diff --git a/.gitattributes b/.gitattributes
index 16abb938292..04ffb4ef1a7 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -2,6 +2,7 @@
 # Set default behavior to automatically normalize line endings.
 ###############################################################################
 * text=auto
+spec/*.sh text eol=lf
 
 ###############################################################################
 # Set default behavior for command prompt diff.
@@ -66,4 +67,4 @@
 # Executable files - preserve execution permissions on Unix systems
 # (https://git-scm.com/docs/gitattributes#_executable)
 ###############################################################################
-runtime/*.exe binary
\ No newline at end of file
+runtime/*.exe binary
diff --git a/.github/workflows/buildtest.yml b/.github/workflows/buildtest.yml
index 6ee6ad43231..869f44a7deb 100644
--- a/.github/workflows/buildtest.yml
+++ b/.github/workflows/buildtest.yml
@@ -1,93 +1,96 @@
----
-name: Run Tests
+name: Run Build Diff Tests
 on:
   pull_request:
-    branches:
-      - dev
+    branches: [dev]
+  schedule:
+    - cron: '17 4 * * *'
   workflow_dispatch:
+    inputs:
+      mode:
+        description: Compare builds or prepare the shared baseline
+        type: choice
+        options: [compare, prepare]
+        default: compare
+      base_ref:
+        description: Base branch or commit
+        default: dev
+permissions:
+  contents: read
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: build-diff-${{ github.event_name == 'pull_request' && github.ref || 'baseline' }}
   cancel-in-progress: true
 jobs:
   run_build_diff:
     runs-on: ubuntu-latest
+    timeout-minutes: 20
+    env:
+      BASE_ONLY: ${{ (github.event_name == 'schedule' || inputs.mode == 'prepare') && '1' || '0' }}
     steps:
-      - name: Checkout HEAD
+      - name: Checkout test code
         uses: actions/checkout@v4
-      - name: Fetch Dev branch
-        id: get-dev-ref
-        run: |
-            git fetch --depth=1 origin dev
-            echo "devref=$(git rev-parse origin/dev)" >> $GITHUB_OUTPUT
-      - name: Download Dev branch cache
-        id: download-dev-ref-cache
-        uses: dawidd6/action-download-artifact@3ecf4024886f219d9290351234889bfb45d1b9da
-        with:
-          name: cache-devref-${{ steps.get-dev-ref.outputs.devref }}
-          path: /tmp/cache/
-          if_no_artifact_found: warn
-          search_artifacts: true
-      # Dev ref cache contains the build list and build xmls. Use that one to keep tests reproducible
-      - name: Update static builds list from cache
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'true' }}
-        run: cat /tmp/cache/builds.txt > spec/builds.txt
-      - name: Download latest build list
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' }}
-        id: download-build-list
-        uses: dawidd6/action-download-artifact@3ecf4024886f219d9290351234889bfb45d1b9da
-        with:
-          name: builds.txt
-          path: /tmp/latestbuildlist/
-          workflow: updatebuildlist.yml
-          if_no_artifact_found: warn
-          search_artifacts: true
-      - name: Update static builds list
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' && steps.download-build-list.outputs.found_artifact == 'true' }}
-        run: cat /tmp/latestbuildlist/builds.txt > spec/builds.txt
-      - name: Download latest build xmls
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' }}
-        uses: dawidd6/action-download-artifact@3ecf4024886f219d9290351234889bfb45d1b9da
         with:
-          name: build-xmls
-          path: /tmp/cache/
-          if_no_artifact_found: warn
-          search_artifacts: true
-      - name: Calculate build xmls and differences between them
+          persist-credentials: false
+      - name: Select base and download the corpus once
+        id: inputs
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.sha || inputs.base_ref || 'dev' }}
+        shell: bash
         run: |
-            mkdir /tmp/cache || true # Make sure /tmp/cache exists. Ignore exit code
-            chmod -R 777 /tmp/cache && docker compose run -v '/tmp/cache/:/cache' -e 'CACHEDIR=/cache' busted-diff | tee /tmp/dockerlog
-      - name: Generate artefact
-        run: |
-          sed -n '/Runtime comparison for/,/Savefile Diff for/{/Savefile Diff for/!p;}' /tmp/dockerlog > /tmp/artefact
-          sed -n '/Savefile Diff for/, $p' /tmp/dockerlog >> /tmp/artefact
-          [ -s /tmp/artefact ] || rm /tmp/artefact
-      - name: Upload artefact
-        uses: actions/upload-artifact@v4
+          git fetch --depth=1 origin "$BASE_REF"
+          export DEVREF=$(git rev-parse FETCH_HEAD)
+          source spec/BuildCache.sh
+          echo "base=$DEV_SHA" >> "$GITHUB_OUTPUT"
+          echo "key=$CACHE_KEY" >> "$GITHUB_OUTPUT"
+          echo "corpus=$CORPUS_FILE" >> "$GITHUB_OUTPUT"
+          echo "Baseline cache: $CACHE_KEY"
+      - name: Find or restore the calculated base
+        id: cache
+        uses: actions/cache/restore@v4
+        continue-on-error: true
         with:
-          name: build-diff-output
-          path: /tmp/artefact
-      - name: Save used build list into cache
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' }}
-        run: cp spec/builds.txt /tmp/cache/
-      - name: Move xmls found in builds.txt to a new directory
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' && steps.download-build-list.outputs.found_artifact == 'true' }}
+          key: ${{ steps.inputs.outputs.key }}
+          path: /tmp/pob-cache/${{ steps.inputs.outputs.key }}
+          lookup-only: ${{ env.BASE_ONLY == '1' }}
+      - name: Calculate builds
+        id: calculate
+        if: env.BASE_ONLY != '1' || steps.cache.outputs.cache-hit != 'true'
+        env:
+          DEVREF: ${{ steps.inputs.outputs.base }}
+          CORPUS_FILE: ${{ steps.inputs.outputs.corpus }}
+          CACHE_KEY: ${{ steps.inputs.outputs.key }}
+        shell: bash
         run: |
-          mkdir new-build-xmls
-          while IFS= read -r line; do
-              FILENAME="/tmp/cache/${line//[^a-zA-Z0-9]/}.xml"
-              if [ -f "$FILENAME" ]; then
-                  mv "$FILENAME" "./new-build-xmls/"
-              fi
-          done < "spec/builds.txt"
-      - name: Upload new build xmls
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' && steps.download-build-list.outputs.found_artifact == 'true' }}
-        uses: actions/upload-artifact@v4
+          mkdir -p /tmp/pob-cache
+          result=0
+          docker compose run --rm -v /tmp/pob-cache:/cache \
+            -v "$CORPUS_FILE:/corpus.json:ro" -e CORPUS_FILE=/corpus.json \
+            busted-diff | tee /tmp/dockerlog || result=$?
+          test -f "/tmp/pob-cache/$CACHE_KEY/$DEVREF"
+          echo "base_complete=true" >> "$GITHUB_OUTPUT"
+          exit "$result"
+      - name: Save the completed base
+        if: ${{ !cancelled() && steps.calculate.outputs.base_complete == 'true' && steps.cache.outputs.cache-hit != 'true' }}
+        uses: actions/cache/save@v4
+        continue-on-error: true
         with:
-          name: build-xmls
-          path: './new-build-xmls/*'
-      - name: Upload dev ref cache
-        if: ${{ steps.download-dev-ref-cache.outputs.found_artifact == 'false' }}
+          key: ${{ steps.inputs.outputs.key }}
+          path: /tmp/pob-cache/${{ steps.inputs.outputs.key }}
+      - name: Collect comparison output
+        if: always() && env.BASE_ONLY != '1' && steps.calculate.outcome != 'skipped'
+        shell: bash
+        run: |
+          if [ -f /tmp/dockerlog ]; then
+            sed -n '/^## /,$p' /tmp/dockerlog > /tmp/build-diff-output
+            [ -s /tmp/build-diff-output ] || rm -f /tmp/build-diff-output
+            sed -n '/^## Build comparison summary$/,$p' /tmp/dockerlog >> "$GITHUB_STEP_SUMMARY"
+          fi
+          if [ ! -s "$GITHUB_STEP_SUMMARY" ]; then
+            printf '## Build comparison summary\n\nCalculation or comparison did not finish. See the **Calculate builds** log for the error.\n' >> "$GITHUB_STEP_SUMMARY"
+          fi
+      - name: Upload comparison output
+        if: always() && env.BASE_ONLY != '1'
         uses: actions/upload-artifact@v4
         with:
-          name: cache-devref-${{ steps.get-dev-ref.outputs.devref }}
-          path: /tmp/cache/
+          name: build-diff-output
+          path: /tmp/build-diff-output
+          if-no-files-found: ignore
diff --git a/.github/workflows/updatebuildlist.yml b/.github/workflows/updatebuildlist.yml
deleted file mode 100644
index ea841152b01..00000000000
--- a/.github/workflows/updatebuildlist.yml
+++ /dev/null
@@ -1,37 +0,0 @@
----
-name: Update build list for tests
-on:
-  schedule:
-    - cron: '20 4 * * *'
-  workflow_dispatch:
-jobs:
-  update-builds-list:
-    runs-on: ubuntu-22.04
-    steps:
-      - name: Checkout HEAD
-        uses: actions/checkout@v4
-      - name: Install moreutils
-        run: sudo apt-get install -y moreutils
-      - name: Download latest build list
-        uses: dawidd6/action-download-artifact@3ecf4024886f219d9290351234889bfb45d1b9da
-        with:
-          name: builds.txt
-          if_no_artifact_found: warn
-          path: /tmp/latestbuildlist/
-      - name: Update list
-        run: >
-            cat spec/builds.txt /tmp/latestbuildlist/builds.txt
-            <({ curl "https://pobarchives.com/api/builds?q=latest" & curl "https://pobarchives.com/api/builds?q=trending"; }
-            | jq -r '.builds[].build_info.build_link')
-            | tail -n 500
-            | sort -u
-            | sponge builds.txt
-      - name: Print new builds list
-        run: cat builds.txt
-      - name: Save new build list
-        uses: actions/upload-artifact@v4
-        with:
-          name: builds.txt
-          path: builds.txt
-          overwrite: true
-          retention-days: 3
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ba990338634..a85475b9584 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -305,7 +305,7 @@ While both can be ran locally it's recommended to use the provided docker image
 PoB uses the [Busted](https://lunarmodules.github.io/busted/) framework to run its tests. Tests are stored under `spec/System` and run automatically when a PR is modified.
 More tests can be added to this folder to test specific functionality, or new test builds can be added to ensure nothing changed that wasn't intended. 
 
