diff --git a/__test__/warpbuild-mirror.test.ts b/__test__/warpbuild-mirror.test.ts
index e6250e9..1d860f4 100644
--- a/__test__/warpbuild-mirror.test.ts
+++ b/__test__/warpbuild-mirror.test.ts
@@ -106,24 +106,34 @@ describe('warpbuild mirror cache', () => {
       )
     })
 
-    it('serves any fetch-depth and fetch-tags (mirror is full history)', () => {
+    it('engages for depth 0/1, defers explicit shallow (>=2) to upstream', () => {
       expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 0}))).toBeNull()
-      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 50}))).toBeNull()
+      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 1}))).toBeNull()
+      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 2}))).toContain(
+        'explicit shallow depth'
+      )
+      expect(getMirrorCacheSkipReason(settingsFor({fetchDepth: 50}))).toContain(
+        'explicit shallow depth'
+      )
       expect(
         getMirrorCacheSkipReason(settingsFor({fetchTags: true}))
       ).toBeNull()
     })
 
-    it('skips filters, sparse and lfs checkouts', () => {
+    it('skips filters and sparse checkouts', () => {
       expect(getMirrorCacheSkipReason(settingsFor({filter: 'blob:none'}))).toBe(
         'a fetch filter is configured'
       )
       expect(
         getMirrorCacheSkipReason(settingsFor({sparseCheckout: ['src']}))
       ).toBe('sparse checkout is configured')
-      expect(getMirrorCacheSkipReason(settingsFor({lfs: true}))).toBe(
-        'lfs is enabled (lfs objects are not in the mirror)'
-      )
+    })
+
+    it('engages for lfs (git objects from the mirror, lfs binaries fetched on top)', () => {
+      expect(getMirrorCacheSkipReason(settingsFor({lfs: true}))).toBeNull()
+      expect(
+        getMirrorCacheSkipReason(settingsFor({lfs: true, fetchDepth: 1}))
+      ).toBeNull()
     })
   })
 
diff --git a/dist/index.js b/dist/index.js
index 04eb13a..943b39f 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -41786,6 +41786,7 @@ async function requestBranchUpload(repoKey, ref, sha) {
 
 
 
