fix: improve Claude Sonnet 4 1M context window beta header handling

- Fixed beta header array initialization to prevent mutation of model betas
- Added proper deduplication of beta headers to avoid duplicates
- Ensured context-1m-2025-08-07 beta is properly combined with prompt caching beta
- Added comprehensive tests for 1M context window feature

Fixes #7229

src/api/providers/__tests__/anthropic.spec.ts

@@ -264,5 +264,138 @@ describe("AnthropicHandler", () => {
 			expect(result.reasoningBudget).toBeUndefined()
 			expect(result.temperature).toBe(0)
 		})
+
+		it("should enable 1M context window when anthropicBeta1MContext is true for Claude Sonnet 4", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-4-20250514",
+				anthropicBeta1MContext: true,
+			})
+
+			const model = handler.getModel()
+
+			// Should have 1M context window when enabled
+			expect(model.info.contextWindow).toBe(1_000_000)
+			// Should use tier pricing for >200K context
+			expect(model.info.inputPrice).toBe(6.0)
+			expect(model.info.outputPrice).toBe(22.5)
+			expect(model.info.cacheWritesPrice).toBe(7.5)
+			expect(model.info.cacheReadsPrice).toBe(0.6)
+		})
+
+		it("should use default context window when anthropicBeta1MContext is false for Claude Sonnet 4", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-4-20250514",
+				anthropicBeta1MContext: false,
+			})
+
+			const model = handler.getModel()
+
+			// Should use default context window (200k)
+			expect(model.info.contextWindow).toBe(200_000)
+			// Should use default pricing for ≤200K context
+			expect(model.info.inputPrice).toBe(3.0)
+			expect(model.info.outputPrice).toBe(15.0)
+			expect(model.info.cacheWritesPrice).toBe(3.75)
+			expect(model.info.cacheReadsPrice).toBe(0.3)
+		})
+
+		it("should not affect context window for non-Claude Sonnet 4 models", () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-3-5-sonnet-20241022",
+				anthropicBeta1MContext: true,
+			})
+
+			const model = handler.getModel()
+
+			// Should use default context window for non-Sonnet 4 models
+			expect(model.info.contextWindow).toBe(200_000)
+		})
+	})
+
+	describe("createMessage with 1M context beta", () => {
+		it("should include context-1m-2025-08-07 beta header when enabled for Claude Sonnet 4", async () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-4-20250514",
+				anthropicBeta1MContext: true,
+			})
+
+			const systemPrompt = "You are a helpful assistant."
+			const messages: Anthropic.Messages.MessageParam[] = [{ role: "user", content: "Hello" }]
+
+			const stream = handler.createMessage(systemPrompt, messages)
+			const chunks: any[] = []
+			for await (const chunk of stream) {
+				chunks.push(chunk)
+			}
+
+			// Verify that the create method was called with the correct beta headers
+			expect(mockCreate).toHaveBeenCalledWith(
+				expect.objectContaining({
+					model: "claude-sonnet-4-20250514",
+				}),
+				expect.objectContaining({
+					headers: expect.objectContaining({
+						"anthropic-beta": expect.stringContaining("context-1m-2025-08-07"),
+					}),
+				}),
+			)
+
+			// Verify that both betas are included
+			const callArgs = mockCreate.mock.calls[0]
+			const headers = callArgs[1]?.headers
+			expect(headers?.["anthropic-beta"]).toContain("prompt-caching-2024-07-31")
+			expect(headers?.["anthropic-beta"]).toContain("context-1m-2025-08-07")
+		})
+
+		it("should not include context-1m beta header when disabled for Claude Sonnet 4", async () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-sonnet-4-20250514",
+				anthropicBeta1MContext: false,
+			})
+
+			const systemPrompt = "You are a helpful assistant."
+			const messages: Anthropic.Messages.MessageParam[] = [{ role: "user", content: "Hello" }]
+
+			const stream = handler.createMessage(systemPrompt, messages)
+			const chunks: any[] = []
+			for await (const chunk of stream) {
+				chunks.push(chunk)
+			}
+
+			// Verify that the create method was called without the 1M context beta
+			const callArgs = mockCreate.mock.calls[0]
+			const headers = callArgs[1]?.headers
+			expect(headers?.["anthropic-beta"]).toContain("prompt-caching-2024-07-31")
+			expect(headers?.["anthropic-beta"]).not.toContain("context-1m-2025-08-07")
+		})
+
+		it("should handle thinking models with 1M context beta correctly", async () => {
+			const handler = new AnthropicHandler({
+				apiKey: "test-api-key",
+				apiModelId: "claude-3-7-sonnet-20250219:thinking",
+				anthropicBeta1MContext: true, // This shouldn't affect non-Sonnet 4 models
+			})
+
+			const systemPrompt = "You are a helpful assistant."
+			const messages: Anthropic.Messages.MessageParam[] = [{ role: "user", content: "Hello" }]
+
+			const stream = handler.createMessage(systemPrompt, messages)
+			const chunks: any[] = []
+			for await (const chunk of stream) {
+				chunks.push(chunk)
+			}
+
+			// Verify that the 1M context beta is NOT included for non-Sonnet 4 models
+			const callArgs = mockCreate.mock.calls[0]
+			const headers = callArgs[1]?.headers
+			expect(headers?.["anthropic-beta"]).toContain("output-128k-2025-02-19")
+			expect(headers?.["anthropic-beta"]).toContain("prompt-caching-2024-07-31")
+			expect(headers?.["anthropic-beta"]).not.toContain("context-1m-2025-08-07")
+		})
 	})
 })

src/api/providers/anthropic.ts

@@ -43,11 +43,17 @@ export class AnthropicHandler extends BaseProvider implements SingleCompletionHa
 	): ApiStream {
 		let stream: AnthropicStream<Anthropic.Messages.RawMessageStreamEvent>
 		const cacheControl: CacheControlEphemeral = { type: "ephemeral" }
-		let { id: modelId, betas = [], maxTokens, temperature, reasoning: thinking } = this.getModel()
+		let { id: modelId, betas: modelBetas, maxTokens, temperature, reasoning: thinking } = this.getModel()
+
+		// Initialize betas array properly
+		const betas: string[] = modelBetas ? [...modelBetas] : []
 
 		// Add 1M context beta flag if enabled for Claude Sonnet 4
 		if (modelId === "claude-sonnet-4-20250514" && this.options.anthropicBeta1MContext) {
-			betas.push("context-1m-2025-08-07")
+			// Only add if not already present
+			if (!betas.includes("context-1m-2025-08-07")) {
+				betas.push("context-1m-2025-08-07")
+			}
 		}
 
 		switch (modelId) {
@@ -118,8 +124,12 @@ export class AnthropicHandler extends BaseProvider implements SingleCompletionHa
 							case "claude-3-5-haiku-20241022":
 							case "claude-3-opus-20240229":
 							case "claude-3-haiku-20240307":
-								betas.push("prompt-caching-2024-07-31")
-								return { headers: { "anthropic-beta": betas.join(",") } }
+								// Only add prompt caching beta if not already present
+								if (!betas.includes("prompt-caching-2024-07-31")) {
+									betas.push("prompt-caching-2024-07-31")
+								}
+								// Only set headers if we have betas to include
+								return betas.length > 0 ? { headers: { "anthropic-beta": betas.join(",") } } : undefined
 							default:
 								return undefined
 						}