diff a/test/hotspot/jtreg/gc/shenandoah/options/TestCompactHeaderTLABSize.java b/test/hotspot/jtreg/gc/shenandoah/options/TestCompactHeaderTLABSize.java
--- /dev/null
+++ b/test/hotspot/jtreg/gc/shenandoah/options/TestCompactHeaderTLABSize.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test id=compact-headers
+ * @summary With compact object headers the max TLAB size must be strictly below a
+ *          region, so a region-sized object can never be served from a TLAB (which
+ *          would bypass the humongous path). The cap is only observable indirectly:
+ *          a region-sized -XX:TLABSize is rejected at startup under compact headers,
+ *          but accepted without them (where the max TLAB equals the region size).
+ * @bug 8387285
+ * @requires vm.gc.Shenandoah
+ * @library /test/lib
+ * @modules java.base/jdk.internal.misc
+ *          java.management
+ * @run driver TestCompactHeaderTLABSize
+ */
+
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+
+public class TestCompactHeaderTLABSize {
+
+    // Minimum Shenandoah region size; the only size at which the un-capped max TLAB
+    // would otherwise equal the region size.
+    static final String REGION_SIZE = "256K";
+
+    public static void main(String[] args) throws Exception {
+        // With compact object headers, the max TLAB is capped one word below the
+        // region, so requesting a region-sized TLAB must be rejected at startup.
+        OutputAnalyzer coh = ProcessTools.executeLimitedTestJava(
+                "-XX:+UnlockExperimentalVMOptions",
+                "-XX:+UnlockDiagnosticVMOptions",
+                "-XX:+UseShenandoahGC",
+                "-XX:+UseCompactObjectHeaders",
+                "-XX:ShenandoahRegionSize=" + REGION_SIZE,
+                "-XX:TLABSize=" + REGION_SIZE,
+                "-Xmx512m",
+                "-version");
+        coh.shouldContain("must be less than or equal to ergonomic TLAB maximum size");
+        coh.shouldHaveExitValue(1);
+
+        // Without compact object headers there is no hash-code expansion, so the max
+        // TLAB equals the region size and the same region-sized TLAB is accepted.
+        OutputAnalyzer noCoh = ProcessTools.executeLimitedTestJava(
+                "-XX:+UnlockExperimentalVMOptions",
+                "-XX:+UnlockDiagnosticVMOptions",
+                "-XX:+UseShenandoahGC",
+                "-XX:-UseCompactObjectHeaders",
+                "-XX:ShenandoahRegionSize=" + REGION_SIZE,
+                "-XX:TLABSize=" + REGION_SIZE,
+                "-Xmx512m",
+                "-version");
+        noCoh.shouldHaveExitValue(0);
+
+        // Under compact headers, a TLAB one word below the region is still accepted:
+        // the cap is exactly one (object-aligned) word, not an arbitrary reduction.
+        OutputAnalyzer atCap = ProcessTools.executeLimitedTestJava(
+                "-XX:+UnlockExperimentalVMOptions",
+                "-XX:+UnlockDiagnosticVMOptions",
+                "-XX:+UseShenandoahGC",
+                "-XX:+UseCompactObjectHeaders",
+                "-XX:ShenandoahRegionSize=" + REGION_SIZE,
+                "-XX:TLABSize=" + (256 * 1024 - 8), // one HeapWord below the region
+                "-Xmx512m",
+                "-version");
+        atCap.shouldHaveExitValue(0);
+    }
+}