feat(prompt): align DeepSeek prompt assembly with tokenizer-style turns

internal/adapter/claude/standard_request.go

@@ -36,7 +36,7 @@ func normalizeClaudeRequest(store ConfigReader, req map[string]any) (claudeNorma
 		thinkingEnabled = false
 		searchEnabled = false
 	}
-	finalPrompt := deepseek.MessagesPrepare(toMessageMaps(dsPayload["messages"]))
+	finalPrompt := deepseek.MessagesPrepareWithThinking(toMessageMaps(dsPayload["messages"]), thinkingEnabled)
 	toolNames := extractClaudeToolNames(toolsRequested)
 	if len(toolNames) == 0 && len(toolsRequested) > 0 {
 		toolNames = []string{"__any_tool__"}

internal/adapter/gemini/convert_request.go

@@ -28,7 +28,7 @@ func normalizeGeminiRequest(store ConfigReader, routeModel string, req map[strin
 	}
 
 	toolsRaw := convertGeminiTools(req["tools"])
-	finalPrompt, toolNames := openai.BuildPromptForAdapter(messagesRaw, toolsRaw, "")
+	finalPrompt, toolNames := openai.BuildPromptForAdapter(messagesRaw, toolsRaw, "", thinkingEnabled)
 	passThrough := collectGeminiPassThrough(req)
 
 	return util.StandardRequest{

internal/adapter/openai/prompt_build.go

@@ -5,22 +5,22 @@ import (
 	"ds2api/internal/util"
 )
 
-func buildOpenAIFinalPrompt(messagesRaw []any, toolsRaw any, traceID string) (string, []string) {
-	return buildOpenAIFinalPromptWithPolicy(messagesRaw, toolsRaw, traceID, util.DefaultToolChoicePolicy())
+func buildOpenAIFinalPrompt(messagesRaw []any, toolsRaw any, traceID string, thinkingEnabled bool) (string, []string) {
+	return buildOpenAIFinalPromptWithPolicy(messagesRaw, toolsRaw, traceID, util.DefaultToolChoicePolicy(), thinkingEnabled)
 }
 
-func buildOpenAIFinalPromptWithPolicy(messagesRaw []any, toolsRaw any, traceID string, toolPolicy util.ToolChoicePolicy) (string, []string) {
+func buildOpenAIFinalPromptWithPolicy(messagesRaw []any, toolsRaw any, traceID string, toolPolicy util.ToolChoicePolicy, thinkingEnabled bool) (string, []string) {
 	messages := normalizeOpenAIMessagesForPrompt(messagesRaw, traceID)
 	toolNames := []string{}
 	if tools, ok := toolsRaw.([]any); ok && len(tools) > 0 {
 		messages, toolNames = injectToolPrompt(messages, tools, toolPolicy)
 	}
-	return deepseek.MessagesPrepare(messages), toolNames
+	return deepseek.MessagesPrepareWithThinking(messages, thinkingEnabled), toolNames
 }
 
 // BuildPromptForAdapter exposes the OpenAI-compatible prompt building flow so
 // other protocol adapters (for example Gemini) can reuse the same tool/history
 // normalization logic and remain behavior-compatible with chat/completions.
-func BuildPromptForAdapter(messagesRaw []any, toolsRaw any, traceID string) (string, []string) {
-	return buildOpenAIFinalPrompt(messagesRaw, toolsRaw, traceID)
+func BuildPromptForAdapter(messagesRaw []any, toolsRaw any, traceID string, thinkingEnabled bool) (string, []string) {
+	return buildOpenAIFinalPrompt(messagesRaw, toolsRaw, traceID, thinkingEnabled)
 }

internal/adapter/openai/prompt_build_test.go

@@ -40,7 +40,7 @@ func TestBuildOpenAIFinalPrompt_HandlerPathIncludesToolRoundtripSemantics(t *tes
 		},
 	}
 
-	finalPrompt, toolNames := buildOpenAIFinalPrompt(messages, tools, "")
+	finalPrompt, toolNames := buildOpenAIFinalPrompt(messages, tools, "", false)
 	if len(toolNames) != 1 || toolNames[0] != "get_weather" {
 		t.Fatalf("unexpected tool names: %#v", toolNames)
 	}
@@ -73,7 +73,7 @@ func TestBuildOpenAIFinalPrompt_VercelPreparePathKeepsFinalAnswerInstruction(t *
 		},
 	}
 
