoning_parts and isinstance(content, list):
+            # DeepSeek V4 Pro (and compatible providers) return content as a
+            # list of typed blocks, e.g.:
+            #   [{"type": "thinking", "thinking": "..."}, {"type": "output", ...}]
+            # Without this branch the thinking text is silently dropped and the
+            # next turn fails with HTTP 400 ("thinking must be passed back").
+            # Refs #21944.
+            for block in content:
+                if isinstance(block, dict) and block.get("type") == "thinking":
+                    thinking_text = block.get("thinking") or block.get("text") or ""
+                    thinking_text = thinking_text.strip()
+                    if thinking_text and thinking_text not in reasoning_parts:
+                        reasoning_parts.append(thinking_text)
         if not reasoning_parts and isinstance(content, str) and content:
             inline_patterns = (
                 r"<think>(.*?)</think>",
diff --git a/tests/run_agent/test_run_agent.py b/tests/run_agent/test_run_agent.py
index 6df71b51f90..5bc485e0711 100644
--- a/tests/run_agent/test_run_agent.py
+++ b/tests/run_agent/test_run_agent.py
@@ -517,6 +517,42 @@ class TestExtractReasoning:
         msg = _mock_assistant_msg(content=content)
         assert agent._extract_reasoning(msg) == expected
 
+    def test_content_list_thinking_blocks_extracted(self, agent):
+        """DeepSeek V4 Pro returns content as a typed-block list (issue #21944).
+
+        Without this branch thinking text is silently dropped → HTTP 400 on
+        the next turn ("thinking must be passed back to the API").
+        """
+        msg = _mock_assistant_msg(
+            content=[
+                {"type": "thinking", "thinking": "deep analysis here"},
+                {"type": "output", "text": "final answer"},
+            ]
+        )
+        result = agent._extract_reasoning(msg)
+        assert result == "deep analysis here"
+
+    def test_content_list_non_thinking_blocks_ignored(self, agent):
+        """Non-thinking blocks in a content list must not be treated as reasoning."""
+        msg = _mock_assistant_msg(
+            content=[
+                {"type": "text", "text": "just a regular response"},
+            ]
+        )
+        assert agent._extract_reasoning(msg) is None
+
+    def test_content_list_thinking_prefers_structured_field(self, agent):
+        """Structured ``reasoning`` field wins over content-list thinking blocks."""
+        msg = _mock_assistant_msg(
+            reasoning="from structured field",
+            content=[
+                {"type": "thinking", "thinking": "from content list"},
+            ],
+        )
+        result = agent._extract_reasoning(msg)
+        # structured field was found first → content-list branch skipped
+        assert result == "from structured field"
+
 
 class TestCleanSessionContent:
     def test_none_passthrough(self):