-### Running tests
+To run the build difference tests, first fetch `origin/dev`, then run `docker compose run --rm busted-diff`. Set `DEVREF` to compare against another branch or commit. The test uses the same PoB Codes builds and checked-in fixtures on both revisions, and reports stat, saved-XML and timing differences. Calculated-stat differences fail the check and appear in the GitHub run summary; the full report remains in the log and `build-diff-output` artifact. Base results are reused from the Docker `build-cache` volume when the base, corpus and test tools match; `docker compose down --volumes` clears that cache. GitHub Actions prepares the shared baseline nightly, while PRs calculate both sides themselves if it is unavailable.
 
 1. Install [Docker](https://www.docker.com/get-started)
 2. Run `docker-compose up` from the command line
diff --git a/docker-compose.yml b/docker-compose.yml
index b01cfb360ab..cc6e46fa4ff 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -1,9 +1,24 @@
 services:
   busted-tests:
     #build: .
-    image: ghcr.io/pathofbuildingcommunity/pathofbuilding-tests:latest
+    image: &test-image ghcr.io/pathofbuildingcommunity/pathofbuilding-tests@sha256:171dc3da232b8c874882e4ae3b3aa4a6e130a6c9450a31904b312435a6bf5daf
     environment:
       HOME: /tmp
+      LUA_PATH: "/usr/share/lua/5.1/?.lua;/usr/share/lua/5.1/?/init.lua;;"
+      LUA_CPATH: "/usr/lib/lua/5.1/?.so;;"
+    # The official image supplies LuaJIT; these are the existing script dependencies.
+    entrypoint: &test-entrypoint
+      - /bin/sh
+      - -ec
+      - |
+        apk add --no-cache git parallel libxml2-utils lua5.1-posix lua5.1-curl zlib-dev
+        luarocks install lua-zlib 1.4-0
+        if [ -n "$${CACHEDIR:-}" ]; then
+          mkdir -p "$$CACHEDIR"
+          chown -R nobody:nobody "$$CACHEDIR"
+        fi
+        exec su -p nobody -s /bin/sh -c "$$*"
+      - --
     container_name: pathofbuilding-tests
     command: busted --lua=luajit
     security_opt:
@@ -13,3 +28,27 @@ services:
     working_dir: /workdir
     volumes:
       - ./:/workdir:ro
+  busted-diff:
+    #build: .
+    image: *test-image
+    environment: #Where in the container the folders are stored
+      WORKDIR: /workdir
+      HOME: /tmp
+      CACHEDIR: /cache
+      BASE_ONLY: ${BASE_ONLY:-0}
+      BUILD_JOBS: ${BUILD_JOBS:-2}
+      DEVREF: ${DEVREF:-origin/dev}
+      HEADREF: ${HEADREF:-}
+      LUA_PATH: "/usr/share/lua/5.1/?.lua;/usr/share/lua/5.1/?/init.lua;;"
+      LUA_CPATH: "/usr/lib/lua/5.1/?.so;;"
+    container_name: busted-diff
+    entrypoint: *test-entrypoint
+    command: ["dos2unix < /workdir/spec/BuildDiff.sh | /bin/sh"]
+    security_opt:
+      - no-new-privileges:true
+    working_dir: /workdir
+    volumes:
+      - ./:/workdir:ro
+      - build-cache:/cache
+volumes:
+  build-cache:
diff --git a/spec/BuildCache.sh b/spec/BuildCache.sh
new file mode 100644
index 00000000000..f388b0ab657
--- /dev/null
+++ b/spec/BuildCache.sh
@@ -0,0 +1,14 @@
+# Shared by GitHub Actions and the local runner so cache identity is identical.
+DEV_SHA=$(git rev-parse "${DEVREF:-origin/dev}^{commit}")
+if [ -z "${CORPUS_FILE:-}" ]; then
+    CORPUS_FILE=$(mktemp)
+    curl --fail --show-error --silent --max-time 60 \
+        https://api.pob.codes/test-builds/corpus -o "$CORPUS_FILE"
+    chmod a+r "$CORPUS_FILE" # The container reads this public feed as nobody.
+fi
+corpus_hash=$(sha256sum "$CORPUS_FILE" | cut -d ' ' -f 1)
+test_hash=$(git ls-files -z -- .busted docker-compose.yml src/HeadlessWrapper.lua \
+    spec/BuildCache.sh spec/BuildDiff.sh spec/BuildStats.lua spec/FetchTestBuilds.lua \
+    spec/GenerateBuilds.lua spec/TestBuilds | xargs -0 git hash-object -- | sha256sum | cut -d ' ' -f 1)
+CACHE_KEY="pob-builds-v1-$DEV_SHA-$corpus_hash-$test_hash"
+export DEV_SHA CORPUS_FILE CACHE_KEY
diff --git a/spec/BuildDiff.sh b/spec/BuildDiff.sh
index 180960a11d1..640361e0533 100644
--- a/spec/BuildDiff.sh
+++ b/spec/BuildDiff.sh
@@ -1,73 +1,87 @@
 #!/bin/sh
+set -eo pipefail
 umask 0
 
-# If external cache dir has not been defined keep it inside the container
-if [[ -z "$CACHEDIR" ]]
-then
-    mkdir /tmp/cachedir
-    export CACHEDIR="/tmp/cachedir"
-fi
-
-# Copy mounted workdir to allow for changes during test run
-rm -rf /tmp/workdir && mkdir /tmp/workdir && cp -rf "$WORKDIR"/. /tmp/workdir/ && cd /tmp/workdir
-
+# Work on a copy: checking out the base must not change the user's checkout.
+rm -rf /tmp/workdir
+mkdir /tmp/workdir
+cp -rf "$WORKDIR"/. /tmp/workdir/
+cd /tmp/workdir
 git config --global --add safe.directory /tmp/workdir
 git config --global --add advice.detachedHead false
-
-if [[ ! -z "$HEADREF" ]]
-then
-    git diff --no-color "$HEADREF" -- /tmp/workdir/.busted /tmp/workdir/src/HeadlessWrapper.lua /tmp/workdir/spec/ > /tmp/HeadPatch &&
-    git reset --hard "$HEADREF" && git clean -fd && git apply --allow-empty /tmp/HeadPatch
+if [ -n "$HEADREF" ]; then
+    git diff --binary "$HEADREF" -- .busted src/HeadlessWrapper.lua spec/ > /tmp/HeadPatch
+    git reset --hard "$HEADREF"
+    git clean -fd
+    git apply --allow-empty --index /tmp/HeadPatch
 fi
-
 headsha=$(git rev-parse HEAD)
-devsha=$(git rev-parse "$DEVREF")
-
-rm -rf /tmp/headsha && mkdir /tmp/headsha
-rm /tmp/workdir/src/Settings.xml
-cat /tmp/workdir/spec/builds.txt | dos2unix | parallel --will-cite --ungroup --pipe -N50 'LINKSBATCH="$(mktemp){#}"; cat > $LINKSBATCH; BUILDLINKS="$LINKSBATCH" BUILDCACHEPREFIX="/tmp/headsha" busted --lua=luajit -r generate' && \
-BUILDCACHEPREFIX='/tmp/headsha' busted --lua=luajit -r generate && date > "/tmp/headsha/$headsha" && echo "[+] Build cache computed for $headsha (headsha)" || exit $?
 
