Clamp GPT-5 max output tokens to 20% of context window (#8495)

Author: daniel-lxs

src/shared/__tests__/api.spec.ts

@@ -194,18 +194,17 @@ describe("getModelMaxOutputTokens", () => {
 		expect(result).toBe(20_000) // Should use model.maxTokens since it's exactly at 20%
 	})
 
-	test("should bypass 20% cap for GPT-5 models and use exact configured max tokens", () => {
+	test("should apply 20% cap for GPT-5 models like other models", () => {
 		const model: ModelInfo = {
 			contextWindow: 200_000,
 			supportsPromptCache: false,
-			maxTokens: 128_000, // 64% of context window, normally would be capped
+			maxTokens: 128_000, // 64% of context window, should be capped
 		}
 
 		const settings: ProviderSettings = {
 			apiProvider: "openai",
 		}
 
-		// Test various GPT-5 model IDs
 		const gpt5ModelIds = ["gpt-5", "gpt-5-turbo", "GPT-5", "openai/gpt-5-preview", "gpt-5-32k", "GPT-5-TURBO"]
 
 		gpt5ModelIds.forEach((modelId) => {
@@ -215,8 +214,8 @@ describe("getModelMaxOutputTokens", () => {
 				settings,
 				format: "openai",
 			})
-			// Should use full 128k tokens, not capped to 20% (40k)
-			expect(result).toBe(128_000)
+			// Should be capped to 20% of context window: 200_000 * 0.2 = 40_000
+			expect(result).toBe(40_000)
 		})
 	})
 
@@ -246,23 +245,11 @@ describe("getModelMaxOutputTokens", () => {
 		})
 	})
 
-	test("should handle GPT-5 models with various max token configurations", () => {
+	test("should cap GPT-5 models to min(model.maxTokens, 20% of contextWindow)", () => {
 		const testCases = [
-			{
-				maxTokens: 128_000,
-				contextWindow: 200_000,
-				expected: 128_000, // Uses full 128k
-			},
-			{
-				maxTokens: 64_000,
-				contextWindow: 200_000,
-				expected: 64_000, // Uses configured 64k
-			},
-			{
-				maxTokens: 256_000,
-				contextWindow: 400_000,
-				expected: 256_000, // Uses full 256k even though it's 64% of context
-			},
+			{ maxTokens: 128_000, contextWindow: 200_000, expected: 40_000 },
+			{ maxTokens: 64_000, contextWindow: 200_000, expected: 40_000 },
+			{ maxTokens: 256_000, contextWindow: 400_000, expected: 80_000 },
 		]
 
 		testCases.forEach(({ maxTokens, contextWindow, expected }) => {

src/shared/api.ts

@@ -116,17 +116,7 @@ export const getModelMaxOutputTokens = ({
 	}
 
 	// If model has explicit maxTokens, clamp it to 20% of the context window
-	// Exception: GPT-5 models should use their exact configured max output tokens
 	if (model.maxTokens) {
-		// Check if this is a GPT-5 model (case-insensitive)
-		const isGpt5Model = modelId.toLowerCase().includes("gpt-5")
-
-		// GPT-5 models bypass the 20% cap and use their full configured max tokens
-		if (isGpt5Model) {
-			return model.maxTokens
-		}
-
-		// All other models are clamped to 20% of context window
 		return Math.min(model.maxTokens, Math.ceil(model.contextWindow * 0.2))
 	}