diff --git a/api/organisations/usage_reporting/mappers.py b/api/organisations/usage_reporting/mappers.py
index 38292a749d70..cb12d567f6b0 100644
--- a/api/organisations/usage_reporting/mappers.py
+++ b/api/organisations/usage_reporting/mappers.py
@@ -17,10 +17,9 @@
 MAX_PROJECT_USAGE_ROWS = 5_000
 
 
-def map_signature_to_control_plane_auth_token(signature: str) -> str:
-    return (
-        base64.urlsafe_b64encode(signature.encode("utf-8")).decode("ascii").rstrip("=")
-    )
+def map_signature_to_control_plane_auth_token(signature_b64: str) -> str:
+    raw_signature = base64.b64decode(signature_b64)
+    return base64.urlsafe_b64encode(raw_signature).decode("ascii").rstrip("=")
 
 
 def map_usage_data_to_total_api_calls(usage_data: list[UsageData]) -> int:
diff --git a/api/organisations/usage_reporting/services.py b/api/organisations/usage_reporting/services.py
index 6474bba2afea..7f5d41369d68 100644
--- a/api/organisations/usage_reporting/services.py
+++ b/api/organisations/usage_reporting/services.py
@@ -31,12 +31,12 @@ def push_snapshot(
     *,
     base_url: str,
     snapshot: UsageSnapshot,
-    signature: str,
+    signature_b64: str,
 ) -> None:
     url = f"{base_url.rstrip('/')}{USAGE_ENDPOINT_PATH}"
     headers = {
         "Authorization": (
-            f"Bearer {map_signature_to_control_plane_auth_token(signature)}"
+            f"Bearer {map_signature_to_control_plane_auth_token(signature_b64)}"
         ),
         "Content-Type": "application/json",
     }
@@ -69,7 +69,7 @@ def push_usage_snapshots() -> None:
                 push_snapshot(
                     base_url=base_url,
                     snapshot=map_organisation_to_usage_snapshot(organisation),
-                    signature=organisation.licence.signature,
+                    signature_b64=organisation.licence.signature,
                 )
             except Exception:
                 logger.exception("snapshot.errored")
diff --git a/api/tests/unit/organisations/usage_reporting/test_services.py b/api/tests/unit/organisations/usage_reporting/test_services.py
index d88a2490eba4..f27cbd1e003b 100644
--- a/api/tests/unit/organisations/usage_reporting/test_services.py
+++ b/api/tests/unit/organisations/usage_reporting/test_services.py
@@ -91,7 +91,7 @@ def test_push_snapshot__status_code__logs_expected_event(
     push_snapshot(
         base_url="https://cp.example.com/",
         snapshot=snapshot,
-        signature="sig",
+        signature_b64="c2ln",
     )
 
     # Then
@@ -105,21 +105,21 @@ def test_push_snapshot__valid_snapshot__sends_bearer_authed_post(
     # Given
     mocked_post = mocker.patch("organisations.usage_reporting.services.requests.post")
     mocked_post.return_value.status_code = 201
-    signature = "abc+/=def=="
+    signature_b64 = "++++ABE="
 
     # When
     push_snapshot(
         base_url="https://cp.example.com/",
         snapshot=snapshot,
-        signature=signature,
+        signature_b64=signature_b64,
     )
 
     # Then
     mocked_post.assert_called_once()
     (url,), kwargs = mocked_post.call_args
     assert url == "https://cp.example.com/v1/public/usage"
-    # The unpadded base64url token the Control Plane receives on the wire
-    assert kwargs["headers"]["Authorization"] == "Bearer YWJjKy89ZGVmPT0"
+    # The signature's raw bytes, unpadded base64url-encoded for the wire.
+    assert kwargs["headers"]["Authorization"] == "Bearer ----ABE"
     assert kwargs["headers"]["Content-Type"] == "application/json"
     assert json.loads(kwargs["data"]) == {
         "timestamp": "2026-06-18T08:00:00Z",
@@ -200,10 +200,10 @@ def test_push_usage_snapshots__licensed_organisations__pushes_each(
     # Then
     assert mocked_push.call_count == 2
     mocked_push.assert_any_call(
-        base_url="https://cp.example.com", snapshot=snapshot, signature="sig-1"
+        base_url="https://cp.example.com", snapshot=snapshot, signature_b64="sig-1"
     )
     mocked_push.assert_any_call(
-        base_url="https://cp.example.com", snapshot=snapshot, signature="sig-2"
+        base_url="https://cp.example.com", snapshot=snapshot, signature_b64="sig-2"
     )
 
 
@@ -233,5 +233,5 @@ def test_push_usage_snapshots__one_organisation_raises__continues(
     # Then - first organisation failed (logged) but second still pushed
     assert log.has("snapshot.errored", level="error")
     mocked_push.assert_called_once_with(
-        base_url="https://cp.example.com", snapshot=snapshot, signature="sig-2"
+        base_url="https://cp.example.com", snapshot=snapshot, signature_b64="sig-2"
     )