-if [[ ! -f "$CACHEDIR/$devsha" ]] # Output of builds outdated or nonexistent
-then
-	rm -rf "$CACHEDIR"/*.build
-
-    # Keep new changes to tests related files
-    git diff --no-color "$DEVREF" -- /tmp/workdir/.busted /tmp/workdir/src/HeadlessWrapper.lua /tmp/workdir/spec/ > /tmp/DevPatch && \
-    git reset --hard "$DEVREF" && git clean -fd && git apply --allow-empty /tmp/DevPatch && \
-    cat /tmp/workdir/spec/builds.txt | dos2unix | parallel --will-cite --ungroup --pipe -N50 'LINKSBATCH="$(mktemp){#}"; cat > $LINKSBATCH; BUILDLINKS="$LINKSBATCH" BUILDCACHEPREFIX="$CACHEDIR" busted --lua=luajit -r generate' && \
-    BUILDCACHEPREFIX="$CACHEDIR" busted --lua=luajit -r generate && date > "$CACHEDIR/$devsha" && echo "[+] Build cache computed for $devsha (devsha)" || exit $?
+# The same response and test tools identify the inputs and calculated base.
+. ./spec/BuildCache.sh
+export CACHEDIR="${CACHEDIR:-/tmp/cachedir}/$CACHE_KEY"
+mkdir -p "$CACHEDIR"
+echo "[+] Baseline cache: $CACHE_KEY"
+if [ ! -f "$CACHEDIR/$DEV_SHA" ]; then
+    rm -f "$CACHEDIR"/*.build "$CACHEDIR"/*.time
+    cp "$CORPUS_FILE" "$CACHEDIR/corpus.json"
+    luajit spec/FetchTestBuilds.lua "$CACHEDIR"
 fi
+cp "$CACHEDIR/builds.txt" spec/builds.txt
 
-for runTime in "$CACHEDIR"/*.time
-do
-    BASENAME=$(basename "$runTime")
-
-    DIFFOUTPUT=$(luajit spec/DiffRuntime.lua "/tmp/headsha/$BASENAME" "$runTime" "$BASENAME") || {
-        echo "## Runtime comparison for $BASENAME"
-        echo '```'
-        echo "$DIFFOUTPUT"
-        echo '```'
-    }
-done
-
-for build in "$CACHEDIR"/*.build
-do
-    BASENAME=$(basename "$build")
+# Restart PoB after each batch of 50, then calculate the checked-in fixtures.
+calculate() {
+    export BUILDCACHEPREFIX="$1"
+    mkdir -p "$BUILDCACHEPREFIX"
+    cat spec/builds.txt | dos2unix | parallel --jobs "${BUILD_JOBS:-2}" --halt now,fail=1 --will-cite --ungroup --pipe -N50 \
+        'batch=$(mktemp); cat > "$batch"; BUILDLINKS="$batch" busted --lua=luajit -r generate'
+    busted --lua=luajit -r generate
+    expected=$(( $(wc -l < spec/builds.txt) + $(find spec/TestBuilds -maxdepth 1 -name '*.xml' | wc -l) ))
+    actual=$(find "$BUILDCACHEPREFIX" -maxdepth 1 -name '*.build' | wc -l)
+    [ "$actual" -eq "$expected" ] || { echo "Expected $expected saved builds; got $actual" >&2; exit 1; }
+    echo "[+] Calculated $actual builds into $BUILDCACHEPREFIX"
+}
 
-    # Only print the header if there is a diff to display
-    DIFFOUTPUT=$(diff <(xmllint --exc-c14n "$build") <(xmllint --exc-c14n "/tmp/headsha/$BASENAME")) || {
-        echo "## Savefile Diff for $BASENAME"
-        echo '```diff'
-        echo "$DIFFOUTPUT"
-        echo '```'
-    }
+# Normal PR/local runs calculate head. The nightly job only prepares the base.
+if [ "${BASE_ONLY:-0}" != 1 ]; then
+    rm -rf /tmp/headsha
+    rm -f src/Settings.xml
+    calculate /tmp/headsha
+fi
+if [ ! -f "$CACHEDIR/$DEV_SHA" ]; then
+    # Carry the same test harness across revisions, without copying game calculations.
+    git diff --binary "$DEV_SHA" -- .busted src/HeadlessWrapper.lua spec/ > /tmp/DevPatch
+    git reset --hard "$DEV_SHA"
+    git clean -fd
+    git apply --allow-empty --index /tmp/DevPatch
+    calculate "$CACHEDIR"
+    date > "$CACHEDIR/$DEV_SHA"
+    echo "[+] Base calculated: $DEV_SHA"
+else
+    echo "[+] Base reused: $DEV_SHA"
+fi
+[ "${BASE_ONLY:-0}" != 1 ] || exit 0
 
