diff --git a/README.md b/README.md
index 220a252..47e1274 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,11 @@ The frontend communicates with the StableRoute API backend.
 
 - **`NEXT_PUBLIC_STABLEROUTE_API_BASE`**: Specifies the base URL of the StableRoute API backend (defaults to `http://localhost:3001` if unset).
 
+The API base must be an absolute `http` or `https` URL. Shared API helpers also
+require request paths to start with `/`, which keeps frontend requests on the
+configured StableRoute API origin and avoids leaking requests to arbitrary
+hosts through misconfiguration or absolute-path call sites.
+
 ### API Endpoints Consumed
 
 - **`/api/v1/pairs`**: Lists registered pairs (`GET`) and registers new pairs (`POST`).
diff --git a/src/lib/__tests__/apiClient.test.ts b/src/lib/__tests__/apiClient.test.ts
index eaa0423..5bf37d5 100644
--- a/src/lib/__tests__/apiClient.test.ts
+++ b/src/lib/__tests__/apiClient.test.ts
@@ -1,4 +1,30 @@
-import { apiFetch, registerAuthErrorHandler } from "../apiClient";
+import {
+  apiDelete,
+  apiFetch,
+  apiPatch,
+  apiPost,
+  registerAuthErrorHandler,
+} from "../apiClient";
+
+function loadApiClientWithBase(base: string) {
+  const previousBase = process.env.NEXT_PUBLIC_STABLEROUTE_API_BASE;
+  try {
+    jest.resetModules();
+    process.env.NEXT_PUBLIC_STABLEROUTE_API_BASE = base;
+    let loadedApiClient: typeof import("../apiClient") | undefined;
+    jest.isolateModules(() => {
+      loadedApiClient = require("../apiClient") as typeof import("../apiClient");
+    });
+    return loadedApiClient!;
+  } finally {
+    if (previousBase === undefined) {
+      delete process.env.NEXT_PUBLIC_STABLEROUTE_API_BASE;
+    } else {
+      process.env.NEXT_PUBLIC_STABLEROUTE_API_BASE = previousBase;
+    }
+    jest.resetModules();
+  }
+}
 
 function mockResponse(status: number, body: string, contentType = "application/json") {
   global.fetch = jest.fn().mockResolvedValue({
@@ -15,6 +41,12 @@ describe("apiFetch", () => {
   it("returns parsed JSON on success", async () => {
     mockResponse(200, JSON.stringify({ id: 1 }));
     await expect(apiFetch("/test")).resolves.toEqual({ id: 1 });
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:3001/test",
+      expect.objectContaining({
+        headers: expect.objectContaining({ "Content-Type": "application/json" }),
+      })
+    );
   });
 
