diff --git a/.github/workflows/build_check_cache.yml b/.github/workflows/build_check_cache.yml
index 31fdd6cb28..46244dcb23 100644
--- a/.github/workflows/build_check_cache.yml
+++ b/.github/workflows/build_check_cache.yml
@@ -20,19 +20,37 @@ on:
         required: false
         type: boolean
   
+# Read by scripts/ci_deps_cache.sh.
+env:
+  REPO: ${{ github.repository }}
+  WORKFLOW_REF: ${{ github.workflow_ref }}
+  BASE_REF: ${{ github.base_ref }}
+
 jobs:
   check_cache: # determines if there is a cache and outputs variables used in caching process
     name: Check Cache
-    runs-on: ${{ inputs.os }}
+    # Hosted macOS runners are scarce, so their lookup runs on Linux.
+    runs-on: ${{ startsWith(inputs.os, 'macos-') && 'ubuntu-24.04' || inputs.os }}
     outputs:
       cache-key: ${{ steps.set_outputs.outputs.cache-key }}
       cache-path: ${{ steps.set_outputs.outputs.cache-path }}
       valid-cache: ${{ steps.cache_deps.outputs.cache-hit }}
+      macos-path: ${{ steps.macos_path.outputs.path }}
+      key-inputs: ${{ steps.wait_check.outputs.key-inputs }}
+      wait-for: ${{ steps.wait_check.outputs.wait-for }}
+      wait-since: ${{ steps.wait_check.outputs.wait-since }}
     steps:
       - name: Checkout
         uses: actions/checkout@v7
         with:
           lfs: 'false'
+          # The wait check compares the merge commit with its first parent.
+          fetch-depth: ${{ startsWith(inputs.os, 'macos-') && 2 || 1 }}
+          # Fetches only deps/, which the key hashes, and the wait script.
+          sparse-checkout: |
+            /deps/
+            /scripts/ci_deps_cache.sh
+          sparse-checkout-cone-mode: false
         
       - name: set outputs
         id: set_outputs
@@ -40,31 +58,89 @@ jobs:
           # Anything that changes how the tree is built belongs in the key, or a job
           # restores one it cannot use. Linux amd64 passes no arch deliberately, so
           # 'linux-clang' keeps the cache it already has.
-          cache-os: ${{ runner.os == 'macOS' && format('{0}-{1}', inputs.os, inputs.arch) || (runner.os == 'Windows' && format('windows-{0}-{1}', inputs.arch, inputs.compiler) || format('linux-clang{0}', inputs.arch && format('-{0}', inputs.arch) || '')) }}
+          cache-os: ${{ (runner.os == 'macOS' || startsWith(inputs.os, 'macos-')) && format('{0}-{1}', inputs.os, inputs.arch) || (runner.os == 'Windows' && format('windows-{0}-{1}', inputs.arch, inputs.compiler) || format('linux-clang{0}', inputs.arch && format('-{0}', inputs.arch) || '')) }}
           # The Windows ARM64 deps build in build-arm64, all others under build;
           # build_deps.yml and build_orca.yml pass the Windows directory to build_win.bat.
-          dep-folder-name: ${{ runner.os == 'macOS' && format('/{0}', inputs.arch) || (runner.os == 'Windows' && inputs.arch == 'arm64') && '-arm64/OrcaSlicer_dep' || '/OrcaSlicer_dep' }}
+          dep-folder-name: ${{ (runner.os == 'macOS' || startsWith(inputs.os, 'macos-')) && format('/{0}', inputs.arch) || (runner.os == 'Windows' && inputs.arch == 'arm64') && '-arm64/OrcaSlicer_dep' || '/OrcaSlicer_dep' }}
           output-cmd: ${{ runner.os == 'Windows' && '$env:GITHUB_OUTPUT' || '"$GITHUB_OUTPUT"'}}
         run: |
           echo cache-key=${{ env.cache-os }}-cache-orcaslicer_deps-build-${{ hashFiles('deps/**') }} >> ${{ env.output-cmd }}
-          echo cache-path=${{ github.workspace }}/deps/build${{ env.dep-folder-name }} >> ${{ env.output-cmd }}
+          # Relative to the workspace, which differs from the build's when this job runs on Linux.