-    # Dedicated output diff
-    DIFFOUTPUT=$(luajit spec/DiffOutput.lua "/tmp/headsha/$BASENAME" "$build") || {
-        echo "## Output Diff for $BASENAME"
-        echo '```'
-        echo "$DIFFOUTPUT"
-        echo '```'
-    }
+# Keep the full report, collecting stat differences for the final summary/check.
+: > /tmp/build-stat-diffs
+report() {
+    title=$1; language=$2; shift 2
+    if output=$("$@"); then return; else status=$?; fi
+    [ "$status" -eq 1 ] && [ -n "$output" ] || return "$status"
+    printf '## %s\n```%s\n%s\n```\n' "$title" "$language" "$output"
+    case "$title" in
+        "Output Diff for "*) printf '## %s\n%s\n' "$title" "$output" >> /tmp/build-stat-diffs ;;
+    esac
+}
+compared=0
+for base in "$CACHEDIR"/*.build; do
+    name=$(basename "$base" .build)
+    report "Runtime comparison for $name.time" '' luajit spec/DiffRuntime.lua "/tmp/headsha/$name.time" "$CACHEDIR/$name.time" "$name.time"
+    xmllint --exc-c14n "$base" > /tmp/base.xml
+    xmllint --exc-c14n "/tmp/headsha/$name.build" > /tmp/head.xml
+    report "Savefile Diff for $name.build" diff diff /tmp/base.xml /tmp/head.xml
+    report "Output Diff for $name.build" '' luajit spec/DiffOutput.lua "/tmp/headsha/$name.build" "$base"
+    compared=$((compared + 1))
 done
+echo "[+] Compared $compared builds: $DEV_SHA -> $headsha"
+luajit spec/BuildSummary.lua /tmp/build-stat-diffs "$compared" "$DEV_SHA" "$headsha"
diff --git a/spec/BuildStats.lua b/spec/BuildStats.lua
new file mode 100644
index 00000000000..dbf8e70bbe0
--- /dev/null
+++ b/spec/BuildStats.lua
@@ -0,0 +1,33 @@
+-- Append calculated stats to the normal saved XML. Item/gem source references
+-- describe objects, not calculated stats, and can lead back into the object graph.
+return function(xml, output, element)
+	local stats, active = {}, {}
+	local function collect(values, prefix)
+		if active[values] or rawget(values, "Object") == values then return end
+		active[values] = true
+		for key, value in pairs(values) do
+			if type(key) == "string" or type(key) == "number" then
+				local name = prefix .. key
+				if type(value) == "table" then
+					if key ~= "sourceItem" and key ~= "sourceGem" and not (prefix == "" and key == "Minion") then
+						collect(value, name .. ".")
+					end
+				elseif type(value) == "number" or type(value) == "string" or type(value) == "boolean" then
+					stats[name] = tostring(value)
+				end
+			end
+		end
+		active[values] = nil
+	end
+	collect(output, "")
+	-- Preserve stats already emitted by PoB's normal save path.
+	for _, child in ipairs(xml) do
+		if child.elem == element then stats[child.attrib.stat] = nil end
+	end
+	local names = {}
+	for name in pairs(stats) do names[#names + 1] = name end
+	table.sort(names)
+	for _, name in ipairs(names) do
+		xml[#xml + 1] = { elem = element, attrib = { stat = name, value = stats[name] } }
+	end
+end
diff --git a/spec/BuildSummary.lua b/spec/BuildSummary.lua
new file mode 100644
index 00000000000..f45556cc13a
--- /dev/null
+++ b/spec/BuildSummary.lua
@@ -0,0 +1,67 @@
+-- Summarize the existing stat report without changing which differences count.
+local function summarize(lines, compared, baseRef, headRef)
+	local builds, stats, build = {}, {}, nil
+	local changedBuilds, changedValues = 0, 0
+	for line in lines do
+		build = line:match("^## Output Diff for (.+)%.build$") or build
+		local stat, actor = line:match("^(.-) Mismatch in (%a+) outputs:")
+		if stat then
+			assert(build, "Stat difference has no build name")
+			local head = assert(lines():match("^%s*head Output: (.*)$"), "Missing head value")
+			local base = assert(lines():match("^%s*dev Output: (.*)$"), "Missing base value")
+			if not builds[build] then
+				builds[build] = true
+				changedBuilds = changedBuilds + 1
+			end
+			local key = actor .. "." .. stat
+			local entry = stats[key] or { key = key, count = 0, build = build, base = base, head = head }
+			entry.count = entry.count + 1
+			stats[key] = entry
+			changedValues = changedValues + 1
+		end
+	end
+	local function escape(value)
+		return tostring(value):gsub("&", "&amp;"):gsub("<", "&lt;"):gsub(">", "&gt;"):gsub("|", "&#124;"):gsub("`", "&#96;")
+	end
+	local rows = {}
+	for _, entry in pairs(stats) do rows[#rows + 1] = entry end
+	-- Put common player results first; this affects presentation only.
+	local priority = { ["player.Life"] = 1, ["player.Mana"] = 2, ["player.EnergyShield"] = 3,
+		["player.TotalDPS"] = 4, ["player.CombinedDPS"] = 5, ["player.FullDPS"] = 6 }
+	table.sort(rows, function(a, b)
+		local ap, bp = priority[a.key] or 7, priority[b.key] or 7
+		if ap ~= bp then return ap < bp end
+		if a.count ~= b.count then return a.count > b.count end
+		return a.key < b.key
+	end)
+	local output = {
+		"## Build comparison summary", "",
+		changedBuilds > 0 and "**Changes found: review required.**" or "**No calculated-stat differences found.**", "",
+		string.format("Compared **%d builds**. **%d builds** have calculated-stat differences (**%d changed values**).", compared, changedBuilds, changedValues), "",
+		"Base: `" .. escape(baseRef) .. "`. PR: `" .. escape(headRef) .. "`.", "",
+	}
+	if #rows > 0 then
+		output[#output + 1] = string.format("Showing %d of %d changed stats, with one example for each. Common player results appear first; other stats are ordered by affected build count.", math.min(25, #rows), #rows)
+		output[#output + 1] = ""
+		output[#output + 1] = "| Stat | Affected builds | Example build | Before | After |"
+		output[#output + 1] = "| --- | ---: | --- | ---: | ---: |"
+		for index = 1, math.min(25, #rows) do
+			local entry = rows[index]
+			local name = entry.build
+			if #name == 64 and name:match("^%x+$") then name = name:sub(1, 12) end
+			output[#output + 1] = string.format("| %s | %d | %s | %s | %s |", escape(entry.key), entry.count, escape(name), escape(entry.base), escape(entry.head))
+		end
+		output[#output + 1] = ""
+	end
+	output[#output + 1] = "Calculated-stat differences fail this check and may be intentional or unintended. Saved-XML and timing differences remain informational."
+	output[#output + 1] = ""
+	output[#output + 1] = "All differences are in the calculation log and the **build-diff-output** artifact. API build identifiers above are shortened to 12 characters for searching the full report."
+	return table.concat(output, "\n"), changedBuilds
+end
+
+if arg and arg[0]:match("BuildSummary%.lua$") then
