diff --git a/packages/core/src/__tests__/session-manager/restore.test.ts b/packages/core/src/__tests__/session-manager/restore.test.ts
index f955bb509..bf9691b4a 100644
--- a/packages/core/src/__tests__/session-manager/restore.test.ts
+++ b/packages/core/src/__tests__/session-manager/restore.test.ts
@@ -197,12 +197,13 @@ describe("restore", () => {
     expect(mockRuntime.create).toHaveBeenCalled();
   });
 
-  it("allows restoring merged sessions", async () => {
+  it("refuses to restore sessions whose PR has merged (would silently re-spawn against the default branch)", async () => {
     const ws = "/tmp/mock-ws/app-1";
     writeMetadata(sessionsDir, "app-1", {
       worktree: ws,
       branch: "main",
       status: "merged",
+      pr: "https://github.com/acme/repo/pull/1723",
       project: "my-app",
       runtimeHandle: makeHandle("rt-old"),
     });
@@ -222,8 +223,7 @@ describe("restore", () => {
     };
 
     const sm = createSessionManager({ config, registry });
-    const restored = await sm.restore("app-1");
-    expect(restored.id).toBe("app-1");
+    await expect(sm.restore("app-1")).rejects.toThrow(/is merged/);
   });
 
   it("throws SessionNotRestorableError for working sessions", async () => {
diff --git a/packages/core/src/session-manager.ts b/packages/core/src/session-manager.ts
index 1cec42a94..8ebe2aca8 100644
--- a/packages/core/src/session-manager.ts
+++ b/packages/core/src/session-manager.ts
@@ -2801,6 +2801,23 @@ export function createSessionManager(deps: SessionManagerDeps): OpenCodeSessionM
       throw new SessionNotRestorableError(sessionId, reason);
     }
 
+    // 3a. Refuse to silently revive a session whose PR has reached a terminal
+    //     state. The original branch has typically been deleted on merge, so
+    //     `session.branch` may have drifted to the project default branch
+    //     between the kill and the restore. Re-spawning would associate the
+    //     session with whatever PR `detectPR` finds on that branch — including
+    //     unrelated fork PRs (see #1724). If the user wants a fresh session
+    //     in this workspace, they should spawn a new one.
+    const prState = session.lifecycle.pr.state;
+    if (prState === "merged" || prState === "closed") {
+      const prRef = session.lifecycle.pr.url ?? session.pr?.url ?? raw["pr"] ?? "(unknown)";
+      throw new SessionNotRestorableError(
+        sessionId,
+        `PR ${prRef} is ${prState}; the session's work is complete. ` +
+          `Spawn a new session for follow-up work instead of restoring.`,
+      );
+    }
+
     // 4. Validate required plugins (plugins already resolved above for enrichment)
     if (!plugins.runtime) {
       throw new Error(`Runtime plugin '${project.runtime ?? config.defaults.runtime}' not found`);
@@ -2967,16 +2984,8 @@ export function createSessionManager(deps: SessionManagerDeps): OpenCodeSessionM
     restoredLifecycle.runtime.handle = handle;
     restoredLifecycle.runtime.lastObservedAt = now;
 
-    // Reset terminal PR state so the lifecycle manager doesn't immediately
-    // re-terminate the session. The old PR is done — if the agent creates
-    // a new one, PR auto-detect will pick it up.
-    if (restoredLifecycle.pr.state === "merged" || restoredLifecycle.pr.state === "closed") {
-      restoredLifecycle.pr.state = "none";
-      restoredLifecycle.pr.reason = "cleared_on_restore";
-      restoredLifecycle.pr.number = null;
-      restoredLifecycle.pr.url = null;
-      restoredLifecycle.pr.lastObservedAt = null;
-    }
+    // Terminal PR state (merged/closed) is rejected up-front in step 3a, so
+    // by here the lifecycle.pr is non-terminal and is preserved as-is.
 
     updateMetadata(sessionsDir, sessionId, {
       ...buildLifecycleMetadataPatch(restoredLifecycle),