fix: make token counting API requests asynchronous for Gemini/Anthropic

- Modified countTokens() in GeminiHandler to return tiktoken estimate immediately
- Modified countTokens() in AnthropicHandler to return tiktoken estimate immediately
- API calls now happen asynchronously in the background without blocking inference
- Added tests to verify asynchronous behavior and immediate returns
- Vertex provider automatically inherits the fix from GeminiHandler

Fixes #3666

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

@@ -265,4 +265,62 @@ describe("AnthropicHandler", () => {
 			expect(result.temperature).toBe(0)
 		})
 	})
+
+	describe("countTokens", () => {
+		it("should return fallback count immediately without waiting for API", async () => {
+			// Mock the countTokens API to take a long time
+			const mockCountTokens = vitest.fn().mockImplementation(() => {
+				return new Promise((resolve) => {
+					setTimeout(() => resolve({ input_tokens: 100 }), 1000)
+				})
+			})
+
+			;(handler as any).client.messages.countTokens = mockCountTokens
+
+			// Mock the base class countTokens to return a known value
+			const baseSpy = vitest.spyOn(handler.constructor.prototype.__proto__, "countTokens")
+			baseSpy.mockResolvedValue(50)
+
+			const content: Anthropic.Messages.ContentBlockParam[] = [{ type: "text", text: "Test content" }]
+
+			const startTime = Date.now()
+			const result = await handler.countTokens(content)
+			const endTime = Date.now()
+
+			// Should return immediately (less than 100ms)
+			expect(endTime - startTime).toBeLessThan(100)
+
+			// Should return the fallback count
+			expect(result).toBe(50)
+
+			// Should have called the base class method
+			expect(baseSpy).toHaveBeenCalledWith(content)
+
+			// Should have started the async API call
+			expect(mockCountTokens).toHaveBeenCalled()
+
+			baseSpy.mockRestore()
+		})
+
+		it("should handle async API errors gracefully", async () => {
+			// Mock the countTokens API to throw an error
+			const mockCountTokens = vitest.fn().mockRejectedValue(new Error("API Error"))
+			;(handler as any).client.messages.countTokens = mockCountTokens
+
+			// Mock the base class countTokens
+			const baseSpy = vitest.spyOn(handler.constructor.prototype.__proto__, "countTokens")
+			baseSpy.mockResolvedValue(75)
+
+			const content: Anthropic.Messages.ContentBlockParam[] = [{ type: "text", text: "Test content" }]
+
+			// Should not throw even if async call fails
+			const result = await handler.countTokens(content)
+			expect(result).toBe(75)
+
+			// Wait a bit to ensure async error is handled
+			await new Promise((resolve) => setTimeout(resolve, 100))
+
+			baseSpy.mockRestore()
+		})
+	})
 })

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

@@ -247,4 +247,62 @@ describe("GeminiHandler", () => {
 			expect(cost).toBeUndefined()
 		})
 	})
+
+	describe("countTokens", () => {
+		it("should return fallback count immediately without waiting for API", async () => {
+			// Mock the countTokens API to take a long time
+			const mockCountTokens = vitest.fn().mockImplementation(() => {
+				return new Promise((resolve) => {
+					setTimeout(() => resolve({ totalTokens: 100 }), 1000)
+				})
+			})
+
+			handler["client"].models.countTokens = mockCountTokens
+
+			// Mock the base class countTokens to return a known value
+			const baseSpy = vitest.spyOn(handler.constructor.prototype.__proto__, "countTokens")
+			baseSpy.mockResolvedValue(50)
+
+			const content: Anthropic.Messages.ContentBlockParam[] = [{ type: "text", text: "Test content" }]
+
+			const startTime = Date.now()
+			const result = await handler.countTokens(content)
+			const endTime = Date.now()
+
+			// Should return immediately (less than 100ms)
+			expect(endTime - startTime).toBeLessThan(100)
+
+			// Should return the fallback count
+			expect(result).toBe(50)
+
+			// Should have called the base class method
+			expect(baseSpy).toHaveBeenCalledWith(content)
+
+			// Should have started the async API call
+			expect(mockCountTokens).toHaveBeenCalled()
+
+			baseSpy.mockRestore()
+		})
+
+		it("should handle async API errors gracefully", async () => {
+			// Mock the countTokens API to throw an error
+			const mockCountTokens = vitest.fn().mockRejectedValue(new Error("API Error"))
+			handler["client"].models.countTokens = mockCountTokens
+
+			// Mock the base class countTokens
+			const baseSpy = vitest.spyOn(handler.constructor.prototype.__proto__, "countTokens")
+			baseSpy.mockResolvedValue(75)
+
+			const content: Anthropic.Messages.ContentBlockParam[] = [{ type: "text", text: "Test content" }]
+
+			// Should not throw even if async call fails
+			const result = await handler.countTokens(content)
+			expect(result).toBe(75)
+
+			// Wait a bit to ensure async error is handled
+			await new Promise((resolve) => setTimeout(resolve, 100))
+
+			baseSpy.mockRestore()
+		})
+	})
 })

