diff --git a/autogpt_platform/backend/backend/copilot/config.py b/autogpt_platform/backend/backend/copilot/config.py
index 8792717cad..36644de680 100644
--- a/autogpt_platform/backend/backend/copilot/config.py
+++ b/autogpt_platform/backend/backend/copilot/config.py
@@ -156,9 +156,10 @@ class ChatConfig(BaseSettings):
         "history compression. Falls back to compression when unavailable.",
     )
     claude_agent_fallback_model: str = Field(
-        default="claude-sonnet-4-6",
+        default="",
         description="Fallback model when the primary model is unavailable (e.g. 529 "
-        "overloaded). The SDK automatically retries with this cheaper model.",
+        "overloaded). The SDK automatically retries with this cheaper model. "
+        "Empty string disables the fallback (no --fallback-model flag passed to CLI).",
     )
     claude_agent_max_turns: int = Field(
         default=50,
diff --git a/autogpt_platform/backend/backend/copilot/sdk/p0_guardrails_test.py b/autogpt_platform/backend/backend/copilot/sdk/p0_guardrails_test.py
index 9305320fea..17b54797b8 100644
--- a/autogpt_platform/backend/backend/copilot/sdk/p0_guardrails_test.py
+++ b/autogpt_platform/backend/backend/copilot/sdk/p0_guardrails_test.py
@@ -86,15 +86,14 @@ class TestResolveFallbackModel:
         assert result == "claude-sonnet-4.5-20250514"
 
     def test_default_value(self):
-        """Default fallback model resolves to a valid string."""
+        """Default fallback model resolves to None (disabled by default)."""
         cfg = _make_config()
         with patch(f"{_SVC}.config", cfg):
             from backend.copilot.sdk.service import _resolve_fallback_model
 
             result = _resolve_fallback_model()
 
-        assert result is not None
-        assert "sonnet" in result.lower() or "claude" in result.lower()
+        assert result is None
 
 
 # ---------------------------------------------------------------------------
@@ -198,8 +197,7 @@ class TestConfigDefaults:
 
     def test_fallback_model_default(self):
         cfg = _make_config()
-        assert cfg.claude_agent_fallback_model
-        assert "sonnet" in cfg.claude_agent_fallback_model.lower()
+        assert cfg.claude_agent_fallback_model == ""
 
     def test_max_turns_default(self):
         cfg = _make_config()