diff --git a/src/server/features/ai-search/services/promptExplorer.ts b/src/server/features/ai-search/services/promptExplorer.ts
index 1aaf5dd..39404e2 100644
--- a/src/server/features/ai-search/services/promptExplorer.ts
+++ b/src/server/features/ai-search/services/promptExplorer.ts
@@ -123,10 +123,12 @@ async function runModel(
   return reapplyHighlightBrand(shaped, args.highlightBrand);
 }
 
-// DataForSEO's Claude catalog caps at the 4.0 family (no Sonnet 4.5+ yet).
+// Each value must be a member of ACCEPTED_LLM_MODEL_NAMES in dataforseo/ai.ts,
+// which mirrors DataForSEO's llm_responses/models catalog. DataForSEO dropped
+// the Claude Sonnet 4.0 family, so we target the 4.5 alias (latest dated 4.5).
 const MODEL_NAMES: Record<PromptExplorerModel, string> = {
   chat_gpt: "gpt-5",
-  claude: "claude-sonnet-4-0",
+  claude: "claude-sonnet-4-5",
   gemini: "gemini-2.5-pro",
   perplexity: "sonar-reasoning-pro",
 };
diff --git a/src/server/lib/dataforseo/ai.ts b/src/server/lib/dataforseo/ai.ts
index 104f052..0401daa 100644
--- a/src/server/lib/dataforseo/ai.ts
+++ b/src/server/lib/dataforseo/ai.ts
@@ -310,6 +310,24 @@ export async function fetchLlmCrossAggregatedMetrics(
 
 type LlmResponseModelSlug = "chat_gpt" | "claude" | "gemini" | "perplexity";
 
+/**
+ * Accepted `model_name` values per slug, mirroring DataForSEO's
+ * `/ai_optimization/{model}/llm_responses/models` catalog (verified 2026-06-30).
+ * We validate against this before dispatching because DataForSEO BILLS a task
+ * that fails with `Invalid Field: 'model_name'` — a stale or mistyped model name
+ * would otherwise pay for a guaranteed-rejected call. DataForSEO resolves a
+ * basic alias (e.g. `claude-sonnet-4-5`) to its latest dated version.
+ */
+const ACCEPTED_LLM_MODEL_NAMES: Record<
+  LlmResponseModelSlug,
+  ReadonlySet<string>
+> = {
+  chat_gpt: new Set(["gpt-5"]),
+  claude: new Set(["claude-sonnet-4-5", "claude-sonnet-4-6"]),
+  gemini: new Set(["gemini-2.5-pro"]),
+  perplexity: new Set(["sonar-reasoning-pro", "sonar-pro", "sonar"]),
+};
+
 type LlmResponsesInput = {
   userPrompt: string;
   modelSlug: LlmResponseModelSlug;
@@ -348,6 +366,15 @@ function buildPerplexityLlmResponseRequest(
 export async function fetchLlmResponse(
   input: LlmResponsesInput,
 ): Promise<DataforseoApiResponse<LlmResponseResult>> {
+  // Fail fast on an unknown model_name: DataForSEO charges for tasks that fail
+  // with `Invalid Field: 'model_name'`, so we must never dispatch one.
+  if (!ACCEPTED_LLM_MODEL_NAMES[input.modelSlug].has(input.modelName)) {
+    throw new AppError(
+      "VALIDATION_ERROR",
+      `Unsupported DataForSEO model_name "${input.modelName}" for ${input.modelSlug}`,
+    );
+  }
+
   // DataForSEO's Gemini endpoint rejects `web_search_country_iso_code` with a
   // 40501 "Invalid Field" error. The other three models accept it.
   const supportsCountry = input.modelSlug !== "gemini";
diff --git a/src/server/lib/dataforseo/endpoints.test.ts b/src/server/lib/dataforseo/endpoints.test.ts
index 9233d2e..47b4974 100644
--- a/src/server/lib/dataforseo/endpoints.test.ts
+++ b/src/server/lib/dataforseo/endpoints.test.ts
@@ -356,3 +356,25 @@ describe("DataForSEO SDK-backed endpoints", () => {
     ]);
   });
 });
+
+describe("fetchLlmResponse model_name validation", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("rejects an unknown model_name before dispatching a paid LLM task", async () => {
+    const fetchMock = vi.fn<typeof fetch>();
+    vi.stubGlobal("fetch", fetchMock);
+
+    await expect(
+      fetchLlmResponse({
+        userPrompt: "What is OpenSEO?",
+        modelSlug: "claude",
+        // DataForSEO dropped this from its catalog; it must never be dispatched.
+        modelName: "claude-sonnet-4-0",
+      }),
+    ).rejects.toThrow(/Unsupported DataForSEO model_name/);
+
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});