diff --git a/internal/openai/prompt.go b/internal/openai/prompt.go
index d8d66ab..8918e05 100644
--- a/internal/openai/prompt.go
+++ b/internal/openai/prompt.go
@@ -237,6 +237,13 @@ func toolsPrompt(tools []ToolDef, toolChoice json.RawMessage) string {
 		"",
 		`{"tool_calls": [{"id": "call_xxx", "type": "function", "function": {"name": "tool_name", "arguments": "{\"param\": \"value\"}"}}]}`,
 		"",
+		// DeepSeek 模型有时会退回自己的 DSML 标记方言（<|DSML|> calls> /
+		// invoke / parameter），网关不解析该方言，整段会被当普通文本透传，
+		// 客户端收不到 tool_calls——指令层显式禁止，只认上面的 JSON。
+		"Never call tools with DSML/XML-style markup (for example `<|DSML|> calls>` or " +
+			"`<|DSML|> invoke name=\"...\">`). Those tags are discarded — only the " +
+			"JSON format above is accepted.",
+		"",
 		"Available tools:",
 	}
 	for i, tool := range tools {
diff --git a/internal/openai/prompt_test.go b/internal/openai/prompt_test.go
index f629609..bd0be05 100644
--- a/internal/openai/prompt_test.go
+++ b/internal/openai/prompt_test.go
@@ -36,6 +36,24 @@ func TestBuildPromptWithTools(t *testing.T) {
 	}
 }
 
+// 红测试：上游模型曾用 DeepSeek 原生 DSML 标记（<|DSML|> calls> / invoke /
+// parameter）代替 JSON 输出工具调用，网关三层（流式、解析、续写）都只认
+// JSON，整段标记被当普通文本发给客户端。指令层必须显式禁止该方言。
+func TestToolsPromptForbidsDSML(t *testing.T) {
+	prompt := BuildPrompt([]ChatMessage{
+		{Role: "user", Content: MessageContent{Text: "Hi"}},
+	}, PromptOptions{Tools: []ToolDef{{
+		Type:     "function",
+		Function: Function{Name: "write", Description: "Write a file"},
+	}}})
+	if !strings.Contains(prompt, "DSML") {
+		t.Error("tool instructions must explicitly forbid DSML markup")
+	}
+	if !strings.Contains(prompt, "Never") {
+		t.Error("tool instructions must state the DSML ban as a prohibition")
+	}
+}
+
 func TestBuildPromptAppendsSamplingConstraints(t *testing.T) {
 	temp := 0.4
 	max := 512