diff --git a/dataretrieval/waterdata/chunking.py b/dataretrieval/waterdata/chunking.py
index ab079070..04a25171 100644
--- a/dataretrieval/waterdata/chunking.py
+++ b/dataretrieval/waterdata/chunking.py
@@ -871,13 +871,22 @@ def __init__(
         self.canonical_url: str | None = None
 
         axes = _extract_axes(args)
-        # No chunkable axes → skip ``build_request`` entirely; the
-        # common Water Data call shape shouldn't pay for an unused
-        # request prep on the passthrough hot path. The fetcher
-        # will run with the user's args verbatim; if that produces
-        # an over-budget URL, the server (or httpx itself) rejects.
         if not axes:
-            return
+            # No chunkable axis: nothing to split. If the single request fits,
+            # run it verbatim (the common passthrough). If it's over budget we
+            # can't chunk it (e.g. a single large CQL ``IN`` clause with no
+            # top-level ``OR``, or one oversized value), so raise an actionable
+            # error instead of shipping it for the server to reject with an
+            # opaque HTTP 414. ``_safe_request_bytes`` treats an un-constructable
+            # URL (httpx.InvalidURL, > 64 KB) as over budget.
+            if _safe_request_bytes(build_request, args, url_limit) <= url_limit:
+                return
+            raise RequestTooLarge(
+                f"Request exceeds {url_limit} bytes (URL + body) and has no "
+                f"chunkable multi-value argument to split (e.g. a single large "
+                f"CQL `IN` clause, or one oversized value). Narrow the query, "
+                f"simplify the filter, or split the call manually."
+            )
 
         # Constructing the initial request can itself trip
         # ``httpx.InvalidURL`` (URL > 64 KB) — that's the canonical
diff --git a/tests/waterdata_chunking_test.py b/tests/waterdata_chunking_test.py
index a435a4de..98ed70c9 100644
--- a/tests/waterdata_chunking_test.py
+++ b/tests/waterdata_chunking_test.py
@@ -161,15 +161,24 @@ def test_extract_axes_skips_singletons_and_never_chunk_params():
 
 
 def test_chunk_plan_returns_passthrough_when_no_chunkable_axes():
-    """Scalar args with nothing to chunk → passthrough, even at a
-    URL limit the request technically exceeds (the server may 414,
-    but ``ChunkPlan`` has nothing to split)."""
+    """Scalar args with nothing to chunk and a request within the limit →
+    passthrough (no axes)."""
     args = {"monitoring_location_id": "scalar-only"}
-    plan = ChunkPlan(args, _fake_build, url_limit=10)
+    plan = ChunkPlan(args, _fake_build, url_limit=1000)
     assert plan.axes == []
     assert plan.total == 1
 
 
+def test_chunk_plan_raises_when_unchunkable_request_exceeds_limit():
+    """A request with nothing to chunk that still exceeds the byte limit (e.g.
+    a single large CQL ``IN`` clause with no top-level ``OR``) raises
+    RequestTooLarge instead of being shipped for the server to reject with an
+    opaque HTTP 414."""
+    args = {"monitoring_location_id": "scalar-only"}
+    with pytest.raises(RequestTooLarge):
+        ChunkPlan(args, _fake_build, url_limit=10)
+
+
 def test_chunk_plan_greedy_halving_targets_largest_axis_chunk():
     """The biggest chunk across all axes halves first — when one list
     axis dominates URL bytes, only it gets split until it stops being