diff --git a/datafusion/sqllogictest/src/accounting_pool.rs b/datafusion/sqllogictest/src/accounting_pool.rs
index a9d2db9f12261..308044f79adb0 100644
--- a/datafusion/sqllogictest/src/accounting_pool.rs
+++ b/datafusion/sqllogictest/src/accounting_pool.rs
@@ -40,7 +40,7 @@ use std::sync::Arc;
 /// untracked allocation — by definition, since DF's pool didn't see it.
 ///
 /// 800% high, but that's what it takes to pass the SLT suite right now. Goal should be ~10%
-const HEADROOM_FACTOR: f64 = 8.0;
+const HEADROOM_FACTOR: f64 = 5.0;
 
 pub struct AccountingMemoryPool {
     inner: Arc<dyn MemoryPool>,
diff --git a/datafusion/sqllogictest/test_files/group_by_spill_row_decode.slt b/datafusion/sqllogictest/test_files/group_by_spill_row_decode.slt
new file mode 100644
index 0000000000000..1117f2d67c070
--- /dev/null
+++ b/datafusion/sqllogictest/test_files/group_by_spill_row_decode.slt
@@ -0,0 +1,65 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# `GroupedHashAggregateStream` spill path: `GroupValuesRows::emit` →
+# `RowConverter::convert_rows` → `decode_column` → `decode_string` /
+# `decode_binary` allocates per-column buffers without
+# `MemoryReservation::try_grow`.
+#
+# Plain single-column `Utf8` routes to `GroupValuesBytes`, not
+# `GroupValuesRows`. Pairing the wide Utf8 key with a small `List<Int>`
+# column ("schema poisoner") makes the schema fall outside
+# `multi_group_by::supported_type`, forcing `GroupValuesRows`. The wide
+# Utf8 column carries the data; its decode hits `decode_string` →
+# `decode_binary` — the exact path the production OOM at one
+# DataFusion-based log analytics deployment took on 2026-05-20.
+#
+# Pool=1M is in the goldilocks zone: above DF's tracked-alloc floor
+# (agg enters spill), below the framework's slack ceiling on the
+# decode buffer.
+#
+# Surfaced by apache/datafusion#22626. Sibling: nested_loop_join_spill.slt.
+
+statement ok
+SET datafusion.execution.target_partitions = 1
+
+statement ok
+CREATE TABLE utf8_keys AS
+SELECT cast(v AS varchar) || repeat('x', 200) AS k,
+       make_array(1) AS _force_rows
+FROM generate_series(1, 50000) AS t(v)
+
+statement ok
+SET datafusion.runtime.memory_limit = '1M'
+
+query I nosort
+SELECT count(*) FROM (
+  SELECT k, _force_rows, count(*) AS c
+  FROM utf8_keys
+  GROUP BY k, _force_rows
+)
+----
+50000
+
+statement ok
+RESET datafusion.runtime.memory_limit
+
+statement ok
+DROP TABLE utf8_keys
+
+statement ok
+SET datafusion.execution.target_partitions = 4