diff --git a/.github/workflows/deploy.yml b/.github/workflows/deploy.yml
index 9ec04c6..60842ac 100644
--- a/.github/workflows/deploy.yml
+++ b/.github/workflows/deploy.yml
@@ -29,6 +29,9 @@ jobs:
           repository: jackyzha0/quartz
           ref: v5
           path: site
+      - name: Apply SPA basepath patch
+        working-directory: site
+        run: python ../publish/quartz-patches/apply.py .
       - name: Apply publish overlay
         run: cp publish/quartz.config.yaml site/quartz.config.yaml
       - uses: actions/setup-node@v6
diff --git a/publish/quartz-patches/apply.py b/publish/quartz-patches/apply.py
new file mode 100644
index 0000000..c2def24
--- /dev/null
+++ b/publish/quartz-patches/apply.py
@@ -0,0 +1,86 @@
+"""Patch quartz spa.inline.ts so relative URL clicks resolve from a directory base.
+
+Quartz's SPA click handler calls `new URL(href)`. When the URL bar is missing
+a trailing slash, the browser treats the last path segment as a file, so
+`../../../../tags/archive` from `/Threadwell/x/y/` (no slash) resolves to
+`/Threadwell/x/tags/archive` instead of `/Threadwell/tags/archive`. SPA routing
+loses the `/Threadwell/` base path on GitHub Pages project deployments.
+
+Fix: compute `SPA_BASE_HREF` from `window.location`, always ending with `/`,
+and resolve relative URLs against it.
+
+Idempotent: safe to re-run.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+TARGET = Path(
+    "quartz/components/scripts/spa.inline.ts"
+)
+
+INSERT_AFTER = (
+    "const isElement = (target: EventTarget | null): target is Element =>\n"
+    "  (target as Node)?.nodeType === NODE_TYPE_ELEMENT"
+)
+
+INSERT_BLOCK = (
+    "const SPA_BASE_HREF = (() => {\n"
+    "  if (typeof window === \"undefined\") return \"\"\n"
+    "  const { origin, pathname } = window.location\n"
+    "  if (pathname.endsWith(\"/\")) return origin + pathname\n"
+    "  const last = pathname.split(\"/\").pop() ?? \"\"\n"
+    "  if (last.includes(\".\")) return origin + pathname\n"
+    "  return origin + pathname.replace(/[^/]*$/, \"\")\n"
+    "})()\n\n"
+    "const isElement"
+)
+
+REPLACE_GETOPTS = (
+    "  const { href } = a\n"
+    "  if (!isLocalUrl(href)) return\n"
+    "  return { url: new URL(href), scroll: "
+    "\"routerNoscroll\" in a.dataset ? false : undefined }\n"
+    "}"
+)
+
+REPLACE_WITH = (
+    "  const { href } = a\n"
+    "  if (!isLocalUrl(href)) return\n"
+    "  const url = SPA_BASE_HREF ? new URL(href, SPA_BASE_HREF) : new URL(href)\n"
+    "  return { url, scroll: "
+    "\"routerNoscroll\" in a.dataset ? false : undefined }\n"
+    "}"
+)
+
+
+def patch(path: Path) -> bool:
+    if not path.exists():
+        print(f"missing {path}")
+        return False
+    text = path.read_text(encoding="utf-8")
+    if "SPA_BASE_HREF" in text:
+        print(f"already patched {path}")
+        return True
+    if INSERT_AFTER not in text:
+        print(f"anchor missing in {path}")
+        return False
+    if REPLACE_GETOPTS not in text:
+        print(f"getOpts block missing in {path}")
+        return False
+    text = text.replace(INSERT_AFTER, INSERT_BLOCK, 1)
+    text = text.replace(REPLACE_GETOPTS, REPLACE_WITH, 1)
+    path.write_text(text, encoding="utf-8")
+    print(f"patched {path}")
+    return True
+
+
+def main() -> int:
+    import sys
+
+    base = Path(sys.argv[1]) if len(sys.argv) > 1 else Path(".")
+    return 0 if patch(base / TARGET) else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())