fix: preserve reasoning_content in multi-turn conversation history

The openaiMessage struct and stripSystemParts() were not carrying over
the ReasoningContent field when serializing conversation history for
API requests. This caused thinking models (e.g. kimi-k2.5) to receive
incomplete assistant messages on subsequent turns, resulting in 400
errors from the Moonshot API.

Add the ReasoningContent field to openaiMessage and copy it in
stripSystemParts(). Also add a test to verify reasoning_content is
preserved when sending conversation history.

Fixes #588
Related: #876

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/providers/openai_compat/provider.go

@@ -289,10 +289,11 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 // It mirrors protocoltypes.Message but omits SystemParts, which is an
 // internal field that would be unknown to third-party endpoints.
 type openaiMessage struct {
-	Role       string     `json:"role"`
-	Content    string     `json:"content"`
-	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
-	ToolCallID string     `json:"tool_call_id,omitempty"`
+	Role             string     `json:"role"`
+	Content          string     `json:"content"`
+	ReasoningContent string     `json:"reasoning_content,omitempty"`
+	ToolCalls        []ToolCall `json:"tool_calls,omitempty"`
+	ToolCallID       string     `json:"tool_call_id,omitempty"`
 }
 
 // stripSystemParts converts []Message to []openaiMessage, dropping the
@@ -302,10 +303,11 @@ func stripSystemParts(messages []Message) []openaiMessage {
 	out := make([]openaiMessage, len(messages))
 	for i, m := range messages {
 		out[i] = openaiMessage{
-			Role:       m.Role,
-			Content:    m.Content,
-			ToolCalls:  m.ToolCalls,
-			ToolCallID: m.ToolCallID,
+			Role:             m.Role,
+			Content:          m.Content,
+			ReasoningContent: m.ReasoningContent,
+			ToolCalls:        m.ToolCalls,
+			ToolCallID:       m.ToolCallID,
 		}
 	}
 	return out

pkg/providers/openai_compat/provider_test.go

@@ -146,6 +146,56 @@ func TestProviderChat_ParsesReasoningContent(t *testing.T) {
 	}
 }
 
+func TestProviderChat_PreservesReasoningContentInHistory(t *testing.T) {
+	var requestBody map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := json.NewDecoder(r.Body).Decode(&requestBody); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "")
+
+	// Simulate a multi-turn conversation where the assistant's previous
+	// reply included reasoning_content (e.g. from kimi-k2.5).
+	messages := []Message{
+		{Role: "user", Content: "What is 1+1?"},
+		{Role: "assistant", Content: "2", ReasoningContent: "Let me think... 1+1=2"},
+		{Role: "user", Content: "What about 2+2?"},
+	}
+
+	_, err := p.Chat(t.Context(), messages, nil, "kimi-k2.5", nil)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+
+	// Verify reasoning_content is preserved in the serialized request.
+	reqMessages, ok := requestBody["messages"].([]any)
+	if !ok {
+		t.Fatalf("messages is not []any: %T", requestBody["messages"])
+	}
+	assistantMsg, ok := reqMessages[1].(map[string]any)
+	if !ok {
+		t.Fatalf("assistant message is not map[string]any: %T", reqMessages[1])
+	}
+	if assistantMsg["reasoning_content"] != "Let me think... 1+1=2" {
+		t.Errorf("reasoning_content not preserved in request, got %v", assistantMsg["reasoning_content"])
+	}
+}
+
 func TestProviderChat_HTTPError(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bad request", http.StatusBadRequest)