diff --git a/__test__/warpbuild-mirror.test.ts b/__test__/warpbuild-mirror.test.ts
index 7d280fd..e6250e9 100644
--- a/__test__/warpbuild-mirror.test.ts
+++ b/__test__/warpbuild-mirror.test.ts
@@ -6,26 +6,29 @@ import {
   afterEach,
   afterAll
 } from '@jest/globals'
-import * as exec from '@actions/exec'
 import * as fs from 'fs'
 import * as os from 'os'
 import * as path from 'path'
 import {
   SKIP_NOT_WARPBUILD,
-  assertSafeTarMembers,
-  computeDestinationRef,
+  computeRefKey,
   getMirrorCacheSkipReason,
-  manifestDir,
   resetGitObjects
 } from '../src/warpbuild/mirror-cache.js'
-import {lookupSnapshot} from '../src/warpbuild/backend-api.js'
+import {
+  lookupRestore,
+  requestBaseUpload,
+  requestBranchUpload
+} from '../src/warpbuild/backend-api.js'
 import {IGitSourceSettings} from '../src/git-source-settings.js'
 
 const WB_ENV = [
   'WARPBUILD_RUNNER_VERIFICATION_TOKEN',
   'WARPBUILD_HOST_URL',
   'GITHUB_REPOSITORY_ID',
-  'GITHUB_REPOSITORY'
+  'GITHUB_REPOSITORY',
+  'GITHUB_REF',
+  'GITHUB_BASE_REF'
 ]
 const savedEnv: {[key: string]: string | undefined} = {}
 for (const key of WB_ENV) {
@@ -57,9 +60,11 @@ function setWarpBuildEnv(): void {
   process.env['WARPBUILD_HOST_URL'] = 'https://api.example.dev'
   process.env['GITHUB_REPOSITORY_ID'] = '123456789'
   process.env['GITHUB_REPOSITORY'] = 'octocat/hello-world'
+  delete process.env['GITHUB_REF']
+  delete process.env['GITHUB_BASE_REF']
 }
 
-describe('warpbuild snapshot cache', () => {
+describe('warpbuild mirror cache', () => {
   beforeEach(() => {
     setWarpBuildEnv()
   })
@@ -75,9 +80,7 @@ describe('warpbuild snapshot cache', () => {
   })
 
   describe('getMirrorCacheSkipReason', () => {
-    const winOnly = process.platform === 'win32' ? it.skip : it
-
-    winOnly('returns null for the default checkout shape', () => {
+    it('returns null for the default checkout shape', () => {
       expect(getMirrorCacheSkipReason(settingsFor())).toBeNull()
     })
 
@@ -86,7 +89,7 @@ describe('warpbuild snapshot cache', () => {
       expect(getMirrorCacheSkipReason(settingsFor())).toBe(SKIP_NOT_WARPBUILD)
     })
 
-    it('reports repository: inputs that are not the workflow repo', () => {
+    it('reports repository inputs that are not the workflow repo', () => {
       expect(
         getMirrorCacheSkipReason(settingsFor({repositoryName: 'other-repo'}))
       ).toBe(
@@ -103,19 +106,15 @@ describe('warpbuild snapshot cache', () => {
       )
     })
 
-    it('only serves fetch-depth 1', () => {
-      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 0}))).toBe(
-        'fetch-depth is 0, cache only serves fetch-depth 1'
-      )
-      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 50}))).toBe(
-        'fetch-depth is 50, cache only serves fetch-depth 1'
-      )
+    it('serves any fetch-depth and fetch-tags (mirror is full history)', () => {
+      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 0}))).toBeNull()
+      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 50}))).toBeNull()
+      expect(
+        getMirrorCacheSkipReason(settingsFor({fetchTags: true}))
+      ).toBeNull()
     })
 
-    it('skips fetch-tags, filters, sparse and lfs checkouts', () => {
-      expect(getMirrorCacheSkipReason(settingsFor({fetchTags: true}))).toBe(
-        'fetch-tags is enabled'
-      )
+    it('skips filters, sparse and lfs checkouts', () => {
       expect(getMirrorCacheSkipReason(settingsFor({filter: 'blob:none'}))).toBe(
         'a fetch filter is configured'
       )
@@ -123,18 +122,36 @@ describe('warpbuild snapshot cache', () => {
         getMirrorCacheSkipReason(settingsFor({sparseCheckout: ['src']}))
       ).toBe('sparse checkout is configured')
       expect(getMirrorCacheSkipReason(settingsFor({lfs: true}))).toBe(
-        'lfs is enabled (lfs objects are not in the snapshot)'
+        'lfs is enabled (lfs objects are not in the mirror)'
       )
     })
+  })
 
-    winOnly('accepts sha-only checkouts (empty ref)', () => {
-      expect(getMirrorCacheSkipReason(settingsFor({ref: ''}))).toBeNull()
+  describe('computeRefKey', () => {
+    it('keys on the pushed branch (GITHUB_REF)', () => {
+      process.env['GITHUB_REF'] = 'refs/heads/feature/x'
+      expect(computeRefKey(settingsFor())).toBe('feature/x')
     })
 
-    it('skips unqualified refs', () => {
-      expect(getMirrorCacheSkipReason(settingsFor({ref: 'main'}))).toBe(
-        "ref 'main' has no cacheable destination ref"
-      )
+    it('keys on the base branch for pull requests (merge sha is synthetic)', () => {
+      process.env['GITHUB_REF'] = 'refs/pull/42/merge'
+      process.env['GITHUB_BASE_REF'] = 'main'
+      expect(computeRefKey(settingsFor())).toBe('main')
+    })
+
+    it('is empty for tags and bare pull refs (base only, no roll)', () => {
+      process.env['GITHUB_REF'] = 'refs/tags/v1.2.3'
+      expect(computeRefKey(settingsFor())).toBe('')
+      process.env['GITHUB_REF'] = 'refs/pull/42/merge'
+      expect(computeRefKey(settingsFor())).toBe('')
+    })
+
+    it('is empty for a detached sha checkout', () => {
+      expect(computeRefKey(settingsFor({ref: SHA}))).toBe('')
+    })
+
+    it('passes through an already-short branch name', () => {
+      expect(computeRefKey(settingsFor({ref: 'main'}))).toBe('main')
     })
   })
 
@@ -144,7 +161,6 @@ describe('warpbuild snapshot cache', () => {
         path.join(os.tmpdir(), 'wb-reset-')
       )
       try {
-        // Simulate a partially-restored .git: stray objects, a pack, a shallow file.
         await fs.promises.mkdir(path.join(gitDir, 'objects', 'pack'), {
           recursive: true
         })
@@ -152,179 +168,95 @@ describe('warpbuild snapshot cache', () => {
           path.join(gitDir, 'objects', 'ab'),
           'junk loose object'
         )
-        await fs.promises.writeFile(
-          path.join(gitDir, 'objects', 'pack', 'pack-x.pack'),
-          'junk pack'
-        )
         await fs.promises.writeFile(path.join(gitDir, 'shallow'), 'deadbeef\n')
 
         await resetGitObjects(gitDir)
 
