diff --git a/apps/server/src/mcp/OrchestratorMcpService.test.ts b/apps/server/src/mcp/OrchestratorMcpService.test.ts
index 816a0dcb981d2..aeb535774dfda 100644
--- a/apps/server/src/mcp/OrchestratorMcpService.test.ts
+++ b/apps/server/src/mcp/OrchestratorMcpService.test.ts
@@ -2430,6 +2430,68 @@ describe("OrchestratorMcpService provider resolution", () => {
       }),
   );
 
+  it.effect("an unadvertised model refusal names advertised slugs to retry with", () =>
+    Effect.gen(function* () {
+      const codex = providerSnapshot({
+        instanceId: codexInstanceId,
+        driver: ProviderDriverKind.make("codex"),
+      });
+      const layerDependencies = Layer.mergeAll(
+        NodeServices.layer,
+        NodeCrypto.layer,
+        Layer.mock(ThreadManagementService.ThreadManagementService)({
+          hasReceipt: () => Effect.succeed(false),
+          getThreadRecords: () => Effect.succeed(parentProjection([])),
+        }),
+        ServerSettings.layerTest(),
+        providerRegistryLayer([
+          {
+            ...codex,
+            models: [
+              { slug: "claude-opus-4-1", isLegacy: true },
+              ...Array.from({ length: 9 }, (_, index) => ({ slug: `claude-opus-5-${index + 1}` })),
+            ].map((model) => ({ ...model, name: model.slug, isCustom: false, capabilities: null })),
+          },
+        ]),
+        adapterRegistryLayer([codexInstanceId]),
+        Layer.mock(ProjectService.ProjectService)({}),
+        Layer.mock(SecretRequests.SecretRequests)({}),
+        Layer.mock(RemoteDelegation.RemoteDelegation)({}),
+        Layer.mock(ScheduledTaskService.ScheduledTaskService)({}),
+      );
+
+      yield* Effect.gen(function* () {
+        const service = yield* OrchestratorMcpService.OrchestratorMcpService;
+        const refuse = (model: string) =>
+          service
+            .delegateTask(scope, {
+              task: "Summarize the diff.",
+              target: { providerInstanceId: codexInstanceId, model },
+              mode: "async",
+              clientRequestId: `delegate-unadvertised-${model}`,
+            })
+            .pipe(Effect.flip);
+
+        const prefixed = yield* refuse("anthropic/claude-opus-5-3");
+        assert.equal(prefixed.code, "model_unavailable");
+        assert.equal(
+          prefixed.message,
+          "Model anthropic/claude-opus-5-3 is not advertised by provider codex. " +
+            "Did you mean claude-opus-5-3?",
+        );
+
+        const unmatched = yield* refuse("opus-5.5");
+        assert.equal(
+          unmatched.message,
+          "Model opus-5.5 is not advertised by provider codex. Advertised models: " +
+            "claude-opus-5-1, claude-opus-5-2, claude-opus-5-3, claude-opus-5-4, claude-opus-5-5, " +
+            "claude-opus-5-6, claude-opus-5-7, claude-opus-5-8 and 2 more; " +
+            "orchestrator_capabilities with search finds them.",
+        );
+      }).pipe(Effect.provide(layerOrchestratorService.pipe(Layer.provide(layerDependencies))));
+    }),
+  );
+
   it.effect("driver-only delegation skips an instance the project turned off", () =>
     Effect.gen(function* () {
       const altInstanceId = ProviderInstanceId.make("codex-alt");
diff --git a/apps/server/src/mcp/OrchestratorMcpService.ts b/apps/server/src/mcp/OrchestratorMcpService.ts
index eaa505663c5bc..9d642c8455bc7 100644
--- a/apps/server/src/mcp/OrchestratorMcpService.ts
+++ b/apps/server/src/mcp/OrchestratorMcpService.ts
@@ -354,6 +354,26 @@ function listCapabilityModels(input: {
   return { listed, omitted: visible.length - listed.length };
 }
 
+/** Names advertised slugs to retry with: same last `/` segment first, else a short list. */
+function advertisedModelsHint(models: ReadonlyArray<ServerProviderModel>, requested: string) {
+  const lastSegment = (slug: string) => slug.slice(slug.lastIndexOf("/") + 1).toLowerCase();
+  const wanted = lastSegment(requested);
+  const close = models.filter((model) => lastSegment(model.slug) === wanted);
+  if (close.length > 0) {
+    return `Did you mean ${close
+      .slice(0, LISTED_MODEL_LIMIT)
+      .map((model) => model.slug)
+      .join(", ")}?`;
+  }
+  const listed = models
+    .toSorted((a, b) => Number(a.isLegacy ?? false) - Number(b.isLegacy ?? false))
+    .slice(0, LISTED_MODEL_LIMIT);
+  const omitted = models.length - listed.length;
+  return `Advertised models: ${listed.map((model) => model.slug).join(", ")}${
+    omitted > 0 ? ` and ${omitted} more; orchestrator_capabilities with search finds them` : ""
+  }.`;
+}
+
 /** Option ids and allowed values are all a delegating agent needs; labels are UI copy. */
 function capabilityModelOption(descriptor: ProviderOptionDescriptor): OrchestratorMcpModelOption {
   const defaultValue = getProviderOptionCurrentValue(descriptor);
@@ -377,7 +397,7 @@ export const requireAdvertisedModel = (provider: ServerProvider, model: string)
     ? Effect.fail(
         failure(
           "model_unavailable",
-          `Model ${model} is not advertised by provider ${provider.instanceId}.`,
+          `Model ${model} is not advertised by provider ${provider.instanceId}. ${advertisedModelsHint(provider.models, model)}`,
         ),
       )
     : Effect.void;
diff --git a/apps/server/src/mcp/toolkits/project/handlers.test.ts b/apps/server/src/mcp/toolkits/project/handlers.test.ts
index 938a2eac1b266..c667c9b00b9bf 100644
--- a/apps/server/src/mcp/toolkits/project/handlers.test.ts
+++ b/apps/server/src/mcp/toolkits/project/handlers.test.ts
@@ -399,7 +399,9 @@ it.effect("a launch with a model its provider does not advertise creates no thre
     ] as const) {
       expect(yield* launchWith(instanceId, model)).toMatchObject({
         code: "model_unavailable",
-        message: `Model ${model} is not advertised by provider ${instanceId}.`,
+        message: expect.stringContaining(
+          `Model ${model} is not advertised by provider ${instanceId}. Advertised models: `,
+        ),
       });
     }
     expect(launched).toHaveLength(0);