diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 7640db8b..1bf303df 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -5,6 +5,7 @@ settings:
   excludeLinksFromLockfile: false
 
 overrides:
+  '@astrojs/sitemap': 3.6.1
   '@isaacs/brace-expansion@<=5.0.0': '>=5.0.1'
   ajv@<6.14.0: '>=6.14.0'
   devalue@<=5.6.2: '>=5.6.3'
@@ -485,8 +486,8 @@ packages:
     resolution: {integrity: sha512-q8VwfU/fDZNoDOf+r7jUnMC2//H2l0TuQ6FkGJL8vD8nw/q5KiL3DS1KKBI3QhI9UQhpJ5dc7AtqfbXWuOgLCQ==}
     engines: {node: 18.20.8 || ^20.3.0 || >=22.0.0}
 
-  '@astrojs/sitemap@3.7.2':
-    resolution: {integrity: sha512-PqkzkcZTb5ICiyIR8VoKbIAP/laNRXi5tw616N1Ckk+40oNB8Can1AzVV56lrbC5GKSZFCyJYUVYqVivMisvpA==}
+  '@astrojs/sitemap@3.6.1':
+    resolution: {integrity: sha512-+o+TbxXqQJAOd+HxCjz/5RdAMrRFGjeuO+U6zddUuTO59WqMqXnsc8uveRiEr2Ff+3McZiEne7iG4J5cnuI6kA==}
 
   '@astrojs/starlight@0.37.7':
     resolution: {integrity: sha512-KyBnou8aKIlPJUSNx6a1SN7XyH22oj/VAvTGC+Edld4Bnei1A//pmCRTBvSrSeoGrdUjK0ErFUfaEhhO1bPfDg==}
@@ -3620,6 +3621,9 @@ packages:
   '@types/nlcst@2.0.3':
     resolution: {integrity: sha512-vSYNSDe6Ix3q+6Z7ri9lyWqgGhJTmzRjZRqyq15N0Z/1/UnVsno9G/N40NBijoYx2seFDIl0+B2mgAb9mezUCA==}
 
+  '@types/node@17.0.45':
+    resolution: {integrity: sha512-w+tIMs3rq2afQdsPJlODhoUEKzFP1ayaoyl1CcnwtIlsVe7K7bA1NGm4s3PraqTLlXnbIN84zuBlxBWo1u9BLw==}
+
   '@types/node@22.19.17':
     resolution: {integrity: sha512-wGdMcf+vPYM6jikpS/qhg6WiqSV/OhG+jeeHT/KlVqxYfD40iYJf9/AE1uQxVWFvU7MipKRkRv8NSHiCGgPr8Q==}
 
@@ -9308,9 +9312,9 @@ packages:
   sisteransi@1.0.5:
     resolution: {integrity: sha512-bLGGlR1QxBcynn2d5YmDX4MGjlZvy2MRBDRNHLJ8VI6l6+9FUiyTFNJ0IveOSP0bcXgVDPRcfGqA0pjaqUpfVg==}
 
-  sitemap@9.0.1:
-    resolution: {integrity: sha512-S6hzjGJSG3d6if0YoF5kTyeRJvia6FSTBroE5fQ0bu1QNxyJqhhinfUsXi9fH3MgtXODWvwo2BDyQSnhPQ88uQ==}
-    engines: {node: '>=20.19.5', npm: '>=10.8.2'}
+  sitemap@8.0.3:
+    resolution: {integrity: sha512-9Ew1tR2WYw8RGE2XLy7GjkusvYXy8Rg6y8TYuBuQMfIEdGcWoJpY2Wr5DzsEiL/TKCw56+YKTCCUHglorEYK+A==}
+    engines: {node: '>=14.0.0', npm: '>=6.0.0'}
     hasBin: true
 
   slash@3.0.0:
@@ -10883,17 +10887,17 @@ snapshots:
     dependencies:
       prismjs: 1.30.0
 
-  '@astrojs/sitemap@3.7.2':
+  '@astrojs/sitemap@3.6.1':
     dependencies:
-      sitemap: 9.0.1
+      sitemap: 8.0.3
       stream-replace-string: 2.0.0
-      zod: 4.3.6
+      zod: 3.25.76
 
   '@astrojs/starlight@0.37.7(astro@5.18.1(@types/node@24.12.2)(db0@0.3.4(@electric-sql/pglite@0.4.1)(mysql2@3.15.3))(ioredis@5.10.1)(jiti@2.7.0)(lightningcss@1.32.0)(rollup@4.60.4)(sass@1.99.0)(terser@5.47.1)(typescript@5.9.3)(yaml@2.9.0))':
     dependencies:
       '@astrojs/markdown-remark': 6.3.11
       '@astrojs/mdx': 4.3.14(astro@5.18.1(@types/node@24.12.2)(db0@0.3.4(@electric-sql/pglite@0.4.1)(mysql2@3.15.3))(ioredis@5.10.1)(jiti@2.7.0)(lightningcss@1.32.0)(rollup@4.60.4)(sass@1.99.0)(terser@5.47.1)(typescript@5.9.3)(yaml@2.9.0))
-      '@astrojs/sitemap': 3.7.2
+      '@astrojs/sitemap': 3.6.1
       '@pagefind/default-ui': 1.5.2
       '@types/hast': 3.0.4
       '@types/js-yaml': 4.0.9
@@ -14328,6 +14332,8 @@ snapshots:
     dependencies:
       '@types/unist': 3.0.3
 
+  '@types/node@17.0.45': {}
+
   '@types/node@22.19.17':
     dependencies:
       undici-types: 6.21.0
@@ -21703,9 +21709,9 @@ snapshots:
 
   sisteransi@1.0.5: {}
 
-  sitemap@9.0.1:
+  sitemap@8.0.3:
     dependencies:
-      '@types/node': 24.12.2
+      '@types/node': 17.0.45
       '@types/sax': 1.2.7
       arg: 5.0.2
       sax: 1.6.0
diff --git a/pnpm-workspace.yaml b/pnpm-workspace.yaml
index 209ed10c..22a4a632 100644
--- a/pnpm-workspace.yaml
+++ b/pnpm-workspace.yaml
@@ -20,6 +20,12 @@ onlyBuiltDependencies:
   - protobufjs
 
 overrides:
+  # Pin @astrojs/sitemap to the last 3.6.x (zod v3) release. Starlight 0.37.7 pulls
+  # in @astrojs/sitemap@3.7.x which depends on zod v4, conflicting with Astro 5.18's
+  # zod v3 and crashing the docs build with "Cannot read properties of undefined
+  # (reading '_zod')". 3.6.1 still satisfies Starlight's "^3.3.0" range.
+  # See https://github.com/withastro/astro/issues/15854
+  "@astrojs/sitemap": "3.6.1"
   "@isaacs/brace-expansion@<=5.0.0": ">=5.0.1"
   ajv@<6.14.0: ">=6.14.0"
   devalue@<=5.6.2: ">=5.6.3"