From 78fc2f289664950a6cacaef2e0308781ea15059c Mon Sep 17 00:00:00 2001
From: Max Gale <max@blacksmith.sh>
Date: Thu, 24 Sep 2026 10:54:49 -0400
Subject: [PATCH] cache: propagate github instance slug

---
 packages/cache/__tests__/instanceSlug.test.ts | 50 +++++++++++++++++++
 .../cache/src/internal/cacheHttpClient.ts     |  8 ++-
 2 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 packages/cache/__tests__/instanceSlug.test.ts

diff --git a/packages/cache/__tests__/instanceSlug.test.ts b/packages/cache/__tests__/instanceSlug.test.ts
new file mode 100644
index 0000000000..9b1c35fe5e
--- /dev/null
+++ b/packages/cache/__tests__/instanceSlug.test.ts
@@ -0,0 +1,50 @@
+import * as http from 'http'
+import {AddressInfo} from 'net'
+import fetch from 'node-fetch'
+import {getCacheEntry, reserveCache} from '../src/internal/cacheHttpClient'
+
+jest.mock('node-fetch', () => jest.fn())
+jest.mock('../src/internal/downloadUtils', () => ({}))
+
+test.each([undefined, 'enterprise'])(
+  'sends instance slug on reserve and restore (%s)',
+  async slug => {
+    const previous = {...process.env}
+    let receivedSlug: string | string[] | undefined
+    const server = http.createServer((req, res) => {
+      receivedSlug = req.headers['x-blacksmith-instance-slug']
+      req.resume()
+      res.writeHead(200, {'Content-Type': 'application/json'})
+      res.end(JSON.stringify({cacheId: 1}))
+    })
+    await new Promise<void>(resolve => server.listen(0, '127.0.0.1', resolve))
+    try {
+      if (slug) process.env.BLACKSMITH_INSTANCE_SLUG = slug
+      else delete process.env.BLACKSMITH_INSTANCE_SLUG
+      process.env.BLACKSMITH_CACHE_URL = `http://127.0.0.1:${
+        (server.address() as AddressInfo).port
+      }`
+      process.env.GITHUB_REPO_NAME = 'owner/repo'
+      process.env.BLACKSMITH_CACHE_TOKEN = 'test-token'
+      await reserveCache('key', ['node_modules'])
+      expect(receivedSlug).toBe(slug || 'github')
+      ;(fetch as jest.MockedFunction<typeof fetch>).mockResolvedValue({
+        status: 204
+      } as any)
+      await getCacheEntry(['key'], ['node_modules'])
+      expect(fetch).toHaveBeenCalledWith(
+        expect.any(String),
+        expect.objectContaining({
+          headers: expect.objectContaining({
+            'X-Blacksmith-Instance-Slug': slug || 'github'
+          })
+        })
+      )
+    } finally {
+      process.env = previous
+      await new Promise<void>((resolve, reject) =>
+        server.close(err => (err ? reject(err) : resolve()))
+      )
+    }
+  }
+)
diff --git a/packages/cache/src/internal/cacheHttpClient.ts b/packages/cache/src/internal/cacheHttpClient.ts
index 40a90b7ca8..665ba89bf3 100644
--- a/packages/cache/src/internal/cacheHttpClient.ts
+++ b/packages/cache/src/internal/cacheHttpClient.ts
@@ -64,6 +64,8 @@ function getRequestOptions(): RequestOptions {
   const requestOptions: RequestOptions = {
     headers: {
       Accept: createAcceptHeader('application/json', '6.0-preview.1'),
+      'X-Blacksmith-Instance-Slug':
+        process.env['BLACKSMITH_INSTANCE_SLUG'] || 'github',
       'X-Github-Repo-Name': process.env['GITHUB_REPO_NAME']
     }
   }
@@ -141,6 +143,8 @@ export async function getCacheEntry(
         headers: {
           Accept: createAcceptHeader('application/json', '6.0-preview.1'),
           'X-Github-Repo-Name': repoName || '',
+          'X-Blacksmith-Instance-Slug':
+            process.env['BLACKSMITH_INSTANCE_SLUG'] || 'github',
           Authorization: `Bearer ${cacheToken}`,
           'X-Cache-Region': process.env['BLACKSMITH_REGION'] ?? 'eu-central',
           'User-Agent': 'node-fetch/cache'
@@ -182,7 +186,7 @@ export async function getCacheEntry(
     } catch (error) {
       const isTimeout = error.name === 'AbortError'
       const status = error.response?.status
-      
+
       if ((status && status >= 500) || isTimeout) {
         retries++
         if (retries <= maxRetries) {
@@ -198,7 +202,7 @@ export async function getCacheEntry(
           continue
         }
       }
-      
+
       if (status) {
         throw new Error(`Cache service responded with ${status}`)
       } else if (isTimeout) {