+
 // 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,
@@ -41801,8 +41802,13 @@ 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.
+// Null = attempt the cache; else a reason to log. We engage only where the result matches
+// upstream: full history (fetch-depth 0) or the default depth 1. An explicit shallow depth
+// (>= 2) is a deliberate request we can't honour (the mirror is full history), so we defer
+// to upstream. sparse/filter change the object set we model, so they defer too. LFS does
+// NOT defer: the bundle carries the git objects (including LFS pointer blobs), and the
+// stock `git lfs fetch`/`checkout` steps pull the actual LFS binaries from GitHub on top,
+// exactly as upstream — the mirror only accelerates the git-object half.
 function getMirrorCacheSkipReason(settings) {
     if (!process.env['WARPBUILD_RUNNER_VERIFICATION_TOKEN'] ||
         !process.env['WARPBUILD_HOST_URL']) {
@@ -41822,15 +41828,18 @@ function getMirrorCacheSkipReason(settings) {
     if (!settings.commit || !SHA_PATTERN.test(settings.commit)) {
         return 'no exact commit sha to key on';
     }
+    // Respect an explicit shallow request: depth 0 (all) and the default depth 1 engage the
+    // mirror; anything deeper is a deliberate shallow the mirror can't reproduce → upstream.
+    if (settings.fetchDepth > 1) {
+        return `fetch-depth ${settings.fetchDepth} is an explicit shallow depth; using upstream checkout`;
+    }
     if (settings.filter) {
         return 'a fetch filter is configured';
     }
     if (settings.sparseCheckout) {
         return 'sparse checkout is configured';
     }
-    if (settings.lfs) {
-        return 'lfs is enabled (lfs objects are not in the mirror)';
-    }
+    // LFS intentionally does not skip — see the note above getMirrorCacheSkipReason.
     return null;
 }
 // The durable branch to key the per-branch bundle on. For pull_request events the merge
@@ -41858,6 +41867,14 @@ async function setup(settings) {
     setOutput('cache-hit', mode === 'seeded' ? 'true' : 'false');
     return mode;
 }
+// Called by getSource when a mirror-seeded fetch fails: clear our refs and disengage, so
+// the caller can retry the standard fetch against a clean repo. Also re-sets cache-hit to
+// false since we're no longer serving from cache.
+async function abandon(settings) {
+    plan = { mode: 'off', repoKey: '', refKey: '' };
+    setOutput('cache-hit', 'false');
+    await cleanupWbRefs(settings.repositoryPath);
+}
 async function setupImpl(settings) {
     plan = { mode: 'off', repoKey: '', refKey: '' };
     const skipReason = getMirrorCacheSkipReason(settings);
@@ -41871,6 +41888,9 @@ async function setupImpl(settings) {
     }
     catch (error) {
         warning(`WarpBuild mirror cache unavailable, using standard checkout: ${error}`);
+        // A partial seed may have left refs/wb/*; strip it so the stock fetch runs against a
+        // clean repo (objects just dangle harmlessly).
+        await cleanupWbRefs(settings.repositoryPath);
         return 'off';
     }
     finally {
@@ -41902,6 +41922,14 @@ async function setupInner(settings) {
     }
     // Restore: seed the base, then this branch's delta if present.
     await seedBundle(settings.repositoryPath, lookup.base.url, BASE_REFNS);
+    // Full-history checkout: mirror the base's branches + tags into refs/remotes/origin/* +
+    // refs/tags/* so the resulting .git matches an upstream `fetch-depth: 0` checkout (the
+    // delta fetch then advances the target ref to its current tip). A shallow upstream fetch
+    // (depth 1) populates only the target ref, so there we keep the base internal and let the
+    // delta fetch provide just that ref.
+    if (settings.fetchDepth <= 0) {
+        await exposeBaseRefs(settings.repositoryPath);
+    }
     if (lookup.branch) {
         try {
             await seedBundle(settings.repositoryPath, lookup.branch.url, BRANCH_REFNS);
@@ -41914,20 +41942,42 @@ async function setupInner(settings) {
     plan = { mode: 'seeded', repoKey, refKey };
     return 'seeded';
 }
-// 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.
+// Runs LAST in getSource — the checkout has already succeeded. Uploads the base (cold) or
+// this branch's refreshed delta (seeded). Hardened so it can NEVER fail the step: its own
+// try/catch for normal errors, plus scoped process guards (installed only for the upload
+// window) that turn an *uncaught* error from the git/upload work — e.g. a spawn error that
+// bypasses the promise chain, like the arg-length crash we hit on Windows — into a clean
+// exit(0). Safe because nothing checkout-relevant runs after this point.
 async function contribute(settings) {
+    if (plan.mode !== 'cold-build' && plan.mode !== 'seeded') {
+        return;
+    }
+    const guard = (error) => {
+        warning(`WarpBuild mirror upload error ignored — checkout already complete: ${error}`);
+        process.exit(0);
+    };
+    process.on('uncaughtException', guard);
+    process.on('unhandledRejection', guard);
     try {
         if (plan.mode === 'cold-build') {
             await uploadBaseMirror(settings);
         }
-        else if (plan.mode === 'seeded') {
+        else {
             await uploadBranchDelta(settings);
         }
     }
     catch (error) {
         warning(`WarpBuild mirror upload skipped: ${error}`);
     }
+    finally {
+        // Seeded mode created the internal refs/wb/* namespace; strip it so the customer's
+        // .git carries no mirror fingerprint (matches upstream; won't break push --mirror).
+        if (plan.mode === 'seeded') {
+            await cleanupWbRefs(settings.repositoryPath);
+        }
