diff --git a/README.md b/README.md
index adad85e..6d0a890 100644
--- a/README.md
+++ b/README.md
@@ -181,7 +181,11 @@ These are real outputs from `examples/basic.py`.
 | `router.fitting(task, limits=None) -> list[ModelInfo]` | Returns the models that pass the limits, without calling Jev (free). |
 | `Limits(output_tokens=1024, max_cost_usd=None)` | The output size you expect and an optional cost cap for each call. |
 
-Errors (all subclasses of `RouterError`):
+`Limits` validates its arguments when created: `output_tokens` must be an integer of at least 1,
+and `max_cost_usd` must be `None` (no cost cap) or a nonnegative `int` or `float` (zero is allowed).
+Booleans and NaN are rejected. Invalid limits raise `ValueError` with the field name and required range.
+
+Routing errors (all subclasses of `RouterError`):
 - `NoModelFitsError`: no model passes the limits. The message gives the reason for each model.
 - `UnknownModelError`: a model id isn't on OpenRouter.
 - `RouterError`: no routing key, a network or HTTP failure, or an error returned by Jev (e.g. a rate limit).
diff --git a/src/model_router/models.py b/src/model_router/models.py
index 0ff700c..5e1f0da 100644
--- a/src/model_router/models.py
+++ b/src/model_router/models.py
@@ -36,3 +36,17 @@ def from_openrouter(cls, raw):
 class Limits:
     output_tokens: int = 1024
     max_cost_usd: float | None = None
+
+    def __post_init__(self):
+        if (
+            isinstance(self.output_tokens, bool)
+            or not isinstance(self.output_tokens, int)
+            or self.output_tokens < 1
+        ):
+            raise ValueError("output_tokens must be an integer >= 1")
+        if self.max_cost_usd is not None and (
+            isinstance(self.max_cost_usd, bool)
+            or not isinstance(self.max_cost_usd, (int, float))
+            or not self.max_cost_usd >= 0
+        ):
+            raise ValueError("max_cost_usd must be None or a number >= 0")
diff --git a/tests/test_models.py b/tests/test_models.py
index f415f38..963c8df 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -56,6 +56,28 @@ class LimitsTest(unittest.TestCase):
     def test_defaults(self):
         self.assertEqual(Limits(), Limits(output_tokens=1024, max_cost_usd=None))
 
+    def test_positive_integer_output_tokens_are_accepted(self):
+        for output_tokens in (1, 500, 1024):
+            with self.subTest(output_tokens=output_tokens):
+                self.assertEqual(Limits(output_tokens=output_tokens).output_tokens, output_tokens)
+
+    def test_invalid_output_tokens_raise_value_error(self):
+        for output_tokens in (0, -5, 1.0, 1.5, "1", None, True, False):
+            with self.subTest(output_tokens=output_tokens):
+                with self.assertRaisesRegex(ValueError, "output_tokens must be an integer >= 1"):
+                    Limits(output_tokens=output_tokens)
+
+    def test_optional_nonnegative_cost_is_accepted(self):
+        for max_cost_usd in (None, 0, 0.0, 0.001, 1):
+            with self.subTest(max_cost_usd=max_cost_usd):
+                self.assertEqual(Limits(max_cost_usd=max_cost_usd).max_cost_usd, max_cost_usd)
+
+    def test_invalid_cost_raises_value_error(self):
+        for max_cost_usd in (-1, -0.001, float("nan"), "0", True, False):
+            with self.subTest(max_cost_usd=max_cost_usd):
+                with self.assertRaisesRegex(ValueError, "max_cost_usd must be None or a number >= 0"):
+                    Limits(max_cost_usd=max_cost_usd)
+
     def test_is_immutable(self):
         with self.assertRaises(dataclasses.FrozenInstanceError):
             Limits().output_tokens = 5