diff --git a/docs/packages/http.md b/docs/packages/http.md
index d9264e8..c39b824 100644
--- a/docs/packages/http.md
+++ b/docs/packages/http.md
@@ -222,7 +222,7 @@ try {
 
 | Parameter                  | Type                     | Description                                                                                                                                                                                                               |
 | -------------------------- | ------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| `baseURL`                  | `string`                 | Base URL for all requests                                                                                                                                                                                                 |
+| `baseURL`                  | `string`                 | Base URL for all requests. **Must be absolute** (e.g. `${location.origin}/api`); relative paths fail fast.                                                                                                                |
 | `options.timeout`          | `number \| undefined`    | Request timeout in milliseconds (default: `30000`; pass `0` to disable)                                                                                                                                                   |
 | `options.headers`          | `Record<string, string>` | Default headers                                                                                                                                                                                                           |
 | `options.withCredentials`  | `boolean`                | Send cookies cross-origin (default: `true`)                                                                                                                                                                               |
diff --git a/package-lock.json b/package-lock.json
index 6e95ae2..16f949a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10228,7 +10228,7 @@
         },
         "packages/http": {
             "name": "@script-development/fs-http",
-            "version": "0.4.0",
+            "version": "0.4.1",
             "license": "MIT",
             "dependencies": {
                 "axios": "^1.16.1"
diff --git a/packages/http/CHANGELOG.md b/packages/http/CHANGELOG.md
index e98e0e6..9f04a2e 100644
--- a/packages/http/CHANGELOG.md
+++ b/packages/http/CHANGELOG.md
@@ -1,5 +1,11 @@
 # @script-development/fs-http
 
+## 0.4.1 — 2026-05-29
+
+### Patch Changes
+
+- **Fail-fast guard on relative `baseURL`.** `createHttpService('/api')` now throws a library-attributed `Error` instead of an opaque native `TypeError: Invalid URL`. The new message names the package, names the function, explains that an absolute baseURL is required, and echoes the offending value — so the failure points at the consumer's call site rather than at fs-http internals. Production-bug class previously surfaced only at runtime as opaque `TypeError: Invalid URL` and remained latent in CI when consumers mock `@script-development/fs-http` in integration tests. Closes enforcement queue #21.
+
 ## 0.4.0 — 2026-05-15
 
 ### Breaking Changes
diff --git a/packages/http/package.json b/packages/http/package.json
index b36a82e..adb51b9 100644
--- a/packages/http/package.json
+++ b/packages/http/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@script-development/fs-http",
-    "version": "0.4.0",
+    "version": "0.4.1",
     "description": "Framework-agnostic HTTP service factory with middleware architecture",
     "homepage": "https://packages.script.nl/packages/http",
     "license": "MIT",
diff --git a/packages/http/src/http.ts b/packages/http/src/http.ts
index 255a456..1865e2e 100644
--- a/packages/http/src/http.ts
+++ b/packages/http/src/http.ts
@@ -30,8 +30,26 @@ const unregister = <T>(array: T[], item: T): UnregisterMiddleware => {
     };
 };
 
+/**
+ * Parse the consumer-supplied baseURL with a library-attributed error on failure.
+ * The native `new URL(baseURL)` throws an opaque `TypeError: Invalid URL` that
+ * points at fs-http internals rather than the consumer's call site, latent for
+ * 6 days on entreezuil (PR #40 adoption → PR #96 fix) because integration tests
+ * mocked @script-development/fs-http and the real factory never ran. Fail-fast
+ * here (vs. silent coercion to absolute) prevents the class for every adopter.
+ */
+const parseBaseURL = (baseURL: string): URL => {
+    try {
+        return new URL(baseURL);
+    } catch {
+        throw new Error(
+            `[@script-development/fs-http] createHttpService requires an absolute baseURL (e.g. \`\${location.origin}/api\`). Received: ${JSON.stringify(baseURL)}`,
+        );
+    }
+};
+
 export const createHttpService = (baseURL: string, options?: HttpServiceOptions): HttpService => {
-    const apiUrl = new URL(baseURL);
+    const apiUrl = parseBaseURL(baseURL);
 
     const http = axios.create({
         baseURL: apiUrl.toString(),
diff --git a/packages/http/tests/http.spec.ts b/packages/http/tests/http.spec.ts
index 905ae92..667044a 100644
--- a/packages/http/tests/http.spec.ts
+++ b/packages/http/tests/http.spec.ts
@@ -36,6 +36,46 @@ describe('createHttpService', () => {
         });
     });
 
+    describe('baseURL guard (queue #21)', () => {
+        // Latent for 6 days on entreezuil (PR #40 adoption → PR #96 fix) because integration
+        // tests mock @script-development/fs-http so the real factory never ran. The opaque
+        // native `TypeError: Invalid URL` pointed at fs-http internals rather than the
+        // consumer's createHttpService call site. Library-side fail-fast guard prevents the
+        // class for every future adopter.
+
+        it('throws a library-attributed error when called with a relative path', () => {
+            // Arrange & Act & Assert
+            expect(() => createHttpService('/api')).toThrow(/@script-development\/fs-http/);
+            expect(() => createHttpService('/api')).toThrow(/createHttpService/);
+            expect(() => createHttpService('/api')).toThrow(/absolute baseURL/);
+            expect(() => createHttpService('/api')).toThrow(/"\/api"/);
+        });
+
+        it('throws a library-attributed error when called with an empty string', () => {
+            // Arrange & Act & Assert
+            expect(() => createHttpService('')).toThrow(/@script-development\/fs-http/);
+            expect(() => createHttpService('')).toThrow(/createHttpService/);
+            expect(() => createHttpService('')).toThrow(/absolute baseURL/);
+            expect(() => createHttpService('')).toThrow(/""/);
+        });
+
+        it('throws a library-attributed error for malformed URL strings', () => {
+            // Arrange & Act & Assert — sample of malformed inputs covered by the guard.
+            expect(() => createHttpService('not a url')).toThrow(/@script-development\/fs-http/);
+            expect(() => createHttpService('not a url')).toThrow(/"not a url"/);
+
+            expect(() => createHttpService('http://')).toThrow(/@script-development\/fs-http/);
+            expect(() => createHttpService('http://')).toThrow(/"http:\/\/"/);
+        });
+
+        it('does NOT throw for valid absolute URLs (happy-path regression guard)', () => {
+            // Arrange & Act & Assert — these all succeed because the guard parses successfully.
+            expect(() => createHttpService('http://localhost')).not.toThrow();
+            expect(() => createHttpService('https://example.com/api')).not.toThrow();
+            expect(() => createHttpService('https://api.example.com')).not.toThrow();
+        });
+    });
+
     describe('default options', () => {
         it('creates axios instance with correct defaults', async () => {
             // Arrange