+        process.removeListener('uncaughtException', guard);
+        process.removeListener('unhandledRejection', guard);
+    }
 }
 // Download a bundle and fetch its refs into refNs/* so its objects seed the local repo
 // (and count as "haves" for the delta negotiation). The bundle is a git-validated file;
@@ -41950,6 +42000,44 @@ async function seedBundle(repoPath, url, refNs) {
         await external_fs_namespaceObject.promises.rm(tmp, { force: true });
     }
 }
+// Copy the seeded base's branches + tags out to the namespaces upstream uses
+// (refs/remotes/origin/* + refs/tags/*), so a full-history checkout's .git matches
+// upstream. Runs before the delta fetch, while the target ref is still absent, so the
+// `create` directives don't collide. update-ref --stdin keeps it Windows arg-length safe.
+async function exposeBaseRefs(repoPath) {
+    let out = '';
+    await exec_exec('git', [
+        '-C',
+        repoPath,
+        'for-each-ref',
+        '--format=create refs/%(refname:lstrip=3) %(objectname)',
+        `${BASE_REFNS}/remotes/origin`,
+        `${BASE_REFNS}/tags`
+    ], { silent: true, listeners: { stdout: (d) => (out += d.toString()) } });
+    if (!out.trim()) {
+        return;
+    }
+    await exec_exec('git', ['-C', repoPath, 'update-ref', '--stdin'], {
+        input: Buffer.from(out)
+    });
+}
+// Remove the internal refs/wb/* namespace. Objects stay (reachable from the real refs), so
+// the checkout is unaffected; only the mirror's bookkeeping refs go. Best-effort.
+async function cleanupWbRefs(repoPath) {
+    try {
+        let out = '';
+        await exec_exec('git', ['-C', repoPath, 'for-each-ref', '--format=delete %(refname)', 'refs/wb'], { silent: true, listeners: { stdout: (d) => (out += d.toString()) } });
+        if (!out.trim()) {
+            return;
+        }
+        await exec_exec('git', ['-C', repoPath, 'update-ref', '--stdin'], {
+            input: Buffer.from(out)
+        });
+    }
+    catch (error) {
+        core_debug(`WarpBuild ref cleanup skipped: ${error}`);
+    }
+}
 // Cold-build: after the forced full fetch the repo holds all branches under
 // refs/remotes/origin/* and all tags. Bundle exactly those (not `--all`, which would
 // also sweep up the ephemeral triggering PR merge ref) as the base and upload.
@@ -42078,7 +42166,9 @@ async function httpPut(url, file) {
     }
 }
 function tempBundlePath(tag) {
-    return external_path_namespaceObject.join(external_os_namespaceObject.tmpdir(), `wb-${tag}-${process.pid}-${Date.now()}.bundle`);
+    // Unpredictable name so a bundle write can't be pre-empted by a symlink planted on a
+    // shared tmpdir.
+    return external_path_namespaceObject.join(external_os_namespaceObject.tmpdir(), `wb-${tag}-${external_crypto_namespaceObject.randomBytes(12).toString('hex')}.bundle`);
 }
 // Kept for callers/tests: reset .git/objects to the empty `git init` shape.
 async function resetGitObjects(gitDir) {
@@ -42224,15 +42314,28 @@ async function getSource(settings) {
         else if (settings.sparseCheckout) {
             fetchOptions.filter = 'blob:none';
         }
+        let warpbuildFetchDone = false;
         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.`);
+            // objects. No depth — the base is full history, so this stays non-shallow. If it
+            // fails for any reason, disengage cleanly and fall through to the standard fetch.
+            try {
+                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.`);
+                }
+                warpbuildFetchDone = true;
             }
+            catch (error) {
+                warning(`WarpBuild mirror delta fetch failed; falling back to standard checkout: ${error}`);
+                await abandon(settings);
+                warpbuildMode = 'off';
+            }
+        }
+        if (warpbuildFetchDone) {
+            // The seeded delta fetch already brought in the target commit.
         }
         else if (warpbuildMode === 'cold-build' || settings.fetchDepth <= 0) {
             // Fetch all branches and tags
@@ -42300,8 +42403,6 @@ async function getSource(settings) {
         startGroup('Checking out the ref');
         await git.checkout(checkoutInfo.ref, checkoutInfo.startPoint);
         endGroup();
-        // WarpBuild snapshot cache: upload the fetch result on a miss
-        await contribute(settings);
         // Submodules
         if (settings.submodules) {
             // Temporarily override global config
@@ -42328,6 +42429,16 @@ async function getSource(settings) {
         setOutput('commit', commitSHA.trim());
         // Check for incorrect pull request merge commit
         await checkCommitInfo(settings.authToken, commitInfo, settings.repositoryOwner, settings.repositoryName, settings.ref, settings.commit, settings.githubServerUrl);
+        // WarpBuild mirror: best-effort upload, run LAST in getSource so it can never affect
+        // the checkout result. Belt-and-suspenders around contribute()'s own try/catch and the
+        // scoped process guard it installs (which ignores any uncaught upload error now that
+        // the checkout is already complete).
+        try {
+            await contribute(settings);
+        }
+        catch (error) {
+            warning(`WarpBuild mirror upload skipped: ${error}`);
+        }
     }
     finally {
         // Remove auth
diff --git a/src/git-source-provider.ts b/src/git-source-provider.ts
index fbdf9d7..df892aa 100644
--- a/src/git-source-provider.ts
+++ b/src/git-source-provider.ts
@@ -190,17 +190,32 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       fetchOptions.filter = 'blob:none'
     }
 
