diff --git a/exist-core/src/main/java/org/exist/xquery/XQueryContext.java b/exist-core/src/main/java/org/exist/xquery/XQueryContext.java
index 9f5e0bbf7ab..9eac6a95e1d 100644
--- a/exist-core/src/main/java/org/exist/xquery/XQueryContext.java
+++ b/exist-core/src/main/java/org/exist/xquery/XQueryContext.java
@@ -509,6 +509,13 @@ public XQueryContext(final XQueryContext copyFrom) {
                 LOG.warn("Failed to copy namespace declaration for prefix '{}'", prefix, ex);
             }
         }
+
+        // Copy in-scope namespaces registered via declareInScopeNamespace. Otherwise QName.parse()
+        // will raise error XPST0081 when resolving path-step names in any of those.
+        for (final Map.Entry<String, String> entry : copyFrom.inScopeNamespaces.entrySet()) {
+            inScopeNamespaces.put(entry.getKey(), entry.getValue());
+            inScopePrefixes.put(entry.getValue(), entry.getKey());
+        }
     }
 
 
diff --git a/exist-core/src/test/java/org/exist/xquery/InScopeNamespaceCompileTest.java b/exist-core/src/test/java/org/exist/xquery/InScopeNamespaceCompileTest.java
new file mode 100644
index 00000000000..1f3e1f28c81
--- /dev/null
+++ b/exist-core/src/test/java/org/exist/xquery/InScopeNamespaceCompileTest.java
@@ -0,0 +1,100 @@
+/*
+ * eXist-db Open Source Native XML Database
+ * Copyright (C) 2001 The eXist-db Authors
+ *
+ * info@exist-db.org
+ * http://www.exist-db.org
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package org.exist.xquery;
+
+import java.io.StringReader;
+
+import org.exist.EXistException;
+import org.exist.security.PermissionDeniedException;
+import org.exist.storage.BrokerPool;
+import org.exist.test.ExistEmbeddedServer;
+import org.exist.xquery.parser.XQueryLexer;
+import org.exist.xquery.parser.XQueryParser;
+import org.exist.xquery.parser.XQueryTreeParser;
+
+import antlr.collections.AST;
+
+import org.junit.ClassRule;
+import org.junit.Test;
+
+/**
+ * Verifies that namespaces registered on the static context via
+ * {@link XQueryContext#declareInScopeNamespace(String, String)} before
+ * compilation are visible to path-step QName resolution. The XQTS runner
+ * relies on this to apply &lt;environment&gt;/&lt;namespace&gt; declarations
+ * (e.g. the "atomic" environment) without rewriting test queries.
+ *
+ * <p>Regression test for set-operator XPST0081 failures discovered in the
+ * XQ 3.1 develop gap analysis (op-intersect, op-union, op-except).
+ */
+public class InScopeNamespaceCompileTest {
+
+    @ClassRule
+    public static final ExistEmbeddedServer server = new ExistEmbeddedServer(true, true);
+
+    private static final String ATOMIC_NS = "http://www.w3.org/XQueryTest";
+
+    private void compileWithAtomicPrefix(final String query) throws Exception {
+        final BrokerPool pool = BrokerPool.getInstance();
+        final XQueryContext context = new XQueryContext(pool);
+        context.declareInScopeNamespace("atomic", ATOMIC_NS);
+
+        final XQueryLexer lexer = new XQueryLexer(context, new StringReader(query));
+        final XQueryParser xparser = new XQueryParser(lexer);
+        xparser.xpath();
+        if (xparser.foundErrors()) {
+            throw new AssertionError(xparser.getErrorMessage());
+        }
+        final AST ast = xparser.getAST();
+        final XQueryTreeParser treeParser = new XQueryTreeParser(context);
+        final PathExpr expr = new PathExpr(context);
+        treeParser.xpath(ast, expr);
+        if (treeParser.foundErrors()) {
+            throw new AssertionError(treeParser.getErrorMessage());
+        }
+    }
+
+    @Test
+    public void intersectResolvesPrefix()
+            throws EXistException, PermissionDeniedException, Exception {
+        // mirrors XQTS fn-intersect-node-args-016
+        compileWithAtomicPrefix("(/atomic:root/atomic:integer) intersect (/atomic:root/atomic:integer)");
+    }
+
+    @Test
+    public void unionResolvesPrefix() throws Exception {
+        // mirrors XQTS fn-union-node-args-016
+        compileWithAtomicPrefix("(/atomic:root/atomic:integer) union (/atomic:root/atomic:integer)");
+    }
+
+    @Test
+    public void exceptResolvesPrefix() throws Exception {
+        // mirrors XQTS fn-except-node-args-016
+        compileWithAtomicPrefix("(/atomic:root/atomic:integer) except (/atomic:root/atomic:integer)");
+    }
+
+    @Test
+    public void plainPathResolvesPrefix() throws Exception {
+        // baseline: a plain path expression must work too
+        compileWithAtomicPrefix("/atomic:root/atomic:integer");
+    }
+}