diff --git a/README.md b/README.md
index 8d8470b..a3713a0 100644
--- a/README.md
+++ b/README.md
@@ -183,11 +183,12 @@ Configurable middleware arguments include:
 **validator**
 
 - Type: `Callable[[str], bool]`
-- Default: `is_valid_uuid4` (
-  found [here](https://github.com/snok/asgi-correlation-id/blob/main/asgi_correlation_id/middleware.py#L17))
-- Description: The validator function is used when reading incoming HTTP header values. By default, we discard non-UUID
-  formatted header values, to enforce correlation ID uniqueness. If you prefer to allow any header value, you can set
-  this setting to `None`, or pass your own validator.
+- Default: `is_valid_uuid4`
+- Description: The validator function is used when reading incoming HTTP header values. By default, we accept any
+  32-character hex string that can be parsed as a UUID — this includes standard UUIDv4s, nginx-generated request IDs
+  (`$request_id`), and OpenTelemetry trace IDs. Non-hex or malformed values are rejected and a new ID is generated
+  instead. If you prefer to allow any header value, set this to `None`. For stricter UUIDv4 validation, pass your own
+  validator.
 
 **transformer**
 
diff --git a/asgi_correlation_id/middleware.py b/asgi_correlation_id/middleware.py
index a43d8c5..4ecc008 100644
--- a/asgi_correlation_id/middleware.py
+++ b/asgi_correlation_id/middleware.py
@@ -17,10 +17,16 @@
 
 def is_valid_uuid4(uuid_: str) -> bool:
     """
-    Check whether a string is a valid v4 uuid.
+    Check whether a string can be parsed as a UUID.
+
+    This is a loose check that accepts any 32-character hex string,
+    including IDs generated by nginx ($request_id) and OpenTelemetry
+    trace IDs. It does not strictly validate UUIDv4 version/variant bits.
+
+    For stricter validation, pass a custom ``validator`` to the middleware.
     """
     try:
-        return UUID(uuid_).version == 4
+        return bool(UUID(uuid_, version=4))
     except ValueError:
         return False
 
diff --git a/tests/test_middleware.py b/tests/test_middleware.py
index 1367589..8ca0598 100644
--- a/tests/test_middleware.py
+++ b/tests/test_middleware.py
@@ -174,9 +174,14 @@ async def test_custom_generator():
 
 
 def test_is_valid_uuid4():
+    # Standard UUIDv4 strings
     assert is_valid_uuid4('3758c31e-1177-4540-ba33-0109c405579a') is True
     assert is_valid_uuid4('9e6454c4-21d5-4e4a-a66a-b28f15576414') is True
     assert is_valid_uuid4('9e6454c421d54e4aa66ab28f15576414') is True
+    # Nginx-style 16 random bytes as hex (not strictly UUIDv4, but should be accepted)
+    assert is_valid_uuid4('c10f7ebebd95e5bb8749430d3485370c') is True
+    # All zeros (accepted by loose check)
+    assert is_valid_uuid4('00000000000000000000000000000000') is True
+    # Invalid strings
     assert is_valid_uuid4('foo') is False
     assert is_valid_uuid4('9e6454c4-21d5-4e4a-a66a-b28f15576414-1') is False
-    assert is_valid_uuid4('00000000000000000000000000000000') is False