diff --git a/src/plyngent/lmproto/anthropic/model.py b/src/plyngent/lmproto/anthropic/model.py
index 41135cd..b3e140d 100644
--- a/src/plyngent/lmproto/anthropic/model.py
+++ b/src/plyngent/lmproto/anthropic/model.py
@@ -67,9 +67,13 @@ class AnthropicThinkingConfig(Struct, omit_defaults=True):
     ``budget_tokens`` is the reasoning budget the model may spend before it
     answers; the API requires it to be at least 1024 and smaller than
     ``max_tokens`` (see ``agent.messages_bridge``, which keeps both valid).
+
+    ``type`` carries no default: ``omit_defaults`` would drop a defaulted one,
+    and the server reads the block as malformed without it — DeepSeek answers
+    ``thinking: missing field ``type``.
     """
 
-    type: Literal["enabled", "disabled"] = "enabled"
+    type: Literal["enabled", "disabled"]
     budget_tokens: int | Unset = UNSET
 
 
diff --git a/tests/test_lmproto/test_anthropic.py b/tests/test_lmproto/test_anthropic.py
index b4ed462..26f71b9 100644
--- a/tests/test_lmproto/test_anthropic.py
+++ b/tests/test_lmproto/test_anthropic.py
@@ -11,6 +11,7 @@ from plyngent.lmproto.anthropic.model import (
     AnthropicMessagesParam,
     AnthropicMessageStop,
     AnthropicResponseText,
+    AnthropicThinkingConfig,
     AnthropicUsage,
     AnthropicUserMessage,
     AnthropicWebSearchToolResultContent,
@@ -114,6 +115,17 @@ def test_messages_param_encodes_server_tool_dicts() -> None:
     assert data["tools"] == [{"type": "web_search_20250305", "name": "web_search", "max_uses": 3}]
 
 
+def test_messages_param_keeps_the_thinking_type_on_the_wire() -> None:
+    """``omit_defaults`` must not drop the block's ``type`` discriminator."""
+    param = AnthropicMessagesParam(
+        model="deepseek-flash",
+        messages=[AnthropicUserMessage(content="hi")],
+        thinking=AnthropicThinkingConfig(type="enabled", budget_tokens=16384),
+    )
+    data = msgspec.json.decode(msgspec.json.encode(param))
+    assert data["thinking"] == {"type": "enabled", "budget_tokens": 16384}
+
+
 async def test_client_messages_create(monkeypatch: pytest.MonkeyPatch) -> None:
     client = AnthropicClient(AnthropicConfig(api_key="sk-test", base_url="https://example/v1"))