diff --git a/smart_router.py b/smart_router.py
index 14b90c03..feccc4eb 100644
--- a/smart_router.py
+++ b/smart_router.py
@@ -228,9 +228,14 @@ class SmartRouter:
         return min(available, key=lambda p: p.score(self.strategy))
 
     def get_model_for_provider(
-        self, provider: Provider, claude_model: str
+        self,
+        provider: Provider,
+        claude_model: str,
+        is_large_request: bool = False,
     ) -> str:
         """Map a Claude model name to the provider's actual model."""
+        if is_large_request:
+            return provider.big_model
         is_large = any(
             keyword in claude_model.lower()
             for keyword in ["opus", "sonnet", "large", "big"]
@@ -289,7 +294,11 @@ class SmartRouter:
             )
 
         provider = min(available, key=lambda p: p.score(self.strategy))
-        model = self.get_model_for_provider(provider, claude_model)
+        model = self.get_model_for_provider(
+            provider,
+            claude_model,
+            is_large_request=large,
+        )
 
         logger.debug(
             f"SmartRouter: routing to {provider.name}/{model} "
diff --git a/test_smart_router.py b/test_smart_router.py
index 97b73863..651b94f1 100644
--- a/test_smart_router.py
+++ b/test_smart_router.py
@@ -13,6 +13,11 @@ from smart_router import SmartRouter, Provider
 
 # ── Fixtures ──────────────────────────────────────────────────────────────────
 
+
+@pytest.fixture(autouse=True)
+def fake_api_key(monkeypatch):
+    monkeypatch.setenv("FAKE_KEY", "test-key")
+
 def make_provider(name, healthy=True, configured=True,
                   latency=100.0, cost=0.002, errors=0, requests=0):
     p = Provider(
@@ -122,6 +127,13 @@ def test_get_model_large_request():
     assert model == "openai-big"
 
 
+def test_get_model_large_message_overrides_claude_label():
+    p = make_provider("openai")
+    r = make_router()
+    model = r.get_model_for_provider(p, "claude-haiku", is_large_request=True)
+    assert model == "openai-big"
+
+
 def test_get_model_small_request():
     p = make_provider("openai")
     r = make_router()
@@ -140,6 +152,16 @@ async def test_route_returns_best_provider():
     assert result["provider"] == "cheap"
 
 
+@pytest.mark.asyncio
+async def test_route_uses_big_model_for_large_message_bodies():
+    p = make_provider("openai")
+    r = make_router(providers=[p])
+    result = await r.route([
+        {"role": "user", "content": "x" * 3001},
+    ], "claude-haiku")
+    assert result["model"] == "openai-big"
+
+
 @pytest.mark.asyncio
 async def test_route_raises_when_no_providers():
     p = make_provider("a", healthy=False)