diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f88664fb53..361a93d49a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -102,6 +102,11 @@ jobs:
       full: ${{ steps.paths.outputs.full }}
       # Code changed on a pull request that has no approved-for-ci label yet: only Quick compile.
       quick: ${{ steps.paths.outputs.quick }}
+      # An approved fork pull request whose approval is this very run: its heavy jobs run on the
+      # trusted self-hosted-fast runners, in private copies of the warm directories (fork-scratch.sh).
+      # Only the `labeled` event qualifies, so a later push to the fork's branch (unreviewed code)
+      # falls back to the fork routing above until a maintainer labels it again. `local` stays false.
+      fork_heavy: ${{ github.event_name == 'pull_request' && github.event.action == 'labeled' && github.event.label.name == 'approved-for-ci' && github.event.pull_request.head.repo.full_name != github.repository }}
       packages: ${{ steps.paths.outputs.packages }}
       oracle: ${{ steps.paths.outputs.oracle }}
       # Whether the other Linux jobs use the self-hosted runners too.
@@ -282,10 +287,13 @@ jobs:
     name: Native Linux Clippy
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ${{ needs.changes.outputs.local == 'true' && 'self-hosted-fast' || 'ubuntu-24.04' }}
+    runs-on: ${{ needs.changes.outputs.fork_heavy == 'true' && 'self-hosted-fast' || needs.changes.outputs.local == 'true' && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Private build, Cargo and rustup directories (approved fork pull request)
+        if: needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh clippy
       - name: Warm build directory (self-hosted runner)
         if: runner.environment == 'self-hosted' && !(github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository)
         run: scripts/ci/warm-target.sh clippy
@@ -308,12 +316,15 @@ jobs:
           scope=(--workspace)
           if [ "$FULL" != true ]; then scope=(); for p in $PACKAGES; do scope+=(-p "$p"); done; fi
           cargo clippy "${scope[@]}" --all-targets --profile ci --locked -- -D warnings
+      - name: Delete the private directories
+        if: always() && needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh --clean
 
   build-tests:
     name: Build tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ${{ (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository && (endsWith(format('{0}', github.event.pull_request.number), '7') || endsWith(format('{0}', github.event.pull_request.number), '8') || endsWith(format('{0}', github.event.pull_request.number), '9'))) && 'self-hosted-untrusted' || 'ubuntu-24.04' }}
+    runs-on: ${{ needs.changes.outputs.fork_heavy == 'true' && 'self-hosted-fast' || (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository && (endsWith(format('{0}', github.event.pull_request.number), '7') || endsWith(format('{0}', github.event.pull_request.number), '8') || endsWith(format('{0}', github.event.pull_request.number), '9'))) && 'self-hosted-untrusted' || 'ubuntu-24.04' }}
     # Fork builds compile inside gVisor on shared boxes; give them room to finish and upload.
     timeout-minutes: ${{ (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository && (endsWith(format('{0}', github.event.pull_request.number), '7') || endsWith(format('{0}', github.event.pull_request.number), '8') || endsWith(format('{0}', github.event.pull_request.number), '9'))) && 90 || (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository) && 60 || 45 }}
     env:
@@ -323,6 +334,9 @@ jobs:
       SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Private build, Cargo and rustup directories (approved fork pull request)
+        if: needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh
       - name: Size parallel compiles to the job's memory
         run: scripts/ci/build-jobs.sh
       - uses: dtolnay/rust-toolchain@stable
@@ -355,11 +369,14 @@ jobs:
       - name: Compiler cache statistics
         if: always()
         run: sccache --show-stats
+      - name: Delete the private directories
+        if: always() && needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh --clean
 
   tests:
     name: Native Linux tests (${{ matrix.shard }}/4)
     needs: [changes, build-tests]
-    runs-on: ${{ needs.changes.outputs.local == 'true' && 'self-hosted-fast' || ((github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository && (endsWith(format('{0}', github.event.pull_request.number), '7') || endsWith(format('{0}', github.event.pull_request.number), '8') || endsWith(format('{0}', github.event.pull_request.number), '9'))) && 'self-hosted-untrusted' || 'ubuntu-24.04') }}
+    runs-on: ${{ needs.changes.outputs.fork_heavy == 'true' && 'self-hosted-fast' || needs.changes.outputs.local == 'true' && 'self-hosted-fast' || ((github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository && (endsWith(format('{0}', github.event.pull_request.number), '7') || endsWith(format('{0}', github.event.pull_request.number), '8') || endsWith(format('{0}', github.event.pull_request.number), '9'))) && 'self-hosted-untrusted' || 'ubuntu-24.04') }}
     timeout-minutes: 60
     strategy:
       fail-fast: false
@@ -374,6 +391,9 @@ jobs:
       SOLIDCRAFT_REQUIRE_GPU: "1"
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Private build, Cargo and rustup directories (approved fork pull request)
+        if: needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh
       - name: Install a software Vulkan driver (lavapipe)
         # Preinstalled on the self-hosted runner, which has no sudo.
         if: runner.environment == 'github-hosted'
@@ -425,6 +445,9 @@ jobs:
           name: junit-${{ matrix.shard }}
           path: target/nextest/ci/junit.xml
           retention-days: 7
+      - name: Delete the private directories
+        if: always() && needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh --clean
 
