diff --git a/.claude/rules/cross-platform.md b/.claude/rules/cross-platform.md
index f9f0c47ec0..f316b158fe 100644
--- a/.claude/rules/cross-platform.md
+++ b/.claude/rules/cross-platform.md
@@ -21,6 +21,22 @@ When the gated code uses no platform-specific API and only *running* it is platf
 
 When gated code changes, update the comment that explains why the gate is needed. A stale explanation can lead someone to widen or copy an unnecessary gate.
 
+## Verify before landing
+
+Windows is not in CI, so the person (or agent) writing platform-specific code is the only line of defense. `cargo check` and `cargo clippy` do not link, so you can type-check the Windows build from any OS without an MSVC toolchain:
+
+```bash
+rustup target add x86_64-pc-windows-msvc   # once
+cargo clippy -p <crate> --all-targets --target x86_64-pc-windows-msvc -- -D warnings
+```
+
+`--all-targets` matters: the breakage is usually in tests (bd-j0pgiips, bd-lhi82470). This catches `std::os::unix` misuse (E0433/E0599) and, under `-D warnings`, cfg-gated dead code. Run it for every crate you touched; run the same command without `--target` for the host platform.
+
+Caveats:
+
+- Crates whose build scripts compile C for the target (e.g. `ring`, `v8`) may not cross-check from unix. If a crate you touched can't cross-check, say so in the PR so a Windows user can run the native check: `cargo clippy -p <crate> --all-targets -- -D warnings`.
+- Cross-checking proves compilation only. Runtime behavior (file locking, line endings, process signals) still needs a Windows run — flag tests that exercise those for manual verification.
+
 ## File paths
 
 - Use `std::path::Path`/`PathBuf`, never hardcode `/` or `\` separators
diff --git a/AGENTS.md b/AGENTS.md
index 330303038f..957a9161a6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -422,7 +422,7 @@ The changelog is rendered in the About section of the hub-client UI.
 - **CRITICAL**: If you'll be writing tests, read the special instructions on file claude-notes/instructions/testing.md
 - **CRITICAL**: For hub-client changes, passing tests alone is NOT sufficient. You must also verify that `npm run build:all` (from `hub-client/`) succeeds before claiming work is done. The production build (`tsc -b && vite build`) is stricter than `tsc --noEmit` and `vitest` — it uses project references mode and catches errors the other tools miss.
 - **CRITICAL**: For any CLI- or user-visible feature, passing tests alone is NOT sufficient. See **End-to-end verification before declaring success** below.
-- **Windows**: Some crates must be manually excluded from tests. See claude-notes/instructions/windows-dev.md for details.
+- **Windows**: Windows is not in CI. When your change uses platform-specific APIs (`std::os::unix`/`std::os::windows`), adds `#[cfg(...)]` gates, or touches test helpers, cross-check before landing: `cargo clippy -p <crate> --all-targets --target x86_64-pc-windows-msvc -- -D warnings` (works from any OS; no linking). See `.claude/rules/cross-platform.md` and claude-notes/instructions/windows-dev.md.
 
 ## End-to-end verification before declaring success
 
diff --git a/claude-notes/instructions/windows-dev.md b/claude-notes/instructions/windows-dev.md
index a6c32fc7ae..4e638945ae 100644
--- a/claude-notes/instructions/windows-dev.md
+++ b/claude-notes/instructions/windows-dev.md
@@ -36,3 +36,16 @@ significantly.
 ## CI
 
 Windows is not currently in the CI matrix. All Windows testing is manual.
+
+Until a Windows leg exists, the pre-landing gate is a cross-check, which works
+from any OS because `cargo check`/`cargo clippy` do not link:
+
+```bash
+rustup target add x86_64-pc-windows-msvc   # once
+cargo clippy -p <crate> --all-targets --target x86_64-pc-windows-msvc -- -D warnings
+```
+
+Run it for every crate you touch when your change uses `std::os::unix` /
+`std::os::windows`, adds `#[cfg(...)]` gates, or touches test helpers. The full
+rules and patterns are in `.claude/rules/cross-platform.md`; failure history:
+bd-j0pgiips, bd-lhi82470, bd-5tdzaf3t.
diff --git a/crates/quarto-mcp-launcher/src/lib.rs b/crates/quarto-mcp-launcher/src/lib.rs
index 1d2e59fffd..8acdf956de 100644
--- a/crates/quarto-mcp-launcher/src/lib.rs
+++ b/crates/quarto-mcp-launcher/src/lib.rs
@@ -504,10 +504,26 @@ mod tests {
         );
     }
 
+    /// Building a non-UTF-8 `OsString` is platform-specific, so the
+    /// fixture is a `cfg` pair and the test itself stays portable
+    /// (`.claude/rules/cross-platform.md`; bd-j0pgiips).
+    #[cfg(unix)]
+    fn non_utf8_os_string() -> std::ffi::OsString {
+        use std::os::unix::ffi::OsStringExt;
+        std::ffi::OsString::from_vec(vec![0xff, 0xfe])
+    }
+
+    /// A lone surrogate is ill-formed UTF-16, so `to_str()` returns
+    /// `None` for it just as it does for non-UTF-8 bytes on unix.
+    #[cfg(windows)]
+    fn non_utf8_os_string() -> std::ffi::OsString {
+        use std::os::windows::ffi::OsStringExt;
+        std::ffi::OsString::from_wide(&[0xD800])
+    }
+
     #[test]
     fn q2_path_injection_skips_non_utf8_exe_paths() {
-        use std::os::unix::ffi::OsStringExt;
-        let weird = std::ffi::OsString::from_vec(vec![0xff, 0xfe]);
+        let weird = non_utf8_os_string();
         assert_eq!(q2_path_for_child(None, Some(PathBuf::from(weird))), None);
     }
 }