diff --git a/src/agents/harness/runtime-plugin.test.ts b/src/agents/harness/runtime-plugin.test.ts
index b9b4b04452bc..9b9865261464 100644
--- a/src/agents/harness/runtime-plugin.test.ts
+++ b/src/agents/harness/runtime-plugin.test.ts
@@ -62,6 +62,20 @@ describe("harness runtime plugins", () => {
     expect(pluginRegistry.agentHarnesses).toHaveLength(1);
   });
 
+  it("explains how to recover when the selected harness registration is missing", async () => {
+    await expect(
+      ensureSelectedAgentHarnessPlugin({
+        provider: "openai",
+        modelId: "gpt-5.5",
+        agentHarnessRuntimeOverride: "codex",
+        workspaceDir: "/tmp/workspace",
+        pluginRegistry: createEmptyPluginRegistry(),
+      }),
+    ).rejects.toThrow(
+      'Agent harness runtime "codex" is unavailable because its plugin registration is missing from this prepared run. Enable or reinstall the plugin that provides this runtime, restart the Gateway, then retry.',
+    );
+  });
+
   it("force-activates a default-disabled harness owner selected for a run", () => {
     const plan = resolveAgentRuntimePluginLoadPlan({
       config: {},
diff --git a/src/agents/harness/runtime-plugin.ts b/src/agents/harness/runtime-plugin.ts
index 01d0afd0e6de..980ae147c5a2 100644
--- a/src/agents/harness/runtime-plugin.ts
+++ b/src/agents/harness/runtime-plugin.ts
@@ -131,6 +131,8 @@ export async function ensureSelectedAgentHarnessPlugin(params: {
   }
 
   if (!params.pluginRegistry?.agentHarnesses.some((entry) => entry.harness.id === runtime)) {
-    throw new Error(`Agent harness runtime "${runtime}" is not present in the prepared registry.`);
+    throw new Error(
+      `Agent harness runtime "${runtime}" is unavailable because its plugin registration is missing from this prepared run. Enable or reinstall the plugin that provides this runtime, restart the Gateway, then retry.`,
+    );
   }
 }
diff --git a/src/agents/prepared-model-runtime.inbound-registry.ts b/src/agents/prepared-model-runtime.inbound-registry.ts
index 44e1a632242d..0c8e7d49c60d 100644
--- a/src/agents/prepared-model-runtime.inbound-registry.ts
+++ b/src/agents/prepared-model-runtime.inbound-registry.ts
@@ -54,10 +54,12 @@ export function prepareWorkspacePluginRegistries(
   runtimePluginRegistry?: PluginRegistry;
   inboundPluginRegistry?: PluginRegistry;
 } {
-  if (input.readOnly) {
+  // Read-only catalog owners stay runtime-free, but setup probes carry an exact harness selection.
+  // That selected registry must belong to the generation instead of leaking from an outer scope.
+  if (input.readOnly && !input.runtimePluginSelections) {
     return {};
   }
-  const inboundPluginRegistry = loadInboundRegistry?.(input);
+  const inboundPluginRegistry = input.readOnly ? undefined : loadInboundRegistry?.(input);
   const runtimePluginRegistry =
     input.runtimePluginSelections || !inboundPluginRegistry
       ? loadAgentRuntimePluginRegistryHandle({
diff --git a/src/agents/prepared-model-runtime.test.ts b/src/agents/prepared-model-runtime.test.ts
index acacd9058df7..94610a557bff 100644
--- a/src/agents/prepared-model-runtime.test.ts
+++ b/src/agents/prepared-model-runtime.test.ts
@@ -48,6 +48,24 @@ describe("prepared model runtime snapshots", () => {
   it("keeps an isolated setup probe exact after a gateway replacement", async () => {
     mocks.configuredAgentIds = ["default"];
     const stagedConfig = { agents: { defaults: { model: "openai/gpt-5.6" } } };
+    const selectedPluginRegistry = createEmptyPluginRegistry();
+    selectedPluginRegistry.agentHarnesses.push({
+      pluginId: "codex",
+      source: "test",
+      harness: {
+        id: "codex",
+        label: "Codex",
+        supports: () => ({ supported: true }),
+        runAttempt: async () => {
+          throw new Error("unused");
+        },
+      },
+    });
+    mocks.loadAgentRuntimePluginRegistryHandle.mockImplementation((params) =>
+      (params as { selections?: unknown }).selections
+        ? selectedPluginRegistry
+        : createEmptyPluginRegistry(),
+    );
     await refreshPreparedModelRuntimeSnapshots({}, { gatewayLifecycle: true });
     markPreparedModelRuntimeSnapshotsStale("test isolated probe replacement", {
       waitForReplacement: true,
@@ -58,6 +76,7 @@ describe("prepared model runtime snapshots", () => {
       agentDir: "/tmp/setup-probe-agent",
       inheritedAuthDir: "/tmp/setup-probe-agent",
       workspaceDir: "/tmp/setup-probe-workspace",
+      runtimePluginSelections: [{ provider: "openai", modelId: "gpt-5.6", runtime: "codex" }],
     });
     await Promise.resolve();
     expect(mocks.ensureOpenClawModelsJson).toHaveBeenCalledTimes(1);
@@ -72,6 +91,14 @@ describe("prepared model runtime snapshots", () => {
       agentDir: "/tmp/setup-probe-agent",
       workspaceDir: "/tmp/setup-probe-workspace",
     });
+    expect(lease.snapshot.pluginRegistry?.agentHarnesses.map((entry) => entry.harness.id)).toEqual([
+      "codex",
+    ]);
+    expect(mocks.loadAgentRuntimePluginRegistryHandle).toHaveBeenCalledWith(
+      expect.objectContaining({
+        selections: [{ provider: "openai", modelId: "gpt-5.6", runtime: "codex" }],
+      }),
+    );
     lease.release();
   });