feat: normalize OpenAI base URL to support both formats

Fixes #10337

- Add normalizeOpenAiBaseUrl utility to handle both URL formats
  (with and without /v1 suffix)
- Update openai-native.ts to use the URL normalization function
- Add UI guidance text explaining supported URL formats
- Add translation string for the base URL hint
- Add comprehensive tests for URL normalization

src/api/providers/openai-native.ts

@@ -25,6 +25,7 @@ import { getModelParams } from "../transform/model-params"
 import { BaseProvider } from "./base-provider"
 import type { SingleCompletionHandler, ApiHandlerCreateMessageMetadata } from "../index"
 import { isMcpTool } from "../../utils/mcp-name"
+import { normalizeOpenAiBaseUrl } from "../../utils/normalizeOpenAiBaseUrl"
 
 export type OpenAiNativeModel = ReturnType<OpenAiNativeHandler["getModel"]>
 
@@ -68,7 +69,13 @@ export class OpenAiNativeHandler extends BaseProvider implements SingleCompletio
 			this.options.enableResponsesReasoningSummary = true
 		}
 		const apiKey = this.options.openAiNativeApiKey ?? "not-provided"
-		this.client = new OpenAI({ baseURL: this.options.openAiNativeBaseUrl, apiKey })
+		// Normalize the base URL to handle both formats (with and without /v1 suffix).
+		// The OpenAI SDK expects a base URL that may or may not include /v1.
+		// We normalize to ensure consistent behavior regardless of user input format.
+		const normalizedBaseUrl = this.options.openAiNativeBaseUrl
+			? normalizeOpenAiBaseUrl(this.options.openAiNativeBaseUrl, "https://api.openai.com") + "/v1"
+			: undefined
+		this.client = new OpenAI({ baseURL: normalizedBaseUrl, apiKey })
 	}
 
 	private normalizeUsage(usage: any, model: OpenAiNativeModel): ApiStreamUsageChunk | undefined {
@@ -509,7 +516,10 @@ export class OpenAiNativeHandler extends BaseProvider implements SingleCompletio
 		messages?: Anthropic.Messages.MessageParam[],
 	): ApiStream {
 		const apiKey = this.options.openAiNativeApiKey ?? "not-provided"
-		const baseUrl = this.options.openAiNativeBaseUrl || "https://api.openai.com"
+		// Normalize the base URL to handle both formats (with and without /v1 suffix).
+		// This ensures consistent URL construction regardless of whether the user provides
+		// "https://my-host.example" or "https://my-host.example/v1"
+		const baseUrl = normalizeOpenAiBaseUrl(this.options.openAiNativeBaseUrl, "https://api.openai.com")
 		const url = `${baseUrl}/v1/responses`
 
 		// Create AbortController for cancellation

src/utils/__tests__/normalizeOpenAiBaseUrl.spec.ts

@@ -0,0 +1,131 @@
+import { normalizeOpenAiBaseUrl } from "../normalizeOpenAiBaseUrl"
+
+describe("normalizeOpenAiBaseUrl", () => {
+	const defaultUrl = "https://api.openai.com"
+
+	describe("when baseUrl is empty or undefined", () => {
+		it("should return the normalized default URL when baseUrl is undefined", () => {
+			expect(normalizeOpenAiBaseUrl(undefined, defaultUrl)).toBe(defaultUrl)
+		})
+
+		it("should return the normalized default URL when baseUrl is empty string", () => {
+			expect(normalizeOpenAiBaseUrl("", defaultUrl)).toBe(defaultUrl)
+		})
+
+		it("should return the normalized default URL when baseUrl is whitespace", () => {
+			expect(normalizeOpenAiBaseUrl("   ", defaultUrl)).toBe(defaultUrl)
+		})
+	})
+
+	describe("when baseUrl does not have /v1 suffix", () => {
+		it("should return the URL as-is for a simple URL", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should remove trailing slash", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should remove multiple trailing slashes", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example///", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should handle URL with port", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example:8080", defaultUrl)).toBe(
+				"https://my-host.example:8080",
+			)
+		})
+
+		it("should handle URL with path", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/api", defaultUrl)).toBe(
+				"https://my-host.example/api",
+			)
+		})
+	})
+
+	describe("when baseUrl has /v1 suffix", () => {
+		it("should strip /v1 suffix", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/v1", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should strip /v1/ suffix with trailing slash", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/v1/", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should strip /v1 suffix with multiple trailing slashes", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/v1///", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should handle URL with port and /v1 suffix", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example:8080/v1", defaultUrl)).toBe(
+				"https://my-host.example:8080",
+			)
+		})
+
+		it("should handle URL with path and /v1 suffix", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/api/v1", defaultUrl)).toBe(
+				"https://my-host.example/api",
+			)
+		})
+
+		it("should be case-insensitive for /v1 suffix", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/V1", defaultUrl)).toBe("https://my-host.example")
+		})
+	})
+
+	describe("when default URL has /v1 suffix", () => {
+		it("should normalize default URL by stripping /v1", () => {
+			expect(normalizeOpenAiBaseUrl(undefined, "https://api.openai.com/v1")).toBe("https://api.openai.com")
+		})
+
+		it("should normalize default URL by stripping /v1/", () => {
+			expect(normalizeOpenAiBaseUrl("", "https://api.openai.com/v1/")).toBe("https://api.openai.com")
+		})
+	})
+
+	describe("edge cases", () => {
+		it("should handle localhost URLs", () => {
+			expect(normalizeOpenAiBaseUrl("http://localhost:4000/v1", defaultUrl)).toBe("http://localhost:4000")
+		})
+
+		it("should handle IP address URLs", () => {
+			expect(normalizeOpenAiBaseUrl("http://192.168.1.100:8080/v1", defaultUrl)).toBe("http://192.168.1.100:8080")
+		})
+
+		it("should trim whitespace from URL", () => {
+			expect(normalizeOpenAiBaseUrl("  https://my-host.example/v1  ", defaultUrl)).toBe("https://my-host.example")
+		})
+
+		it("should not remove v1 from the middle of the path", () => {
+			expect(normalizeOpenAiBaseUrl("https://my-host.example/v1/api", defaultUrl)).toBe(
+				"https://my-host.example/v1/api",
+			)
+		})
+
+		it("should not remove v1 if it is part of the hostname", () => {
+			expect(normalizeOpenAiBaseUrl("https://api-v1.example.com", defaultUrl)).toBe("https://api-v1.example.com")
+		})
+	})
+
+	describe("real-world scenarios", () => {
+		it("should handle official OpenAI URL without /v1", () => {
+			expect(normalizeOpenAiBaseUrl("https://api.openai.com", defaultUrl)).toBe("https://api.openai.com")
+		})
+
+		it("should handle official OpenAI URL with /v1", () => {
+			expect(normalizeOpenAiBaseUrl("https://api.openai.com/v1", defaultUrl)).toBe("https://api.openai.com")
+		})
+
+		it("should handle Azure OpenAI URLs", () => {
+			expect(normalizeOpenAiBaseUrl("https://myinstance.openai.azure.com", defaultUrl)).toBe(
+				"https://myinstance.openai.azure.com",
+			)
+		})
+
+		it("should handle custom proxy URLs", () => {
+			expect(normalizeOpenAiBaseUrl("https://proxy.example.com/openai/v1", defaultUrl)).toBe(
+				"https://proxy.example.com/openai",
+			)
+		})
+	})
+})