+    let warpbuildFetchDone = false
     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.`
+      // objects. No depth — the base is full history, so this stays non-shallow. If it
+      // fails for any reason, disengage cleanly and fall through to the standard fetch.
+      try {
+        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.`
+          )
+        }
+        warpbuildFetchDone = true
+      } catch (error) {
+        core.warning(
+          `WarpBuild mirror delta fetch failed; falling back to standard checkout: ${error}`
         )
+        await warpbuildMirror.abandon(settings)
+        warpbuildMode = 'off'
       }
+    }
+
+    if (warpbuildFetchDone) {
+      // The seeded delta fetch already brought in the target commit.
     } else if (warpbuildMode === 'cold-build' || settings.fetchDepth <= 0) {
       // Fetch all branches and tags
       let refSpec = refHelper.getRefSpecForAllHistory(
@@ -287,9 +302,6 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
     await git.checkout(checkoutInfo.ref, checkoutInfo.startPoint)
     core.endGroup()
 
-    // WarpBuild snapshot cache: upload the fetch result on a miss
-    await warpbuildMirror.contribute(settings)
-
     // Submodules
     if (settings.submodules) {
       // Temporarily override global config
@@ -332,6 +344,16 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       settings.commit,
       settings.githubServerUrl
     )
+
+    // WarpBuild mirror: best-effort upload, run LAST in getSource so it can never affect
+    // the checkout result. Belt-and-suspenders around contribute()'s own try/catch and the
+    // scoped process guard it installs (which ignores any uncaught upload error now that
+    // the checkout is already complete).
+    try {
+      await warpbuildMirror.contribute(settings)
+    } catch (error) {
+      core.warning(`WarpBuild mirror upload skipped: ${error}`)
+    }
   } finally {
     // Remove auth
     if (authHelper) {
diff --git a/src/warpbuild/mirror-cache.ts b/src/warpbuild/mirror-cache.ts
index 42d85c0..3ed93ad 100644
--- a/src/warpbuild/mirror-cache.ts
+++ b/src/warpbuild/mirror-cache.ts
@@ -2,6 +2,7 @@
 import * as core from '@actions/core'
 import * as exec from '@actions/exec'
 import * as io from '@actions/io'
+import * as crypto from 'crypto'
 import * as fs from 'fs'
 import * as os from 'os'
 import * as path from 'path'
@@ -40,8 +41,13 @@ interface Plan {
 }
 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.
+// Null = attempt the cache; else a reason to log. We engage only where the result matches
+// upstream: full history (fetch-depth 0) or the default depth 1. An explicit shallow depth
+// (>= 2) is a deliberate request we can't honour (the mirror is full history), so we defer
+// to upstream. sparse/filter change the object set we model, so they defer too. LFS does
+// NOT defer: the bundle carries the git objects (including LFS pointer blobs), and the
+// stock `git lfs fetch`/`checkout` steps pull the actual LFS binaries from GitHub on top,
+// exactly as upstream — the mirror only accelerates the git-object half.
 export function getMirrorCacheSkipReason(
   settings: IGitSourceSettings
 ): string | null {
@@ -68,15 +74,18 @@ export function getMirrorCacheSkipReason(
   if (!settings.commit || !SHA_PATTERN.test(settings.commit)) {
     return 'no exact commit sha to key on'
   }
+  // Respect an explicit shallow request: depth 0 (all) and the default depth 1 engage the
+  // mirror; anything deeper is a deliberate shallow the mirror can't reproduce → upstream.
+  if (settings.fetchDepth > 1) {
+    return `fetch-depth ${settings.fetchDepth} is an explicit shallow depth; using upstream checkout`
+  }
   if (settings.filter) {
     return 'a fetch filter is configured'
   }
   if (settings.sparseCheckout) {
     return 'sparse checkout is configured'
   }
-  if (settings.lfs) {
-    return 'lfs is enabled (lfs objects are not in the mirror)'
-  }
+  // LFS intentionally does not skip — see the note above getMirrorCacheSkipReason.
   return null
 }
 