   it("returns undefined for 204", async () => {
@@ -52,6 +84,117 @@ describe("apiFetch", () => {
     expect(err).toBeInstanceOf(Error);
     expect(err.status).toBe(400);
   });
+
+  it("uses a valid configured http API base", async () => {
+    const { apiFetch: configuredApiFetch } = loadApiClientWithBase("https://api.example.test");
+    mockResponse(200, JSON.stringify({ id: 2 }));
+
+    await expect(configuredApiFetch("/test")).resolves.toEqual({ id: 2 });
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "https://api.example.test/test",
+      expect.any(Object)
+    );
+  });
+
+  it("rejects non-http API base schemes without leaking the raw value", () => {
+    const rawBase = "javascript:alert(1)";
+
+    try {
+      loadApiClientWithBase(rawBase);
+      throw new Error("expected loadApiClientWithBase to throw");
+    } catch (error) {
+      const message = (error as Error).message;
+      expect(message).toBe("Invalid StableRoute API base URL configuration");
+      expect(message).not.toContain(rawBase);
+    }
+  });
+
+  it("rejects unparseable API base values without leaking the raw value", () => {
+    const rawBase = "not a url";
+
+    try {
+      loadApiClientWithBase(rawBase);
+      throw new Error("expected loadApiClientWithBase to throw");
+    } catch (error) {
+      const message = (error as Error).message;
+      expect(message).toBe("Invalid StableRoute API base URL configuration");
+      expect(message).not.toContain(rawBase);
+    }
+  });
+
+  it("rejects absolute request paths before fetch", async () => {
+    global.fetch = jest.fn();
+
+    await expect(apiFetch("https://evil.example/api")).rejects.toThrow(
+      "API request path must be a relative path starting with /"
+    );
+
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it("rejects protocol-relative request paths before fetch", async () => {
+    global.fetch = jest.fn();
+
+    await expect(apiFetch("//evil.example/api")).rejects.toThrow(
+      "API request path must be a relative path starting with /"
+    );
+
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it("rejects backslash paths that would resolve to another origin", async () => {
+    global.fetch = jest.fn();
+
+    await expect(apiFetch("/\\evil.example/api")).rejects.toThrow(
+      "API request path must stay on the configured API origin"
+    );
+
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it("posts JSON through the shared wrapper", async () => {
+    mockResponse(200, JSON.stringify({ ok: true }));
+
+    await expect(apiPost("/test", { label: "prod" })).resolves.toEqual({ ok: true });
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:3001/test",
+      expect.objectContaining({
+        method: "POST",
+        body: JSON.stringify({ label: "prod" }),
+      })
+    );
+  });
+
+  it("patches JSON through the shared wrapper", async () => {
+    mockResponse(200, JSON.stringify({ ok: true }));
+
+    await expect(apiPatch("/test", { fee_bps: 10 })).resolves.toEqual({ ok: true });
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:3001/test",
+      expect.objectContaining({
+        method: "PATCH",
+        body: JSON.stringify({ fee_bps: 10 }),
+      })
+    );
+  });
+
+  it("deletes through the shared wrapper", async () => {
+    global.fetch = jest.fn().mockResolvedValue({
+      status: 204,
+      ok: true,
+      text: () => Promise.resolve(""),
+    } as unknown as Response);
+
+    await expect(apiDelete("/test")).resolves.toBeUndefined();
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:3001/test",
+      expect.objectContaining({ method: "DELETE" })
+    );
+  });
 });
 
 describe("registerAuthErrorHandler", () => {
diff --git a/src/lib/apiClient.ts b/src/lib/apiClient.ts
index 5df2924..7f35ea3 100644
--- a/src/lib/apiClient.ts
+++ b/src/lib/apiClient.ts
@@ -1,6 +1,32 @@
 const API_BASE =
   process.env.NEXT_PUBLIC_STABLEROUTE_API_BASE ?? "http://localhost:3001";
 
+function parseApiBase(value: string): URL {
+  let url: URL;
+  try {
+    url = new URL(value);
+  } catch {
+    throw new Error("Invalid StableRoute API base URL configuration");
+  }
+  if (url.protocol !== "http:" && url.protocol !== "https:") {
+    throw new Error("Invalid StableRoute API base URL configuration");
+  }
+  return url;
+}
+
+const API_BASE_URL = parseApiBase(API_BASE);
+
+function resolveApiUrl(path: string): string {
+  if (!path.startsWith("/") || path.startsWith("//")) {
+    throw new Error("API request path must be a relative path starting with /");
+  }
+  const url = new URL(path, API_BASE_URL);
+  if (url.origin !== API_BASE_URL.origin) {
+    throw new Error("API request path must stay on the configured API origin");
+  }
+  return url.toString();
+}
+
 export type ApiError = {
   error: string;
   message: string;
@@ -22,7 +48,7 @@ export async function apiFetch<T>(
   path: string,
   init: RequestInit = {}
 ): Promise<T> {
-  const res = await fetch(`${API_BASE}${path}`, {
+  const res = await fetch(resolveApiUrl(path), {
     headers: { "Content-Type": "application/json", ...(init.headers ?? {}) },
     ...init,
   });