fix(site): allow single-label AI provider endpoints (#28122)

Closes #27980.

Reduces the `baseUrl` field validation in `ProviderForm.tsx` to only
validate non-empty input. The previous validation was not in line with
`validateAIProviderBaseURL` in `codersdk/aiproviders.go`. This was
blocking users from adding providers with a short-form hostname (e.g.
`http://localhost:8080/v1`).

> Generated by Coder Agents, reviewed by a human.

site/src/pages/AISettingsPage/ProvidersPage/AddProviderPage/AddProviderPageView.stories.tsx

@@ -1,6 +1,8 @@
 import type { Meta, StoryObj } from "@storybook/react-vite";
-import { within } from "storybook/test";
+import { expect, spyOn, userEvent, waitFor, within } from "storybook/test";
 import { reactRouterParameters } from "storybook-addon-remix-react-router";
+import { API } from "#/api/api";
+import { mockApiError } from "#/testHelpers/entities";
 import { withToaster } from "#/testHelpers/storybook";
 import { addableProviders } from "../components/addableProviderTypes";
 import AddProviderPageView from "./AddProviderPageView";
@@ -62,3 +64,36 @@ export const AddCopilot: Story = {
 		await canvas.findByText("Add a GitHub Copilot provider");
 	},
 };
+
+// Server base_url errors must render inline on the Endpoint input, not only in the top-of-form ErrorAlert.
+export const WithBaseUrlValidationError: Story = {
+	args: {
+		provider: addableProviders.find((p) => p.value === "openai-compat")!,
+	},
+	beforeEach: () => {
+		spyOn(API, "createAIProvider").mockRejectedValue(
+			mockApiError({
+				message: "Invalid AI provider request.",
+				validations: [{ field: "base_url", detail: "server base_url error" }],
+			}),
+		);
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		await userEvent.type(await canvas.findByLabelText(/^name/i), "localai");
+		await userEvent.type(
+			canvas.getByLabelText(/^endpoint\s*\*?$/i),
+			"http://localai:8080/v1",
+		);
+		await userEvent.type(canvas.getByLabelText(/api key/i), "sk-local");
+		const submitButton = canvas.getByRole("button", { name: /add provider/i });
+		await waitFor(() => expect(submitButton).toBeEnabled());
+		await userEvent.click(submitButton);
+		const endpointInput = canvas.getByLabelText(/^endpoint\s*\*?$/i);
+		await waitFor(() =>
+			expect(endpointInput).toHaveAttribute("aria-invalid", "true"),
+		);
+		// Guard the visible diagnostic too, not just the invalid state.
+		expect(await canvas.findAllByText("server base_url error")).toHaveLength(2);
+	},
+};

site/src/pages/AISettingsPage/ProvidersPage/components/ProviderForm.stories.tsx

@@ -762,3 +762,52 @@ export const UnsavedChangesPrompt: Story = {
 		).toBeInTheDocument();
 	},
 };
+
+// Regression coverage for #27980. The create form must accept endpoints with
+// single-label hosts such as http://localai:8080/v1.
+export const AddOpenAICompatSingleLabelHost: Story = {
+	args: {
+		initialValues: {
+			type: "openai-compat",
+			name: "localai",
+			baseUrl: "http://localai:8080/v1",
+			enabled: true,
+		},
+	},
+	play: async ({ canvasElement, args }) => {
+		const canvas = within(canvasElement);
+		const apiKeyInput = await canvas.findByLabelText(/api key/i);
+		await userEvent.type(apiKeyInput, "sk-local");
+
+		const submitButton = canvas.getByRole("button", { name: /add provider/i });
+
+		await waitFor(() => expect(submitButton).toBeEnabled());
+
+		await userEvent.click(submitButton);
+		await waitFor(() =>
+			expect(args.onSubmit).toHaveBeenCalledWith(
+				expect.objectContaining({ baseUrl: "http://localai:8080/v1" }),
+			),
+		);
+	},
+};
+
+export const AddOpenAICompatEmptyEndpointBlocked: Story = {
+	args: {
+		initialValues: {
+			type: "openai-compat",
+			name: "localai",
+			baseUrl: "",
+			enabled: true,
+		},
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		const apiKeyInput = await canvas.findByLabelText(/api key/i);
+		await userEvent.type(apiKeyInput, "sk-local");
+
+		const submitButton = canvas.getByRole("button", { name: /add provider/i });
+
+		await waitFor(() => expect(submitButton).toBeDisabled());
+	},
+};

site/src/pages/AISettingsPage/ProvidersPage/components/ProviderForm.tsx

@@ -45,7 +45,6 @@ export type ProviderFormValues = {
 	enabled: boolean;
 };
 
-const HTTP_SCHEME_REGEX = /^https?:\/\//i;
 // AWS Bedrock InvokeModel URL, e.g. https://bedrock-runtime.{region}.amazonaws.com
 const BEDROCK_INVOKE_MODEL_URL_REGEX =
 	/^https:\/\/bedrock-runtime\.([a-z0-9-]+)\.amazonaws\.com\/?$/i;
@@ -166,10 +165,8 @@ const makeOpenAiAnthropicSchema = (editing: boolean) =>
 		name: makeNameSchema(editing),
 		displayName: makeDisplayNameSchema(editing),
 		icon: Yup.string(),
-		baseUrl: Yup.string()
-			.url("Endpoint must be a valid URL")
-			.matches(HTTP_SCHEME_REGEX, "Endpoint must use http or https.")
-			.required("Endpoint is required"),
+		// URL shape is validated by the backend; the form only checks presence.
+		baseUrl: Yup.string().required("Endpoint is required"),
 		apiKey: editing
 			? Yup.string()
 			: Yup.string().required("API key is required"),
@@ -201,7 +198,6 @@ const makeBedrockSchema = (editing: boolean) =>
 			.oneOf(["invoke-model", "mantle"] as const)
 			.required(),
 		baseUrl: Yup.string()
-			.url("Endpoint must be a valid URL")
 			.when("protocol", {
 				is: "mantle",
 				then: (schema) =>
@@ -257,10 +253,7 @@ const makeCopilotSchema = (editing: boolean) =>
 		name: makeNameSchema(editing),
 		displayName: makeDisplayNameSchema(editing),
 		icon: Yup.string(),
-		baseUrl: Yup.string()
-			.url("Endpoint must be a valid URL")
-			.matches(HTTP_SCHEME_REGEX, "Endpoint must use http or https.")
-			.required("Endpoint is required"),
+		baseUrl: Yup.string().required("Endpoint is required"),
 		enabled: Yup.boolean(),
 	});
 
@@ -503,7 +496,9 @@ export const ProviderForm: FC<ProviderFormProps> = ({
 						{iconField}
 						<FormField
 							required
-							field={getFieldHelpers("baseUrl")}
+							field={getFieldHelpers("baseUrl", {
+								backendFieldName: "base_url",
+							})}
 							label="Endpoint"
 							description={
 								typeSelectValue === "copilot" ? (
@@ -587,7 +582,9 @@ export const ProviderForm: FC<ProviderFormProps> = ({
 						</div>
 						<FormField
 							required
-							field={getFieldHelpers("baseUrl")}
+							field={getFieldHelpers("baseUrl", {
+								backendFieldName: "base_url",
+							})}
 							label="Endpoint"
 							description={
 								<>