diff --git a/sdk/packages/llms/src/providers/routing/anthropic-compatible.ts b/sdk/packages/llms/src/providers/routing/anthropic-compatible.ts
index 6f15144120..41b3a63998 100644
--- a/sdk/packages/llms/src/providers/routing/anthropic-compatible.ts
+++ b/sdk/packages/llms/src/providers/routing/anthropic-compatible.ts
@@ -581,10 +581,16 @@ export function buildGatewayReasoningOptions(
 					explicitBudgetTokens: request.reasoning?.budgetTokens,
 				})
 			: request.reasoning?.budgetTokens;
+	const shouldSendDisabledReasoning =
+		request.reasoning?.enabled === false &&
+		// FIXME: temporary OpenRouter compatibility patch for Claude Fable.
+		// Remove once OpenRouter normalizes disabled reasoning like Vercel does,
+		// or replace with a systematic policy for models that reject it.
+		!request.modelId.toLowerCase().includes("claude-fable");
 	const reasoning: Record<string, unknown> = {
 		...(request.reasoning?.enabled === true
 			? { enabled: true }
-			: request.reasoning?.enabled === false
+			: shouldSendDisabledReasoning
 				? { enabled: false }
 				: request.reasoning?.effort
 					? { enabled: true }
diff --git a/sdk/packages/llms/src/providers/routing/provider-options.test.ts b/sdk/packages/llms/src/providers/routing/provider-options.test.ts
index 615909be04..df9a4c6a7c 100644
--- a/sdk/packages/llms/src/providers/routing/provider-options.test.ts
+++ b/sdk/packages/llms/src/providers/routing/provider-options.test.ts
@@ -1101,6 +1101,20 @@ describe("composeAiSdkProviderOptions: family/provider thinking patches", () =>
 			context: { family: "kimi-k2.6" },
 			expect: [{ bucket: "cline", has: { reasoning: { enabled: false } } }],
 		},
+		{
+			name: "cline Claude Fable reasoning.enabled=false uses lowest supported reasoning",
+			request: {
+				providerId: "cline",
+				modelId: "anthropic/claude-fable-5",
+				reasoning: { enabled: false },
+			},
+			expect: [
+				{
+					bucket: "cline",
+					has: { reasoning: { max_tokens: 1024 } },
+				},
+			],
+		},
 		// OpenRouter owns the reasoning object regardless of Moonshot family.
 		{
 			name: "openrouter non-K2.6 Moonshot Kimi reasoning.enabled=false -> reasoning.exclude",