+	local output, changed = summarize(io.lines(assert(arg[1])), assert(tonumber(arg[2])), assert(arg[3]), assert(arg[4]))
+	print(output)
+	os.exit(changed > 0 and 1 or 0)
+end
+return summarize
diff --git a/spec/DiffOutput.lua b/spec/DiffOutput.lua
index 5f3228f21c0..949c36ba44e 100644
--- a/spec/DiffOutput.lua
+++ b/spec/DiffOutput.lua
@@ -8,11 +8,13 @@ local function buildOutputMap(filecontent)
 	local playerOutput = {}
     local minionOutput = {}
 	for line in splitLines(filecontent) do
-		local key, val = line:match('PlayerStat stat="(.-)" value="(.-)"')
+		local stat = line:match('<PlayerStat%s+([^>]+)')
+		local key, val = stat and stat:match('stat="(.-)"'), stat and stat:match('value="(.-)"')
         if key then
 		    playerOutput[key] = val
 		else
-			local key,val = line:match('MinionStat stat="(.-)" value="(.-)"')
+			local stat = line:match('<MinionStat%s+([^>]+)')
+			local key, val = stat and stat:match('stat="(.-)"'), stat and stat:match('value="(.-)"')
 			if key then
 				minionOutput[key] = val
 			end
diff --git a/spec/FetchTestBuilds.lua b/spec/FetchTestBuilds.lua
new file mode 100644
index 00000000000..82123f141ba
--- /dev/null
+++ b/spec/FetchTestBuilds.lua
@@ -0,0 +1,21 @@
+-- Store the API corpus alongside the calculated base cache, outside Git.
+package.path = "runtime/lua/?.lua;" .. package.path
+local json = require("dkjson")
+local base64 = require("base64")
+local zlib = require("zlib")
+local cache = assert(arg[1])
+local input = assert(io.open(cache .. "/corpus.json", "r"))
+local corpus = assert(json.decode(input:read("*a")))
+input:close()
+assert(corpus.schemaVersion == 2 and #corpus.builds > 0 and #corpus.builds == corpus.count, "Invalid build corpus")
+local list = assert(io.open(cache .. "/builds.txt", "w"))
+for _, entry in ipairs(corpus.builds) do
+    assert(#entry.sha256 == 64 and entry.sha256:match("^%x+$"), "Invalid build filename")
+    local xml = zlib.inflate()(base64.decode(entry.code:gsub("-", "+"):gsub("_", "/")))
+    local output = assert(io.open(cache .. "/" .. entry.sha256 .. ".xml", "w"))
+    output:write(xml)
+    output:close()
+    list:write(entry.sha256, "\n")
+end
+list:close()
+print("[+] Downloaded " .. corpus.count .. " builds from corpus " .. corpus.snapshotId)
diff --git a/spec/GenerateBuilds.lua b/spec/GenerateBuilds.lua
index 9ed6c9de924..2b4d76e03ef 100644
--- a/spec/GenerateBuilds.lua
+++ b/spec/GenerateBuilds.lua
@@ -1,89 +1,53 @@
-local function fetchBuilds(path)
-    local lastDLtime = GetTime()
-    local co = coroutine.create(function(path)
-        if os.getenv("BUILDLINKS") then
-            local fileHnd, errMsg = io.open(os.getenv("BUILDLINKS"), "r")
-            if not fileHnd then error(errMsg) end
-            local fileText = fileHnd:read("*a")
-            fileHnd:close()
-            for line in splitLines(fileText) do
-                if line ~= "" then
-                    for j = 1, #buildSites.websiteList do
-                        if line:match(buildSites.websiteList[j].matchURL) then
-                            local filename = line:gsub('%W', '')
-
-                            -- Load from cache if downloaded already
-                            local fileHnd = io.open( (os.getenv("CACHEDIR") or "/tmp") .. "/" .. filename .. ".xml", "r")
-                            if fileHnd then
-                                coroutine.yield({ xml = fileHnd:read("*a"), filename = filename, link = line })
-                                fileHnd:close()
-                            else
-                                -- Throttle build downloads to 15 per 10 seconds
-                                local timeSinceLastDL = GetTime() - lastDLtime
-                                if timeSinceLastDL < 666 then
-                                    posix.nanosleep(0, (666 - timeSinceLastDL) * 1000000)
-                                end
-                                buildSites.DownloadBuild(line, buildSites.websiteList[j], function(isSuccess, data)
-                                    lastDLtime = GetTime()
-                                    if isSuccess then
-                                        local xml = Inflate(common.base64.decode(data:gsub("-", "+"):gsub("_", "/")))
-                                        local xmlHnd = io.open((os.getenv("CACHEDIR") or "/tmp") .. "/" .. filename .. ".xml", "w")
-                                        xmlHnd:write(xml)
-                                        xmlHnd:close()
-                                        coroutine.yield({ xml = xml, filename = filename, link = line })
-                                    else
-                                        print("Failed to download build: " .. line)
-                                    end
-                                end)
-                            end
-                            break
-                        elseif j == #buildSites.websiteList then
-                            print("Failed to match provider for: " .. line)
-                        end
-                    end
-                end
-            end
-        else
-            for file in lfs.dir(path) do
-                if file ~= "." and file ~= ".." then
-                    local f = path .. '/' .. file
-                    local attr = lfs.attributes(f)
-                    assert(type(attr) == "table")
-                    if attr.mode ~= "directory" and file:match("^.+(%..+)$") == ".xml" then
-                        local fileHnd, errMsg = io.open(f, "r")
-                        if not fileHnd then error(errMsg) end
-                        local fileText = fileHnd:read("*a")
-                        fileHnd:close()
-                        coroutine.yield({ xml = fileText, filename = file })
-                    end
-                end
-            end
-        end
-    end)
-    return function()
-        local ok, result = coroutine.resume(co, path)
-        if not ok then error(result) end
-        return result
-    end
+-- Use the same test exporter on both revisions, including bases whose normal
+-- SaveDB does not support the old fullPlayerStat/fullMinionStat options.
+local appendStats = dofile("../spec/BuildStats.lua")
+local saveBuild = build.Save
+function build:Save(xml)
+	saveBuild(self, xml)
+	appendStats(xml, self.calcsTab.mainOutput, "PlayerStat")
+	if self.calcsTab.mainOutput.Minion then
+		appendStats(xml, self.calcsTab.mainOutput.Minion, "MinionStat")
+	end
 end
 
