diff --git a/src/resources/extensions/gsd/dispatch-guard.ts b/src/resources/extensions/gsd/dispatch-guard.ts
index c687f1b30..568f872d0 100644
--- a/src/resources/extensions/gsd/dispatch-guard.ts
+++ b/src/resources/extensions/gsd/dispatch-guard.ts
@@ -107,10 +107,27 @@ export function getPriorSliceCompletionBlocker(
         // it may be a cross-milestone reference handled elsewhere.
       }
     } else {
+      // Positional fallback is only a heuristic for legacy slices with no
+      // declared dependencies. Skip any earlier slice that depends on the
+      // target, directly or transitively, or we can deadlock a valid zero-dep
+      // slice behind its own downstream dependents (#3720).
+      const reverseDependents = new Set<string>();
+      let changed = true;
+      while (changed) {
+        changed = false;
+        for (const slice of slices) {
+          if (reverseDependents.has(slice.id)) continue;
+          if (slice.depends.some((depId) => depId === targetSid || reverseDependents.has(depId))) {
+            reverseDependents.add(slice.id);
+            changed = true;
+          }
+        }
+      }
+
       const targetIndex = slices.findIndex((slice) => slice.id === targetSid);
       const incomplete = slices
         .slice(0, targetIndex)
-        .find((slice) => !slice.done);
+        .find((slice) => !slice.done && !reverseDependents.has(slice.id));
       if (incomplete) {
         return `Cannot dispatch ${unitType} ${unitId}: earlier slice ${targetMid}/${incomplete.id} is not complete.`;
       }
diff --git a/src/resources/extensions/gsd/tests/dispatch-guard.test.ts b/src/resources/extensions/gsd/tests/dispatch-guard.test.ts
index 1989a0195..04c167b08 100644
--- a/src/resources/extensions/gsd/tests/dispatch-guard.test.ts
+++ b/src/resources/extensions/gsd/tests/dispatch-guard.test.ts
@@ -145,6 +145,33 @@ test("dispatch guard falls back to positional ordering when no dependencies decl
   );
 });
 
+test("dispatch guard ignores positionally-earlier reverse dependents for zero-dependency slices (#3720)", (t) => {
+  const repo = setupRepo();
+  t.after(() => teardownRepo(repo));
+
+  mkdirSync(join(repo, ".gsd", "milestones", "M015"), { recursive: true });
+
+  insertMilestone({ id: "M015", title: "Reverse dependency fallback" });
+  insertSlice({ id: "S03", milestoneId: "M015", title: "Complete prerequisite", status: "complete", depends: [], sequence: 0 });
+  insertSlice({ id: "S04", milestoneId: "M015", title: "Depends on S04A", status: "pending", depends: ["S03", "S04A"], sequence: 0 });
+  insertSlice({ id: "S04A", milestoneId: "M015", title: "No explicit deps", status: "pending", depends: [], sequence: 0 });
+
+  writeFileSync(join(repo, ".gsd", "milestones", "M015", "M015-ROADMAP.md"), "# M015\n");
+
+  // S04A has no declared dependencies and should not be blocked by S04, because
+  // S04 itself depends on S04A. With sequence=0, DB ordering falls back to id.
+  assert.equal(
+    getPriorSliceCompletionBlocker(repo, "main", "execute-task", "M015/S04A/T02"),
+    null,
+  );
+
+  // The reverse direction is still blocked normally.
+  assert.equal(
+    getPriorSliceCompletionBlocker(repo, "main", "execute-task", "M015/S04/T01"),
+    "Cannot dispatch execute-task M015/S04/T01: dependency slice M015/S04A is not complete.",
+  );
+});
+
 test("dispatch guard allows slice with all declared dependencies complete", (t) => {
   const repo = setupRepo();
   t.after(() => teardownRepo(repo));