diff --git a/apps/api/requirements.txt b/apps/api/requirements.txt
index d40b65d8..b1d3c461 100644
--- a/apps/api/requirements.txt
+++ b/apps/api/requirements.txt
@@ -40,7 +40,8 @@ opentelemetry-instrumentation-logging>=0.41b0
 # Langfuse (LLMOps Observability)
 # Phase 15.1: LLM 呼叫追蹤、成本監控、Prompt 版本管理
 # ==========================================================================
-langfuse>=2.0.0
+# 2026-04-01 Claude Code: 鎖定 v2.x — v4.x 改用 OTLP，與 langfuse_client.py 的 client.trace() 不相容
+langfuse>=2.0.0,<4.0.0
 
 # Development
 pytest>=7.4.0