diff --git a/app/api/query.py b/app/api/query.py
index 207c27f..6dbbadd 100644
--- a/app/api/query.py
+++ b/app/api/query.py
@@ -763,17 +763,27 @@ def _rewrite_user_sql_for_bigquery_query(
     # Skip 4: BQ project not configured.
     try:
         bq = get_bq_access()
-        project = bq.projects.data
+        data_project = bq.projects.data
+        # The first arg to `bigquery_query()` is the **execution / billing**
+        # project — the project under which the BQ job runs and is billed.
+        # In cross-project deployments the SA may only have
+        # `serviceusage.services.use` on the billing project, so passing
+        # the data project there returns 403 USER_PROJECT_DENIED. Match
+        # the convention used everywhere else in the codebase (v2_scan /
+        # v2_sample / v2_schema / extractor): backtick paths use the
+        # **data** project, `bigquery_query()` first-arg uses the
+        # **billing** project. For single-project deploys the two are
+        # identical so the fix is a no-op there.
+        billing_project = bq.projects.billing or data_project
     except Exception:
         return user_sql, False
-    if not project:
+    if not data_project:
         return user_sql, False
 
-    # Rewrite identifiers, then wrap the whole thing in bigquery_query().
-    # The DuckDB BQ extension's UDF expects (<billing-project-string>,
-    # <inner-sql-string>); we use the data project so the inner SQL's
-    # backticked paths resolve to the same project.
-    inner_sql = _rewrite_bq_table_refs_to_native(user_sql, name_lookups, project)
+    # Rewrite identifiers using the DATA project — backtick paths
+    # `<data-project>.<dataset>.<table>` resolve to the same logical
+    # source no matter which project bills the query.
+    inner_sql = _rewrite_bq_table_refs_to_native(user_sql, name_lookups, data_project)
 
     # Embed the inner SQL using DuckDB's dollar-quoted string literal form
     # (`$tag$ ... $tag$`). Naive `replace("'", "''")` doubling misses
@@ -790,11 +800,11 @@ def _rewrite_user_sql_for_bigquery_query(
     if DOLLAR_TAG in inner_sql:
         escaped_inner = inner_sql.replace("'", "''")
         rewritten = (
-            f"SELECT * FROM bigquery_query('{project}', '{escaped_inner}')"
+            f"SELECT * FROM bigquery_query('{billing_project}', '{escaped_inner}')"
         )
     else:
         rewritten = (
-            f"SELECT * FROM bigquery_query('{project}', "
+            f"SELECT * FROM bigquery_query('{billing_project}', "
             f"{DOLLAR_TAG}{inner_sql}{DOLLAR_TAG})"
         )
     return rewritten, True
diff --git a/tests/test_query_remote_rewrite.py b/tests/test_query_remote_rewrite.py
index 224b2e1..bc84c51 100644
--- a/tests/test_query_remote_rewrite.py
+++ b/tests/test_query_remote_rewrite.py
@@ -67,11 +67,16 @@ def _register_local(conn, *, table_id, name, source_type="keboola"):
     )
 
 
-def _set_bq_project(monkeypatch, project="test-prj"):
-    """Stub get_bq_access so the rewriter sees a real-looking project ID."""
+def _set_bq_project(monkeypatch, project="test-prj", billing=None):
+    """Stub get_bq_access so the rewriter sees a real-looking project ID.
+
+    `project` configures the data project (used in backtick paths).
+    `billing` (when provided) configures a different billing project so
+    cross-project deployments can be exercised; defaults to `project`
+    for the single-project case."""
     from connectors.bigquery.access import BqAccess, BqProjects, get_bq_access
     bq = BqAccess(
-        BqProjects(billing=project, data=project),
+        BqProjects(billing=billing or project, data=project),
         client_factory=lambda projects: object(),
     )
     monkeypatch.setattr(
@@ -546,6 +551,38 @@ def test_endpoint_falls_back_to_original_sql_on_bq_parse_error(
     assert calls["sqls"][1] == "SELECT (count(*))::INT FROM ue"
 
 
+def test_rewriter_uses_billing_project_for_bigquery_query_first_arg(
+    seeded_registry, monkeypatch,
+):
+    """Devin-review BUG #1: `bigquery_query()` first arg is the
+    **billing** project (where BQ jobs are billed + executed), backtick
+    paths use the **data** project. In cross-project deploys the SA
+    has `serviceusage.services.use` only on the billing project, so
+    using the data project as billing → 403 USER_PROJECT_DENIED.
+
+    Match the existing convention in v2_scan / v2_sample / v2_schema /
+    extractor.
+    """
+    from app.api.query import _rewrite_user_sql_for_bigquery_query
+    _register_bq_remote(
+        seeded_registry, table_id="bq.fin.ue", name="ue",
+        bucket="fin", source_table="ue",
+    )
+    _set_bq_project(monkeypatch, project="data-prj", billing="billing-prj")
+
+    rewritten, did_rewrite = _rewrite_user_sql_for_bigquery_query(
+        "SELECT count(*) FROM ue",
+        seeded_registry,
+    )
+    assert did_rewrite is True
+    # First arg of bigquery_query must be the billing project.
+    assert "bigquery_query('billing-prj'" in rewritten
+    # Backtick path must use the data project.
+    assert "`data-prj.fin.ue`" in rewritten
+    # And the data project must NOT appear as the first arg.
+    assert "bigquery_query('data-prj'" not in rewritten
+
+
 def test_rewriter_skips_when_bq_row_bucket_contains_dot(
     seeded_registry, monkeypatch,
 ):