diff --git a/src/services/api/openai/convertMessages.ts b/src/services/api/openai/convertMessages.ts
index 1dbf1791f..8a95b7a85 100644
--- a/src/services/api/openai/convertMessages.ts
+++ b/src/services/api/openai/convertMessages.ts
@@ -250,9 +250,13 @@ function convertInternalAssistantMessage(
         },
       })
     } else if (block.type === 'thinking' && preserveReasoning) {
-      // DeepSeek thinking mode: preserve reasoning_content for tool call iterations
+      // DeepSeek thinking mode: always preserve reasoning_content,
+      // including the empty-string case. DeepSeek v4 may return
+      // reasoning_content: "" when the model answers directly, and the
+      // empty value must be echoed back in the next request — otherwise
+      // DeepSeek returns 400 ("reasoning_content ... must be passed back").
       const thinkingText = (block as Record<string, unknown>).thinking
-      if (typeof thinkingText === 'string' && thinkingText) {
+      if (typeof thinkingText === 'string') {
         reasoningParts.push(thinkingText)
       }
     }
diff --git a/src/services/api/openai/streamAdapter.ts b/src/services/api/openai/streamAdapter.ts
index 2b248a1d6..c8c60255e 100644
--- a/src/services/api/openai/streamAdapter.ts
+++ b/src/services/api/openai/streamAdapter.ts
@@ -90,10 +90,13 @@ export async function* adaptOpenAIStreamToAnthropic(
 
     if (!delta) continue
 
-    // Handle reasoning_content → Anthropic thinking block
-    // DeepSeek and compatible providers send delta.reasoning_content
+    // Handle reasoning_content → Anthropic thinking block.
+    // Empty string is a valid signal: DeepSeek v4 thinking mode sometimes
+    // returns reasoning_content: "" when the model answers directly. The
+    // empty thinking block must round-trip back to the API in subsequent
+    // requests, otherwise DeepSeek rejects with 400.
     const reasoningContent = (delta as any).reasoning_content
-    if (reasoningContent != null && reasoningContent !== '') {
+    if (reasoningContent != null) {
       if (!thinkingBlockOpen) {
         currentContentIndex++
         thinkingBlockOpen = true
@@ -110,14 +113,16 @@ export async function* adaptOpenAIStreamToAnthropic(
         } as BetaRawMessageStreamEvent
       }
 
-      yield {
-        type: 'content_block_delta',
-        index: currentContentIndex,
-        delta: {
-          type: 'thinking_delta',
-          thinking: reasoningContent,
-        },
-      } as BetaRawMessageStreamEvent
+      if (reasoningContent !== '') {
+        yield {
+          type: 'content_block_delta',
+          index: currentContentIndex,
+          delta: {
+            type: 'thinking_delta',
+            thinking: reasoningContent,
+          },
+        } as BetaRawMessageStreamEvent
+      }
     }
 
     // Handle text content