   action-matrix:
     # Every ordinary action on every face, edge, vertex and body of the design corpus, against
@@ -522,7 +545,7 @@ jobs:
     name: Doc tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ${{ needs.changes.outputs.local == 'true' && 'self-hosted-fast' || 'ubuntu-24.04' }}
+    runs-on: ${{ needs.changes.outputs.fork_heavy == 'true' && 'self-hosted-fast' || needs.changes.outputs.local == 'true' && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 45
     env:
       CARGO_INCREMENTAL: "0"
@@ -531,6 +554,9 @@ jobs:
       SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Private build, Cargo and rustup directories (approved fork pull request)
+        if: needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh doc-tests
       - name: Size parallel compiles to the job's memory
         run: scripts/ci/build-jobs.sh
       - name: Warm build directory (self-hosted runner)
@@ -570,6 +596,9 @@ jobs:
             exit 0
           fi
           cargo test "${scope[@]}" --profile ci --locked --doc
+      - name: Delete the private directories
+        if: always() && needs.changes.outputs.fork_heavy == 'true'
+        run: scripts/ci/fork-scratch.sh --clean
 
   oracle:
     name: Reference tests
@@ -671,6 +700,9 @@ jobs:
 
   # Preserve the existing check name for branch protection, including docs-only PRs.
   ci:
+    # On the skipped run another label starts, this job is named differently, so that skipped job
+    # can never stand in for the required `ci` check of the pull request's real run.
+    name: ${{ github.event.action == 'labeled' && github.event.label.name != 'approved-for-ci' && 'ci (ignored label)' || 'ci' }}
     needs: [changes, checks, quick-compile, clippy, build-tests, tests, doc-tests, oracle, windows-fxc, windows-startup]
     if: always() && (github.event_name != 'pull_request' || !github.event.pull_request.draft) && (github.event.action != 'labeled' || github.event.label.name == 'approved-for-ci')
     runs-on: ${{ needs.changes.outputs.local == 'true' && 'self-hosted-light' || 'ubuntu-24.04' }}
diff --git a/scripts/ci/fork-scratch.sh b/scripts/ci/fork-scratch.sh
new file mode 100755
index 0000000000..a430d551e2
--- /dev/null
+++ b/scripts/ci/fork-scratch.sh
@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Self-hosted runner, approved fork pull requests only: give the job private copies of the build
+# directory, Cargo home and rustup home, so the job never writes to the shared warm directories
+# that main and merge-queue runs reuse (a build script or test from a fork could otherwise leave
+# files there). The copies start as the warm state (reflinks where the filesystem has them) and
+# `fork-scratch.sh --clean` deletes them at the end of the job.
+# Usage: fork-scratch.sh [NAME]   NAME = the warm-target.sh directory to start the build from
+#        fork-scratch.sh --clean
+# This keeps shared state out of reach of ordinary writes. It is not a sandbox: fork code still
+# runs as the runner's user, which is why the workflow starts it only from a `labeled` event a
+# maintainer triggered, never from a push.
+set -euo pipefail
+base=${RUNNER_TEMP:?}/fork-scratch
+if [ "${1:-}" = --clean ]; then
+  rm -rf "$base"
+  exit 0
+fi
+if [ "${GITHUB_EVENT_NAME:-}" != pull_request ]; then
+  echo "fork-scratch: only for pull request runs" >&2
+  exit 1
+fi
+name=${1:-}
+rm -rf "$base"
+mkdir -p "$base"
+if [ -n "$name" ]; then
+  warm=/home/runner/work/cargo-target/$name
+  if [ -d "$warm" ]; then
+    cp -a --reflink=auto "$warm" "$base/target"
+  else
+    mkdir -p "$base/target"
+  fi
+  echo "CARGO_TARGET_DIR=$base/target" >> "$GITHUB_ENV"
+fi
+cargo_home=${CARGO_HOME:-$HOME/.cargo}
+rustup_home=${RUSTUP_HOME:-$HOME/.rustup}
+mkdir -p "$base/cargo-home" "$base/rustup-home"
+[ -d "$cargo_home" ] && cp -a --reflink=auto "$cargo_home/." "$base/cargo-home/"
+[ -d "$rustup_home" ] && cp -a --reflink=auto "$rustup_home/." "$base/rustup-home/"
+{
+  echo "CARGO_HOME=$base/cargo-home"
+  echo "RUSTUP_HOME=$base/rustup-home"
+} >> "$GITHUB_ENV"
+echo "$base/cargo-home/bin" >> "$GITHUB_PATH"
+echo "fork-scratch: private copies in $base ($(du -sh "$base" | cut -f1))"