-for testBuild in fetchBuilds("../spec/TestBuilds") do
-    local filePath = (os.getenv("BUILDCACHEPREFIX") or "/tmp") .. "/" .. testBuild.filename
-    local startTime = GetTime()
-
-    -- Compute the build
-    print("[+] Computing " .. filePath)
-    loadBuildFromXML(testBuild.xml)
-    local calcDuration = GetTime() - startTime
-    print("[-] Computed " .. filePath .. " in " .. calcDuration .. "ms")
+-- The API adapter supplies local XML files. A missing file is a failed test,
+-- not a reason to try another download provider or silently skip the build.
+local inputs = {}
+if os.getenv("BUILDLINKS") then
+	local list = assert(io.open(os.getenv("BUILDLINKS"), "r"))
+	for name in list:lines() do
+		name = name:gsub("\r$", "")
+		assert(#name == 64 and name:match("^%x+$"), "Invalid corpus build name")
+		inputs[#inputs + 1] = { filename = name, path = assert(os.getenv("CACHEDIR")) .. "/" .. name .. ".xml" }
+	end
+	list:close()
+else
+	for name in lfs.dir("../spec/TestBuilds") do
+		if name:match("%.xml$") then
+			inputs[#inputs + 1] = { filename = name, path = "../spec/TestBuilds/" .. name }
+		end
+	end
+end
 
