diff --git a/packages/client-runtime/src/gateway/runtimePort.ts b/packages/client-runtime/src/gateway/runtimePort.ts
index 80c6caa81f921..391a1c4b8ab86 100644
--- a/packages/client-runtime/src/gateway/runtimePort.ts
+++ b/packages/client-runtime/src/gateway/runtimePort.ts
@@ -406,6 +406,8 @@ export function gatewayThreadProjection(projection: OrchestrationV2ThreadProject
       })),
     createdAt: iso(thread.createdAt),
     updatedAt: iso(thread.updatedAt),
+    archivedAt: iso(thread.archivedAt),
+    deletedAt: iso(thread.deletedAt),
   };
 }
 
diff --git a/packages/contracts/src/mcpGateway.ts b/packages/contracts/src/mcpGateway.ts
index b63756fb21bf3..7df63a4f38530 100644
--- a/packages/contracts/src/mcpGateway.ts
+++ b/packages/contracts/src/mcpGateway.ts
@@ -66,6 +66,8 @@ export const McpGatewayRelayThread = Schema.Struct({
   ),
   settledAt: RelaySettledAt,
   settlementSource: Schema.optional(Schema.NullOr(Schema.Literals(["manual", "inactivity"]))),
+  archivedAt: RelayIso,
+  deletedAt: RelayIso,
   runs: Schema.Array(
     Schema.Struct({
       id: RunId,
diff --git a/packages/mcp-gateway/src/tools.test.ts b/packages/mcp-gateway/src/tools.test.ts
index cbc5271fd7515..745c79246d55d 100644
--- a/packages/mcp-gateway/src/tools.test.ts
+++ b/packages/mcp-gateway/src/tools.test.ts
@@ -528,6 +528,21 @@ describe("gateway chat tools", () => {
     const context = {
       port: {
         ...base,
+        listThreads: async (environmentId: string) => ({
+          snapshotAt: "2026-09-02T00:00:00.000Z",
+          items:
+            environmentId === "local"
+              ? [{ id: "coordinator" }, { id: "other-parent" }, { id: "archived-parent" }]
+              : [],
+        }),
+        getThread: async (environmentId: string, threadId: string) => {
+          if (environmentId !== "local" || !["other-parent", "archived-parent"].includes(threadId))
+            throw new Error("Failed to load thread");
+          const thread = await base.getThread(environmentId, threadId);
+          return threadId === "archived-parent"
+            ? { ...thread, archivedAt: "2026-09-01T00:00:00.000Z" }
+            : thread;
+        },
         createThread: async (request: Parameters<typeof base.createThread>[0]) => {
           parents.push([request.parentThreadId, request.parentEnvironmentId]);
           return base.createThread(request);
@@ -577,6 +592,28 @@ describe("gateway chat tools", () => {
       ["coordinator", "local"],
       ["coordinator", undefined],
     ]);
+
+    // An MCP request namespace copied out of a task ID is not a chat on either environment.
+    const fake = "mcp:b33476a4-892a-4ec5-a923-2a98b0bd14e5";
+    for (const [input, key] of [
+      [
+        {
+          environmentId: "remote",
+          projectId: "remote-project",
+          parentThreadId: fake,
+          parentEnvironmentId: "local",
+        },
+        "fake-remote",
+      ],
+      [{ parentThreadId: fake }, "fake-local"],
+      [{ parentThreadId: "archived-parent" }, "archived"],
+    ] as const) {
+      await expect(create(input, key)).rejects.toMatchObject({
+        code: "invalid_input",
+        message: `parentThreadId ${input.parentThreadId} is not a chat on environment local; omit parentThreadId to nest under the calling chat.`,
+      });
+    }
+    expect(parents).toHaveLength(5);
   });
 
   it.each(["constructor", "toString", "__proto__"])(
diff --git a/packages/mcp-gateway/src/tools.ts b/packages/mcp-gateway/src/tools.ts
index b96290380005b..e32636d19ae8e 100644
--- a/packages/mcp-gateway/src/tools.ts
+++ b/packages/mcp-gateway/src/tools.ts
@@ -1994,6 +1994,25 @@ export async function callGatewayTool(
               ? input.parentEnvironmentId.trim()
               : undefined
             : invocation.caller?.environmentId;
+      // Checked here: the target environment cannot verify a parent on another environment.
+      if (typeof input.parentThreadId === "string" && parentThreadId !== undefined) {
+        const parentEnvironment = parentEnvironmentId ?? environmentId;
+        const parent = await getThreadOrNotFound(context, parentEnvironment, parentThreadId).catch(
+          (error: unknown) => {
+            if (error instanceof GatewayError && error.code === "thread_not_found")
+              return undefined;
+            throw error;
+          },
+        );
+        if (parent === undefined || parent.archivedAt != null || parent.deletedAt != null) {
+          throw new GatewayError({
+            code: "invalid_input",
+            message: `parentThreadId ${parentThreadId} is not a chat on environment ${parentEnvironment}; omit parentThreadId to nest under the calling chat.`,
+            retryable: false,
+            environmentId: parentEnvironment,
+          });
+        }
+      }
       const profileName = typeof input.profile === "string" ? input.profile.trim() : "";
       const profileIdInput = typeof input.profileId === "string" ? input.profileId.trim() : "";
       const legacyIdentity = {