diff --git a/src/reflection-store.ts b/src/reflection-store.ts
index 38da5ce7..577a6b36 100644
Binary files a/src/reflection-store.ts and b/src/reflection-store.ts differ
diff --git a/test/isOwnedByAgent.test.mjs b/test/isOwnedByAgent.test.mjs
new file mode 100644
index 00000000..b23e4c4b
--- /dev/null
+++ b/test/isOwnedByAgent.test.mjs
@@ -0,0 +1,65 @@
+// isOwnedByAgent unit tests — Issue #448 fix verification
+import { describe, it } from "node:test";
+import assert from "node:assert";
+
+// From reflection-store.ts: isOwnedByAgent function for isolated testing
+function isOwnedByAgent(metadata, agentId) {
+  const owner = typeof metadata.agentId === "string" ? metadata.agentId.trim() : "";
+
+  const itemKind = metadata.itemKind;
+
+  // derived: no main fallback, empty owner -> completely invisible
+  if (itemKind === "derived") {
+    if (!owner) return false;
+    return owner === agentId;
+  }
+
+  // invariant / legacy / mapped: maintain original main fallback
+  if (!owner) return true;
+  return owner === agentId || owner === "main";
+}
+
+describe("isOwnedByAgent — derived ownership fix (Issue #448)", () => {
+  describe("itemKind === 'derived'", () => {
+    it("main's derived -> main visible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "main" }, "main"), true);
+    });
+    it("main's derived -> sub-agent invisible (core bug fix)", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "main" }, "sub-agent-A"), false);
+    });
+    it("agent-x's derived -> agent-x visible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "agent-x" }, "agent-x"), true);
+    });
+    it("agent-x's derived -> agent-y invisible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "agent-x" }, "agent-y"), false);
+    });
+    it("derived + empty owner -> completely invisible (guard)", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "" }, "main"), false);
+      assert.strictEqual(isOwnedByAgent({ itemKind: "derived", agentId: "" }, "sub-agent"), false);
+    });
+  });
+
+  describe("itemKind === 'invariant' (maintain fallback)", () => {
+    it("main's invariant -> sub-agent visible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "invariant", agentId: "main" }, "sub-agent-A"), true);
+    });
+    it("agent-x's invariant -> agent-x visible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "invariant", agentId: "agent-x" }, "agent-x"), true);
+    });
+    it("agent-x's invariant -> agent-y invisible", () => {
+      assert.strictEqual(isOwnedByAgent({ itemKind: "invariant", agentId: "agent-x" }, "agent-y"), false);
+    });
+  });
+
+  describe("legacy / mapped (no itemKind, maintain fallback)", () => {
+    it("main legacy -> sub-agent visible", () => {
+      assert.strictEqual(isOwnedByAgent({ agentId: "main" }, "sub-agent-A"), true);
+    });
+    it("agent-x legacy -> agent-x visible", () => {
+      assert.strictEqual(isOwnedByAgent({ agentId: "agent-x" }, "agent-x"), true);
+    });
+    it("agent-x legacy -> agent-y invisible", () => {
+      assert.strictEqual(isOwnedByAgent({ agentId: "agent-x" }, "agent-y"), false);
+    });
+  });
+});
\ No newline at end of file