-    -- Save the computed build xml. Include full minion and player outputs.
-    local buildHnd = io.open(filePath .. ".build", "w+")
-    buildHnd:write(build:SaveDB("Cache", {fullPlayerStat = true, fullMinionStat = true} ))
-    buildHnd:close()
+for _, input in ipairs(inputs) do
+	local file = assert(io.open(input.path, "r"))
+	local xml = file:read("*a")
+	file:close()
+	local document, err = common.xml.ParseXML(xml)
+	assert(document and not err and document[1] and document[1].elem == "PathOfBuilding", "Invalid build XML: " .. input.filename)
+	local filePath = (os.getenv("BUILDCACHEPREFIX") or "/tmp") .. "/" .. input.filename
+	local startTime = GetTime()
+	print("[+] Computing " .. filePath)
+	loadBuildFromXML(xml)
+	assert(not build.abortSave and type(build.calcsTab.mainOutput.Life) == "number", "No calculated output: " .. input.filename)
+	local calcDuration = GetTime() - startTime
+	print("[-] Computed " .. filePath .. " in " .. calcDuration .. "ms")
 
-    -- Save the amount of time calculation of this build took
-    local timeHnd = io.open(filePath .. ".time", "w+")
-    timeHnd:write(calcDuration)
-    timeHnd:close()
+	local saved = assert(build:SaveDB("Cache"), "Could not save " .. input.filename)
+	local buildFile = assert(io.open(filePath .. ".build", "w"))
+	buildFile:write(saved)
+	buildFile:close()
+	local timeFile = assert(io.open(filePath .. ".time", "w"))
+	timeFile:write(calcDuration)
+	timeFile:close()
 end
diff --git a/spec/System/BuildStats_spec.lua b/spec/System/BuildStats_spec.lua
new file mode 100644
index 00000000000..2ad6464a7a1
--- /dev/null
+++ b/spec/System/BuildStats_spec.lua
@@ -0,0 +1,29 @@
+describe("Full build stat export", function()
+	local appendStats = dofile("../spec/BuildStats.lua")
+	it("keeps both nested stat paths and minion stats", function()
+		local xml = { { elem = "PlayerStat", attrib = { stat = "Life", value = "100" } } }
+		local output = { Life = 100, MainHand = { Damage = 10 }, OffHand = { Damage = 20 }, Minion = { Life = 50 } }
+		appendStats(xml, output, "PlayerStat")
+		appendStats(xml, output.Minion, "MinionStat")
+		assert.same({
+			{ elem = "PlayerStat", attrib = { stat = "Life", value = "100" } },
+			{ elem = "PlayerStat", attrib = { stat = "MainHand.Damage", value = "10" } },
+			{ elem = "PlayerStat", attrib = { stat = "OffHand.Damage", value = "20" } },
+			{ elem = "MinionStat", attrib = { stat = "Life", value = "50" } },
+		}, xml)
+	end)
+	it("avoids runtime object references and cycles without dropping shared stats", function()
+		local object = { internal = 99 }
+		object.Object = object
+		local shared = { Damage = 10 }
+		local output = { A = shared, B = shared, ObjectReference = object, Requirement = { value = 42, sourceItem = object, sourceGem = { internal = 99 } }, [object] = true }
+		output.Cycle = output
+		local xml = {}
+		appendStats(xml, output, "PlayerStat")
+		assert.same({
+			{ elem = "PlayerStat", attrib = { stat = "A.Damage", value = "10" } },
+			{ elem = "PlayerStat", attrib = { stat = "B.Damage", value = "10" } },
+			{ elem = "PlayerStat", attrib = { stat = "Requirement.value", value = "42" } },
+		}, xml)
+	end)
+end)
diff --git a/spec/System/BuildSummary_spec.lua b/spec/System/BuildSummary_spec.lua
new file mode 100644
index 00000000000..957f3fbd422
--- /dev/null
+++ b/spec/System/BuildSummary_spec.lua
@@ -0,0 +1,45 @@
+describe("Build comparison summary", function()
+	local summarize = dofile("../spec/BuildSummary.lua")
+	local function lines(text)
+		return (text .. "\n"):gmatch("([^\n]*)\n")
+	end
+	it("reports identical calculations without a mismatch", function()
+		local output, changed = summarize(lines(""), 5, "base", "head")
+		assert.equals(0, changed)
+		assert.matches("No calculated-stat differences found", output, 1, true)
+		assert.matches("Compared **5 builds**", output, 1, true)
+	end)
+	it("counts affected builds once and keeps player, minion and missing values", function()
+		local output, changed = summarize(lines([[## Output Diff for A.build
+Life Mismatch in player outputs:
+	head Output: 118
+	dev Output: 92
+Life Mismatch in minion outputs:
+	head Output: 20
+	dev Output: nil
+## Output Diff for B.build
+Life Mismatch in player outputs:
+	head Output: 200
+	dev Output: 100]]), 5, "base", "head")
+		assert.equals(2, changed)
+		assert.matches("3 changed values", output, 1, true)
+		assert.matches("| player.Life | 2 | A | 92 | 118 |", output, 1, true)
+		assert.matches("| minion.Life | 1 | A | nil | 20 |", output, 1, true)
+	end)
+	it("limits display rows without dropping mismatches and escapes table content", function()
+		local input = { "## Output Diff for A|<B>.build" }
+		for index = 1, 30 do
+			input[#input + 1] = string.format("Stat%d Mismatch in player outputs:\n\thead Output: 2\n\tdev Output: 1", index)
+		end
+		local output, changed = summarize(lines(table.concat(input, "\n")), 1, "base", "head")
+		assert.equals(1, changed)
+		assert.matches("Showing 25 of 30 changed stats", output, 1, true)
+		assert.matches("30 changed values", output, 1, true)
+		assert.matches("A&#124;&lt;B&gt;", output, 1, true)
+	end)
+	it("rejects a truncated difference instead of reporting a clean comparison", function()
+		assert.has_error(function()
+			summarize(lines("## Output Diff for A.build\nLife Mismatch in player outputs:\n"), 1, "base", "head")
+		end)
+	end)
+end)
diff --git a/spec/System/TestTradeQueryRequests_spec.lua b/spec/System/TestTradeQueryRequests_spec.lua
index f05068d8fa2..f57ce72d151 100644
--- a/spec/System/TestTradeQueryRequests_spec.lua
+++ b/spec/System/TestTradeQueryRequests_spec.lua
@@ -46,12 +46,10 @@ describe("TradeQueryRequests", function()
 		-- Pass: Dequeues and processes valid item
 		-- Fail: Queue unchanged, indicating timing/insertion bug, blocking trade searches
 		it("processes search queue item", function()
-			local orig_launch = launch
-			launch = {
-				DownloadPage = function(url, onComplete, opts)
-					onComplete({ body = "{}", header = "HTTP/1.1 200 OK" }, nil)
-				end
-			}
+			local download = stub(launch, "DownloadPage", function(self, url, onComplete, opts)
+				onComplete({ body = "{}", header = "HTTP/1.1 200 OK" }, nil)
+			end)
+			finally(function() download:revert() end)
 			table.insert(requests.requestQueue.search, {
 				url = "test",
 				callback = function() end,
@@ -63,7 +61,6 @@ describe("TradeQueryRequests", function()
 			mock_limiter.NextRequestTime = mock_next_time
 			requests:ProcessQueue()
 			assert.are.equal(#requests.requestQueue.search, 0)
-			launch = orig_launch
 		end)
 