-	finalPrompt, _ := buildOpenAIFinalPrompt(messages, tools, "")
+	finalPrompt, _ := buildOpenAIFinalPrompt(messages, tools, "", false)
 	if !strings.Contains(finalPrompt, "Remember: Output ONLY the <tool_calls>...</tool_calls> XML block when calling tools.") {
 		t.Fatalf("vercel prepare finalPrompt missing final tool-call anchor instruction: %q", finalPrompt)
 	}

internal/adapter/openai/standard_request.go

@@ -24,7 +24,7 @@ func normalizeOpenAIChatRequest(store ConfigReader, req map[string]any, traceID
 		responseModel = resolvedModel
 	}
 	toolPolicy := util.DefaultToolChoicePolicy()
-	finalPrompt, toolNames := buildOpenAIFinalPromptWithPolicy(messagesRaw, req["tools"], traceID, toolPolicy)
+	finalPrompt, toolNames := buildOpenAIFinalPromptWithPolicy(messagesRaw, req["tools"], traceID, toolPolicy, thinkingEnabled)
 	toolNames = ensureToolDetectionEnabled(toolNames, req["tools"])
 	passThrough := collectOpenAIChatPassThrough(req)
 
@@ -74,7 +74,7 @@ func normalizeOpenAIResponsesRequest(store ConfigReader, req map[string]any, tra
 	if err != nil {
 		return util.StandardRequest{}, err
 	}
-	finalPrompt, toolNames := buildOpenAIFinalPromptWithPolicy(messagesRaw, req["tools"], traceID, toolPolicy)
+	finalPrompt, toolNames := buildOpenAIFinalPromptWithPolicy(messagesRaw, req["tools"], traceID, toolPolicy, thinkingEnabled)
 	toolNames = ensureToolDetectionEnabled(toolNames, req["tools"])
 	if !toolPolicy.IsNone() {
 		toolPolicy.Allowed = namesToSet(toolNames)

internal/deepseek/prompt.go

@@ -5,3 +5,7 @@ import "ds2api/internal/prompt"
 func MessagesPrepare(messages []map[string]any) string {
 	return prompt.MessagesPrepare(messages)
 }
+
+func MessagesPrepareWithThinking(messages []map[string]any, thinkingEnabled bool) string {
+	return prompt.MessagesPrepareWithThinking(messages, thinkingEnabled)
+}

internal/prompt/messages.go

@@ -10,6 +10,7 @@ import (
 var markdownImagePattern = regexp.MustCompile(`!\[(.*?)\]\((.*?)\)`)
 
 const (
+	beginSentenceMarker   = "<｜begin▁of▁sentence｜>"
 	systemMarker          = "<｜System｜>"
 	userMarker            = "<｜User｜>"
 	assistantMarker       = "<｜Assistant｜>"
@@ -17,9 +18,15 @@ const (
 	endSentenceMarker     = "<｜end▁of▁sentence｜>"
 	endToolResultsMarker  = "<｜end▁of▁toolresults｜>"
 	endInstructionsMarker = "<｜end▁of▁instructions｜>"
+	openThinkMarker       = "<think>"
+	closeThinkMarker      = "</think>"
 )
 
 func MessagesPrepare(messages []map[string]any) string {
+	return MessagesPrepareWithThinking(messages, false)
+}
+
+func MessagesPrepareWithThinking(messages []map[string]any, thinkingEnabled bool) string {
 	type block struct {
 		Role string
 		Text string
@@ -41,11 +48,14 @@ func MessagesPrepare(messages []map[string]any) string {
 		}
 		merged = append(merged, msg)
 	}
-	parts := make([]string, 0, len(merged))
+	parts := make([]string, 0, len(merged)+2)
+	parts = append(parts, beginSentenceMarker)
+	lastRole := ""
 	for _, m := range merged {
+		lastRole = m.Role
 		switch m.Role {
 		case "assistant":
-			parts = append(parts, formatRoleBlock(assistantMarker, m.Text, endSentenceMarker))
+			parts = append(parts, formatRoleBlock(assistantMarker, closeThinkMarker+m.Text, endSentenceMarker))
 		case "tool":
 			if strings.TrimSpace(m.Text) != "" {
 				parts = append(parts, formatRoleBlock(toolMarker, m.Text, endToolResultsMarker))
@@ -62,6 +72,13 @@ func MessagesPrepare(messages []map[string]any) string {
 			}
 		}
 	}
+	if lastRole != "assistant" {
+		thinkPrefix := closeThinkMarker
+		if thinkingEnabled {
+			thinkPrefix = openThinkMarker
+		}
+		parts = append(parts, assistantMarker+thinkPrefix)
+	}
 	out := strings.Join(parts, "\n\n")
 	return markdownImagePattern.ReplaceAllString(out, `[${1}](${2})`)
 }