@@ -107,6 +116,15 @@ export async function setup(settings: IGitSourceSettings): Promise<MirrorMode> {
   return mode
 }
 
+// Called by getSource when a mirror-seeded fetch fails: clear our refs and disengage, so
+// the caller can retry the standard fetch against a clean repo. Also re-sets cache-hit to
+// false since we're no longer serving from cache.
+export async function abandon(settings: IGitSourceSettings): Promise<void> {
+  plan = {mode: 'off', repoKey: '', refKey: ''}
+  core.setOutput('cache-hit', 'false')
+  await cleanupWbRefs(settings.repositoryPath)
+}
+
 async function setupImpl(settings: IGitSourceSettings): Promise<MirrorMode> {
   plan = {mode: 'off', repoKey: '', refKey: ''}
   const skipReason = getMirrorCacheSkipReason(settings)
@@ -121,6 +139,9 @@ async function setupImpl(settings: IGitSourceSettings): Promise<MirrorMode> {
     core.warning(
       `WarpBuild mirror cache unavailable, using standard checkout: ${error}`
     )
+    // A partial seed may have left refs/wb/*; strip it so the stock fetch runs against a
+    // clean repo (objects just dangle harmlessly).
+    await cleanupWbRefs(settings.repositoryPath)
     return 'off'
   } finally {
     core.endGroup()
@@ -159,6 +180,14 @@ async function setupInner(settings: IGitSourceSettings): Promise<MirrorMode> {
 
   // Restore: seed the base, then this branch's delta if present.
   await seedBundle(settings.repositoryPath, lookup.base.url, BASE_REFNS)
+  // Full-history checkout: mirror the base's branches + tags into refs/remotes/origin/* +
+  // refs/tags/* so the resulting .git matches an upstream `fetch-depth: 0` checkout (the
+  // delta fetch then advances the target ref to its current tip). A shallow upstream fetch
+  // (depth 1) populates only the target ref, so there we keep the base internal and let the
+  // delta fetch provide just that ref.
+  if (settings.fetchDepth <= 0) {
+    await exposeBaseRefs(settings.repositoryPath)
+  }
   if (lookup.branch) {
     try {
       await seedBundle(settings.repositoryPath, lookup.branch.url, BRANCH_REFNS)
@@ -175,17 +204,40 @@ async function setupInner(settings: IGitSourceSettings): Promise<MirrorMode> {
   return 'seeded'
 }
 
