diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 56d4517..3cb6928 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,6 +41,20 @@ jobs:
           npm run manifest
       - name: Run verification suite
         run: npm run check
+      - name: Verify Pages deployment portability
+        shell: bash
+        run: |
+          set -euo pipefail
+          TMP_DIR="$(mktemp -d)"
+          trap 'rm -rf "$TMP_DIR"' EXIT
+          python3 scripts/build_pages.py \
+            --source site \
+            --output "$TMP_DIR/current"
+          diff -qr site "$TMP_DIR/current"
+          python3 scripts/build_pages.py \
+            --source site \
+            --output "$TMP_DIR/portable" \
+            --public-url "https://portable.example/frontier-decision-engine/"
       - name: Run required CodeQL analysis
         uses: github/codeql-action/analyze@2892aa5e19bbd11bc0cff5427e3b750a04d9e3c2 # v4.38.2
         with:
diff --git a/.github/workflows/pages.yml b/.github/workflows/pages.yml
index b27e573..443c4d7 100644
--- a/.github/workflows/pages.yml
+++ b/.github/workflows/pages.yml
@@ -57,10 +57,19 @@ jobs:
             exit 1
           fi
           echo "Released site payload unchanged for $TAG."
+      - name: Configure Pages
+        id: pages
+        uses: actions/configure-pages@45bfe0192ca1faeb007ade9deae92b16b8254a0d # v6.0.0
+      - name: Build deployment artifact
+        run: |
+          python3 scripts/build_pages.py \
+            --source site \
+            --output _site \
+            --public-url "${{ steps.pages.outputs.base_url }}"
       - name: Upload static site
         uses: actions/upload-pages-artifact@fc324d3547104276b827a68afc52ff2a11cc49c9 # v5.0.0
         with:
-          path: site
+          path: _site
 
   deploy:
     needs: build
@@ -76,8 +85,6 @@ jobs:
     outputs:
       page_url: ${{ steps.deployment.outputs.page_url }}
     steps:
-      - name: Configure Pages
-        uses: actions/configure-pages@45bfe0192ca1faeb007ade9deae92b16b8254a0d # v6.0.0
       - name: Deploy
         id: deployment
         uses: actions/deploy-pages@368f82528645a54fb793d4d04e342629a3f51346 # v5.0.1
diff --git a/scripts/build_pages.py b/scripts/build_pages.py
new file mode 100644
index 0000000..480aea3
--- /dev/null
+++ b/scripts/build_pages.py
@@ -0,0 +1,179 @@
+#!/usr/bin/env python3
+from __future__ import annotations
+
+import argparse
+import hashlib
+import json
+import re
+import shutil
+from pathlib import Path
+from urllib.parse import urlparse
+
+MANIFEST_NAME = "manifest.sha256"
+OG_URL = re.compile(r'(<meta property="og:url" content=")([^"]+)(">)')
+CANONICAL = re.compile(r'(<link rel="canonical" href=")([^"]+)(">)')
+ALLOWED_CHANGED_FILES = {"index.html", "start.html", MANIFEST_NAME}
+
+
+def normalize_public_url(value: str) -> str:
+    candidate = str(value).strip()
+    parsed = urlparse(candidate)
+    if (
+        parsed.scheme != "https"
+        or not parsed.hostname
+        or parsed.username
+        or parsed.password
+        or parsed.query
+        or parsed.fragment
+    ):
+        raise ValueError(
+            "public URL must be an absolute HTTPS URL without credentials, query, or fragment"
+        )
+    return candidate.rstrip("/") + "/"
+
+
+def extract_one(text: str, pattern: re.Pattern[str], label: str) -> str:
+    matches = list(pattern.finditer(text))
+    if len(matches) != 1:
+        raise RuntimeError(
+            f"{label} field count changed: expected 1, observed {len(matches)}"
+        )
+    return normalize_public_url(matches[0].group(2))
+
+
+def replace_one(text: str, pattern: re.Pattern[str], public_url: str, label: str) -> str:
+    matches = list(pattern.finditer(text))
+    if len(matches) != 1:
+        raise RuntimeError(
+            f"{label} field count changed: expected 1, observed {len(matches)}"
+        )
+    return pattern.sub(
+        lambda match: f"{match.group(1)}{public_url}{match.group(3)}",
+        text,
+        count=1,
+    )
+
+
+def manifest_text(root: Path) -> str:
+    rows = []
+    for path in sorted(
+        p for p in root.rglob("*") if p.is_file() and p.name != MANIFEST_NAME