-        // shallow gone; objects/ holds only the two empty init dirs.
         expect(fs.existsSync(path.join(gitDir, 'shallow'))).toBe(false)
         expect(fs.existsSync(path.join(gitDir, 'objects', 'ab'))).toBe(false)
         expect(
           (await fs.promises.readdir(path.join(gitDir, 'objects'))).sort()
         ).toEqual(['info', 'pack'])
-        expect(
-          await fs.promises.readdir(path.join(gitDir, 'objects', 'pack'))
-        ).toEqual([])
-        expect(
-          await fs.promises.readdir(path.join(gitDir, 'objects', 'info'))
-        ).toEqual([])
       } finally {
         await fs.promises.rm(gitDir, {recursive: true, force: true})
       }
     })
-
-    it('is a no-op-safe on an already-clean .git', async () => {
-      const gitDir = await fs.promises.mkdtemp(
-        path.join(os.tmpdir(), 'wb-reset-')
-      )
-      try {
-        await expect(resetGitObjects(gitDir)).resolves.toBeUndefined()
-        expect(
-          (await fs.promises.readdir(path.join(gitDir, 'objects'))).sort()
-        ).toEqual(['info', 'pack'])
-      } finally {
-        await fs.promises.rm(gitDir, {recursive: true, force: true})
-      }
-    })
-  })
-
-  describe('assertSafeTarMembers', () => {
-    async function makeTar(
-      dir: string,
-      members: string[],
-      writeFiles = true
-    ): Promise<string> {
-      if (writeFiles) {
-        for (const m of members) {
-          const abs = path.join(dir, m)
-          await fs.promises.mkdir(path.dirname(abs), {recursive: true})
-          await fs.promises.writeFile(abs, 'x')
-        }
-      }
-      const tar = path.join(dir, 'out.tar')
-      await exec.exec('tar', ['-cf', tar, '-C', dir, ...members])
-      return tar
-    }
-
-    it('accepts objects/ and shallow members', async () => {
-      const dir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'wb-tar-'))
-      try {
-        const tar = await makeTar(dir, ['objects/pack/p.pack', 'shallow'])
-        await expect(assertSafeTarMembers(tar)).resolves.toBeUndefined()
-      } finally {
-        await fs.promises.rm(dir, {recursive: true, force: true})
-      }
-    })
-
-    it('rejects members outside objects/ and shallow (e.g. hooks)', async () => {
-      const dir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'wb-tar-'))
-      try {
-        const tar = await makeTar(dir, ['objects/x', 'hooks/pre-commit'])
-        await expect(assertSafeTarMembers(tar)).rejects.toThrow(
-          /unexpected snapshot tar member/
-        )
-      } finally {
-        await fs.promises.rm(dir, {recursive: true, force: true})
-      }
-    })
-
-    it('rejects path traversal members', async () => {
-      const dir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'wb-tar-'))
-      try {
-        await fs.promises.mkdir(path.join(dir, 'sub'))
-        const tar = path.join(dir, 'evil.tar')
-        // -P preserves the ../ member instead of stripping it.
-        await fs.promises.writeFile(path.join(dir, 'evil'), 'x')
-        await exec.exec('tar', [
-          '-cPf',
-          tar,
-          '-C',
-          path.join(dir, 'sub'),
-          '../evil'
-        ])
-        await expect(assertSafeTarMembers(tar)).rejects.toThrow(
-          /unexpected snapshot tar member/
-        )
-      } finally {
-        await fs.promises.rm(dir, {recursive: true, force: true})
-      }
-    })
-  })
-
-  describe('computeDestinationRef', () => {
-    it('maps the refs the fetch would have created', () => {
-      expect(computeDestinationRef('refs/heads/main')).toBe(
-        'refs/remotes/origin/main'
-      )
-      expect(computeDestinationRef('refs/pull/42/merge')).toBe(
-        'refs/remotes/pull/42/merge'
-      )
-      expect(computeDestinationRef('refs/tags/v1.2.3')).toBe('refs/tags/v1.2.3')
-      expect(computeDestinationRef('')).toBe('')
-      expect(computeDestinationRef('main')).toBeNull()
-    })
   })
 
   describe('backend api http contract', () => {
     const realFetch = globalThis.fetch
     let lastUrl = ''
+    let lastInit: RequestInit | undefined
 
     afterEach(() => {
       globalThis.fetch = realFetch
     })
 
     function stubFetch(status: number, body: unknown): void {
-      globalThis.fetch = (async (input: unknown) => {
+      globalThis.fetch = (async (input: unknown, init?: RequestInit) => {
         lastUrl = String(input)
+        lastInit = init
         return new Response(JSON.stringify(body), {status})
       }) as typeof fetch
     }
 
-    it('maps 200 to hit and keys by sha', async () => {
-      stubFetch(200, {url: 'https://s3/x', size_bytes: 42, created_at: 'now'})
-      const result = await lookupSnapshot('123', SHA)
-      expect(result.kind).toBe('hit')
-      expect(lastUrl).toContain(`sha=${SHA}`)
+    it('lookupRestore maps 200 to a restore plan keyed by ref', async () => {
+      stubFetch(200, {
+        base: {url: 'https://s3/base', size_bytes: 42},
+        branch: {url: 'https://s3/branch'}
+      })
+      const result = await lookupRestore('123', 'main')
+      expect(result.kind).toBe('restore')
+      if (result.kind === 'restore') {
+        expect(result.base.url).toBe('https://s3/base')
+        expect(result.branch?.url).toBe('https://s3/branch')
+      }
+      expect(lastUrl).toContain('ref=main')
     })
 
-    it('maps 404 to miss (agent uploads after the job)', async () => {
-      stubFetch(404, {sub_code: 'NFE_GITMIRROR'})
-      expect((await lookupSnapshot('123', SHA)).kind).toBe('miss')
-    })
-
-    it('maps 403 to disabled (backend kill switch)', async () => {
-      stubFetch(403, {sub_code: 'PDE_GITMIRROR_DISABLED'})
-      expect((await lookupSnapshot('123', SHA)).kind).toBe('disabled')
-    })
-
-    it('maps other statuses and network failures to error', async () => {
-      stubFetch(500, {})
-      expect((await lookupSnapshot('123', SHA)).kind).toBe('error')
-      globalThis.fetch = (async () => {
-        throw new Error('boom')
-      }) as typeof fetch
-      expect((await lookupSnapshot('123', SHA)).kind).toBe('error')
-    })
-  })
-
-  describe('manifestDir', () => {
-    const saved = process.env['RUNNER_TEMP']
-    afterEach(() => {
-      if (saved === undefined) {
-        delete process.env['RUNNER_TEMP']
-      } else {
-        process.env['RUNNER_TEMP'] = saved
+    it('lookupRestore maps a null branch (base but no delta yet)', async () => {
+      stubFetch(200, {base: {url: 'https://s3/base'}, branch: null})
+      const result = await lookupRestore('123', 'main')
+      expect(result.kind).toBe('restore')
+      if (result.kind === 'restore') {
+        expect(result.branch).toBeNull()
       }
     })
 
-    it('places manifests under RUNNER_TEMP/wb-snapshots', () => {
-      process.env['RUNNER_TEMP'] = path.join(os.tmpdir(), 'runner-temp-x')
-      expect(manifestDir()).toBe(
-        path.join(os.tmpdir(), 'runner-temp-x', 'wb-snapshots')
+    it('lookupRestore maps 404 to cold, 403 to disabled, 500 to error', async () => {
+      stubFetch(404, {sub_code: 'NFE_GITMIRROR'})
+      expect((await lookupRestore('123', 'main')).kind).toBe('cold')
+      stubFetch(403, {sub_code: 'PDE_GITMIRROR_DISABLED'})
+      expect((await lookupRestore('123', 'main')).kind).toBe('disabled')
+      stubFetch(500, {})
+      expect((await lookupRestore('123', 'main')).kind).toBe('error')
+    })
+
+    it('lookupRestore maps network failure to error', async () => {
+      globalThis.fetch = (async () => {
+        throw new Error('boom')
+      }) as typeof fetch
+      expect((await lookupRestore('123', 'main')).kind).toBe('error')
+    })
+
+    it('requestBaseUpload maps 200 to a grant, 409 to locked', async () => {
+      stubFetch(200, {url: 'https://s3/put-base'})
+      const grant = await requestBaseUpload('123')
+      expect(grant.kind).toBe('grant')
+      if (grant.kind === 'grant') {
+        expect(grant.url).toBe('https://s3/put-base')
+      }
+      expect(lastInit?.method).toBe('POST')
+      stubFetch(409, {sub_code: 'FVE_GITMIRROR_LOCKED'})
+      expect((await requestBaseUpload('123')).kind).toBe('locked')
+    })
+
+    it('requestBranchUpload maps 200 to a grant, 403 to disabled', async () => {
+      stubFetch(200, {url: 'https://s3/put-branch'})
+      expect((await requestBranchUpload('123', 'main', SHA)).kind).toBe('grant')
+      stubFetch(403, {})
+      expect((await requestBranchUpload('123', 'main', SHA)).kind).toBe(
+        'disabled'
       )
     })
   })
diff --git a/dist/index.js b/dist/index.js
index 0a42bda..3bbc325 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -41697,9 +41697,11 @@ const promises_namespaceObject = __WEBPACK_EXTERNAL_createRequire(import.meta.ur
 ;// CONCATENATED MODULE: ./src/warpbuild/backend-api.ts
 /* eslint-disable i18n-text/no-en -- upstream convention */
 
