diff --git a/scripts/lib/mcode-tools-artifact.mjs b/scripts/lib/mcode-tools-artifact.mjs
index 98868d243..0c4fe695a 100644
--- a/scripts/lib/mcode-tools-artifact.mjs
+++ b/scripts/lib/mcode-tools-artifact.mjs
@@ -2,6 +2,7 @@ import { createHash } from 'node:crypto';
 import { mkdir, readFile, writeFile } from 'node:fs/promises';
 import path from 'node:path';
 import { Readable } from 'node:stream';
+import { setTimeout as sleep } from 'node:timers/promises';
 import { Parser } from 'tar';
 
 export const MCODE_TOOLS_ARTIFACT = Object.freeze({
@@ -51,15 +52,52 @@ export async function extractMcodeToolsArtifact(bytes) {
   return { cli, manifest: manifestBytes, notices };
 }
 
+const transientNetworkCodes = new Set([
+  'ECONNRESET', 'ECONNREFUSED', 'EPIPE', 'ETIMEDOUT', 'EAI_AGAIN', 'ENETUNREACH',
+  'UND_ERR_CONNECT_TIMEOUT', 'UND_ERR_HEADERS_TIMEOUT', 'UND_ERR_BODY_TIMEOUT', 'UND_ERR_SOCKET',
+]);
+const transientHttpStatuses = new Set([408, 429, 500, 502, 503, 504]);
+
+export async function downloadMcodeToolsArtifact(fetchImpl = fetch, { wait = sleep, warn = console.warn } = {}) {
+  const attempts = 3;
+  for (let attempt = 1; attempt <= attempts; attempt++) {
+    let retryAfterMs = 0;
+    try {
+      // A new deadline covers both connection setup and the complete response body.
+      const response = await fetchImpl(MCODE_TOOLS_ARTIFACT.url, { signal: AbortSignal.timeout(120_000) });
+      if (!response.ok) {
+        const retryAfter = response.headers.get('retry-after');
+        if (retryAfter !== null) {
+          const milliseconds = /^\d+$/.test(retryAfter) ? Number(retryAfter) * 1000 : Date.parse(retryAfter) - Date.now();
+          if (!Number.isNaN(milliseconds)) retryAfterMs = Math.min(30_000, Math.max(0, milliseconds));
+        }
+        await response.body?.cancel().catch(() => {});
+        const error = new Error(`Cannot download public mcode-tools artifact: HTTP ${response.status}`);
+        error.status = response.status;
+        throw error;
+      }
+      return Buffer.from(await response.arrayBuffer());
+    } catch (error) {
+      const code = error.cause?.code ?? error.code;
+      const retryable = transientHttpStatuses.has(error.status) ||
+        transientNetworkCodes.has(code) || error.name === 'TimeoutError';
+      if (!retryable) throw error;
+      if (attempt === attempts)
+        throw new Error(`Cannot download public mcode-tools artifact after ${attempts} attempts`, { cause: error });
+      const delayMs = Math.max(1000 * 2 ** (attempt - 1), retryAfterMs);
+      warn(`[mcode-tools] Download attempt ${attempt}/${attempts} failed (${code ?? error.message}); retrying in ${delayMs} ms.`);
+      await wait(delayMs);
+    }
+  }
+}
+
 export async function copyMcodeToolsArtifact(root, outdir, fetchImpl = fetch) {
   const cache = path.join(root, '.cache', 'artifacts', 'code-0.3.11.tgz');
   let bytes;
   try { bytes = await readFile(cache); }
   catch (error) {
     if (error.code !== 'ENOENT') throw error;
-    const response = await fetchImpl(MCODE_TOOLS_ARTIFACT.url, { signal: AbortSignal.timeout(120_000) });
-    if (!response.ok) throw new Error(`Cannot download public mcode-tools artifact: HTTP ${response.status}`);
-    bytes = Buffer.from(await response.arrayBuffer());
+    bytes = await downloadMcodeToolsArtifact(fetchImpl);
   }
   const artifact = await extractMcodeToolsArtifact(bytes);
   await mkdir(path.dirname(cache), { recursive: true });
diff --git a/test/source-sync.test.mjs b/test/source-sync.test.mjs
index 8a92c52f8..2833c05fc 100644
--- a/test/source-sync.test.mjs
+++ b/test/source-sync.test.mjs
@@ -19,6 +19,93 @@ import { releaseManifest } from '../scripts/package-cli-release.mjs';
 import { validateReleaseReports } from '../scripts/publish-cli-release.mjs';
 import { compareVersions, releaseCli } from '../scripts/release-cli.mjs';
 import { compareRuns, validateRun, validateRequest, validateToolOutput, median, selectScenarios } from '../scripts/perf/report.mjs';
+import { copyMcodeToolsArtifact, downloadMcodeToolsArtifact, MCODE_TOOLS_ARTIFACT } from '../scripts/lib/mcode-tools-artifact.mjs';
+
+test('artifact download recovers from TLS reset and interrupted response bodies', async () => {
+  const reset = new TypeError('fetch failed', { cause: Object.assign(new Error('connection reset'), { code: 'ECONNRESET' }) });