+    ):
+        relative = path.relative_to(root).as_posix()
+        rows.append(f"{hashlib.sha256(path.read_bytes()).hexdigest()}  {relative}")
+    return "\n".join(rows) + "\n"
+
+
+def file_map(root: Path) -> dict[str, bytes]:
+    return {
+        path.relative_to(root).as_posix(): path.read_bytes()
+        for path in sorted(p for p in root.rglob("*") if p.is_file())
+    }
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--source", type=Path, default=Path("site"))
+    parser.add_argument("--output", type=Path, default=Path("_site"))
+    parser.add_argument("--public-url", default=None)
+    args = parser.parse_args()
+
+    source = args.source.resolve()
+    output = args.output.resolve()
+    if not source.is_dir():
+        raise SystemExit(f"STOP - source directory not found: {source}")
+    if source == output or source in output.parents or output in source.parents:
+        raise SystemExit("STOP - source and output directories must not contain each other")
+
+    source_index = (source / "index.html").read_text(encoding="utf-8")
+    source_start = (source / "start.html").read_text(encoding="utf-8")
+    source_urls = [
+        extract_one(source_index, OG_URL, "index og:url"),
+        extract_one(source_index, CANONICAL, "index canonical"),
+        extract_one(source_start, CANONICAL, "start canonical"),
+    ]
+    if len(set(source_urls)) != 1:
+        raise SystemExit("STOP - source self URL fields disagree")
+
+    source_public_url = source_urls[0]
+    try:
+        target_public_url = normalize_public_url(args.public_url or source_public_url)
+    except ValueError as exc:
+        raise SystemExit(f"STOP - {exc}") from exc
+
+    source_files = file_map(source)
+
+    if output.exists():
+        shutil.rmtree(output)
+    shutil.copytree(source, output)
+
+    output_index = (output / "index.html").read_text(encoding="utf-8")
+    output_index = replace_one(
+        output_index, OG_URL, target_public_url, "index og:url"
+    )
+    output_index = replace_one(
+        output_index, CANONICAL, target_public_url, "index canonical"
+    )
+    (output / "index.html").write_text(
+        output_index, encoding="utf-8", newline="\n"
+    )
+
+    output_start = (output / "start.html").read_text(encoding="utf-8")
+    output_start = replace_one(
+        output_start, CANONICAL, target_public_url, "start canonical"
+    )
+    (output / "start.html").write_text(
+        output_start, encoding="utf-8", newline="\n"
+    )
+
+    observed_urls = [
+        extract_one(output_index, OG_URL, "output index og:url"),
+        extract_one(output_index, CANONICAL, "output index canonical"),
+        extract_one(output_start, CANONICAL, "output start canonical"),
+    ]
+    if set(observed_urls) != {target_public_url}:
+        raise SystemExit("STOP - generated deployment self URLs disagree")
+
+    (output / MANIFEST_NAME).write_text(
+        manifest_text(output), encoding="utf-8", newline="\n"
+    )
+
+    output_files = file_map(output)
+    if set(source_files) != set(output_files):
+        raise SystemExit("STOP - deployment artifact file set differs from source")
+
+    changed = {
+        path for path in source_files if source_files[path] != output_files[path]
+    }
+    expected_changed = (
+        set()
+        if target_public_url == source_public_url
+        else ALLOWED_CHANGED_FILES
+    )
+    if changed != expected_changed:
+        raise SystemExit(
+            "STOP - deployment artifact changed outside the envelope allowlist: "
+            + json.dumps(
+                {
+                    "expected": sorted(expected_changed),
+                    "observed": sorted(changed),
+                }
+            )
+        )
+
+    print(
+        json.dumps(
+            {
+                "source_public_url": source_public_url,
+                "target_public_url": target_public_url,
+                "changed_files": sorted(changed),
+                "status": "PASS",
+            },
+            sort_keys=True,
+        )
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())