diff --git a/src/server/sessions/piSessionService.test.ts b/src/server/sessions/piSessionService.test.ts
index 543105a..cb72342 100644
--- a/src/server/sessions/piSessionService.test.ts
+++ b/src/server/sessions/piSessionService.test.ts
@@ -1136,7 +1136,9 @@ describe("PiSessionService", () => {
           getHeader: () => ({ parentSession: parentFile }),
           getEntries: () => [{ type: "custom", customType: "pi-web.subsession.spawned", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1" } }],
         });
-        const parentManager = fakeSessionManager("/workspace");
+        const parentManager = fakeSessionManager("/workspace", {
+          getEntries: () => [{ type: "custom", customType: "pi-web.subsession.link", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1", spawnedSessionFile: childFile, cwd: "/workspace-feature" } }],
+        });
         const child = fakeRuntime("child-1", { sessionFile: childFile, sessionManager: childManager });
         const parent = fakeRuntime("parent-1", { sessionFile: parentFile, sessionManager: parentManager });
         const runtimes = [child.runtime, parent.runtime];
@@ -1174,6 +1176,121 @@ describe("PiSessionService", () => {
       }
     });
 
+    it("notifies the validated parent file instead of an active prefix-matched parent id", async () => {
+      const tempDir = await mkdtemp(join(tmpdir(), "pi-web-subsession-prefix-parent-"));
+      const parentFile = join(tempDir, "parent.jsonl");
+      const forkParentFile = join(tempDir, "parent-fork.jsonl");
+      const childFile = join(tempDir, "child.jsonl");
+      await writeFile(parentFile, `${JSON.stringify({ type: "session", version: 3, id: "parent-1", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace" })}\n`, "utf8");
+      await writeFile(forkParentFile, `${JSON.stringify({ type: "session", version: 3, id: "parent-1-fork", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace" })}\n`, "utf8");
+      await writeFile(childFile, `${JSON.stringify({ type: "session", version: 3, id: "child-1", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace-feature", parentSession: parentFile })}\n`, "utf8");
+
+      try {
+        const childManager = fakeSessionManager("/workspace-feature", {
+          getHeader: () => ({ parentSession: parentFile }),
+          getEntries: () => [{ type: "custom", customType: "pi-web.subsession.spawned", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1" } }],
+        });
+        const parentManager = fakeSessionManager("/workspace", {
+          getEntries: () => [{ type: "custom", customType: "pi-web.subsession.link", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1", spawnedSessionFile: childFile, cwd: "/workspace-feature" } }],
+        });
+        const forkManager = fakeSessionManager("/workspace");
+        const fork = fakeRuntime("parent-1-fork", { sessionFile: forkParentFile, sessionManager: forkManager });
+        const child = fakeRuntime("child-1", { sessionFile: childFile, sessionManager: childManager });
+        const parent = fakeRuntime("parent-1", { sessionFile: parentFile, sessionManager: parentManager });
+        const runtimes = [fork.runtime, child.runtime, parent.runtime];
+        let index = 0;
+        const open = vi.fn((path: string) => {
+          if (path === parentFile) return parentManager;
+          if (path === forkParentFile) return forkManager;
+          return childManager;
+        });
+        const service = new PiSessionService(new CapturingSessionEventHub(), {
+          createAgentRuntime: () => {
+            const runtime = runtimes[index] ?? parent.runtime;
+            index += 1;
+            return Promise.resolve(runtime);
+          },
+          sessionManager: {
+            create: () => forkManager,
+            list: (cwd: string) => Promise.resolve(cwd === "/workspace"
+              ? [{ ...sessionRecord("parent-1-fork", "/workspace"), path: forkParentFile }]
+              : [{ ...sessionRecord("child-1", "/workspace-feature"), path: childFile, parentSessionPath: parentFile }]),
+            listAll: () => Promise.resolve([]),
+            open,
+          },
+          archiveStore: emptyArchiveStore(),
+          heartbeatIntervalMs: 60_000,
+        });
+
+        await service.status(sessionRef("parent-1-fork", "/workspace"));
+        await service.status(sessionRef("child-1", "/workspace-feature"));
+        child.session.isStreaming = true;
+        child.emit({ type: "agent_start" });
+        child.session.isStreaming = false;
+        child.emit({ type: "agent_end" });
+        await new Promise((resolve) => setTimeout(resolve, 20));
+
+        expect(fork.calls.sendCustomMessage).toHaveLength(0);
+        expect(parent.calls.sendCustomMessage).toHaveLength(1);
