diff --git a/src/api/providers/__tests__/deepseek.spec.ts b/src/api/providers/__tests__/deepseek.spec.ts
index 50cabfa922..6353af5efd 100644
--- a/src/api/providers/__tests__/deepseek.spec.ts
+++ b/src/api/providers/__tests__/deepseek.spec.ts
@@ -96,6 +96,60 @@ describe("DeepSeekHandler", () => {
 			expect(handler.getModel().id).toBe(mockOptions.apiModelId)
 		})
 
+		it("should throw error if API key contains non-ASCII characters", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: "sk-test中文characters",
+				})
+			}).toThrow("Invalid DeepSeek API key: contains non-ASCII character at position 8")
+		})
+
+		it("should throw error with helpful message for non-ASCII characters", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: "sk-test-😀-key",
+				})
+			}).toThrow(/API keys must contain only ASCII characters/)
+		})
+
+		it("should accept valid ASCII-only API keys", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: "sk-test-1234567890-abcdefghijklmnopqrstuvwxyz-ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+				})
+			}).not.toThrow()
+		})
+
+		it("should accept API keys with standard symbols", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: "sk-test_key-123!@#$%^&*()_+-=[]{}|;:,.<>?/",
+				})
+			}).not.toThrow()
+		})
+
+		it("should not validate when API key is not provided", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: undefined,
+				})
+			}).not.toThrow()
+		})
+
+		it("should not validate when API key is 'not-provided'", () => {
+			expect(() => {
+				new DeepSeekHandler({
+					...mockOptions,
+					deepSeekApiKey: "not-provided",
+				})
+			}).not.toThrow()
+		})
+
 		it.skip("should throw error if API key is missing", () => {
 			expect(() => {
 				new DeepSeekHandler({
diff --git a/src/api/providers/deepseek.ts b/src/api/providers/deepseek.ts
index de119de6db..4fb24abc43 100644
--- a/src/api/providers/deepseek.ts
+++ b/src/api/providers/deepseek.ts
@@ -9,9 +9,13 @@ import { OpenAiHandler } from "./openai"
 
 export class DeepSeekHandler extends OpenAiHandler {
 	constructor(options: ApiHandlerOptions) {
+		// Validate API key before passing to parent constructor
+		const apiKey = options.deepSeekApiKey ?? "not-provided"
+		DeepSeekHandler.validateApiKey(apiKey)
+
 		super({
 			...options,
-			openAiApiKey: options.deepSeekApiKey ?? "not-provided",
+			openAiApiKey: apiKey,
 			openAiModelId: options.apiModelId ?? deepSeekDefaultModelId,
 			openAiBaseUrl: options.deepSeekBaseUrl ?? "https://api.deepseek.com",
 			openAiStreamingEnabled: true,
@@ -19,6 +23,26 @@ export class DeepSeekHandler extends OpenAiHandler {
 		})
 	}
 
+	/**
+	 * Validates that the API key contains only ASCII characters.
+	 * Non-ASCII characters in API keys cause ByteString conversion errors.
+	 */
+	private static validateApiKey(apiKey: string): void {
+		if (apiKey && apiKey !== "not-provided") {
+			// Check for non-ASCII characters
+			for (let i = 0; i < apiKey.length; i++) {
+				const charCode = apiKey.charCodeAt(i)
+				if (charCode > 255) {
+					throw new Error(
+						`Invalid DeepSeek API key: contains non-ASCII character at position ${i + 1}. ` +
+							`API keys must contain only ASCII characters (letters, numbers, and standard symbols). ` +
+							`Please check your API key for any accidental non-ASCII characters or spaces.`,
+					)
+				}
+			}
+		}
+	}
+
 	override getModel() {
 		const id = this.options.apiModelId ?? deepSeekDefaultModelId
 		const info = deepSeekModels[id as keyof typeof deepSeekModels] || deepSeekModels[deepSeekDefaultModelId]