-// Client for backend-core's git-mirror download-url endpoint, authed by the runner
-// verification token. Contract: 200 = presigned URL (hit); 404 = miss; 403 = unservable
-// org; else = fall back. The upload half runs in the warpbuild-agent after the job.
+// Client for backend-core's git-mirror endpoints, authed by the runner verification
+// token. The action seeds from a bare base mirror + a per-branch delta bundle (both
+// presigned GETs), delta-fetches the tip from GitHub, then uploads the refreshed
+// per-branch bundle — or, on a cold repo, the base — via a presigned PUT. Every call
+// fails closed to stock checkout.
 const API_TIMEOUT_MS = 10_000;
 function backend_api_baseUrl() {
     return (process.env['WARPBUILD_HOST_URL'] || '').replace(/\/+$/, '');
@@ -41707,30 +41709,71 @@ function backend_api_baseUrl() {
 function authHeader() {
     return `Bearer ${process.env['WARPBUILD_RUNNER_VERIFICATION_TOKEN'] || ''}`;
 }
-async function lookupSnapshot(repoKey, sha) {
+function backend_api_endpoint(p) {
+    return `${backend_api_baseUrl()}/api/v1/git-mirrors/${p}`;
+}
+async function lookupRestore(repoKey, ref) {
     try {
-        const res = await fetch(`${backend_api_baseUrl()}/api/v1/git-mirrors/download-url?repo_key=${encodeURIComponent(repoKey)}&sha=${encodeURIComponent(sha)}`, {
+        const res = await fetch(`${backend_api_endpoint('restore-url')}?repo_key=${encodeURIComponent(repoKey)}&ref=${encodeURIComponent(ref)}`, {
             headers: { authorization: authHeader() },
             signal: AbortSignal.timeout(API_TIMEOUT_MS)
         });
         if (res.status === 200) {
-            return { kind: 'hit', info: (await res.json()) };
+            const body = (await res.json());
+            return { kind: 'restore', base: body.base, branch: body.branch ?? null };
         }
         if (res.status === 404) {
-            return { kind: 'miss' };
+            return { kind: 'cold' };
         }
         if (res.status === 403) {
-            core_debug(`[wb-cache] download-url answered 403 (disabled)`);
+            core_debug('[wb-cache] restore-url answered 403 (disabled)');
             return { kind: 'disabled' };
         }
-        core_debug(`[wb-cache] download-url answered ${res.status}`);
+        core_debug(`[wb-cache] restore-url answered ${res.status}`);
         return { kind: 'error' };
     }
     catch (error) {
-        core_debug(`[wb-cache] download-url failed: ${error}`);
+        core_debug(`[wb-cache] restore-url failed: ${error}`);
         return { kind: 'error' };
     }
 }
+async function requestUpload(p, body) {
+    try {
+        const res = await fetch(backend_api_endpoint(p), {
+            method: 'POST',
+            headers: {
+                authorization: authHeader(),
+                'content-type': 'application/json'
+            },
+            body: JSON.stringify(body),
+            signal: AbortSignal.timeout(API_TIMEOUT_MS)
+        });
+        if (res.status === 200) {
+            const b = (await res.json());
+            return { kind: 'grant', url: b.url };
+        }
+        if (res.status === 409) {
+            return { kind: 'locked' };
+        }
+        if (res.status === 403) {
+            return { kind: 'disabled' };
+        }
+        core_debug(`[wb-cache] ${p} answered ${res.status}`);
+        return { kind: 'error' };
+    }
+    catch (error) {
+        core_debug(`[wb-cache] ${p} failed: ${error}`);
+        return { kind: 'error' };
+    }
+}
+// Cold repo: request the single-flight grant to build + upload the base mirror.
+async function requestBaseUpload(repoKey) {
+    return requestUpload('base/upload-url', { repo_key: repoKey });
+}
+// Warm repo: request the grant to overwrite this branch's delta bundle.
+async function requestBranchUpload(repoKey, ref, sha) {
+    return requestUpload('branch/upload-url', { repo_key: repoKey, ref, sha });
+}
 
 ;// CONCATENATED MODULE: ./src/warpbuild/mirror-cache.ts
 /* eslint-disable i18n-text/no-en, import/no-unresolved -- upstream conventions; no TS import resolver configured */
@@ -41743,23 +41786,29 @@ async function lookupSnapshot(repoKey, sha) {
 
 
 
