diff --git a/run_agent.py b/run_agent.py
index 43c367e460f..b567b965458 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -7868,7 +7868,17 @@ class AIAgent:
             api_msg["reasoning_content"] = existing
             return
 
-        # 2. DeepSeek / Kimi thinking mode: tool-call turns that lack
+        # 2. Healthy session: promote 'reasoning' field to 'reasoning_content'
+        # for providers that use the internal 'reasoning' key.
+        # This must happen BEFORE the DeepSeek/Kimi tool-call check so that
+        # genuine reasoning content is not overwritten by the empty-string
+        # fallback (#15812 regression in PR #15478).
+        normalized_reasoning = source_msg.get("reasoning")
+        if isinstance(normalized_reasoning, str) and normalized_reasoning:
+            api_msg["reasoning_content"] = normalized_reasoning
+            return
+
+        # 3. DeepSeek / Kimi thinking mode: tool-call turns that lack
         # reasoning_content are "poisoned history" — a prior provider (MiniMax,
         # etc.) left them empty. DeepSeek returns HTTP 400 if reasoning_content
         # is absent on replay; inject "" to satisfy the provider's requirement
@@ -7884,13 +7894,6 @@ class AIAgent:
             api_msg["reasoning_content"] = ""
             return
 
-        # 3. Healthy session: promote 'reasoning' field to 'reasoning_content'
-        # for providers that use the internal 'reasoning' key.
-        normalized_reasoning = source_msg.get("reasoning")
-        if isinstance(normalized_reasoning, str) and normalized_reasoning:
-            api_msg["reasoning_content"] = normalized_reasoning
-            return
-
         # 4. DeepSeek / Kimi thinking mode: all assistant messages need
         # reasoning_content. Inject "" to satisfy the provider's requirement
         # when no explicit reasoning content is present.