diff --git a/packages/cli/e2e/upload.test.js b/packages/cli/e2e/upload.test.js
index 9d092755..1fd784bd 100644
--- a/packages/cli/e2e/upload.test.js
+++ b/packages/cli/e2e/upload.test.js
@@ -4,7 +4,10 @@ import { getRequiredEnv, run } from "./utils.js";
 
 getRequiredEnv("ARGOS_TOKEN");
 
-test("upload returns a full build URL", { timeout: 15_000 }, () => {
+// This test uploads the full __fixtures__ directory, which includes a 10MB PNG
+// stress fixture. That file is sharp-optimized, hashed, and uploaded to S3 over
+// a real network connection, so a generous timeout is required to avoid flakes.
+test("upload returns a full build URL", { timeout: 30_000 }, () => {
   const buildName = `argos-cli-e2e-node-${process.env.NODE_VERSION}-${process.env.OS}`;
   const uploadResult = run([
     "upload",
diff --git a/packages/core/src/s3.ts b/packages/core/src/s3.ts
index eb4d04b8..28837e04 100644
--- a/packages/core/src/s3.ts
+++ b/packages/core/src/s3.ts
@@ -11,6 +11,58 @@ interface PresignedPostUploadInput extends UploadInput {
   fields: Record<string, string>;
 }
 
+/**
+ * On failure, S3 responds with an XML body describing the error, e.g.:
+ *
+ *   <?xml version="1.0" encoding="UTF-8"?>
+ *   <Error>
+ *     <Code>AccessDenied</Code>
+ *     <Message>Request has expired</Message>
+ *     ...
+ *   </Error>
+ *
+ * Extract the `Code` and `Message` so the user gets the actual reason for the
+ * failure instead of a generic HTTP status. Returns `null` when the body
+ * cannot be read or does not look like an S3 error document.
+ */
+async function readS3ErrorMessage(response: Response): Promise<string | null> {
+  try {
+    const body = await response.text();
+    // Trim captures: pretty-printed XML can leave whitespace/newlines around
+    // the inner text. `[\s\S]` matches across lines in case the value wraps.
+    const code = body.match(/<Code>([\s\S]*?)<\/Code>/)?.[1]?.trim();
+    const message = body.match(/<Message>([\s\S]*?)<\/Message>/)?.[1]?.trim();
+    if (code && message) {
+      return `${code}: ${message}`;
+    }
+    return message || code || null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Build an error for a failed upload, including the S3 error details when
+ * available so the user can understand why the upload was rejected.
+ */
+async function createUploadError(
+  url: string,
+  response: Response,
+): Promise<Error> {
+  const detail = await readS3ErrorMessage(response);
+  // `statusText` is often empty in Node (e.g. HTTP/2), so only append it when
+  // present to avoid a trailing space like "403 ".
+  const status = response.statusText
+    ? `${response.status} ${response.statusText}`
+    : `${response.status}`;
+  return new Error(
+    `Failed to upload file to ${url}: ${status}${detail ? ` — ${detail}` : ""}`,
+  );
+}
+
+/**
+ * Upload a file to S3 using a presigned PUT URL.
+ */
 export async function uploadFile(input: UploadInput): Promise<void> {
   const file = await readFile(input.path);
   const response = await fetch(input.url, {
@@ -22,17 +74,22 @@ export async function uploadFile(input: UploadInput): Promise<void> {
     body: new Uint8Array(file),
   });
   if (!response.ok) {
-    throw new Error(
-      `Failed to upload file to ${input.url}: ${response.status} ${response.statusText}`,
-    );
+    throw await createUploadError(input.url, response);
   }
 }
 
+/**
+ * Upload a file to S3 using a presigned POST. Unlike a presigned PUT, this
+ * sends the file as multipart form data alongside the policy `fields`
+ * provided by S3.
+ */
 export async function uploadFileWithPresignedPost(
   input: PresignedPostUploadInput,
 ): Promise<void> {
   const file = await readFile(input.path);
   const formData = new FormData();
+  // The presigned policy fields (key, policy, signature, etc.) must be
+  // appended before the file part for S3 to accept the upload.
   for (const [key, value] of Object.entries(input.fields)) {
     formData.append(key, value);
   }
@@ -48,8 +105,6 @@ export async function uploadFileWithPresignedPost(
     body: formData,
   });
   if (!response.ok) {
-    throw new Error(
-      `Failed to upload file to ${input.url}: ${response.status} ${response.statusText}`,
-    );
+    throw await createUploadError(input.url, response);
   }
 }