-// WarpBuild checkout snapshot cache: SHA-keyed tars of what the stock shallow fetch
-// produces. Hit = restore + skip the fetch; miss = upload after checkout. Keys are
-// immutable (no expiry). Fail-open: any error degrades to stock behavior.
+// WarpBuild checkout cache: a bare base mirror + per-branch orderless delta bundles on
+// S3. We seed the base (+ this branch's delta) before the fetch, so the GitHub fetch is
+// only the tip delta — or nothing. Each run overwrites its branch's bundle (base-relative,
+// so order-free); a cold repo single-flights the base build. Fail-open: any error →
+// standard checkout.
 const DOWNLOAD_TIMEOUT_MS = 15 * 60_000;
-// Logged at debug (normal state outside WarpBuild); other reasons log at info.
+const UPLOAD_TIMEOUT_MS = 15 * 60_000;
+// Logged at info when off; the WARPBUILD_* env is only present on our runners.
 const SKIP_NOT_WARPBUILD = 'not running on a WarpBuild runner (WARPBUILD_* env not present)';
 const SHA_PATTERN = /^[0-9a-f]{40}([0-9a-f]{24})?$/;
-let decision = 'off';
-// Null = attempt the cache; else a reason to log. Only the default checkout shape
-// is served.
+// Seeded bundles land here, out of the user's ref space; excluded objects for deltas.
+const BASE_REFNS = 'refs/wb/base';
+const BRANCH_REFNS = 'refs/wb/branch';
+const UPLOAD_TIP_REF = 'refs/wb/tip';
+let plan = { mode: 'off', repoKey: '', refKey: '' };
+// Null = attempt the cache; else a reason to log. The mirror serves full history for any
+// depth, so fetch-depth is not gated; sparse/lfs/filter change the object set we model.
 function getMirrorCacheSkipReason(settings) {
     if (!process.env['WARPBUILD_RUNNER_VERIFICATION_TOKEN'] ||
         !process.env['WARPBUILD_HOST_URL']) {
         return SKIP_NOT_WARPBUILD;
     }
-    const repoKey = process.env['GITHUB_REPOSITORY_ID'] || '';
-    if (!repoKey) {
+    if (!process.env['GITHUB_REPOSITORY_ID']) {
         return 'GITHUB_REPOSITORY_ID is not set';
     }
     const checkoutRepo = `${settings.repositoryOwner}/${settings.repositoryName}`;
@@ -41773,12 +41822,6 @@ function getMirrorCacheSkipReason(settings) {
     if (!settings.commit || !SHA_PATTERN.test(settings.commit)) {
         return 'no exact commit sha to key on';
     }
-    if (settings.fetchDepth !== 1) {
-        return `fetch-depth is ${settings.fetchDepth}, cache only serves fetch-depth 1`;
-    }
-    if (settings.fetchTags) {
-        return 'fetch-tags is enabled';
-    }
     if (settings.filter) {
         return 'a fetch filter is configured';
     }
@@ -41786,51 +41829,49 @@ function getMirrorCacheSkipReason(settings) {
         return 'sparse checkout is configured';
     }
     if (settings.lfs) {
-        return 'lfs is enabled (lfs objects are not in the snapshot)';
-    }
-    if (settings.ref && computeDestinationRef(settings.ref) === null) {
-        return `ref '${settings.ref}' has no cacheable destination ref`;
+        return 'lfs is enabled (lfs objects are not in the mirror)';
     }
     return null;
 }
-// The local ref the fetch would have created ('' = none needed, null = uncacheable).
-function computeDestinationRef(ref) {
-    if (!ref) {
+// The durable branch to key the per-branch bundle on. For pull_request events the merge
+// SHA is synthetic, so we key on the base branch; otherwise the pushed branch. '' for
+// tags / detached HEAD → seed the base only, upload nothing.
+function computeRefKey(settings) {
+    const baseRef = process.env['GITHUB_BASE_REF']; // set on pull_request events
+    if (baseRef) {
+        return baseRef;
+    }
+    const ref = process.env['GITHUB_REF'] || settings.ref || '';
+    if (ref.startsWith('refs/heads/')) {
+        return ref.substring('refs/heads/'.length);
+    }
+    if (ref.startsWith('refs/tags/') || ref.startsWith('refs/pull/')) {
         return '';
     }
-    const upper = ref.toUpperCase();
-    if (upper.startsWith('REFS/HEADS/')) {
-        return `refs/remotes/origin/${ref.substring('refs/heads/'.length)}`;
-    }
-    if (upper.startsWith('REFS/PULL/')) {
-        return `refs/remotes/pull/${ref.substring('refs/pull/'.length)}`;
-    }
-    if (upper.startsWith('REFS/TAGS/')) {
-        return ref;
-    }
-    return null;
+    // Already a short branch name, or empty.
+    return SHA_PATTERN.test(ref) ? '' : ref;
 }
-// Runs right after `git init`; true = restored (caller skips the fetch). Never throws.
-// Sets the `cache-hit` action output (true only on a restore).
+// Runs after `git init`, before the fetch. Returns the mode the fetch/contribute steps
+// branch on. Never throws. Sets the `cache-hit` output (true only when seeded from cache).
 async function setup(settings) {
-    const restored = await setupImpl(settings);
-    setOutput('cache-hit', restored ? 'true' : 'false');
-    return restored;
+    const mode = await setupImpl(settings);
+    setOutput('cache-hit', mode === 'seeded' ? 'true' : 'false');
+    return mode;
 }
 async function setupImpl(settings) {
-    decision = 'off';
+    plan = { mode: 'off', repoKey: '', refKey: '' };
     const skipReason = getMirrorCacheSkipReason(settings);
     if (skipReason) {
-        info(`WarpBuild snapshot cache skipped: ${skipReason}`);
-        return false;
+        info(`WarpBuild mirror cache skipped: ${skipReason}`);
+        return 'off';
     }
-    startGroup('WarpBuild: checkout snapshot cache');
+    startGroup('WarpBuild: checkout mirror cache');
     try {
         return await setupInner(settings);
     }
     catch (error) {
-        warning(`WarpBuild snapshot cache unavailable, using standard checkout: ${error}`);
-        return false;
+        warning(`WarpBuild mirror cache unavailable, using standard checkout: ${error}`);
+        return 'off';
     }
     finally {
         endGroup();
@@ -41838,140 +41879,196 @@ async function setupImpl(settings) {
 }
 async function setupInner(settings) {
     const repoKey = process.env['GITHUB_REPOSITORY_ID'];
-    const sha = settings.commit;
-    // The restore/upload paths shell out to `tar`; without it, fall back cleanly.
-    if (!(await which('tar', false))) {
-        info('tar not found on PATH; using standard checkout');
-        return false;
-    }
-    const lookup = await lookupSnapshot(repoKey, sha);
+    const refKey = computeRefKey(settings);
+    plan = { mode: 'off', repoKey, refKey };
+    const lookup = await lookupRestore(repoKey, refKey);
     if (lookup.kind === 'disabled') {
-        info('Snapshot cache is disabled by the backend for this organization');
-        return false;
+        info('Mirror cache is disabled by the backend for this organization');
+        return 'off';
     }
     if (lookup.kind === 'error') {
-        info('Snapshot cache backend unavailable; using standard checkout');
-        return false;
+        info('Mirror cache backend unavailable; using standard checkout');
+        return 'off';
     }
-    if (lookup.kind === 'miss') {
-        info(`Cache miss for ${sha}: the standard fetch will run and its result will be uploaded`);
-        decision = 'miss';
-        return false;
+    if (lookup.kind === 'cold') {
+        const grant = await requestBaseUpload(repoKey);
+        if (grant.kind === 'grant') {
+            info('Cold repo: building the base mirror this run (full fetch, then upload)');
+            plan = { mode: 'cold-build', repoKey, refKey, baseUploadUrl: grant.url };
+            return 'cold-build';
+        }
+        info('Cold repo: base build already in progress elsewhere; using standard checkout');
+        return 'off';
     }
-    info(`Cache hit for ${sha}: restoring snapshot (${lookup.info.size_bytes} bytes)`);
-    if (!(await restoreSnapshot(settings, lookup.info.url, sha))) {
-        return false;
+    // Restore: seed the base, then this branch's delta if present.
+    await seedBundle(settings.repositoryPath, lookup.base.url, BASE_REFNS);
+    if (lookup.branch) {
+        try {
+            await seedBundle(settings.repositoryPath, lookup.branch.url, BRANCH_REFNS);
+        }
+        catch (error) {
+            info(`Branch delta seed skipped (${error}); base only`);
+        }
     }
-    info('Snapshot restored; skipping the GitHub fetch entirely');
-    return true;
+    info(`Seeded base mirror${lookup.branch ? ' + branch delta' : ''}; the GitHub fetch will be a tip delta`);
+    plan = { mode: 'seeded', repoKey, refKey };
+    return 'seeded';
 }
-// Runs after checkout; on a miss records a manifest so the WarpBuild agent uploads the
-// snapshot after the job exits (keeping tar+upload off the customer's billed time).
+// Runs after checkout, in the same step (`.git` still pristine). Uploads the base (cold)
+// or this branch's refreshed delta (seeded). Best-effort — never fails the checkout.
 async function contribute(settings) {
-    if (decision !== 'miss') {
-        return;
-    }
     try {
-        await writeSnapshotManifest(settings);
+        if (plan.mode === 'cold-build') {
+            await uploadBaseMirror(settings);
+        }
+        else if (plan.mode === 'seeded') {
+            await uploadBranchDelta(settings);
+        }
     }
     catch (error) {
-        warning(`Snapshot manifest not written: ${error}`);
+        warning(`WarpBuild mirror upload skipped: ${error}`);
     }
 }
-// Refuse any tar member outside objects/ or shallow, absolute, or with a `..`
-// component — the tar is remote and extracted into .git, so a crafted member could
-// escape (e.g. hooks/, ../) and run code during checkout.
-async function assertSafeTarMembers(tar) {
-    let listing = '';
-    await exec_exec('tar', ['-tf', tar], {
-        silent: true,
-        listeners: { stdout: (d) => (listing += d.toString()) }
-    });
-    for (const raw of listing.split('\n')) {
-        const member = raw.trim();
-        if (!member) {
-            continue;
-        }
-        const top = member.replace(/^\.\//, '').split('/')[0];
-        if (member.startsWith('/') ||
-            member.split('/').includes('..') ||
-            (top !== 'objects' && top !== 'shallow')) {
-            throw new Error(`unexpected snapshot tar member: ${member}`);
-        }
-    }
-}
-async function restoreSnapshot(settings, url, sha) {
-    const gitDir = external_path_namespaceObject.join(settings.repositoryPath, '.git');
-    const tmpTar = external_path_namespaceObject.join(external_os_namespaceObject.tmpdir(), `wb-snapshot-${process.pid}.tar`);
+// Download a bundle and fetch its refs into refNs/* so its objects seed the local repo
+// (and count as "haves" for the delta negotiation). The bundle is a git-validated file;
+// nothing is extracted into .git directly.
+async function seedBundle(repoPath, url, refNs) {
+    const tmp = tempBundlePath('seed');
     try {
-        const res = await fetch(url, {
-            signal: AbortSignal.timeout(DOWNLOAD_TIMEOUT_MS)
-        });
-        if (!res.ok || !res.body) {
-            throw new Error(`snapshot download answered ${res.status}`);
-        }
-        await (0,promises_namespaceObject.pipeline)(external_stream_namespaceObject.Readable.fromWeb(res.body), external_fs_namespaceObject.createWriteStream(tmpTar));
-        await assertSafeTarMembers(tmpTar);
-        await exec_exec('tar', ['-xf', tmpTar, '-C', gitDir]);
-        const check = await exec_exec('git', ['-C', settings.repositoryPath, 'cat-file', '-e', `${sha}^{commit}`], { ignoreReturnCode: true });
-        if (check !== 0) {
-            throw new Error(`restored snapshot does not contain ${sha}`);
-        }
-        // The ref the skipped fetch would have created; upstream's verification needs it.
-        const dstRef = computeDestinationRef(settings.ref);
-        if (dstRef) {
-            await exec_exec('git', [
-                '-C',
-                settings.repositoryPath,
-                'update-ref',
-                dstRef,
-                sha
-            ]);
-        }
-        return true;
-    }
-    catch (error) {
-        warning(`Snapshot restore failed: ${error}`);
-        await resetGitObjects(gitDir);
-        return false;
+        await downloadTo(url, tmp);
+        await exec_exec('git', [
+            '-C',
+            repoPath,
+            'fetch',
+            '--quiet',
+            '--no-tags',
+            tmp,
+            `+refs/*:${refNs}/*`
+        ]);
     }
     finally {
-        await external_fs_namespaceObject.promises.rm(tmpTar, { force: true });
+        await external_fs_namespaceObject.promises.rm(tmp, { force: true });
     }
 }
-// Reset .git/objects (and drop any restored shallow) to the empty shape `git init`
-// creates, so a failed restore leaves no trace for the fallback fetch.
+// Cold-build: after the forced full fetch the repo holds all branches under
+// refs/remotes/origin/* and all tags. Bundle exactly those (not `--all`, which would
+// also sweep up the ephemeral triggering PR merge ref) as the base and upload.
+async function uploadBaseMirror(settings) {
+    if (!plan.baseUploadUrl) {
+        return;
+    }
+    const tmp = tempBundlePath('base');
+    try {
+        await exec_exec('git', [
+            '-C',
+            settings.repositoryPath,
+            'bundle',
+            'create',
+            tmp,
+            '--remotes=origin',
+            '--tags'
+        ]);
+        await httpPut(plan.baseUploadUrl, tmp);
+        info('Uploaded base mirror');
+    }
+    finally {
+        await external_fs_namespaceObject.promises.rm(tmp, { force: true });
+    }
+}
+// Seeded: build the base-relative delta (target sha, excluding everything in the base)
+// and overwrite this branch's bundle. Guarded by a per-branch lock server-side.
+async function uploadBranchDelta(settings) {
+    if (!plan.refKey) {
+        return; // tag / detached HEAD: nothing to roll
+    }
+    const repoPath = settings.repositoryPath;
+    const sha = settings.commit;
+    const grant = await requestBranchUpload(plan.repoKey, plan.refKey, sha);
+    if (grant.kind !== 'grant') {
+        info(`Branch delta upload skipped (${grant.kind})`);
+        return;
+    }
+    const excludes = await baseRefExcludes(repoPath);
+    if (excludes.length === 0) {
+        info('No base refs to diff against; branch delta skipped');
+        return;
+    }
+    const tmp = tempBundlePath('branch');
+    try {
+        await exec_exec('git', ['-C', repoPath, 'update-ref', UPLOAD_TIP_REF, sha]);
+        await exec_exec('git', [
+            '-C',
+            repoPath,
+            'bundle',
+            'create',
+            tmp,
+            UPLOAD_TIP_REF,
+            ...excludes
+        ]);
+        await httpPut(grant.url, tmp);
+        info(`Uploaded branch delta for '${plan.refKey}'`);
+    }
+    finally {
+        await external_fs_namespaceObject.promises.rm(tmp, { force: true });
+        await exec_exec('git', ['-C', repoPath, 'update-ref', '-d', UPLOAD_TIP_REF], {
+            ignoreReturnCode: true
+        });
+    }
+}
+// `^refname` for every seeded base ref — the exclusion set that makes the delta bundle
+// base-relative (and therefore order-free).
+async function baseRefExcludes(repoPath) {
+    let out = '';
+    await exec_exec('git', ['-C', repoPath, 'for-each-ref', '--format=^%(refname)', BASE_REFNS], { silent: true, listeners: { stdout: (d) => (out += d.toString()) } });
+    return out
+        .split('\n')
+        .map(s => s.trim())
+        .filter(Boolean);
+}
+async function downloadTo(url, dest) {
+    const res = await fetch(url, {
+        signal: AbortSignal.timeout(DOWNLOAD_TIMEOUT_MS)
+    });
+    if (!res.ok || !res.body) {
+        throw new Error(`download answered ${res.status}`);
+    }
+    await (0,promises_namespaceObject.pipeline)(external_stream_namespaceObject.Readable.fromWeb(res.body), external_fs_namespaceObject.createWriteStream(dest));
+}
+async function httpPut(url, file) {
+    const stat = await external_fs_namespaceObject.promises.stat(file);
+    const res = await fetch(url, {
+        method: 'PUT',
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any -- node stream body
+        body: external_fs_namespaceObject.createReadStream(file),
+        duplex: 'half',
+        headers: { 'content-length': String(stat.size) },
+        signal: AbortSignal.timeout(UPLOAD_TIMEOUT_MS)
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any -- duplex not yet typed
+    });
+    if (!res.ok) {
+        throw new Error(`PUT answered ${res.status}`);
+    }
+}
+function tempBundlePath(tag) {
+    return external_path_namespaceObject.join(external_os_namespaceObject.tmpdir(), `wb-${tag}-${process.pid}-${Date.now()}.bundle`);
+}
+// Kept for callers/tests: reset .git/objects to the empty `git init` shape.
 async function resetGitObjects(gitDir) {
-    await external_fs_namespaceObject.promises.rm(external_path_namespaceObject.join(gitDir, 'objects'), {
+    await fs.promises.rm(path.join(gitDir, 'objects'), {
         recursive: true,
         force: true
     });
-    await external_fs_namespaceObject.promises.rm(external_path_namespaceObject.join(gitDir, 'shallow'), { force: true });
-    await external_fs_namespaceObject.promises.mkdir(external_path_namespaceObject.join(gitDir, 'objects', 'info'), {
+    await fs.promises.rm(path.join(gitDir, 'shallow'), { force: true });
+    await fs.promises.mkdir(path.join(gitDir, 'objects', 'info'), {
         recursive: true
     });
-    await external_fs_namespaceObject.promises.mkdir(external_path_namespaceObject.join(gitDir, 'objects', 'pack'), {
+    await fs.promises.mkdir(path.join(gitDir, 'objects', 'pack'), {
         recursive: true
     });
 }
-// Directory the agent scans post-job for snapshots to upload. Under RUNNER_TEMP, which
-// is <runner>/_work/_temp — host-visible to the agent (it knows the runner dir).
-function manifestDir() {
-    const runnerTemp = process.env['RUNNER_TEMP'] || external_os_namespaceObject.tmpdir();
-    return external_path_namespaceObject.join(runnerTemp, 'wb-snapshots');
-}
-// One manifest per cache miss: the agent tars git_dir/objects and uploads it keyed by sha.
-async function writeSnapshotManifest(settings) {
-    const sha = settings.commit;
-    const manifest = {
-        repo_key: process.env['GITHUB_REPOSITORY_ID'],
-        sha,
-        git_dir: external_path_namespaceObject.join(settings.repositoryPath, '.git')
-    };
-    const dir = manifestDir();
-    await external_fs_namespaceObject.promises.mkdir(dir, { recursive: true });
-    await external_fs_namespaceObject.promises.writeFile(external_path_namespaceObject.join(dir, `${sha}.json`), JSON.stringify(manifest));
-    info(`Cache miss recorded for ${sha}; the WarpBuild agent will upload the snapshot after the job`);
+// Kept so tests referencing io stay valid; the mirror path no longer shells to `tar`.
+async function hasGit() {
+    return Boolean(await io.which('git', false));
 }
 
