diff --git a/client.py b/client.py
index a81a66db..12121c2a 100644
--- a/client.py
+++ b/client.py
@@ -472,6 +472,12 @@ def create_client(
         elif "ANTHROPIC_BASE_URL" in sdk_env:
             print(f"   - GLM Mode: Using {sdk_env['ANTHROPIC_BASE_URL']}")
 
+    # ADDED: Confirm subscription-only mode for default Claude provider
+    # This prevents accidental API credit consumption when user has a Claude Code subscription
+    if not is_alternative_api and not base_url:
+        print("   ✓ Claude Code Subscription Mode: API credit usage DISABLED")
+        print("   ✓ Using OAuth token in subscription mode only")
+
     # Create a wrapper for bash_security_hook that passes project_dir via context
     async def bash_hook_with_context(input_data, tool_use_id=None, context=None):
         """Wrapper that injects project_dir into context for security hook."""
@@ -552,6 +558,26 @@ async def pre_compact_hook(
             }
         )
 
+    # CRITICAL SAFETY CHECK: Ensure we're not in API mode for default Claude provider
+    # This prevents accidental API charges when user has a Claude Code subscription.
+    # The check must happen BEFORE creating the SDK client to catch configuration errors early.
+    from registry import get_all_settings
+    all_settings = get_all_settings()
+    provider_id = all_settings.get("api_provider", "claude")
+    if provider_id == "claude" and not is_alternative_api:
+        # Verify no API credentials are being passed that would trigger API billing
+        if sdk_env.get("ANTHROPIC_API_KEY") and sdk_env.get("ANTHROPIC_API_KEY") != "":
+            raise RuntimeError(
+                "SAFETY CHECK FAILED: ANTHROPIC_API_KEY detected for Claude provider! "
+                "This would consume API credits. Check registry settings and environment variables."
+            )
+        if sdk_env.get("ANTHROPIC_AUTH_TOKEN") and sdk_env.get("ANTHROPIC_AUTH_TOKEN") != "":
+            raise RuntimeError(
+                "SAFETY CHECK FAILED: ANTHROPIC_AUTH_TOKEN detected for Claude provider! "
+                "This would consume API credits. Check registry settings and environment variables."
+            )
+        print("   ✓ Safety check passed: No API credentials detected")
+
     # PROMPT CACHING: The Claude Code CLI applies cache_control breakpoints internally.
     # Our system_prompt benefits from automatic caching without explicit configuration.
     # If explicit cache_control is needed, the SDK would need to accept content blocks
diff --git a/registry.py b/registry.py
index 30765198..af821e94 100644
--- a/registry.py
+++ b/registry.py
@@ -710,13 +710,16 @@ def get_effective_sdk_env() -> dict[str, str]:
     provider_id = all_settings.get("api_provider", "claude")
 
     if provider_id == "claude":
-        # Default behavior: forward existing env vars
-        from env_constants import API_ENV_VARS
-        sdk_env: dict[str, str] = {}
-        for var in API_ENV_VARS:
-            value = os.getenv(var)
-            if value:
-                sdk_env[var] = value
+        # MODIFIED: Force subscription-only mode by explicitly disabling API
+        # This prevents the SDK from using OAuth tokens in API mode, which would
+        # charge the user's Anthropic API account instead of using their Claude Code subscription.
+        sdk_env: dict[str, str] = {
+            # Explicitly clear API authentication to prevent unexpected charges
+            "ANTHROPIC_API_KEY": "",
+            "ANTHROPIC_AUTH_TOKEN": "",
+        }
+        # Log that we're in subscription-only mode
+        logger.info("Claude Code subscription mode: API authentication explicitly disabled")
         return sdk_env
 
     # Alternative provider: build env from settings