src/api/providers/anthropic.ts

@@ -278,22 +278,33 @@ export class AnthropicHandler extends BaseProvider implements SingleCompletionHa
 	 * @returns A promise resolving to the token count
 	 */
 	override async countTokens(content: Array<Anthropic.Messages.ContentBlockParam>): Promise<number> {
-		try {
-			// Use the current model
-			const { id: model } = this.getModel()
+		// Immediately return the tiktoken estimate
+		const fallbackCount = super.countTokens(content)
 
-			const response = await this.client.messages.countTokens({
-				model,
-				messages: [{ role: "user", content: content }],
-			})
+		// Start the API call asynchronously (fire and forget)
+		this.countTokensAsync(content).catch((error) => {
+			// Log error but don't throw - we already returned the fallback
+			console.debug("Anthropic async token counting failed:", error)
+		})
 
-			return response.input_tokens
-		} catch (error) {
-			// Log error but fallback to tiktoken estimation
-			console.warn("Anthropic token counting failed, using fallback", error)
+		return fallbackCount
+	}
 
-			// Use the base provider's implementation as fallback
-			return super.countTokens(content)
-		}
+	/**
+	 * Performs the actual API call to count tokens asynchronously
+	 * This method is called in the background and doesn't block the main request
+	 */
+	private async countTokensAsync(content: Array<Anthropic.Messages.ContentBlockParam>): Promise<number> {
+		// Use the current model
+		const { id: model } = this.getModel()
+
+		const response = await this.client.messages.countTokens({
+			model,
+			messages: [{ role: "user", content: content }],
+		})
+
+		// In the future, we could cache this result or use it for telemetry
+		console.debug(`Anthropic token count: API=${response.input_tokens}`)
+		return response.input_tokens
 	}
 }

src/api/providers/gemini.ts

@@ -168,24 +168,38 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 	}
 
 	override async countTokens(content: Array<Anthropic.Messages.ContentBlockParam>): Promise<number> {
-		try {
-			const { id: model } = this.getModel()
+		// Immediately return the tiktoken estimate
+		const fallbackCount = super.countTokens(content)
 
-			const response = await this.client.models.countTokens({
-				model,
-				contents: convertAnthropicContentToGemini(content),
-			})
+		// Start the API call asynchronously (fire and forget)
+		this.countTokensAsync(content).catch((error) => {
+			// Log error but don't throw - we already returned the fallback
+			console.debug("Gemini async token counting failed:", error)
+		})
 
-			if (response.totalTokens === undefined) {
-				console.warn("Gemini token counting returned undefined, using fallback")
-				return super.countTokens(content)
-			}
+		return fallbackCount
+	}
 
-			return response.totalTokens
-		} catch (error) {
-			console.warn("Gemini token counting failed, using fallback", error)
-			return super.countTokens(content)
+	/**
+	 * Performs the actual API call to count tokens asynchronously
+	 * This method is called in the background and doesn't block the main request
+	 */
+	private async countTokensAsync(content: Array<Anthropic.Messages.ContentBlockParam>): Promise<number> {
+		const { id: model } = this.getModel()
+
+		const response = await this.client.models.countTokens({
+			model,
+			contents: convertAnthropicContentToGemini(content),
+		})
+
+		if (response.totalTokens === undefined) {
+			console.debug("Gemini token counting returned undefined")
+			return 0
 		}
+
+		// In the future, we could cache this result or use it for telemetry
+		console.debug(`Gemini token count: API=${response.totalTokens}`)
+		return response.totalTokens
 	}
 
 	public calculateCost({