 ;// CONCATENATED MODULE: ./src/git-source-provider.ts
@@ -42007,7 +42104,7 @@ async function getSource(settings) {
     const git = await getGitCommandManager(settings);
     endGroup();
     let authHelper = null;
-    let warpbuildRestored = false;
+    let warpbuildMode = 'off';
     try {
         if (git) {
             authHelper = createAuthHelper(git, settings);
@@ -42059,7 +42156,7 @@ async function getSource(settings) {
             await git.remoteAdd('origin', repositoryUrl);
             endGroup();
             // WarpBuild snapshot cache: hit = objects restored, fetch below is skipped
-            warpbuildRestored = await setup(settings);
+            warpbuildMode = await setup(settings);
         }
         // Disable automatic garbage collection
         startGroup('Disabling automatic garbage collection');
@@ -42099,10 +42196,17 @@ async function getSource(settings) {
         else if (settings.sparseCheckout) {
             fetchOptions.filter = 'blob:none';
         }
-        if (warpbuildRestored) {
-            info('Skipping fetch: checkout was restored from the snapshot cache');
+        if (warpbuildMode === 'seeded') {
+            // Seeded from the mirror: fetch only the tip delta, negotiating against the seeded
+            // objects. No depth — the base is full history, so this stays non-shallow.
+            const refSpec = getRefSpec(settings.ref, settings.commit);
+            await git.fetch(refSpec, fetchOptions);
+            if (!(await testRef(git, settings.ref, settings.commit))) {
+                throw new Error(`The ref '${settings.ref}' does not point to the expected commit '${settings.commit}'. ` +
+                    `The ref may have been updated after the workflow was triggered.`);
+            }
         }
-        else if (settings.fetchDepth <= 0) {
+        else if (warpbuildMode === 'cold-build' || settings.fetchDepth <= 0) {
             // Fetch all branches and tags
             let refSpec = getRefSpecForAllHistory(settings.ref, settings.commit);
             await git.fetch(refSpec, fetchOptions);
diff --git a/src/git-source-provider.ts b/src/git-source-provider.ts
index 3b5adbc..fbdf9d7 100644
--- a/src/git-source-provider.ts
+++ b/src/git-source-provider.ts
@@ -41,7 +41,7 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
   core.endGroup()
 
   let authHelper: gitAuthHelper.IGitAuthHelper | null = null
-  let warpbuildRestored = false
+  let warpbuildMode: warpbuildMirror.MirrorMode = 'off'
   try {
     if (git) {
       authHelper = gitAuthHelper.createAuthHelper(git, settings)
@@ -134,7 +134,7 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       core.endGroup()
 
       // WarpBuild snapshot cache: hit = objects restored, fetch below is skipped
-      warpbuildRestored = await warpbuildMirror.setup(settings)
+      warpbuildMode = await warpbuildMirror.setup(settings)
     }
 
     // Disable automatic garbage collection
@@ -190,9 +190,18 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       fetchOptions.filter = 'blob:none'
     }
 