 		-- Pass: Does not crash on 401, and passes error message
@@ -79,12 +76,10 @@ Server: cloudflare
 WWW-Authenticate: Bearer realm="pathofexile:production", error="invalid_token", error_description="The access token provided is invalid or has expired"
 Cache-Control: no-store
 Strict-Transport-Security: max-age=63115200; includeSubDomains; preload]]
-		local orig_launch = launch
-			launch = {
-				DownloadPage = function(url, onComplete, opts)
-					onComplete({ body = json, header = header }, nil)
-				end
-			}
+			local download = stub(launch, "DownloadPage", function(self, url, onComplete, opts)
+				onComplete({ body = json, header = header }, "Response code: 401")
+			end)
+			finally(function() download:revert() end)
 			table.insert(requests.requestQueue.search, {
 				url = "test",
 				callback = function(body, msg)
@@ -99,7 +94,6 @@ Strict-Transport-Security: max-age=63115200; includeSubDomains; preload]]
 			mock_limiter.NextRequestTime = mock_next_time
 			requests:ProcessQueue()
 			assert.are.equal(#requests.requestQueue.search, 0)
-			launch = orig_launch
 		end)
 
 		-- Pass: Retries with increasing backoff up to cap, preventing infinite loops
diff --git a/src/Export/spec.lua b/src/Export/spec.lua
index 4b947a5b528..696a2d8d2dc 100644
--- a/src/Export/spec.lua
+++ b/src/Export/spec.lua
@@ -13333,17 +13333,17 @@ return {
 		},
 		[3]={
 			list=false,
-			name="",
+			name="GameObjectRegisterKey",
 			refTo="",
 			type="Int",
-			width=80
+			width=170
 		},
 		[4]={
 			list=false,
-			name="",
-			refTo="",
+			name="SummonEffect",
+			refTo="MiscAnimated",
 			type="Key",
-			width=80
+			width=170
 		},
 		[5]={
 			list=false,
@@ -13382,15 +13382,15 @@ return {
 		},
 		[10]={
 			list=false,
-			name="",
-			refTo="",
+			name="BuffDefinition",
+			refTo="BuffDefinitions",
 			type="Key",
 			width=80
 		},
 		[11]={
 			list=false,
-			name="",
-			refTo="",
+			name="BuffVisual",
+			refTo="BuffVisuals",
 			type="Key",
 			width=80
 		},
diff --git a/src/HeadlessWrapper.lua b/src/HeadlessWrapper.lua
index 5f246c4a8e3..f35f04c3336 100644
--- a/src/HeadlessWrapper.lua
+++ b/src/HeadlessWrapper.lua
@@ -7,26 +7,6 @@
 -- bodies intended for headless use.
 dofile("_SimpleGraphic.def.lua")
 
--- Callbacks
-local callbackTable = { }
-local mainObject
-function runCallback(name, ...)
-	if callbackTable[name] then
-		return callbackTable[name](...)
-	elseif mainObject and mainObject[name] then
-		return mainObject[name](mainObject, ...)
-	end
-end
-function SetCallback(name, func)
-	callbackTable[name] = func
-end
-function GetCallback(name)
-	return callbackTable[name]
-end
-function SetMainObject(obj)
-	mainObject = obj
-end
-
 -- https://stackoverflow.com/questions/19326368/iterate-over-lines-including-blank-lines
 function splitLines(s)
 	if s:sub(-1)~="\n" then s=s.."\n" end
@@ -59,7 +39,6 @@ function GetVirtualScreenSize()
 	return 1920, 1080
 end
 
-<<<<<<< HEAD
 posix = require("posix")
 
 -- Search Handles
@@ -147,9 +126,6 @@ end
 
 dofile("Launch.lua")
 
--- The CI env var will be true when run from github workflows but should be false for other tools using the headless wrapper 
-__mainObject__.continuousIntegrationMode = os.getenv("CI")
-
 function launch:DownloadPage(url, callback, params)
 	params = params or {}
 	local responseHeader = ""
@@ -165,7 +141,7 @@ function launch:DownloadPage(url, callback, params)
 		easy:setopt(curl.OPT_HTTPHEADER, header)
 	end
 	easy:setopt_url(url)
-	easy:setopt(curl.OPT_USERAGENT, "Headless Path of Building" .. (mainObject.continuousIntegrationMode and " CI" or "") .. "/"..launch.versionNumber)
+	easy:setopt(curl.OPT_USERAGENT, "Headless Path of Building" .. "/"..launch.versionNumber)
 	easy:setopt(curl.OPT_ACCEPT_ENCODING, "")
 	if params.body then
 		easy:setopt(curl.OPT_POST, true)