diff --git a/main.go b/main.go
index ee77df1..60f5e2b 100644
--- a/main.go
+++ b/main.go
@@ -835,7 +835,13 @@ type streamDelta struct {
 func cleanMessagesForLLM(msgs []Message) []Message {
   out := make([]Message, len(msgs))
   for i, m := range msgs {
-    out[i] = Message{Role: m.Role, Content: m.Content, ToolCalls: m.ToolCalls, ToolCallID: m.ToolCallID}
+    out[i] = Message{
+      Role:             m.Role,
+      Content:          m.Content,
+      ReasoningContent: m.ReasoningContent,
+      ToolCalls:        m.ToolCalls,
+      ToolCallID:       m.ToolCallID,
+    }
   }
   return out
 }
@@ -855,17 +861,22 @@ func filterText(s string) string {
 
 func sanitizeMessages(msgs []Message) {
   for i := range msgs {
-    if msgs[i].Role == "assistant" && len(msgs[i].ToolCalls) > 0 {
-      for j := range msgs[i].ToolCalls {
-        tc := &msgs[i].ToolCalls[j]
-        tc.Function.Arguments = filterText(tc.Function.Arguments)
-        astr := tc.Function.Arguments
-        var a map[string]any
-        if err := json.Unmarshal([]byte(astr), &a); err != nil || a == nil {
-          fixed, _ := json.Marshal(map[string]string{"invalid_raw": astr})
-          tc.Function.Arguments = string(fixed)
+    if msgs[i].Role == "assistant" {
+      if len(msgs[i].ToolCalls) > 0 {
+        for j := range msgs[i].ToolCalls {
+          tc := &msgs[i].ToolCalls[j]
+          tc.Function.Arguments = filterText(tc.Function.Arguments)
+          astr := tc.Function.Arguments
+          var a map[string]any
+          if err := json.Unmarshal([]byte(astr), &a); err != nil || a == nil {
+            fixed, _ := json.Marshal(map[string]string{"invalid_raw": astr})
+            tc.Function.Arguments = string(fixed)
+          }
         }
       }
+      if msgs[i].ReasoningContent != "" {
+        msgs[i].ReasoningContent = filterText(msgs[i].ReasoningContent)
+      }
     } else if msgs[i].Role == "tool" && msgs[i].Content != nil {
       msgs[i].Content = strp(filterText(*msgs[i].Content))
     }
diff --git a/main_test.go b/main_test.go
index 02cf06a..e4136f0 100644
--- a/main_test.go
+++ b/main_test.go
@@ -7,6 +7,7 @@ import (
   "encoding/json"
   "errors"
   "fmt"
+  "io"
   "net"
   "net/http"
   "net/http/httptest"
@@ -2282,3 +2283,109 @@ func TestSOCKS5ProxySupport(t *testing.T) {
     t.Fatalf("timed out waiting for SOCKS5 handshake through proxy")
   }
 }
+
+func TestCleanMessagesForLLMPreservesReasoningContent(t *testing.T) {
+  msgs := []Message{
+    {Role: "user", Content: strp("hello")},
+    {Role: "assistant", Content: strp("done"), ReasoningContent: "thinking steps"},
+    {Role: "tool", ToolCallID: "tc1", Content: strp("result")},
+  }
+  cleaned := cleanMessagesForLLM(msgs)
+  if len(cleaned) != 3 {
+    t.Fatalf("expected 3 messages, got %d", len(cleaned))
+  }
+  if cleaned[1].ReasoningContent != "thinking steps" {
+    t.Errorf("expected ReasoningContent %q, got %q", "thinking steps", cleaned[1].ReasoningContent)
+  }
+
+  // Verify JSON serialization includes reasoning_content for assistant
+  b, err := json.Marshal(cleaned[1])
+  if err != nil {
+    t.Fatalf("marshal error: %v", err)
+  }
+  if !strings.Contains(string(b), `"reasoning_content":"thinking steps"`) {
+    t.Errorf("expected JSON to contain reasoning_content, got %s", string(b))
+  }
+
+  // Verify JSON serialization omits reasoning_content when empty
+  bUser, err := json.Marshal(cleaned[0])
+  if err != nil {
+    t.Fatalf("marshal error: %v", err)
+  }
+  if strings.Contains(string(bUser), "reasoning_content") {
+    t.Errorf("expected JSON to omit reasoning_content for empty, got %s", string(bUser))
+  }
+}
+
+func TestLLMPreservesReasoningContentInRequestBody(t *testing.T) {
+  var receivedBody []byte
+  srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+    var err error
+    receivedBody, err = io.ReadAll(r.Body)
+    if err != nil {
+      t.Errorf("read body err: %v", err)
+    }
+    w.Header().Set("Content-Type", "application/json")
+    w.Write([]byte(`{"choices":[{"message":{"role":"assistant","content":"reply"}}]}`))
+  }))
+  defer srv.Close()
+
+  cfg := defCfg
+  cfg.Endpoint = srv.URL
+  cfg.Stream = false
+  cfg.APIKey = "-"
+
+  msgs := []Message{
+    {Role: "user", Content: strp("call tool")},
+    {
+      Role:             "assistant",
+      Content:          strp(""),
+      ReasoningContent: "deep thoughts about tool",
+      ToolCalls: []ToolCall{
+        {
+          ID:   "call_1",
+          Type: "function",
+          Function: struct {
+            Name      string `json:"name"`
+            Arguments string `json:"arguments"`
+          }{Name: "shell_exec", Arguments: `{"command":"ls"}`},
+        },
+      },
+    },
+    {Role: "tool", ToolCallID: "call_1", Content: strp("file.txt\n\nexit: 0")},
+  }
+
+  _, _, err := llm(context.Background(), &cfg, msgs, nil)
+  if err != nil {
+    t.Fatalf("unexpected llm error: %v", err)
+  }
+
+  var reqPayload struct {
+    Messages []map[string]any `json:"messages"`
+  }
+  if err := json.Unmarshal(receivedBody, &reqPayload); err != nil {
+    t.Fatalf("unmarshal request payload: %v", err)
+  }
+  if len(reqPayload.Messages) != 3 {
+    t.Fatalf("expected 3 messages in request, got %d", len(reqPayload.Messages))
+  }
+  asstMsg := reqPayload.Messages[1]
+  rc, ok := asstMsg["reasoning_content"].(string)
+  if !ok || rc != "deep thoughts about tool" {
+    t.Errorf("expected assistant message reasoning_content %q, got %v", "deep thoughts about tool", asstMsg["reasoning_content"])
+  }
+}
+
+func TestSanitizeMessagesCleansReasoningContent(t *testing.T) {
+  msgs := []Message{
+    {Role: "assistant", ReasoningContent: "clean\x00\u200B\u00A0reasoning"},
+  }
+  sanitizeMessages(msgs)
+  if strings.ContainsAny(msgs[0].ReasoningContent, "\x00") || strings.Contains(msgs[0].ReasoningContent, "\u200B") {
+    t.Errorf("sanitizeMessages did not strip control/invisible characters from ReasoningContent: %q", msgs[0].ReasoningContent)
+  }
+  if msgs[0].ReasoningContent != "cleanreasoning" {
+    t.Errorf("expected 'cleanreasoning', got %q", msgs[0].ReasoningContent)
+  }
+}
+