-    if (warpbuildRestored) {
-      core.info('Skipping fetch: checkout was restored from the snapshot cache')
-    } else if (settings.fetchDepth <= 0) {
+    if (warpbuildMode === 'seeded') {
+      // Seeded from the mirror: fetch only the tip delta, negotiating against the seeded
+      // objects. No depth — the base is full history, so this stays non-shallow.
+      const refSpec = refHelper.getRefSpec(settings.ref, settings.commit)
+      await git.fetch(refSpec, fetchOptions)
+      if (!(await refHelper.testRef(git, settings.ref, settings.commit))) {
+        throw new Error(
+          `The ref '${settings.ref}' does not point to the expected commit '${settings.commit}'. ` +
+            `The ref may have been updated after the workflow was triggered.`
+        )
+      }
+    } else if (warpbuildMode === 'cold-build' || settings.fetchDepth <= 0) {
       // Fetch all branches and tags
       let refSpec = refHelper.getRefSpecForAllHistory(
         settings.ref,
diff --git a/src/warpbuild/backend-api.ts b/src/warpbuild/backend-api.ts
index 0f8c379..b052abc 100644
--- a/src/warpbuild/backend-api.ts
+++ b/src/warpbuild/backend-api.ts
@@ -1,21 +1,30 @@
 /* eslint-disable i18n-text/no-en -- upstream convention */
 import * as core from '@actions/core'
 
-// Client for backend-core's git-mirror download-url endpoint, authed by the runner
-// verification token. Contract: 200 = presigned URL (hit); 404 = miss; 403 = unservable
-// org; else = fall back. The upload half runs in the warpbuild-agent after the job.
+// Client for backend-core's git-mirror endpoints, authed by the runner verification
+// token. The action seeds from a bare base mirror + a per-branch delta bundle (both
+// presigned GETs), delta-fetches the tip from GitHub, then uploads the refreshed
+// per-branch bundle — or, on a cold repo, the base — via a presigned PUT. Every call
+// fails closed to stock checkout.
 
 const API_TIMEOUT_MS = 10_000
 
-export interface SnapshotDownloadInfo {
+export interface Presigned {
   url: string
-  size_bytes: number
-  created_at: string
+  size_bytes?: number
 }
 
-export type SnapshotLookup =
-  | {kind: 'hit'; info: SnapshotDownloadInfo}
-  | {kind: 'miss'}
+// GET restore-url: what to seed before the delta fetch. 'cold' = no base yet.
+export type RestoreLookup =
+  | {kind: 'restore'; base: Presigned; branch: Presigned | null}
+  | {kind: 'cold'}
+  | {kind: 'disabled'}
+  | {kind: 'error'}
+
+// POST *upload-url: a presigned PUT, or a signal to skip (locked/disabled).
+export type UploadGrant =
+  | {kind: 'grant'; url: string}
+  | {kind: 'locked'}
   | {kind: 'disabled'}
   | {kind: 'error'}
 
@@ -27,34 +36,85 @@ function authHeader(): string {
   return `Bearer ${process.env['WARPBUILD_RUNNER_VERIFICATION_TOKEN'] || ''}`
 }
 
-export async function lookupSnapshot(
+function endpoint(p: string): string {
+  return `${baseUrl()}/api/v1/git-mirrors/${p}`
+}
+
+export async function lookupRestore(
   repoKey: string,
-  sha: string
-): Promise<SnapshotLookup> {
+  ref: string
+): Promise<RestoreLookup> {
   try {
     const res = await fetch(
-      `${baseUrl()}/api/v1/git-mirrors/download-url?repo_key=${encodeURIComponent(
+      `${endpoint('restore-url')}?repo_key=${encodeURIComponent(
         repoKey
-      )}&sha=${encodeURIComponent(sha)}`,
+      )}&ref=${encodeURIComponent(ref)}`,
       {
         headers: {authorization: authHeader()},
         signal: AbortSignal.timeout(API_TIMEOUT_MS)
       }
     )
     if (res.status === 200) {
-      return {kind: 'hit', info: (await res.json()) as SnapshotDownloadInfo}
+      const body = (await res.json()) as {
+        base: Presigned
+        branch: Presigned | null
+      }
+      return {kind: 'restore', base: body.base, branch: body.branch ?? null}
     }
     if (res.status === 404) {
-      return {kind: 'miss'}
+      return {kind: 'cold'}
     }
     if (res.status === 403) {
-      core.debug(`[wb-cache] download-url answered 403 (disabled)`)
+      core.debug('[wb-cache] restore-url answered 403 (disabled)')
       return {kind: 'disabled'}
     }
-    core.debug(`[wb-cache] download-url answered ${res.status}`)
+    core.debug(`[wb-cache] restore-url answered ${res.status}`)
     return {kind: 'error'}
   } catch (error) {
-    core.debug(`[wb-cache] download-url failed: ${error}`)
+    core.debug(`[wb-cache] restore-url failed: ${error}`)
     return {kind: 'error'}
   }
 }
+
+async function requestUpload(p: string, body: object): Promise<UploadGrant> {
+  try {
+    const res = await fetch(endpoint(p), {
+      method: 'POST',
+      headers: {
+        authorization: authHeader(),
+        'content-type': 'application/json'
+      },
+      body: JSON.stringify(body),
+      signal: AbortSignal.timeout(API_TIMEOUT_MS)
+    })
+    if (res.status === 200) {
+      const b = (await res.json()) as {url: string}
+      return {kind: 'grant', url: b.url}
+    }
+    if (res.status === 409) {
+      return {kind: 'locked'}
+    }
+    if (res.status === 403) {
+      return {kind: 'disabled'}
+    }
+    core.debug(`[wb-cache] ${p} answered ${res.status}`)
+    return {kind: 'error'}
+  } catch (error) {
+    core.debug(`[wb-cache] ${p} failed: ${error}`)
+    return {kind: 'error'}
+  }
+}
+
+// Cold repo: request the single-flight grant to build + upload the base mirror.
+export async function requestBaseUpload(repoKey: string): Promise<UploadGrant> {
+  return requestUpload('base/upload-url', {repo_key: repoKey})
+}
+
+// Warm repo: request the grant to overwrite this branch's delta bundle.
+export async function requestBranchUpload(
+  repoKey: string,
+  ref: string,
+  sha: string
+): Promise<UploadGrant> {
+  return requestUpload('branch/upload-url', {repo_key: repoKey, ref, sha})
+}
diff --git a/src/warpbuild/mirror-cache.ts b/src/warpbuild/mirror-cache.ts
index 6b54721..14ddefe 100644
--- a/src/warpbuild/mirror-cache.ts
+++ b/src/warpbuild/mirror-cache.ts
@@ -10,22 +10,38 @@ import {pipeline} from 'stream/promises'
 import {IGitSourceSettings} from '../git-source-settings.js'
 import * as api from './backend-api.js'
 
-// WarpBuild checkout snapshot cache: SHA-keyed tars of what the stock shallow fetch
-// produces. Hit = restore + skip the fetch; miss = upload after checkout. Keys are
-// immutable (no expiry). Fail-open: any error degrades to stock behavior.
+// WarpBuild checkout cache: a bare base mirror + per-branch orderless delta bundles on
+// S3. We seed the base (+ this branch's delta) before the fetch, so the GitHub fetch is
+// only the tip delta — or nothing. Each run overwrites its branch's bundle (base-relative,
+// so order-free); a cold repo single-flights the base build. Fail-open: any error →
+// standard checkout.
 
 const DOWNLOAD_TIMEOUT_MS = 15 * 60_000
+const UPLOAD_TIMEOUT_MS = 15 * 60_000
 
-// Logged at debug (normal state outside WarpBuild); other reasons log at info.
+// Logged at info when off; the WARPBUILD_* env is only present on our runners.
 export const SKIP_NOT_WARPBUILD =
   'not running on a WarpBuild runner (WARPBUILD_* env not present)'
 
 const SHA_PATTERN = /^[0-9a-f]{40}([0-9a-f]{24})?$/
 
-let decision: 'off' | 'miss' = 'off'
+// Seeded bundles land here, out of the user's ref space; excluded objects for deltas.
+const BASE_REFNS = 'refs/wb/base'
+const BRANCH_REFNS = 'refs/wb/branch'
+const UPLOAD_TIP_REF = 'refs/wb/tip'
 
