diff --git a/README.md b/README.md
index 96dfd20..3559265 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,7 @@ Append these query parameters to the URL to customize the look and data of your
 | `height`     | Number  | Sets the height of the SVG in pixels.                           | `300`         | `?height=350`          |
 | `stroke`     | Boolean | Adds an outline stroke to chart segments.                       | `false`       | `?stroke=true`         |
 | `test`       | Boolean | Uses samples data instead of fetching from GitHub API.          | `false`       | `?test=true`           |
+| `error`      | String  | Forces an error SVG with the given message. For testing only.   | —             | `?error=test`          |
 
 #### Example URL
 To get 10 languages, a dark theme, and a custom title:
@@ -110,5 +111,14 @@ vercel dev
 
 > The default endpoint is /api/languages
 
+## Error Responses
+
+All errors return HTTP 200 with an error SVG so they render in GitHub README embeds.
+
+Common error messages:
+- `GitHub API error: {status} {statusText}` — GitHub API request failed
+- `No language data available` — no public repositories found
+- `At least one of GITHUB_USERNAMES or GITHUB_ORGS must be set` — missing environment configuration
+
 ## License
 MIT License - see [LICENSE](./LICENSE) for details.
diff --git a/api/languages/index.ts b/api/languages/index.ts
index 9d4c286..36ff84e 100644
--- a/api/languages/index.ts
+++ b/api/languages/index.ts
@@ -13,10 +13,12 @@ export default async function handler(
     chartType, chartTitle,
     width, height, count,
     selectedTheme, stroke,
-    useTestData
+    useTestData, errorTest
   } = parseQueryParams(req.query as QueryParams);
 
   try {
+    if (errorTest) throw new Error(errorTest);
+
     const rawData              = await fetchLanguageData(useTestData);
     const normalizedData       = processLanguageData(rawData, count);
     const { segments, legend } = generateChartData(normalizedData, selectedTheme, chartType, width, stroke);
diff --git a/src/utils/params.ts b/src/utils/params.ts
index c3bb486..aa431b4 100644
--- a/src/utils/params.ts
+++ b/src/utils/params.ts
@@ -41,6 +41,7 @@ export function parseQueryParams(query: QueryParams) {
       colours:   customColours
     },
     stroke:      query["stroke"] === "true",
-    useTestData: query["test"] === "true"
+    useTestData: query["test"] === "true",
+    errorTest:   sanitize(query["error"] ?? '')
   }
 }
diff --git a/tests/api/languages/index.test.ts b/tests/api/languages/index.test.ts
index ae24dff..db74b4e 100644
--- a/tests/api/languages/index.test.ts
+++ b/tests/api/languages/index.test.ts
@@ -33,15 +33,18 @@ describe("handler", () => {
       send: vi.fn()
     } as unknown as VercelResponse;
 
+    vi.spyOn(console, 'error').mockImplementation(() => {});
+
     vi.mocked(parseQueryParams).mockReturnValue({
-      chartType: "donut",
-      chartTitle: "Languages",
-      width: 600,
-      height: 400,
-      count: 5,
+      chartType:     "donut",
+      chartTitle:    "Languages",
+      width:         600,
+      height:        400,
+      count:         5,
       selectedTheme: mockTheme,
-      stroke: false,
-      useTestData: false
+      stroke:        false,
+      useTestData:   false,
+      errorTest:     ''
     });
   });
 
@@ -106,4 +109,20 @@ describe("handler", () => {
     expect(res.setHeader).toHaveBeenCalledWith("X-Chart-Error", "true");
     expect(res.status).toHaveBeenCalledWith(200);
   });
+
+  it("throws error when errorTest param is set", async () => {
+    vi.mocked(parseQueryParams).mockReturnValue({
+      chartType: "donut", chartTitle: "Languages",
+      width: 600, height: 400, count: 5,
+      selectedTheme: mockTheme, stroke: false,
+      useTestData: false, errorTest: "test error"
+    });
+    const errorSvg = "<svg>error</svg>";
+    vi.mocked(renderError).mockReturnValue(errorSvg);
+
+    await handler(req, res);
+
+    expect(renderError).toHaveBeenCalledWith("test error", 600, 400, mockTheme);
+    expect(res.status).toHaveBeenCalledWith(200);
+  });
 });