diff --git a/internal/providers/openai_extra_headers_test.go b/internal/providers/openai_extra_headers_test.go
index d46593ed..223afa9c 100644
--- a/internal/providers/openai_extra_headers_test.go
+++ b/internal/providers/openai_extra_headers_test.go
@@ -201,3 +201,56 @@ func TestNonKimi_ReasoningContentNotAddedWhenEmpty(t *testing.T) {
 		t.Error("non-kimi providers must not inject empty reasoning_content; key should be absent")
 	}
 }
+
+// TestDeepSeek_ReasoningContentPresentOnLaterToolCallTurns reproduces upstream
+// "The `reasoning_content` in the thinking mode must be passed back to the API."
+// — once a DeepSeek turn has emitted reasoning, a later assistant tool-call turn
+// that emitted none must still carry the key, or the next request is rejected.
+// Trace: 01a04c34-a74b-7be7-973e-2cb66ea68f3d.
+func TestDeepSeek_ReasoningContentPresentOnLaterToolCallTurns(t *testing.T) {
+	p := NewOpenAIProvider("deepseek", "sk", "https://api.deepseek.com/v1", "deepseek-v4-pro").
+		WithProviderType("deepseek")
+
+	body := p.buildRequestBody("deepseek-v4-pro", ChatRequest{
+		Messages: []Message{
+			{Role: "user", Content: "thêm khoản thu 300k"},
+			{Role: "assistant", Thinking: "the user wants an income entry", ToolCalls: []ToolCall{{ID: "call_1", Name: "edit", Arguments: map[string]any{}}}},
+			{Role: "tool", Content: "old_string not found in file", ToolCallID: "call_1"},
+			{Role: "assistant", ToolCalls: []ToolCall{{ID: "call_2", Name: "read_file", Arguments: map[string]any{}}}},
+			{Role: "tool", Content: "# Sổ sách chi tiêu", ToolCallID: "call_2"},
+		},
+	}, true)
+
+	msgs := body["messages"].([]map[string]any)
+	if got := msgs[1]["reasoning_content"]; got != "the user wants an income entry" {
+		t.Errorf("captured reasoning clobbered: got %q", got)
+	}
+	rc, present := msgs[3]["reasoning_content"]
+	if !present {
+		t.Fatalf("assistant turn without reasoning must still carry reasoning_content; got %v", msgs[3])
+	}
+	if rc != "" {
+		t.Errorf("reasoning_content = %q, want empty string when Thinking unset", rc)
+	}
+}
+
+// TestDeepSeek_ReasoningContentAbsentWithoutThinkingMode is the negative control:
+// a conversation that never carried reasoning is not in thinking mode, so the
+// empty-string key must not be injected.
+func TestDeepSeek_ReasoningContentAbsentWithoutThinkingMode(t *testing.T) {
+	p := NewOpenAIProvider("deepseek", "sk", "https://api.deepseek.com/v1", "deepseek-chat").
+		WithProviderType("deepseek")
+
+	body := p.buildRequestBody("deepseek-chat", ChatRequest{
+		Messages: []Message{
+			{Role: "user", Content: "hi"},
+			{Role: "assistant", ToolCalls: []ToolCall{{ID: "call_1", Name: "exec", Arguments: map[string]any{}}}},
+			{Role: "tool", Content: "...", ToolCallID: "call_1"},
+		},
+	}, true)
+
+	msgs := body["messages"].([]map[string]any)
+	if _, present := msgs[1]["reasoning_content"]; present {
+		t.Error("non-thinking conversation must not inject empty reasoning_content")
+	}
+}
diff --git a/internal/providers/openai_request.go b/internal/providers/openai_request.go
index 8cfa6172..55a3eab0 100644
--- a/internal/providers/openai_request.go
+++ b/internal/providers/openai_request.go
@@ -40,6 +40,19 @@ func (p *OpenAIProvider) buildRequestBody(model string, req ChatRequest, stream
 	// Matching OpenClaw TS: model-compat.ts → isOpenAINativeEndpoint().
 	useDevRole := isOpenAINativeEndpoint(p.apiBase)
 
+	// A conversation where some assistant turn captured reasoning is running in
+	// thinking mode, so every replayed assistant message must carry the field —
+	// turns that produced no reasoning of their own included. DeepSeek rejects
+	// the gap with HTTP 400 "The `reasoning_content` in the thinking mode must
+	// be passed back to the API." Trace: 01a04c34-a74b-7be7-973e-2cb66ea68f3d.
+	historyHasReasoning := false
+	for _, m := range inputMessages {
+		if m.Role == "assistant" && m.Thinking != "" {
+			historyHasReasoning = true
+			break
+		}
+	}
+
 	// Convert messages to proper OpenAI wire format.
 	// This is necessary because our internal Message/ToolCall structs don't match
 	// the OpenAI API format (tool_calls need type+function wrapper, arguments as JSON string).
@@ -63,12 +76,14 @@ func (p *OpenAIProvider) buildRequestBody(model string, req ChatRequest, stream
 		// kimi-k2-turbo-preview), assistant tool-call messages MUST carry
 		// reasoning_content even if empty — otherwise upstream returns 400 "thinking
 		// is enabled but reasoning_content is missing in assistant tool call message".
+		// DeepSeek enforces the same rule, but only once the conversation is in
+		// thinking mode (historyHasReasoning).
 		if m.Role == "assistant" && openAIWireAssistantReasoningContent(model) {
 			switch {
 			case m.Thinking != "":
 				msg["reasoning_content"] = m.Thinking
-			case p.providerType == "kimi_coding":
-				// Send empty string rather than omit the field — satisfies Kimi's
+			case p.providerType == "kimi_coding" || historyHasReasoning:
+				// Send empty string rather than omit the field — satisfies the
 				// "must be present" check without inventing reasoning content.
 				msg["reasoning_content"] = ""
 			}