diff --git a/docs/overview.mdx b/docs/overview.mdx
index 3980c0bec..e91085d5e 100644
--- a/docs/overview.mdx
+++ b/docs/overview.mdx
@@ -20,7 +20,7 @@ from openrouter import OpenRouter
 import os
 
 with OpenRouter(
-    api_key=os.getenv("OPENROUTER_API_KEY", ""),
+    api_key=os.getenv("OPENROUTER_API_KEY"),
 ) as open_router:
     res = open_router.chat.send(
         messages=[
diff --git a/src/openrouter/_hooks/registration.py b/src/openrouter/_hooks/registration.py
index cab477877..1f133f1a6 100644
--- a/src/openrouter/_hooks/registration.py
+++ b/src/openrouter/_hooks/registration.py
@@ -1,4 +1,11 @@
-from .types import Hooks
+import os
+
+import httpx
+
+from openrouter import components
+from openrouter.sdkconfiguration import SDKConfiguration
+
+from .types import BeforeRequestContext, BeforeRequestHook, Hooks, SDKInitHook
 
 
 # This file is only ever generated once on the first generation and then is free to be modified.
@@ -6,8 +13,47 @@
 # in this file or in separate files in the hooks folder.
 
 
+class BlankAPIKeyIsUnsetHook(SDKInitHook, BeforeRequestHook):
+    """Treat a blank `api_key` (e.g. from `os.getenv("OPENROUTER_API_KEY", "")`)
+    as not supplied, so the `OPENROUTER_API_KEY` env-var fallback still applies
+    instead of sending an empty `Bearer ` token.
+
+    The env var is read per request and a callable `api_key` is resolved per
+    request, so either can still yield a blank token after init; a blank
+    bearer token is replaced with `OPENROUTER_API_KEY` when that is set, and
+    otherwise the `Authorization` header is dropped.
+    """
+
+    def sdk_init(self, config: SDKConfiguration) -> SDKConfiguration:
+        security = config.security
+        if isinstance(security, components.Security) and not (
+            security.api_key or ""
+        ).strip():
+            config.security = None
+        return config
+
+    def before_request(
+        self, hook_ctx: BeforeRequestContext, request: httpx.Request
+    ) -> httpx.Request:
+        auth = request.headers.get("authorization")
+        if auth is not None:
+            scheme, _, token = auth.strip().partition(" ")
+            if not auth.strip() or (scheme.lower() == "bearer" and not token.strip()):
+                env_key = (os.getenv("OPENROUTER_API_KEY") or "").strip()
+                env_scheme, _, env_token = env_key.partition(" ")
+                if env_scheme.lower() == "bearer":
+                    env_key = env_token.strip()
+                if env_key:
+                    request.headers["authorization"] = f"Bearer {env_key}"
+                else:
+                    del request.headers["authorization"]
+        return request
+
+
 def init_hooks(hooks: Hooks):
-    # pylint: disable=unused-argument
     """Add hooks by calling hooks.register{sdk_init/before_request/after_success/after_error}Hook
     with an instance of a hook that implements that specific Hook interface
     Hooks are registered per SDK instance, and are valid for the lifetime of the SDK instance"""
+    blank_api_key_hook = BlankAPIKeyIsUnsetHook()
+    hooks.register_sdk_init_hook(blank_api_key_hook)
+    hooks.register_before_request_hook(blank_api_key_hook)
diff --git a/tests/test_api_key_resolution.py b/tests/test_api_key_resolution.py
new file mode 100644
index 000000000..d368dcbc4
--- /dev/null
+++ b/tests/test_api_key_resolution.py
@@ -0,0 +1,50 @@
+import httpx
+import pytest
+
+from openrouter import OpenRouter
+
+
+def _authorization_header(**kwargs):
+    seen = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request.headers.get("authorization"))
+        return httpx.Response(200, json={"data": {"count": 0}})
+
+    client = httpx.Client(transport=httpx.MockTransport(handler))
+    try:
+        OpenRouter(client=client, **kwargs).models.count()
+    except Exception:  # pylint: disable=broad-except
+        pass  # response shape is irrelevant; only the request header matters
+    assert seen, "no request was sent"
+    return seen[0]
+
+
+@pytest.mark.parametrize("api_key", ["", "   "])
+def test_blank_api_key_falls_back_to_env_var(monkeypatch, api_key):
+    monkeypatch.setenv("OPENROUTER_API_KEY", "from-env")
+    assert _authorization_header(api_key=api_key) == "Bearer from-env"
+
+
+def test_blank_api_key_without_env_var_sends_no_authorization(monkeypatch):
+    monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
+    assert _authorization_header(api_key="") is None
+
+
+@pytest.mark.parametrize("env_value", ["", "   "])
+def test_blank_api_key_and_blank_env_var_sends_no_authorization(
+    monkeypatch, env_value
+):
+    monkeypatch.setenv("OPENROUTER_API_KEY", env_value)
+    assert _authorization_header(api_key="") is None
+
+
+@pytest.mark.parametrize("env_value", ["from-env", "Bearer from-env"])
+def test_blank_callable_api_key_falls_back_to_env_var(monkeypatch, env_value):
+    monkeypatch.setenv("OPENROUTER_API_KEY", env_value)
+    assert _authorization_header(api_key=lambda: "") == "Bearer from-env"
+
+
+def test_explicit_api_key_wins_over_env_var(monkeypatch):
+    monkeypatch.setenv("OPENROUTER_API_KEY", "from-env")
+    assert _authorization_header(api_key="explicit") == "Bearer explicit"