internal/prompt/messages_test.go

@@ -32,13 +32,16 @@ func TestMessagesPrepareUsesTurnSuffixes(t *testing.T) {
 		{"role": "assistant", "content": "Answer"},
 	}
 	got := MessagesPrepare(messages)
+	if !strings.HasPrefix(got, "<｜begin▁of▁sentence｜>") {
+		t.Fatalf("expected begin-of-sentence marker, got %q", got)
+	}
 	if !strings.Contains(got, "<｜System｜>\nSystem rule<｜end▁of▁instructions｜>") {
 		t.Fatalf("expected system instructions suffix, got %q", got)
 	}
 	if !strings.Contains(got, "<｜User｜>\nQuestion<｜end▁of▁sentence｜>") {
 		t.Fatalf("expected user sentence suffix, got %q", got)
 	}
-	if !strings.Contains(got, "<｜Assistant｜>\nAnswer<｜end▁of▁sentence｜>") {
+	if !strings.Contains(got, "<｜Assistant｜>\n</think>Answer<｜end▁of▁sentence｜>") {
 		t.Fatalf("expected assistant sentence suffix, got %q", got)
 	}
 }
@@ -51,3 +54,11 @@ func TestNormalizeContentArrayFallsBackToContentWhenTextEmpty(t *testing.T) {
 		t.Fatalf("expected fallback to content when text is empty, got %q", got)
 	}
 }
+
+func TestMessagesPrepareWithThinkingEndsWithOpenThink(t *testing.T) {
+	messages := []map[string]any{{"role": "user", "content": "Question"}}
+	got := MessagesPrepareWithThinking(messages, true)
+	if !strings.HasSuffix(got, "<｜Assistant｜><think>") {
+		t.Fatalf("expected thinking suffix, got %q", got)
+	}
+}

internal/util/messages_test.go

@@ -12,7 +12,7 @@ func TestMessagesPrepareBasic(t *testing.T) {
 	if got == "" {
 		t.Fatal("expected non-empty prompt")
 	}
-	if got != "<｜User｜>\nHello<｜end▁of▁sentence｜>" {
+	if got != "<｜begin▁of▁sentence｜>\n\n<｜User｜>\nHello<｜end▁of▁sentence｜>\n\n<｜Assistant｜></think>" {
 		t.Fatalf("unexpected prompt: %q", got)
 	}
 }
@@ -29,10 +29,13 @@ func TestMessagesPrepareRoles(t *testing.T) {
 	if !contains(got, "<｜System｜>\nYou are helper<｜end▁of▁instructions｜>\n\n<｜User｜>\nHi<｜end▁of▁sentence｜>") {
 		t.Fatalf("expected system/user separation in %q", got)
 	}
-	if !contains(got, "<｜User｜>\nHi<｜end▁of▁sentence｜>\n\n<｜Assistant｜>\nHello<｜end▁of▁sentence｜>") {
+	if !contains(got, "<｜begin▁of▁sentence｜>") {
+		t.Fatalf("expected begin marker in %q", got)
+	}
+	if !contains(got, "<｜User｜>\nHi<｜end▁of▁sentence｜>\n\n<｜Assistant｜>\n</think>Hello<｜end▁of▁sentence｜>") {
 		t.Fatalf("expected user/assistant separation in %q", got)
 	}
-	if !contains(got, "<｜Assistant｜>\nHello<｜end▁of▁sentence｜>\n\n<｜Tool｜>\nSearch results<｜end▁of▁toolresults｜>") {
+	if !contains(got, "<｜Assistant｜>\n</think>Hello<｜end▁of▁sentence｜>\n\n<｜Tool｜>\nSearch results<｜end▁of▁toolresults｜>") {
 		t.Fatalf("expected assistant/tool separation in %q", got)
 	}
 	if !contains(got, "<｜Tool｜>\nSearch results<｜end▁of▁toolresults｜>\n\n<｜User｜>\nHow are you<｜end▁of▁sentence｜>") {
@@ -74,7 +77,7 @@ func TestMessagesPrepareArrayTextVariants(t *testing.T) {
 		},
 	}
 	got := MessagesPrepare(messages)
-	if got != "<｜User｜>\nline1\nline2<｜end▁of▁sentence｜>" {
+	if got != "<｜begin▁of▁sentence｜>\n\n<｜User｜>\nline1\nline2<｜end▁of▁sentence｜>\n\n<｜Assistant｜></think>" {
 		t.Fatalf("unexpected content from text variants: %q", got)
 	}
 }

internal/util/util_edge_test.go

@@ -162,7 +162,7 @@ func TestMessagesPrepareMergesConsecutiveSameRole(t *testing.T) {
 		{"role": "user", "content": "World"},
 	}
 	got := MessagesPrepare(messages)
-	if !strings.HasPrefix(got, "<｜User｜>") {
+	if !strings.HasPrefix(got, "<｜begin▁of▁sentence｜>") {
 		t.Fatalf("expected user marker at the start, got %q", got)
 	}
 	if !strings.Contains(got, "Hello") || !strings.Contains(got, "World") {
@@ -193,7 +193,7 @@ func TestMessagesPrepareAssistantMarkers(t *testing.T) {
 	if strings.Count(got, "<｜end▁of▁sentence｜>") != 2 {
 		t.Fatalf("expected both turns to be terminated, got %q", got)
 	}
-	if !strings.Contains(got, "<｜Assistant｜>\nHello!<｜end▁of▁sentence｜>") {
+	if !strings.Contains(got, "<｜Assistant｜>\n</think>Hello!<｜end▁of▁sentence｜>") {
 		t.Fatalf("expected assistant EOS suffix, got %q", got)
 	}
 	if strings.Contains(got, "<system_instructions>") {