diff --git a/src/dedalus_sdk/_client.py b/src/dedalus_sdk/_client.py
index 6117352..8222e28 100644
--- a/src/dedalus_sdk/_client.py
+++ b/src/dedalus_sdk/_client.py
@@ -201,9 +201,11 @@ def _validate_headers(
         params: Mapping[str, object],
         cookies: Mapping[str, str],
     ) -> None:
-        if headers.get("Authorization"):
+        headers = {key.lower(): value for key, value in headers.items()}
+        custom_headers = {key.lower(): value for key, value in custom_headers.items()}
+        if headers.get("authorization"):
             return
-        if isinstance(custom_headers.get("Authorization"), Omit):
+        if isinstance(custom_headers.get("authorization"), Omit):
             return
         if headers.get("x-api-key"):
             return
@@ -441,9 +443,11 @@ def _validate_headers(
         params: Mapping[str, object],
         cookies: Mapping[str, str],
     ) -> None:
-        if headers.get("Authorization"):
+        headers = {key.lower(): value for key, value in headers.items()}
+        custom_headers = {key.lower(): value for key, value in custom_headers.items()}
+        if headers.get("authorization"):
             return
-        if isinstance(custom_headers.get("Authorization"), Omit):
+        if isinstance(custom_headers.get("authorization"), Omit):
             return
         if headers.get("x-api-key"):
             return
diff --git a/tests/test_auth_header_case.py b/tests/test_auth_header_case.py
new file mode 100644
index 0000000..3fa3363
--- /dev/null
+++ b/tests/test_auth_header_case.py
@@ -0,0 +1,120 @@
+import httpx
+import pytest
+
+from dedalus_sdk import AsyncDedalus, Dedalus, omit
+
+HEADERS = [
+    ("Authorization", "Bearer fixture"),
+    ("authorization", "Bearer fixture"),
+    ("AUTHORIZATION", "Bearer fixture"),
+    ("x-api-key", "fixture"),
+    ("X-API-Key", "fixture"),
+    ("X-API-KEY", "fixture"),
+]
+
+
+@pytest.fixture(autouse=True)
+def clear_auth_environment(monkeypatch: pytest.MonkeyPatch) -> None:
+    for name in ("DEDALUS_API_KEY", "DEDALUS_X_API_KEY", "DEDALUS_CUSTOM_HEADERS"):
+        monkeypatch.delenv(name, raising=False)
+
+
+@pytest.mark.parametrize("header,value", HEADERS)
+@pytest.mark.parametrize("location", ["default", "request"])
+def test_sync_accepts_case_insensitive_auth_headers(header: str, value: str, location: str) -> None:
+    seen = []
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={"items": [], "next_cursor": None})
+
+    with Dedalus(
+        base_url="https://fixture.invalid",
+        http_client=httpx.Client(transport=httpx.MockTransport(handle)),
+        default_headers={header: value} if location == "default" else None,
+    ) as client:
+        page = client.machines.list(extra_headers={header: value} if location == "request" else None)
+        assert list(page) == []
+    assert len(seen) == 1
+    assert seen[0].headers[header] == value
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("header,value", HEADERS)
+@pytest.mark.parametrize("location", ["default", "request"])
+async def test_async_accepts_case_insensitive_auth_headers(header: str, value: str, location: str) -> None:
+    seen = []
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={"items": [], "next_cursor": None})
+
+    async with AsyncDedalus(
+        base_url="https://fixture.invalid",
+        http_client=httpx.AsyncClient(transport=httpx.MockTransport(handle)),
+        default_headers={header: value} if location == "default" else None,
+    ) as client:
+        page = await client.machines.list(extra_headers={header: value} if location == "request" else None)
+        assert [item async for item in page] == []
+    assert len(seen) == 1
+    assert seen[0].headers[header] == value
+
+
+@pytest.mark.parametrize("header", ["Authorization", "authorization", "x-api-key", "X-API-Key"])
+def test_sync_accepts_case_insensitive_explicit_omission(header: str) -> None:
+    seen = []
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={"items": []})
+
+    with Dedalus(
+        base_url="https://fixture.invalid", http_client=httpx.Client(transport=httpx.MockTransport(handle))
+    ) as client:
+        client.machines.list(extra_headers={header: omit})
+    assert len(seen) == 1
+    assert "authorization" not in seen[0].headers
+    assert "x-api-key" not in seen[0].headers
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("header", ["Authorization", "authorization", "x-api-key", "X-API-Key"])
+async def test_async_accepts_case_insensitive_explicit_omission(header: str) -> None:
+    seen = []
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={"items": []})
+
+    async with AsyncDedalus(
+        base_url="https://fixture.invalid", http_client=httpx.AsyncClient(transport=httpx.MockTransport(handle))
+    ) as client:
+        await client.machines.list(extra_headers={header: omit})
+    assert len(seen) == 1
+    assert "authorization" not in seen[0].headers
+    assert "x-api-key" not in seen[0].headers
+
+
+def test_sync_still_rejects_missing_auth() -> None:
+    def handle(request: httpx.Request) -> httpx.Response:
+        pytest.fail("An unauthenticated request must not be sent")
+
+    with (
+        Dedalus(
+            base_url="https://fixture.invalid", http_client=httpx.Client(transport=httpx.MockTransport(handle))
+        ) as client,
+        pytest.raises(TypeError, match="Could not resolve authentication"),
+    ):
+        client.machines.list()
+
+
+@pytest.mark.asyncio
+async def test_async_still_rejects_missing_auth() -> None:
+    def handle(request: httpx.Request) -> httpx.Response:
+        pytest.fail("An unauthenticated request must not be sent")
+
+    async with AsyncDedalus(
+        base_url="https://fixture.invalid", http_client=httpx.AsyncClient(transport=httpx.MockTransport(handle))
+    ) as client:
+        with pytest.raises(TypeError, match="Could not resolve authentication"):
+            await client.machines.list()