+        expect(open).toHaveBeenCalledWith(parentFile);
+        await service.dispose();
+      } finally {
+        await rm(tempDir, { recursive: true, force: true });
+      }
+    });
+
+    it("does not relink a copied child with the original session id unless the parent link names the current child file", async () => {
+      const tempDir = await mkdtemp(join(tmpdir(), "pi-web-subsession-copied-child-"));
+      const parentFile = join(tempDir, "parent.jsonl");
+      const originalChildFile = join(tempDir, "original-child.jsonl");
+      const copiedChildFile = join(tempDir, "copied-child.jsonl");
+      await writeFile(parentFile, `${JSON.stringify({ type: "session", version: 3, id: "parent-1", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace" })}\n`, "utf8");
+      await writeFile(originalChildFile, `${JSON.stringify({ type: "session", version: 3, id: "child-1", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace-feature", parentSession: parentFile })}\n`, "utf8");
+      await writeFile(copiedChildFile, `${JSON.stringify({ type: "session", version: 3, id: "child-1", timestamp: "2026-01-01T00:00:00.000Z", cwd: "/workspace-feature", parentSession: parentFile })}\n`, "utf8");
+
+      try {
+        const childManager = fakeSessionManager("/workspace-feature", {
+          getHeader: () => ({ parentSession: parentFile }),
+          getEntries: () => [{ type: "custom", customType: "pi-web.subsession.spawned", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1" } }],
+        });
+        const parentManager = fakeSessionManager("/workspace", {
+          getEntries: () => [{ type: "custom", customType: "pi-web.subsession.link", data: { version: 1, spawnedBySessionId: "parent-1", spawnedSessionId: "child-1", spawnedSessionFile: originalChildFile, cwd: "/workspace-feature" } }],
+        });
+        const child = fakeRuntime("child-1", { sessionFile: copiedChildFile, sessionManager: childManager });
+        const parent = fakeRuntime("parent-1", { sessionFile: parentFile, sessionManager: parentManager });
+        const runtimes = [child.runtime, parent.runtime];
+        let index = 0;
+        const open = vi.fn((path: string) => path === parentFile ? parentManager : childManager);
+        const service = new PiSessionService(new CapturingSessionEventHub(), {
+          createAgentRuntime: () => {
+            const runtime = runtimes[index] ?? parent.runtime;
+            index += 1;
+            return Promise.resolve(runtime);
+          },
+          sessionManager: {
+            create: () => childManager,
+            list: () => Promise.resolve([{ ...sessionRecord("child-1", "/workspace-feature"), path: copiedChildFile, parentSessionPath: parentFile }]),
+            listAll: () => Promise.resolve([]),
+            open,
+          },
+          archiveStore: emptyArchiveStore(),
+          heartbeatIntervalMs: 60_000,
+        });
+
+        await service.status(sessionRef("child-1", "/workspace-feature"));
+        child.session.isStreaming = true;
+        child.emit({ type: "agent_start" });
+        child.session.isStreaming = false;
+        child.emit({ type: "agent_end" });
+        await new Promise((resolve) => setTimeout(resolve, 20));
+
+        expect(parent.calls.sendCustomMessage).toHaveLength(0);
+        await service.dispose();
+      } finally {
+        await rm(tempDir, { recursive: true, force: true });
+      }
+    });
+
     it("does not relink a child marker when the child header points at a different parent id", async () => {
       const tempDir = await mkdtemp(join(tmpdir(), "pi-web-subsession-wrong-parent-"));
       const mismatchedParentFile = join(tempDir, "other-parent.jsonl");
diff --git a/src/server/sessions/piSessionService.ts b/src/server/sessions/piSessionService.ts
index b414534..e2bc43f 100644
--- a/src/server/sessions/piSessionService.ts
+++ b/src/server/sessions/piSessionService.ts
@@ -633,14 +633,35 @@ export class PiSessionService {
     const parentHeader = await readSessionHeaderSummary(parentSessionFile);
     if (parentHeader?.id !== marker.spawnedBySessionId) return;
     const childSessionFile = nonEmptyString(session.sessionFile);
+    if (childSessionFile === undefined) return;
+    const hasReciprocalLink = await this.parentHasReciprocalSubsessionLink(parentSessionFile, marker.spawnedBySessionId, session.sessionId, childSessionFile);
+    if (!hasReciprocalLink) return;
     this.registerSubsession(marker.spawnedBySessionId, {
       childSessionId: session.sessionId,
-      ...(childSessionFile === undefined ? {} : { childSessionFile }),
+      childSessionFile,
       parentSessionFile,
       cwd: session.sessionManager.getCwd(),
     });
   }
 
