diff --git a/apps/web/app/api/chat/_lib/model-selection.ts b/apps/web/app/api/chat/_lib/model-selection.ts
index bd7ded148..cd7f40508 100644
--- a/apps/web/app/api/chat/_lib/model-selection.ts
+++ b/apps/web/app/api/chat/_lib/model-selection.ts
@@ -1,4 +1,5 @@
 import type { AgentModelSelection } from "@open-harness/agent";
+import { resolveAvailableModelId } from "@/lib/model-availability";
 import { type ModelVariant, resolveModelSelection } from "@/lib/model-variants";
 import { DEFAULT_MODEL_ID } from "@/lib/models";
 
@@ -23,8 +24,16 @@ export function resolveChatModelSelection({
     return { id: DEFAULT_MODEL_ID as AgentModelSelection["id"] };
   }
 
+  const availableModelId = resolveAvailableModelId(selection.resolvedModelId);
+  if (availableModelId !== selection.resolvedModelId) {
+    console.warn(
+      `${missingVariantLabel} "${requestedModelId}" resolves to disabled model "${selection.resolvedModelId}". Falling back to default model.`,
+    );
+    return { id: DEFAULT_MODEL_ID as AgentModelSelection["id"] };
+  }
+
   return {
-    id: selection.resolvedModelId as AgentModelSelection["id"],
+    id: availableModelId as AgentModelSelection["id"],
     ...(selection.providerOptionsByProvider
       ? {
           providerOptionsOverrides: selection.providerOptionsByProvider,
diff --git a/apps/web/lib/model-availability.ts b/apps/web/lib/model-availability.ts
new file mode 100644
index 000000000..d6a5e9c5d
--- /dev/null
+++ b/apps/web/lib/model-availability.ts
@@ -0,0 +1,21 @@
+import { DEFAULT_MODEL_ID } from "@/lib/models";
+
+const DISABLED_MODEL_IDS = new Set(["openai/gpt-5.4-pro"]);
+
+export function isModelDisabled(modelId: string): boolean {
+  return DISABLED_MODEL_IDS.has(modelId);
+}
+
+export function filterDisabledModels<T extends { id: string }>(
+  models: T[],
+): T[] {
+  return models.filter((model) => !isModelDisabled(model.id));
+}
+
+export function resolveAvailableModelId(modelId: string): string {
+  if (isModelDisabled(modelId)) {
+    return DEFAULT_MODEL_ID;
+  }
+
+  return modelId;
+}
diff --git a/apps/web/lib/models-with-context.ts b/apps/web/lib/models-with-context.ts
index 7356119b2..3ca1b8c22 100644
--- a/apps/web/lib/models-with-context.ts
+++ b/apps/web/lib/models-with-context.ts
@@ -1,6 +1,7 @@
 import "server-only";
 
 import { gateway } from "ai";
+import { filterDisabledModels } from "./model-availability";
 import type { AvailableModel, AvailableModelCost } from "./models";
 
 const MODELS_DEV_URL = "https://models.dev/api.json";
@@ -173,7 +174,9 @@ export async function fetchAvailableLanguageModels(): Promise<
   AvailableModel[]
 > {
   const { models } = await gateway.getAvailableModels();
-  return models.filter((model) => model.modelType === "language");
+  return filterDisabledModels(
+    models.filter((model) => model.modelType === "language"),
+  );
 }
 
 export async function fetchAvailableLanguageModelsWithContext(): Promise<