diff --git a/packages/api-client/src/index.ts b/packages/api-client/src/index.ts
index 7de0663..1022f1e 100644
--- a/packages/api-client/src/index.ts
+++ b/packages/api-client/src/index.ts
@@ -1,4 +1,4 @@
-import createFetchClient from "openapi-fetch";
+import createFetchClient, { FetchResponse } from "openapi-fetch";
 import type { paths } from "./schema";
 
 export type ArgosAPIClient = ReturnType<typeof createClient>;
@@ -15,3 +15,27 @@ export function createClient(options: { baseUrl?: string; authToken: string }) {
     },
   });
 }
+
+export class APIError extends Error {
+  constructor(message: string) {
+    super(message);
+  }
+}
+
+/**
+ * Handle API errors.
+ */
+export function throwAPIError(
+  fetchResponse: FetchResponse<any, any, any>,
+): never {
+  const { error, response } = fetchResponse;
+  if (
+    error &&
+    typeof error === "object" &&
+    "error" in error &&
+    typeof error.error === "string"
+  ) {
+    throw new APIError(error.error);
+  }
+  throw new APIError(`API error: ${response.status} ${response.statusText}`);
+}
diff --git a/packages/core/src/finalize.ts b/packages/core/src/finalize.ts
index e069fa4..5f660b4 100644
--- a/packages/core/src/finalize.ts
+++ b/packages/core/src/finalize.ts
@@ -1,4 +1,4 @@
-import { createClient } from "@argos-ci/api-client";
+import { createClient, throwAPIError } from "@argos-ci/api-client";
 import { getAuthToken } from "./auth";
 import { readConfig } from "./config";
 
@@ -33,7 +33,7 @@ export async function finalize(params: FinalizeParameters) {
   });
 
   if (finalizeBuildsResult.error) {
-    throw new Error(finalizeBuildsResult.error.error);
+    throwAPIError(finalizeBuildsResult);
   }
 
   return finalizeBuildsResult.data;
diff --git a/packages/core/src/upload.ts b/packages/core/src/upload.ts
index 5c8f13c..ef75ed2 100644
--- a/packages/core/src/upload.ts
+++ b/packages/core/src/upload.ts
@@ -1,4 +1,4 @@
-import { createClient } from "@argos-ci/api-client";
+import { createClient, throwAPIError } from "@argos-ci/api-client";
 import { readConfig } from "./config";
 import { discoverScreenshots } from "./discovery";
 import { optimizeScreenshot } from "./optimize";
@@ -201,7 +201,7 @@ export async function upload(params: UploadParameters) {
   debug("Fetch project");
   const projectResponse = await apiClient.GET("/project");
   if (projectResponse.error) {
-    throw new Error(projectResponse.error.error);
+    throwAPIError(projectResponse);
   }
   const { defaultBaseBranch, hasRemoteContentAccess } = projectResponse.data;
   const referenceBranch = config.referenceBranch || defaultBaseBranch;
@@ -267,7 +267,7 @@ export async function upload(params: UploadParameters) {
   });
 
   if (createBuildResponse.error) {
-    throw new Error(createBuildResponse.error.error);
+    throwAPIError(createBuildResponse);
   }
 
   const result = createBuildResponse.data;
@@ -328,7 +328,7 @@ export async function upload(params: UploadParameters) {
   });
 
   if (uploadBuildResponse.error) {
-    throw new Error(uploadBuildResponse.error.error);
+    throwAPIError(uploadBuildResponse);
   }
 
   return { build: uploadBuildResponse.data.build, screenshots };