-// 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.
+// Runs LAST in getSource — the checkout has already succeeded. Uploads the base (cold) or
+// this branch's refreshed delta (seeded). Hardened so it can NEVER fail the step: its own
+// try/catch for normal errors, plus scoped process guards (installed only for the upload
+// window) that turn an *uncaught* error from the git/upload work — e.g. a spawn error that
+// bypasses the promise chain, like the arg-length crash we hit on Windows — into a clean
+// exit(0). Safe because nothing checkout-relevant runs after this point.
 export async function contribute(settings: IGitSourceSettings): Promise<void> {
+  if (plan.mode !== 'cold-build' && plan.mode !== 'seeded') {
+    return
+  }
+  const guard = (error: unknown): void => {
+    core.warning(
+      `WarpBuild mirror upload error ignored — checkout already complete: ${error}`
+    )
+    process.exit(0)
+  }
+  process.on('uncaughtException', guard)
+  process.on('unhandledRejection', guard)
   try {
     if (plan.mode === 'cold-build') {
       await uploadBaseMirror(settings)
-    } else if (plan.mode === 'seeded') {
+    } else {
       await uploadBranchDelta(settings)
     }
   } catch (error) {
     core.warning(`WarpBuild mirror upload skipped: ${error}`)
+  } finally {
+    // Seeded mode created the internal refs/wb/* namespace; strip it so the customer's
+    // .git carries no mirror fingerprint (matches upstream; won't break push --mirror).
+    if (plan.mode === 'seeded') {
+      await cleanupWbRefs(settings.repositoryPath)
+    }
+    process.removeListener('uncaughtException', guard)
+    process.removeListener('unhandledRejection', guard)
   }
 }
 
@@ -214,6 +266,53 @@ async function seedBundle(
   }
 }
 
+// Copy the seeded base's branches + tags out to the namespaces upstream uses
+// (refs/remotes/origin/* + refs/tags/*), so a full-history checkout's .git matches
+// upstream. Runs before the delta fetch, while the target ref is still absent, so the
+// `create` directives don't collide. update-ref --stdin keeps it Windows arg-length safe.
+async function exposeBaseRefs(repoPath: string): Promise<void> {
+  let out = ''
+  await exec.exec(
+    'git',
+    [
+      '-C',
+      repoPath,
+      'for-each-ref',
+      '--format=create refs/%(refname:lstrip=3) %(objectname)',
+      `${BASE_REFNS}/remotes/origin`,
+      `${BASE_REFNS}/tags`
+    ],
+    {silent: true, listeners: {stdout: (d: Buffer) => (out += d.toString())}}
+  )
+  if (!out.trim()) {
+    return
+  }
+  await exec.exec('git', ['-C', repoPath, 'update-ref', '--stdin'], {
+    input: Buffer.from(out)
+  })
+}
+
+// Remove the internal refs/wb/* namespace. Objects stay (reachable from the real refs), so
+// the checkout is unaffected; only the mirror's bookkeeping refs go. Best-effort.
+async function cleanupWbRefs(repoPath: string): Promise<void> {
+  try {
+    let out = ''
+    await exec.exec(
+      'git',
+      ['-C', repoPath, 'for-each-ref', '--format=delete %(refname)', 'refs/wb'],
+      {silent: true, listeners: {stdout: (d: Buffer) => (out += d.toString())}}
+    )
+    if (!out.trim()) {
+      return
+    }
+    await exec.exec('git', ['-C', repoPath, 'update-ref', '--stdin'], {
+      input: Buffer.from(out)
+    })
+  } catch (error) {
+    core.debug(`WarpBuild ref cleanup skipped: ${error}`)
+  }
+}
+
 // 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.
@@ -363,7 +462,12 @@ async function httpPut(url: string, file: string): Promise<void> {
 }
 
 function tempBundlePath(tag: string): string {
-  return path.join(os.tmpdir(), `wb-${tag}-${process.pid}-${Date.now()}.bundle`)
+  // Unpredictable name so a bundle write can't be pre-empted by a symlink planted on a
+  // shared tmpdir.
+  return path.join(
+    os.tmpdir(),
+    `wb-${tag}-${crypto.randomBytes(12).toString('hex')}.bundle`
+  )
 }
 
 // Kept for callers/tests: reset .git/objects to the empty `git init` shape.