+  private async parentHasReciprocalSubsessionLink(parentSessionFile: string, parentSessionId: string, childSessionId: string, childSessionFile: string): Promise<boolean> {
+    let parentManager: PiSessionManager;
+    try {
+      parentManager = this.sessionManager.open(parentSessionFile);
+    } catch {
+      return false;
+    }
+    const entries = parentManager.getEntries?.() ?? parentManager.getBranch();
+    for (const entry of entries) {
+      const link = parsePersistedParentSubsessionLink(entry);
+      if (link === undefined) continue;
+      if (link.spawnedBySessionId !== parentSessionId || link.spawnedSessionId !== childSessionId) continue;
+      if (link.spawnedSessionFile === undefined || !sessionPathsEqual(link.spawnedSessionFile, childSessionFile)) continue;
+      if (await this.persistedSubsessionLinkMatchesParent(parentSessionFile, link)) return true;
+    }
+    return false;
+  }
+
   private async getOrOpenTrackedSubsession(sessionId: string): Promise<PiAgentSession> {
     const active = this.active.get(sessionId);
     if (active !== undefined) return active.runtime.session;
@@ -657,7 +678,11 @@ export class PiSessionService {
       }
     }
 
-    return this.getOrOpen(sessionId);
+    const listed = link?.cwd === undefined
+      ? (await this.sessionManager.listAll?.() ?? []).find((session) => session.id === sessionId)
+      : (await this.sessionManager.list(link.cwd)).find((session) => session.id === sessionId);
+    if (listed === undefined) throw new Error("Session not found");
+    return (await this.create(this.sessionManager.open(listed.path), listed.cwd)).runtime.session;
   }
 
   private async subsessionSummaryFields(childSessionId: string): Promise<{ cwd: string; status: SubsessionStatus }> {
@@ -706,16 +731,16 @@ export class PiSessionService {
   }
 
   private async getOrOpenParentForSubsession(parentSessionId: string, childSessionId: string): Promise<PiAgentSession> {
-    const active = this.activeForLookup(parentSessionId);
+    const active = this.active.get(parentSessionId);
     if (active !== undefined) return active.runtime.session;
 
     const parentSessionFile = this.subsessionLinks.get(childSessionId)?.parentSessionFile;
-    if (parentSessionFile !== undefined && (await readSessionHeaderSummary(parentSessionFile))?.id === parentSessionId) {
-      const sessionManager = this.sessionManager.open(parentSessionFile);
-      return (await this.create(sessionManager, sessionManager.getCwd())).runtime.session;
+    if (parentSessionFile === undefined) throw new Error(`Parent session ${parentSessionId} is not available for subsession notification`);
+    if ((await readSessionHeaderSummary(parentSessionFile))?.id !== parentSessionId) {
+      throw new Error(`Parent session ${parentSessionId} is not available for subsession notification`);
     }
-
-    return this.getOrOpen(parentSessionId);
+    const sessionManager = this.sessionManager.open(parentSessionFile);
+    return (await this.create(sessionManager, sessionManager.getCwd())).runtime.session;
   }
 
   /**