+          echo cache-path=deps/build${{ env.dep-folder-name }} >> ${{ env.output-cmd }}
             
+      # actions/cache matches the path too, so a Linux lookup for macOS uses the
+      # workspace of a hosted macOS job.
+      - name: macOS cache path
+        id: macos_path
+        if: ${{ startsWith(inputs.os, 'macos-') }}
+        run: echo "path=/Users/runner/work/${GITHUB_REPOSITORY#*/}/${GITHUB_REPOSITORY#*/}/${{ steps.set_outputs.outputs.cache-path }}" >> "$GITHUB_OUTPUT"
+
       - name: load cache
         id: cache_deps
         uses: actions/cache@v6
         with:
-          path: ${{ steps.set_outputs.outputs.cache-path }}
+          path: ${{ steps.macos_path.outputs.path || format('{0}/{1}', github.workspace, steps.set_outputs.outputs.cache-path) }}
           key: ${{ steps.set_outputs.outputs.cache-key }}
           lookup-only: true
         
+      # A pull request that misses the cache can wait for a base branch build of the same deps.
+      - name: check for a base branch build to wait for
+        id: wait_check
+        if: ${{ startsWith(inputs.os, 'macos-') && github.event_name == 'pull_request' && steps.cache_deps.outputs.cache-hit != 'true' }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: bash scripts/ci_deps_cache.sh check
+
+  wait_for_deps:
+    name: Wait for base branch deps
+    needs: [check_cache]
+    if: ${{ needs.check_cache.outputs.wait-for != '' }}
+    runs-on: ubuntu-24.04
+    env:
+      WAIT_MINUTES: 120
+    # Must exceed WAIT_MINUTES.
+    timeout-minutes: 130
+    # Waiting runs queue here, and only the first one holds a runner.
+    concurrency:
+      group: deps-wait-${{ github.base_ref }}-${{ needs.check_cache.outputs.cache-key }}
+      queue: max
+    outputs:
+      hit: ${{ steps.lookup.outputs.cache-hit }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+        with:
+          sparse-checkout: scripts/ci_deps_cache.sh
+          sparse-checkout-cone-mode: false
+
+      - name: wait for the deps cache
+        env:
+          GH_TOKEN: ${{ github.token }}
+          KEY: ${{ needs.check_cache.outputs.cache-key }}
+          KEY_INPUTS: ${{ needs.check_cache.outputs.key-inputs }}
+          WAIT_FOR: ${{ needs.check_cache.outputs.wait-for }}
+          WAIT_SINCE: ${{ needs.check_cache.outputs.wait-since }}
+          ARCH: ${{ inputs.arch }}
+        run: bash scripts/ci_deps_cache.sh wait
+
+      - name: load cache
+        id: lookup
+        uses: actions/cache@v6
+        with:
+          path: ${{ needs.check_cache.outputs.macos-path }}
+          key: ${{ needs.check_cache.outputs.cache-key }}
+          lookup-only: true
+
   build_deps: # call next step
     name: Build Deps
-    needs: [check_cache]
+    needs: [check_cache, wait_for_deps]
+    # A failed wait counts as no cache, so the deps are built here.
+    if: ${{ !cancelled() && needs.check_cache.result == 'success' }}
     uses: ./.github/workflows/build_deps.yml
     with:
       cache-key: ${{ needs.check_cache.outputs.cache-key }}
       cache-path: ${{ needs.check_cache.outputs.cache-path }}
-      valid-cache: ${{ needs.check_cache.outputs.valid-cache == 'true' }}
+      valid-cache: ${{ needs.check_cache.outputs.valid-cache == 'true' || needs.wait_for_deps.outputs.hit == 'true' }}
       os: ${{ inputs.os }}
       arch: ${{ inputs.arch }}
       compiler: ${{ inputs.compiler }}
diff --git a/.github/workflows/build_deps.yml b/.github/workflows/build_deps.yml
index f1cc9a9723..3c778d03e6 100644
--- a/.github/workflows/build_deps.yml
+++ b/.github/workflows/build_deps.yml
@@ -45,7 +45,7 @@ jobs:
       - name: load cached deps
         uses: actions/cache@v6
         with:
-          path: ${{ inputs.cache-path }}
+          path: ${{ github.workspace }}/${{ inputs.cache-path }}
           key: ${{ inputs.cache-key }}
 
       - uses: lukka/get-cmake@latest
@@ -122,7 +122,7 @@ jobs:
               choco install strawberryperl
             }
             # cache-path is the install directory inside the deps build directory.
-            $deps = (Split-Path "${{ inputs.cache-path }}").Replace('\', '/')
+            $deps = (Split-Path "${{ github.workspace }}/${{ inputs.cache-path }}").Replace('\', '/')
             # -l compiles with Visual Studio's clang-cl and -x builds with Ninja; --msvc --msbuild is cl under the Visual Studio generator.
