diff --git a/tradingagents/default_config.py b/tradingagents/default_config.py
index 89b51765..7498d188 100644
--- a/tradingagents/default_config.py
+++ b/tradingagents/default_config.py
@@ -11,7 +11,12 @@ DEFAULT_CONFIG = {
     "llm_provider": "openai",
     "deep_think_llm": "gpt-5.4",
     "quick_think_llm": "gpt-5.4-mini",
-    "backend_url": "https://api.openai.com/v1",
+    # When None, each provider's client falls back to its own default endpoint
+    # (api.openai.com for OpenAI, generativelanguage.googleapis.com for Gemini, ...).
+    # The CLI overrides this per provider when the user picks one. Keeping a
+    # provider-specific URL here would leak (e.g. OpenAI's /v1 was previously
+    # being forwarded to Gemini, producing malformed request URLs).
+    "backend_url": None,
     # Provider-specific thinking configuration
     "google_thinking_level": None,      # "high", "minimal", etc.
     "openai_reasoning_effort": None,    # "medium", "high", "low"