+  const interrupted = new TypeError('terminated', { cause: Object.assign(new Error('socket closed'), { code: 'UND_ERR_SOCKET' }) });
+  const signals = [], delays = [], warnings = [];
+  const result = await downloadMcodeToolsArtifact(async (url, { signal }) => {
+    assert.equal(url, MCODE_TOOLS_ARTIFACT.url);
+    assert.ok(signal instanceof AbortSignal);
+    signals.push(signal);
+    if (signals.length === 1) throw reset;
+    if (signals.length === 2) return new Response(new ReadableStream({
+      start(controller) { controller.enqueue(new Uint8Array([1, 2])); },
+      pull(controller) { controller.error(interrupted); },
+    }));
+    return new Response('complete archive');
+  }, { wait: async ms => delays.push(ms), warn: message => warnings.push(message) });
+  assert.equal(result.toString(), 'complete archive');
+  assert.equal(new Set(signals).size, 3);
+  assert.deepEqual(delays, [1000, 2000]);
+  assert.equal(warnings.length, 2);
+  assert.match(warnings[0], /1\/3.*ECONNRESET/);
+  assert.match(warnings[1], /2\/3.*UND_ERR_SOCKET/);
+});
+
+test('artifact download retries temporary HTTP failures and releases rejected bodies', async () => {
+  for (const status of [408, 429, 500, 502, 503, 504]) {
+    let calls = 0, cancelled = false;
+    const delays = [];
+    const result = await downloadMcodeToolsArtifact(async () => {
+      if (++calls > 1) return new Response('ok');
+      return new Response(new ReadableStream({ cancel() { cancelled = true; } }), { status });
+    }, { wait: async ms => delays.push(ms), warn() {} });
+    assert.equal(result.toString(), 'ok');
+    assert.equal(calls, 2);
+    assert.equal(cancelled, true);
+    assert.deepEqual(delays, [1000]);
+  }
+});
+
+test('artifact download bounds Retry-After delays and rejects permanent failures immediately', async () => {
+  for (const [header, expectedDelay] of [['5', 5000], ['999999', 30000], ['invalid', 1000], ['0', 1000]]) {
+    let calls = 0;
+    const delays = [];
+    await downloadMcodeToolsArtifact(async () => ++calls === 1
+      ? new Response(null, { status: 429, headers: { 'Retry-After': header } }) : new Response('ok'),
+    { wait: async ms => delays.push(ms), warn() {} });
+    assert.deepEqual(delays, [expectedDelay]);
+  }
+  const certificateError = new TypeError('fetch failed', { cause: Object.assign(new Error('certificate expired'), { code: 'CERT_HAS_EXPIRED' }) });
+  for (const failure of [new Response(null, { status: 403 }), new Response(null, { status: 404 }), certificateError]) {
+    let calls = 0;
+    await assert.rejects(downloadMcodeToolsArtifact(async () => {
+      calls++;
+      if (failure instanceof Error) throw failure;
+      return failure;
+    }, { wait: async () => assert.fail('permanent failures must not wait'), warn: () => assert.fail('permanent failures must not retry') }));
+    assert.equal(calls, 1);
+  }
+});
+
+test('artifact download stops after three timeouts and preserves the final cause', async () => {
+  const timeout = new DOMException('The operation timed out', 'TimeoutError');
+  let calls = 0;
+  const delays = [];
+  await assert.rejects(downloadMcodeToolsArtifact(async () => { calls++; throw timeout; },
+    { wait: async ms => delays.push(ms), warn() {} }), error => {
+    assert.match(error.message, /after 3 attempts/);
+    assert.equal(error.cause, timeout);
+    return true;
+  });
+  assert.equal(calls, 3);
+  assert.deepEqual(delays, [1000, 2000]);
+});
+
+test('artifact download never retries or caches an archive that fails integrity', async t => {
+  const root = mkdtempSync(path.join(tmpdir(), 'mcode-artifact-download-'));
+  t.after(() => rmSync(root, { recursive: true, force: true }));
+  let calls = 0;
+  await assert.rejects(copyMcodeToolsArtifact(root, path.join(root, 'dist'), async () => {
+    calls++;
+    return new Response('corrupt archive');
+  }), /integrity mismatch/);
+  assert.equal(calls, 1);
+  assert.equal(existsSync(path.join(root, '.cache', 'artifacts', 'code-0.3.11.tgz')), false);
+  assert.equal(existsSync(path.join(root, 'dist')), false);
+});
 
 test('performance defaults to the 100-round suite; long history requires explicit selection', () => {
   const config = JSON.parse(readFileSync(new URL('../scripts/perf/config.json', import.meta.url), 'utf8'));