diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index 29f2b672..0ae55ad3 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -172,6 +172,11 @@ func applyCodexOAuthTransform(reqBody map[string]any, isCodexCLI bool, isCompact
 		result.PromptCacheKey = strings.TrimSpace(v)
 	}
 
+	// 提取 input 中 role:"system" 消息至 instructions（OAuth 上游不支持 system role）。
+	if extractSystemMessagesFromInput(reqBody) {
+		result.Modified = true
+	}
+
 	// instructions 处理逻辑：根据是否是 Codex CLI 分别调用不同方法
 	if applyInstructions(reqBody, isCodexCLI) {
 		result.Modified = true
@@ -301,6 +306,73 @@ func getNormalizedCodexModel(modelID string) string {
 	return ""
 }
 
+// extractTextFromContent extracts plain text from a content value that is either
+// a Go string or a []any of content-part maps with type:"text".
+func extractTextFromContent(content any) string {
+	switch v := content.(type) {
+	case string:
+		return v
+	case []any:
+		var parts []string
+		for _, part := range v {
+			m, ok := part.(map[string]any)
+			if !ok {
+				continue
+			}
+			if t, _ := m["type"].(string); t == "text" {
+				if text, ok := m["text"].(string); ok {
+					parts = append(parts, text)
+				}
+			}
+		}
+		return strings.Join(parts, "")
+	default:
+		return ""
+	}
+}
+
+// extractSystemMessagesFromInput scans the input array for items with role=="system",
+// removes them, and merges their content into reqBody["instructions"].
+// If instructions is already non-empty, extracted content is prepended with "\n\n".
+// Returns true if any system messages were extracted.
+func extractSystemMessagesFromInput(reqBody map[string]any) bool {
+	input, ok := reqBody["input"].([]any)
+	if !ok || len(input) == 0 {
+		return false
+	}
+
+	var systemTexts []string
+	remaining := make([]any, 0, len(input))
+
+	for _, item := range input {
+		m, ok := item.(map[string]any)
+		if !ok {
+			remaining = append(remaining, item)
+			continue
+		}
+		if role, _ := m["role"].(string); role != "system" {
+			remaining = append(remaining, item)
+			continue
+		}
+		if text := extractTextFromContent(m["content"]); text != "" {
+			systemTexts = append(systemTexts, text)
+		}
+	}
+
+	if len(systemTexts) == 0 {
+		return false
+	}
+
+	extracted := strings.Join(systemTexts, "\n\n")
+	if existing, ok := reqBody["instructions"].(string); ok && strings.TrimSpace(existing) != "" {
+		reqBody["instructions"] = extracted + "\n\n" + existing
+	} else {
+		reqBody["instructions"] = extracted
+	}
+	reqBody["input"] = remaining
+	return true
+}
+
 // applyInstructions 处理 instructions 字段：仅在 instructions 为空时填充默认值。
 func applyInstructions(reqBody map[string]any, isCodexCLI bool) bool {
 	if !isInstructionsEmpty(reqBody) {
diff --git a/backend/internal/service/openai_codex_transform_test.go b/backend/internal/service/openai_codex_transform_test.go
index ae6f8555..b52f0566 100644
--- a/backend/internal/service/openai_codex_transform_test.go
+++ b/backend/internal/service/openai_codex_transform_test.go
@@ -344,6 +344,135 @@ func TestApplyCodexOAuthTransform_StringInputWithToolsField(t *testing.T) {
 	require.Len(t, input, 1)
 }
 
+func TestExtractSystemMessagesFromInput(t *testing.T) {
+	t.Run("no system messages", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{"role": "user", "content": "hello"},
+			},
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.False(t, result)
+		input, ok := reqBody["input"].([]any)
+		require.True(t, ok)
+		require.Len(t, input, 1)
+		_, hasInstructions := reqBody["instructions"]
+		require.False(t, hasInstructions)
+	})
+
+	t.Run("string content system message", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{"role": "system", "content": "You are an assistant."},
+				map[string]any{"role": "user", "content": "hello"},
+			},
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.True(t, result)
+		input, ok := reqBody["input"].([]any)
+		require.True(t, ok)
+		require.Len(t, input, 1)
+		msg, ok := input[0].(map[string]any)
+		require.True(t, ok)
+		require.Equal(t, "user", msg["role"])
+		require.Equal(t, "You are an assistant.", reqBody["instructions"])
+	})
+
+	t.Run("array content system message", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{
+					"role": "system",
+					"content": []any{
+						map[string]any{"type": "text", "text": "Be helpful."},
+					},
+				},
+			},
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.True(t, result)
+		require.Equal(t, "Be helpful.", reqBody["instructions"])
+		input, ok := reqBody["input"].([]any)
+		require.True(t, ok)
+		require.Len(t, input, 0)
+	})
+
+	t.Run("multiple system messages concatenated", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{"role": "system", "content": "First."},
+				map[string]any{"role": "system", "content": "Second."},
+				map[string]any{"role": "user", "content": "hi"},
+			},
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.True(t, result)
+		require.Equal(t, "First.\n\nSecond.", reqBody["instructions"])
+		input, ok := reqBody["input"].([]any)
+		require.True(t, ok)
+		require.Len(t, input, 1)
+	})
+
+	t.Run("mixed system and non-system preserves non-system", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{"role": "user", "content": "hello"},
+				map[string]any{"role": "system", "content": "Sys prompt."},
+				map[string]any{"role": "assistant", "content": "Hi there"},
+			},
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.True(t, result)
+		input, ok := reqBody["input"].([]any)
+		require.True(t, ok)
+		require.Len(t, input, 2)
+		first, ok := input[0].(map[string]any)
+		require.True(t, ok)
+		require.Equal(t, "user", first["role"])
+		second, ok := input[1].(map[string]any)
+		require.True(t, ok)
+		require.Equal(t, "assistant", second["role"])
+	})
+
+	t.Run("existing instructions prepended", func(t *testing.T) {
+		reqBody := map[string]any{
+			"input": []any{
+				map[string]any{"role": "system", "content": "Extracted."},
+				map[string]any{"role": "user", "content": "hi"},
+			},
+			"instructions": "Existing instructions.",
+		}
+		result := extractSystemMessagesFromInput(reqBody)
+		require.True(t, result)
+		require.Equal(t, "Extracted.\n\nExisting instructions.", reqBody["instructions"])
+	})
+}
+
+func TestApplyCodexOAuthTransform_ExtractsSystemMessages(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "gpt-5.1",
+		"input": []any{
+			map[string]any{"role": "system", "content": "You are a coding assistant."},
+			map[string]any{"role": "user", "content": "Write a function."},
+		},
+	}
+
+	result := applyCodexOAuthTransform(reqBody, false, false)
+
+	require.True(t, result.Modified)
+
+	input, ok := reqBody["input"].([]any)
+	require.True(t, ok)
+	require.Len(t, input, 1)
+	msg, ok := input[0].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "user", msg["role"])
+
+	instructions, ok := reqBody["instructions"].(string)
+	require.True(t, ok)
+	require.Equal(t, "You are a coding assistant.", instructions)
+}
+
 func TestIsInstructionsEmpty(t *testing.T) {
 	tests := []struct {
 		name     string