src/utils/normalizeOpenAiBaseUrl.ts

@@ -0,0 +1,47 @@
+/**
+ * Normalizes an OpenAI-compatible base URL to ensure consistent URL construction.
+ *
+ * This function handles various user-provided base URL formats:
+ * - With /v1 suffix: "https://api.openai.com/v1" → "https://api.openai.com"
+ * - Without /v1 suffix: "https://api.openai.com" → "https://api.openai.com"
+ * - With trailing slash: "https://api.openai.com/" → "https://api.openai.com"
+ * - With /v1/ suffix: "https://api.openai.com/v1/" → "https://api.openai.com"
+ *
+ * The normalized URL can then safely have "/v1/..." appended to it.
+ *
+ * @param baseUrl - The user-provided base URL (may or may not include /v1)
+ * @param defaultUrl - The default URL to use if baseUrl is empty
+ * @returns The normalized base URL without trailing /v1 or trailing slash
+ */
+export function normalizeOpenAiBaseUrl(baseUrl: string | undefined, defaultUrl: string): string {
+	// Use default if no custom URL provided
+	if (!baseUrl || baseUrl.trim() === "") {
+		return normalizeUrl(defaultUrl)
+	}
+
+	return normalizeUrl(baseUrl)
+}
+
+/**
+ * Internal helper to normalize a URL by removing trailing /v1 and trailing slashes.
+ */
+function normalizeUrl(url: string): string {
+	let normalized = url.trim()
+
+	// Remove trailing slashes first
+	while (normalized.endsWith("/")) {
+		normalized = normalized.slice(0, -1)
+	}
+
+	// Remove /v1 suffix if present (case-insensitive for robustness)
+	if (normalized.toLowerCase().endsWith("/v1")) {
+		normalized = normalized.slice(0, -3)
+	}
+
+	// Remove any trailing slashes that might be left after removing /v1
+	while (normalized.endsWith("/")) {
+		normalized = normalized.slice(0, -1)
+	}
+
+	return normalized
+}

webview-ui/src/components/settings/providers/OpenAI.tsx

@@ -54,9 +54,12 @@ export const OpenAI = ({ apiConfiguration, setApiConfigurationField, selectedMod
 						value={apiConfiguration?.openAiNativeBaseUrl || ""}
 						type="url"
 						onInput={handleInputChange("openAiNativeBaseUrl")}
-						placeholder="https://api.openai.com/v1"
+						placeholder="https://api.openai.com"
 						className="w-full mt-1"
 					/>
+					<div className="text-sm text-vscode-descriptionForeground -mt-1">
+						{t("settings:providers.openAiBaseUrlHint")}
+					</div>
 				</>
 			)}
 			<VSCodeTextField

webview-ui/src/i18n/locales/en/settings.json

@@ -285,6 +285,7 @@
 		"getVercelAiGatewayApiKey": "Get Vercel AI Gateway API Key",
 		"apiKeyStorageNotice": "API keys are stored securely in VSCode's Secret Storage",
 		"useCustomBaseUrl": "Use custom base URL",
+		"openAiBaseUrlHint": "Both formats are supported: https://my-host.example or https://my-host.example/v1",
 		"useReasoning": "Enable reasoning",
 		"useHostHeader": "Use custom Host header",
 		"useLegacyFormat": "Use legacy OpenAI API format",