fix: keep developer messages in DeepSeek-V4 thinking drop

lmdeploy/deepseek_v4_encoding.py

@@ -584,14 +584,13 @@ def _drop_thinking_messages(messages: list[dict[str, Any]]) -> list[dict[str, An
     message.
 
     Behavior:
-    - Messages with role in ["user", "system", "tool", "latest_reminder"] are always kept.
+    - Messages with role in ["user", "system", "tool", "latest_reminder", "developer"] are always kept.
     - Messages at or after the last user index are always kept.
     - Assistant messages before the last user get reasoning_content removed.
-    - Developer messages before the last user are dropped entirely.
     """
     last_user_idx = find_last_user_index(messages)
     result = []
-    keep_roles = {'user', 'system', 'tool', 'latest_reminder', 'direct_search_results'}
+    keep_roles = {'user', 'system', 'tool', 'latest_reminder', 'direct_search_results', 'developer'}
 
     for idx, msg in enumerate(messages):
         role = msg.get('role')
@@ -601,7 +600,7 @@ def _drop_thinking_messages(messages: list[dict[str, Any]]) -> list[dict[str, An
             msg = copy.copy(msg)
             msg.pop('reasoning_content', None)
             result.append(msg)
-        # developer and other roles before last_user_idx are dropped
+        # other unknown roles before last_user_idx are dropped
 
     return result
 

tests/test_lmdeploy/test_deepseek_v4_encoding.py

@@ -128,6 +128,39 @@ def test_deepseek_v4_task_and_latest_reminder_rendering():
     assert '<｜Assistant｜>' not in prompt
 
 
+def test_deepseek_v4_thinking_drop_keeps_developer_message():
+    messages = [
+        {'role': 'system', 'content': 'You are helpful.'},
+        {'role': 'developer', 'content': 'Always answer in French.'},
+        {'role': 'user', 'content': 'Real question'},
+    ]
+
+    prompt = encode_messages(messages, thinking_mode='thinking')
+
+    assert '<｜User｜>Always answer in French.' in prompt
+    assert '<｜User｜>Real question' in prompt
+    assert prompt.endswith('<｜Assistant｜><think>')
+
+
+def test_deepseek_v4_thinking_drop_keeps_developer_in_multiturn():
+    messages = [
+        {'role': 'user', 'content': 'First question'},
+        {
+            'role': 'assistant',
+            'content': 'First answer',
+            'reasoning_content': 'private chain of thought',
+        },
+        {'role': 'developer', 'content': 'Now answer in French.'},
+        {'role': 'user', 'content': 'Second question'},
+    ]
+
+    prompt = encode_messages(messages, thinking_mode='thinking')
+
+    assert '<｜User｜>Now answer in French.' in prompt
+    assert '<｜User｜>Second question' in prompt
+    assert 'private chain of thought' not in prompt
+
+
 def test_deepseek_v4_parse_completion_text():
     completion = (
         'I should call a tool.</think>\n\n'