-// Null = attempt the cache; else a reason to log. Only the default checkout shape
-// is served.
+export type MirrorMode = 'off' | 'seeded' | 'cold-build'
+
+interface Plan {
+  mode: MirrorMode
+  repoKey: string
+  refKey: string
+  baseUploadUrl?: string // cold-build only
+}
+let plan: Plan = {mode: 'off', repoKey: '', refKey: ''}
+
+// Null = attempt the cache; else a reason to log. The mirror serves full history for any
+// depth, so fetch-depth is not gated; sparse/lfs/filter change the object set we model.
 export function getMirrorCacheSkipReason(
   settings: IGitSourceSettings
 ): string | null {
@@ -35,8 +51,7 @@ export function getMirrorCacheSkipReason(
   ) {
     return SKIP_NOT_WARPBUILD
   }
-  const repoKey = process.env['GITHUB_REPOSITORY_ID'] || ''
-  if (!repoKey) {
+  if (!process.env['GITHUB_REPOSITORY_ID']) {
     return 'GITHUB_REPOSITORY_ID is not set'
   }
   const checkoutRepo = `${settings.repositoryOwner}/${settings.repositoryName}`
@@ -53,12 +68,6 @@ export function getMirrorCacheSkipReason(
   if (!settings.commit || !SHA_PATTERN.test(settings.commit)) {
     return 'no exact commit sha to key on'
   }
-  if (settings.fetchDepth !== 1) {
-    return `fetch-depth is ${settings.fetchDepth}, cache only serves fetch-depth 1`
-  }
-  if (settings.fetchTags) {
-    return 'fetch-tags is enabled'
-  }
   if (settings.filter) {
     return 'a fetch filter is configured'
   }
@@ -66,191 +75,266 @@ export function getMirrorCacheSkipReason(
     return 'sparse checkout is configured'
   }
   if (settings.lfs) {
-    return 'lfs is enabled (lfs objects are not in the snapshot)'
-  }
-  if (settings.ref && computeDestinationRef(settings.ref) === null) {
-    return `ref '${settings.ref}' has no cacheable destination ref`
+    return 'lfs is enabled (lfs objects are not in the mirror)'
   }
   return null
 }
 
-// The local ref the fetch would have created ('' = none needed, null = uncacheable).
-export function computeDestinationRef(ref: string): string | null {
-  if (!ref) {
+// The durable branch to key the per-branch bundle on. For pull_request events the merge
+// SHA is synthetic, so we key on the base branch; otherwise the pushed branch. '' for
+// tags / detached HEAD → seed the base only, upload nothing.
+export function computeRefKey(settings: IGitSourceSettings): string {
+  const baseRef = process.env['GITHUB_BASE_REF'] // set on pull_request events
+  if (baseRef) {
+    return baseRef
+  }
+  const ref = process.env['GITHUB_REF'] || settings.ref || ''
+  if (ref.startsWith('refs/heads/')) {
+    return ref.substring('refs/heads/'.length)
+  }
+  if (ref.startsWith('refs/tags/') || ref.startsWith('refs/pull/')) {
     return ''
   }
-  const upper = ref.toUpperCase()
-  if (upper.startsWith('REFS/HEADS/')) {
-    return `refs/remotes/origin/${ref.substring('refs/heads/'.length)}`
-  }
-  if (upper.startsWith('REFS/PULL/')) {
-    return `refs/remotes/pull/${ref.substring('refs/pull/'.length)}`
-  }
-  if (upper.startsWith('REFS/TAGS/')) {
-    return ref
-  }
-  return null
+  // Already a short branch name, or empty.
+  return SHA_PATTERN.test(ref) ? '' : ref
 }
 
-// Runs right after `git init`; true = restored (caller skips the fetch). Never throws.
-// Sets the `cache-hit` action output (true only on a restore).
-export async function setup(settings: IGitSourceSettings): Promise<boolean> {
-  const restored = await setupImpl(settings)
-  core.setOutput('cache-hit', restored ? 'true' : 'false')
-  return restored
+// Runs after `git init`, before the fetch. Returns the mode the fetch/contribute steps
+// branch on. Never throws. Sets the `cache-hit` output (true only when seeded from cache).
+export async function setup(settings: IGitSourceSettings): Promise<MirrorMode> {
+  const mode = await setupImpl(settings)
+  core.setOutput('cache-hit', mode === 'seeded' ? 'true' : 'false')
+  return mode
 }
 
-async function setupImpl(settings: IGitSourceSettings): Promise<boolean> {
-  decision = 'off'
+async function setupImpl(settings: IGitSourceSettings): Promise<MirrorMode> {
+  plan = {mode: 'off', repoKey: '', refKey: ''}
   const skipReason = getMirrorCacheSkipReason(settings)
   if (skipReason) {
-    core.info(`WarpBuild snapshot cache skipped: ${skipReason}`)
-    return false
+    core.info(`WarpBuild mirror cache skipped: ${skipReason}`)
+    return 'off'
   }
-  core.startGroup('WarpBuild: checkout snapshot cache')
+  core.startGroup('WarpBuild: checkout mirror cache')
   try {
     return await setupInner(settings)
   } catch (error) {
     core.warning(
-      `WarpBuild snapshot cache unavailable, using standard checkout: ${error}`
+      `WarpBuild mirror cache unavailable, using standard checkout: ${error}`
     )
-    return false
+    return 'off'
   } finally {
     core.endGroup()
   }
 }
 
-async function setupInner(settings: IGitSourceSettings): Promise<boolean> {
+async function setupInner(settings: IGitSourceSettings): Promise<MirrorMode> {
   const repoKey = process.env['GITHUB_REPOSITORY_ID'] as string
-  const sha = settings.commit
-
-  // The restore/upload paths shell out to `tar`; without it, fall back cleanly.
-  if (!(await io.which('tar', false))) {
-    core.info('tar not found on PATH; using standard checkout')
-    return false
-  }
-
-  const lookup = await api.lookupSnapshot(repoKey, sha)
+  const refKey = computeRefKey(settings)
+  plan = {mode: 'off', repoKey, refKey}
 
+  const lookup = await api.lookupRestore(repoKey, refKey)
   if (lookup.kind === 'disabled') {
-    core.info('Snapshot cache is disabled by the backend for this organization')
-    return false
+    core.info('Mirror cache is disabled by the backend for this organization')
+    return 'off'
   }
   if (lookup.kind === 'error') {
-    core.info('Snapshot cache backend unavailable; using standard checkout')
-    return false
+    core.info('Mirror cache backend unavailable; using standard checkout')
+    return 'off'
   }
 
-  if (lookup.kind === 'miss') {
+  if (lookup.kind === 'cold') {
+    const grant = await api.requestBaseUpload(repoKey)
+    if (grant.kind === 'grant') {
+      core.info(
+        'Cold repo: building the base mirror this run (full fetch, then upload)'
+      )
+      plan = {mode: 'cold-build', repoKey, refKey, baseUploadUrl: grant.url}
+      return 'cold-build'
+    }
     core.info(
-      `Cache miss for ${sha}: the standard fetch will run and its result will be uploaded`
+      'Cold repo: base build already in progress elsewhere; using standard checkout'
     )
-    decision = 'miss'
-    return false
+    return 'off'
   }
 
-  core.info(
-    `Cache hit for ${sha}: restoring snapshot (${lookup.info.size_bytes} bytes)`
-  )
-  if (!(await restoreSnapshot(settings, lookup.info.url, sha))) {
-    return false
+  // Restore: seed the base, then this branch's delta if present.
+  await seedBundle(settings.repositoryPath, lookup.base.url, BASE_REFNS)
+  if (lookup.branch) {
+    try {
+      await seedBundle(settings.repositoryPath, lookup.branch.url, BRANCH_REFNS)
+    } catch (error) {
+      core.info(`Branch delta seed skipped (${error}); base only`)
+    }
   }
-  core.info('Snapshot restored; skipping the GitHub fetch entirely')
-  return true
+  core.info(
+    `Seeded base mirror${
+      lookup.branch ? ' + branch delta' : ''
+    }; the GitHub fetch will be a tip delta`
+  )
+  plan = {mode: 'seeded', repoKey, refKey}
+  return 'seeded'
 }
 