             $flags = if ("${{ inputs.compiler }}" -eq "clang") { "-l", "-x" } else { "--msvc", "--msbuild" }
             .\build_win.bat -d --arch ${{ inputs.arch }} --deps-dir $deps @flags
diff --git a/.github/workflows/build_orca.yml b/.github/workflows/build_orca.yml
index 5797681d99..32aa726f5b 100644
--- a/.github/workflows/build_orca.yml
+++ b/.github/workflows/build_orca.yml
@@ -44,7 +44,7 @@ jobs:
         if: ${{ !(runner.os == 'macOS' && inputs.macos-combine-only) }}
         uses: actions/cache@v6
         with:
-          path: ${{ inputs.cache-path }}
+          path: ${{ github.workspace }}/${{ inputs.cache-path }}
           key: ${{ inputs.cache-key }}
           fail-on-cache-miss: true
 
@@ -460,7 +460,7 @@ jobs:
         # --tests builds the unit tests too; the unit_tests_windows_* jobs run them.
         run: |
           # cache-path is the install directory inside the deps build directory.
-          $deps = (Split-Path "${{ inputs.cache-path }}").Replace('\', '/')
+          $deps = (Split-Path "${{ github.workspace }}/${{ inputs.cache-path }}").Replace('\', '/')
           # -l compiles with Visual Studio's clang-cl and -x builds with Ninja; --msvc --msbuild is cl under the Visual Studio generator.
           $flags = if ("${{ inputs.compiler }}" -eq "clang") { "-l", "-x" } else { "--msvc", "--msbuild" }
           .\build_win.bat -s --tests -i --arch ${{ inputs.arch }} --build-dir $env:BUILD_DIR --deps-dir $deps @flags
diff --git a/scripts/ci_deps_cache.sh b/scripts/ci_deps_cache.sh
new file mode 100644
index 0000000000..a27b7e2522
--- /dev/null
+++ b/scripts/ci_deps_cache.sh
@@ -0,0 +1,128 @@
+#!/usr/bin/env bash
+# Lets a pull request's macOS deps build wait for a push build of the base
+# branch that builds the same deps, from a Linux job. `check` runs after a cache
+# miss and names a queued or running base build with the same deps/ and .github/
+# to wait for, unless the pull request changes either. `wait` waits until that
+# build saves KEY. actions/cache then confirms the entry can be restored.
+#
+# Environment: GH_TOKEN, REPO, WORKFLOW_REF and BASE_REF, plus KEY_INPUTS,
+# WAIT_FOR, WAIT_SINCE, WAIT_MINUTES, KEY and ARCH for `wait`.
+# Writes key-inputs, wait-for and wait-since to $GITHUB_OUTPUT.
+set -uo pipefail
+
+workflow=${WORKFLOW_REF%%@*}
+workflow=${workflow##*/}
+
+# Must match the job names in build_all.yml, build_check_cache.yml and build_deps.yml.
+check_job_name="build_macos_arch (${ARCH:-}) / Check Cache"
+deps_job_name="build_macos_arch (${ARCH:-}) / Build Deps / Build Deps"
+
+out() { echo "$1" >> "${GITHUB_OUTPUT:-/dev/stdout}"; }
+
+# The cache key depends on deps/ and on the workflows under .github/.
+local_key_inputs() { echo "$(git rev-parse "$1:.github") $(git rev-parse "$1:deps")"; }
+remote_key_inputs() {
+  gh api "repos/$REPO/git/trees/$1" < /dev/null \
+    --jq '[.tree[] | select(.path == ".github" or .path == "deps") | {key: .path, value: .sha}] | from_entries | "\(.[".github"]) \(.deps)"'
+}
+
+# Prints the id of a push build of the base branch that has not finished, is not
+# one of the ids in $1, and has KEY_INPUTS.
