diff --git a/.agents/skills/jetpack-review-pr.md b/.agents/skills/jetpack-review-pr.md
index e56fe6a6fee..fe70d323007 100644
--- a/.agents/skills/jetpack-review-pr.md
+++ b/.agents/skills/jetpack-review-pr.md
@@ -127,6 +127,7 @@ Note: `AGENTS.md` is already loaded in your context via the CLAUDE.md `@AGENTS.m
 | Security patterns | yes | yes | yes |
 | Security threat model | no | if security files | yes |
 | Backward compat | yes | yes | yes |
+| Cross-package version skew (optional-sibling calls) | yes | yes | yes |
 | Error handling | no | yes | yes |
 | Feature gating | no | yes | yes |
 | HTML / a11y / RTL | no | if has_html/has_css | yes |
@@ -170,6 +171,26 @@ Note: `AGENTS.md` is already loaded in your context via the CLAUDE.md `@AGENTS.m
 - **Removed public constants/properties**: Fatal errors in consumers
 - Scan diff for removed lines with `public function`, `function jetpack_`, `do_action(`, `apply_filters(`, `register_rest_route`
 
+*Cross-package version skew — calls into optional sibling packages* (all depths — this catches fatal errors):
+
+A monorepo package may call into another package it does NOT list in its own `composer.json` `require` (e.g. my-jetpack → `SEO\Initializer`, `Search\*`, `VideoPress\*`). On trunk every package is at the same version, so the call resolves and CI passes. But standalone plugins (Social, Boost, VideoPress, Protect, …) each bundle their own `jetpack_vendor/` copies, and jetpack-autoloader loads ONE copy of each shared package across all active plugins — frequently an OLDER version than trunk. The symbol you call may not exist at runtime.
+
+- **`class_exists()` is NOT a sufficient guard** when you then call a method, read a constant, or access a property on that class. The class can load from an older bundled copy that predates the member → `Call to undefined method` fatal. Guard the EXACT symbol you use:
+  - method → `method_exists( $class, 'method' )` or `is_callable()`
+  - constant → `defined( "$class::CONST" )`
+  - function → `function_exists()`
+  - property → `property_exists()`
+- `[blocker]`: a newly-added call into a non-`require`d sibling package guarded only by `class_exists()` (or unguarded). The same PR often adds both the new symbol (in package A) and its caller (in package B), so the diff looks self-consistent — the skew only appears across release trains. Check the guard, not the definition; an internally-consistent diff does not prove the symbol ships together everywhere.
+- Detection:
+  ```bash
+  # Cross-package class references added in this diff:
+  gh pr diff <PR> | grep -nE '^\+.*\\Automattic\\Jetpack\\[A-Z][A-Za-z]+\\'
+  # For each referenced package, confirm it is in THIS project's composer.json "require":
+  grep '"automattic/jetpack-<pkg>"' projects/<type>/<name>/composer.json \
+    || echo "OPTIONAL sibling — any method/constant/property access needs a symbol-level guard"
+  ```
+- Precedent: SOCIAL-515 (`SEO\Initializer::is_optin_available()` fatal on Social 9.0.2 standalone), MYJP-308 (Search product fatal on plugins not bundling jetpack-search). Both: `class_exists()` passed, the method was absent from the bundled copy.
+
 *Feature gating* (standard + thorough):
 - New user-facing features (admin pages, blocks, endpoints) should be gated behind feature flags for staged rollout. Flag if ungated.
 
@@ -254,6 +275,8 @@ grep -r "@automattic/jetpack-<package-name>" projects/*/*/package.json
 
 Flag changes that could break consumers (changed signatures, removed methods, altered return types). List affected downstream projects.
 
+**Upstream direction too:** if the diff *adds* a call into a monorepo package this project does NOT `require`, that call is subject to version skew on standalone installs — apply the *Cross-package version skew* check from step 4.
+
 ### 6. Test and static analysis (thorough only)
 
 Identify the test runner for each project and **always report it**, even when skipped:
@@ -366,6 +389,7 @@ Review depth: **<depth>** (<lines> lines, <N> projects)
 ### Feature Gating
 ### Data / Privacy
 ### Backward Compatibility / Removed Public API
+### Cross-Package Version Skew
 ### Cross-Project Impact
 ### Test Results
 ### Test Coverage Gaps