-// Runs after checkout; on a miss records a manifest so the WarpBuild agent uploads the
-// snapshot after the job exits (keeping tar+upload off the customer's billed time).
+// Runs after checkout, in the same step (`.git` still pristine). Uploads the base (cold)
+// or this branch's refreshed delta (seeded). Best-effort — never fails the checkout.
 export async function contribute(settings: IGitSourceSettings): Promise<void> {
-  if (decision !== 'miss') {
+  try {
+    if (plan.mode === 'cold-build') {
+      await uploadBaseMirror(settings)
+    } else if (plan.mode === 'seeded') {
+      await uploadBranchDelta(settings)
+    }
+  } catch (error) {
+    core.warning(`WarpBuild mirror upload skipped: ${error}`)
+  }
+}
+
+// Download a bundle and fetch its refs into refNs/* so its objects seed the local repo
+// (and count as "haves" for the delta negotiation). The bundle is a git-validated file;
+// nothing is extracted into .git directly.
+async function seedBundle(
+  repoPath: string,
+  url: string,
+  refNs: string
+): Promise<void> {
+  const tmp = tempBundlePath('seed')
+  try {
+    await downloadTo(url, tmp)
+    await exec.exec('git', [
+      '-C',
+      repoPath,
+      'fetch',
+      '--quiet',
+      '--no-tags',
+      tmp,
+      `+refs/*:${refNs}/*`
+    ])
+  } finally {
+    await fs.promises.rm(tmp, {force: true})
+  }
+}
+
+// Cold-build: after the forced full fetch the repo holds all branches under
+// refs/remotes/origin/* and all tags. Bundle exactly those (not `--all`, which would
+// also sweep up the ephemeral triggering PR merge ref) as the base and upload.
+async function uploadBaseMirror(settings: IGitSourceSettings): Promise<void> {
+  if (!plan.baseUploadUrl) {
     return
   }
+  const tmp = tempBundlePath('base')
   try {
-    await writeSnapshotManifest(settings)
-  } catch (error) {
-    core.warning(`Snapshot manifest not written: ${error}`)
-  }
-}
-
-// Refuse any tar member outside objects/ or shallow, absolute, or with a `..`
-// component — the tar is remote and extracted into .git, so a crafted member could
-// escape (e.g. hooks/, ../) and run code during checkout.
-export async function assertSafeTarMembers(tar: string): Promise<void> {
-  let listing = ''
-  await exec.exec('tar', ['-tf', tar], {
-    silent: true,
-    listeners: {stdout: (d: Buffer) => (listing += d.toString())}
-  })
-  for (const raw of listing.split('\n')) {
-    const member = raw.trim()
-    if (!member) {
-      continue
-    }
-    const top = member.replace(/^\.\//, '').split('/')[0]
-    if (
-      member.startsWith('/') ||
-      member.split('/').includes('..') ||
-      (top !== 'objects' && top !== 'shallow')
-    ) {
-      throw new Error(`unexpected snapshot tar member: ${member}`)
-    }
-  }
-}
-
-async function restoreSnapshot(
-  settings: IGitSourceSettings,
-  url: string,
-  sha: string
-): Promise<boolean> {
-  const gitDir = path.join(settings.repositoryPath, '.git')
-  const tmpTar = path.join(os.tmpdir(), `wb-snapshot-${process.pid}.tar`)
-  try {
-    const res = await fetch(url, {
-      signal: AbortSignal.timeout(DOWNLOAD_TIMEOUT_MS)
-    })
-    if (!res.ok || !res.body) {
-      throw new Error(`snapshot download answered ${res.status}`)
-    }
-    await pipeline(
-      Readable.fromWeb(res.body as import('stream/web').ReadableStream),
-      fs.createWriteStream(tmpTar)
-    )
-    await assertSafeTarMembers(tmpTar)
-    await exec.exec('tar', ['-xf', tmpTar, '-C', gitDir])
-
-    const check = await exec.exec(
-      'git',
-      ['-C', settings.repositoryPath, 'cat-file', '-e', `${sha}^{commit}`],
-      {ignoreReturnCode: true}
-    )
-    if (check !== 0) {
-      throw new Error(`restored snapshot does not contain ${sha}`)
-    }
-
-    // The ref the skipped fetch would have created; upstream's verification needs it.
-    const dstRef = computeDestinationRef(settings.ref)
-    if (dstRef) {
-      await exec.exec('git', [
-        '-C',
-        settings.repositoryPath,
-        'update-ref',
-        dstRef,
-        sha
-      ])
-    }
-
-    return true
-  } catch (error) {
-    core.warning(`Snapshot restore failed: ${error}`)
-    await resetGitObjects(gitDir)
-    return false
+    await exec.exec('git', [
+      '-C',
+      settings.repositoryPath,
+      'bundle',
+      'create',
+      tmp,
+      '--remotes=origin',
+      '--tags'
+    ])
+    await httpPut(plan.baseUploadUrl, tmp)
+    core.info('Uploaded base mirror')
   } finally {
-    await fs.promises.rm(tmpTar, {force: true})
+    await fs.promises.rm(tmp, {force: true})
   }
 }
 
-// Reset .git/objects (and drop any restored shallow) to the empty shape `git init`
-// creates, so a failed restore leaves no trace for the fallback fetch.
+// Seeded: build the base-relative delta (target sha, excluding everything in the base)
+// and overwrite this branch's bundle. Guarded by a per-branch lock server-side.
+async function uploadBranchDelta(settings: IGitSourceSettings): Promise<void> {
+  if (!plan.refKey) {
+    return // tag / detached HEAD: nothing to roll
+  }
+  const repoPath = settings.repositoryPath
+  const sha = settings.commit
+
+  const grant = await api.requestBranchUpload(plan.repoKey, plan.refKey, sha)
+  if (grant.kind !== 'grant') {
+    core.info(`Branch delta upload skipped (${grant.kind})`)
+    return
+  }
+
+  const excludes = await baseRefExcludes(repoPath)
+  if (excludes.length === 0) {
+    core.info('No base refs to diff against; branch delta skipped')
+    return
+  }
+
+  const tmp = tempBundlePath('branch')
+  try {
+    await exec.exec('git', ['-C', repoPath, 'update-ref', UPLOAD_TIP_REF, sha])
+    await exec.exec('git', [
+      '-C',
+      repoPath,
+      'bundle',
+      'create',
+      tmp,
+      UPLOAD_TIP_REF,
+      ...excludes
+    ])
+    await httpPut(grant.url, tmp)
+    core.info(`Uploaded branch delta for '${plan.refKey}'`)
+  } finally {
+    await fs.promises.rm(tmp, {force: true})
+    await exec.exec(
+      'git',
+      ['-C', repoPath, 'update-ref', '-d', UPLOAD_TIP_REF],
+      {
+        ignoreReturnCode: true
+      }
+    )
+  }
+}
+
+// `^refname` for every seeded base ref — the exclusion set that makes the delta bundle
+// base-relative (and therefore order-free).
+async function baseRefExcludes(repoPath: string): Promise<string[]> {
+  let out = ''
+  await exec.exec(
+    'git',
+    ['-C', repoPath, 'for-each-ref', '--format=^%(refname)', BASE_REFNS],
+    {silent: true, listeners: {stdout: (d: Buffer) => (out += d.toString())}}
+  )
+  return out
+    .split('\n')
+    .map(s => s.trim())
+    .filter(Boolean)
+}
+
+async function downloadTo(url: string, dest: string): Promise<void> {
+  const res = await fetch(url, {
+    signal: AbortSignal.timeout(DOWNLOAD_TIMEOUT_MS)
+  })
+  if (!res.ok || !res.body) {
+    throw new Error(`download answered ${res.status}`)
+  }
+  await pipeline(
+    Readable.fromWeb(res.body as import('stream/web').ReadableStream),
+    fs.createWriteStream(dest)
+  )
+}
+
+async function httpPut(url: string, file: string): Promise<void> {
+  const stat = await fs.promises.stat(file)
+  const res = await fetch(url, {
+    method: 'PUT',
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any -- node stream body
+    body: fs.createReadStream(file) as any,
+    duplex: 'half',
+    headers: {'content-length': String(stat.size)},
+    signal: AbortSignal.timeout(UPLOAD_TIMEOUT_MS)
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any -- duplex not yet typed
+  } as any)
+  if (!res.ok) {
+    throw new Error(`PUT answered ${res.status}`)
+  }
+}
+
+function tempBundlePath(tag: string): string {
+  return path.join(os.tmpdir(), `wb-${tag}-${process.pid}-${Date.now()}.bundle`)
+}
+
+// Kept for callers/tests: reset .git/objects to the empty `git init` shape.
 export async function resetGitObjects(gitDir: string): Promise<void> {
   await fs.promises.rm(path.join(gitDir, 'objects'), {
     recursive: true,
@@ -265,30 +349,7 @@ export async function resetGitObjects(gitDir: string): Promise<void> {
   })
 }
 
-// Directory the agent scans post-job for snapshots to upload. Under RUNNER_TEMP, which
-// is <runner>/_work/_temp — host-visible to the agent (it knows the runner dir).
-export function manifestDir(): string {
-  const runnerTemp = process.env['RUNNER_TEMP'] || os.tmpdir()
-  return path.join(runnerTemp, 'wb-snapshots')
-}
-
-// One manifest per cache miss: the agent tars git_dir/objects and uploads it keyed by sha.
-async function writeSnapshotManifest(
-  settings: IGitSourceSettings
-): Promise<void> {
-  const sha = settings.commit
-  const manifest = {
-    repo_key: process.env['GITHUB_REPOSITORY_ID'] as string,
-    sha,
-    git_dir: path.join(settings.repositoryPath, '.git')
-  }
-  const dir = manifestDir()
-  await fs.promises.mkdir(dir, {recursive: true})
-  await fs.promises.writeFile(
-    path.join(dir, `${sha}.json`),
-    JSON.stringify(manifest)
-  )
-  core.info(
-    `Cache miss recorded for ${sha}; the WarpBuild agent will upload the snapshot after the job`
-  )
+// Kept so tests referencing io stay valid; the mirror path no longer shells to `tar`.
+export async function hasGit(): Promise<boolean> {
+  return Boolean(await io.which('git', false))
 }