+matching_base_build() {
+  local status runs id sha
+  runs=$(for status in in_progress queued pending waiting requested; do
+    gh api -X GET "repos/$REPO/actions/workflows/$workflow/runs" \
+      -f branch="$BASE_REF" -f event=push -f status="$status" -f per_page=10 < /dev/null \
+      | jq -r '.workflow_runs[] | "\(.id) \(.head_sha)"'
+  done)
+  while read -r id sha; do
+    [ -n "$id" ] || continue
+    [[ " ${1:-} " == *" $id "* ]] && continue
+    if [ "$(remote_key_inputs "$sha")" = "$KEY_INPUTS" ]; then
+      echo "$id"
+      return
+    fi
+  done <<< "$runs"
+}
+
+check() {
+  KEY_INPUTS=$(local_key_inputs HEAD)
+  out "key-inputs=$KEY_INPUTS"
+  if [ "$KEY_INPUTS" != "$(local_key_inputs HEAD^1)" ]; then
+    echo "This pull request changes deps/ or .github/, so it builds its own deps."
+    return
+  fi
+  local id
+  id=$(matching_base_build)
+  if [ -n "$id" ]; then
+    echo "Build $id of $BASE_REF has the same deps/ and .github/. Waiting for its cache."
+    out "wait-for=$id"
+    out "wait-since=$(date +%s)"
+  else
+    echo "No build of $BASE_REF is building these deps."
+  fi
+}
+
+key_saved() {
+  gh cache list -R "$REPO" --ref "refs/heads/$BASE_REF" --key "$KEY" --json key < /dev/null \
+    | jq -e --arg key "$KEY" 'any(.[]; .key == $key)' > /dev/null
+}
+
+wait_for() {
+  # Counts the limit from the check, including time queued behind other waiting
+  # pull requests.
+  local id=$WAIT_FOR seen=$WAIT_FOR deadline=$((${WAIT_SINCE:-$(date +%s)} + WAIT_MINUTES * 60)) jobs job check warned=
+  while [ "$(date +%s)" -lt "$deadline" ]; do
+    if key_saved; then
+      echo "Found $KEY."
+      return
+    fi
+    jobs=$(gh api -X GET "repos/$REPO/actions/runs/$id/jobs" -f filter=latest -f per_page=100 < /dev/null)
+    job=$(jq -r --arg name "$deps_job_name" 'first(.jobs[] | select(.name == $name) | "\(.status) \(.conclusion)") // ""' <<< "$jobs")
+    case "$job" in
+      "completed success" | "completed skipped")
+        # The new entry can take a moment to be listed, and a skipped job means
+        # the build found the cache, possibly under the default branch.
+        for _ in 1 2 3; do
+          key_saved && { echo "Found $KEY."; return; }
+          sleep 20
+        done
+        echo "Build $id of $BASE_REF finished its deps without saving $KEY."
+        return ;;
+      "completed cancelled") ;;
+      completed*)
+        echo "The deps job of build $id of $BASE_REF did not succeed."
+        return ;;
+      # The deps job exists only once the build's cache check has succeeded.
+      "")
+        case "$(gh api "repos/$REPO/actions/runs/$id" --jq '"\(.status) \(.conclusion)"' < /dev/null)" in
+          "completed cancelled") ;;
+          completed*)
+            check=$(jq -r --arg name "$check_job_name" 'first(.jobs[] | select(.name == $name) | .conclusion) // ""' <<< "$jobs")
+            if [ -z "$warned" ] && { [ -z "$check" ] || [ "$check" = success ]; }; then
+              echo "::warning title=Deps wait::Build $id of $BASE_REF has no job named \"$deps_job_name\". If the build jobs were renamed, update scripts/ci_deps_cache.sh. This does not affect this pull request."
+              warned=1
+            fi ;;
+          *)
+            sleep 120
+            continue ;;
+        esac ;;
+      *)
+        sleep 120
+        continue ;;
+    esac
+    # Follows another unfinished build with the same key inputs, such as the
+    # push that replaced a cancelled one.
+    id=$(matching_base_build "$seen")
+    [ -n "$id" ] || break
+    seen="$seen $id"
+    echo "Following build $id of $BASE_REF."
+  done
+  echo "No build of $BASE_REF saved $KEY."
+}
+
+case "${1:-}" in
+  check) check ;;
+  wait) wait_for ;;
+  *) echo "usage: $0 check|wait" >&2; exit 2 ;;
+esac