diff --git a/src/simulator/api-gdext/src/lib.rs b/src/simulator/api-gdext/src/lib.rs
index f1328462..8a1a9923 100644
--- a/src/simulator/api-gdext/src/lib.rs
+++ b/src/simulator/api-gdext/src/lib.rs
@@ -14,6 +14,7 @@ pub mod building_action;
 pub mod building_registry;
 pub mod capture;
 pub mod civics;
+mod mod_host;
 pub mod observation;
 pub mod player_api;
 pub mod replay;
@@ -3826,6 +3827,125 @@ impl GdGameState {
         true
     }
 
+    /// Stage 5d — load and register a single mod from its
+    /// `manifest.json` path. Used by the GDScript `ModLoader` autoload
+    /// at engine boot to walk the user mod directory and surface each
+    /// installed AI controller in [`registered_controller_ids`].
+    ///
+    /// `manifest_path` must be an absolute filesystem path (NOT a
+    /// `res://` URI — the user mod dir lives outside the project tree).
+    /// The payload (`.wasm` or platform-native shared library) is
+    /// resolved relative to the manifest via `manifest.entrypoint`,
+    /// defaulting to `controller.wasm` / `controller.so` / `.dylib` /
+    /// `.dll`.
+    ///
+    /// Returns the registered `controller_id` on success, or an empty
+    /// [`GString`] on any failure (manifest unreadable, parse error,
+    /// validation error, payload missing, sandbox load failure, …).
+    /// Errors are logged once via [`godot_error!`]; the caller's GDScript
+    /// side decides whether to surface a user-visible warning.
+    ///
+    /// `&self` because the registry mutation lives behind the global
+    /// `RwLock` in `mc_player_api::controllers` — this method does not
+    /// touch `GdGameState`'s own state.
+    #[func]
+    fn register_mod_from_path(&self, manifest_path: GString) -> GString {
+        let path_str = manifest_path.to_string();
+        let manifest_path = std::path::PathBuf::from(&path_str);
+
+        // Read + parse + validate manifest.
+        let manifest_bytes = match std::fs::read(&manifest_path) {
+            Ok(b) => b,
+            Err(e) => {
+                godot_error!(
+                    "ModLoader: cannot read manifest {path_str:?}: {e}"
+                );
+                return GString::new();
+            }
+        };
+        let manifest = match mc_mod_host::Manifest::parse(&manifest_bytes) {
+            Ok(m) => m,
+            Err(e) => {
+                godot_error!(
+                    "ModLoader: failed to parse {path_str:?}: {e}"
+                );
+                return GString::new();
+            }
+        };
+        if let Err(e) = manifest.validate() {
+            godot_error!("ModLoader: invalid manifest {path_str:?}: {e}");
+            return GString::new();
+        }
+
+        match manifest.sandbox {
+            mc_mod_host::SandboxKindWire::Wasm => {
+                let manifest_dir = match manifest_path.parent() {
+                    Some(d) => d,
+                    None => {
+                        godot_error!(
+                            "ModLoader: manifest path {path_str:?} has no parent dir"
+                        );
+                        return GString::new();
+                    }
+                };
+                let entry = manifest
+                    .entrypoint
+                    .as_deref()
+                    .unwrap_or("controller.wasm");
+                let wasm_path = manifest_dir.join(entry);
+                let wasm_bytes = match std::fs::read(&wasm_path) {
+                    Ok(b) => b,
+                    Err(e) => {
+                        godot_error!(
+                            "ModLoader: cannot read wasm payload {:?}: {e}",
+                            wasm_path
+                        );
+                        return GString::new();
+                    }
+                };
+                let host = match crate::mod_host::shared_host() {
+                    Ok(h) => h,
+                    Err(e) => {
+                        godot_error!(
+                            "ModLoader: shared WasmHost init failed: {e}"
+                        );
+                        return GString::new();
+                    }
+                };
+                match mc_mod_host::register_wasm_mod(host, &wasm_bytes) {
+                    Ok(id) => {
+                        godot_print!(
+                            "ModLoader: registered '{id}' from {path_str}"
+                        );
+                        GString::from(id)
+                    }
+                    Err(e) => {
+                        godot_error!(
+                            "ModLoader: register_wasm_mod failed for {path_str:?}: {e}"
+                        );
+                        GString::new()
+                    }
+                }
+            }
+            mc_mod_host::SandboxKindWire::Native => {
+                match mc_mod_host::register_native_mod(&manifest_path) {
+                    Ok(id) => {
+                        godot_print!(
+                            "ModLoader: registered '{id}' from {path_str}"
+                        );
+                        GString::from(id)
+                    }
+                    Err(e) => {
+                        godot_error!(
+                            "ModLoader: register_native_mod failed for {path_str:?}: {e}"
+                        );
+                        GString::new()
+                    }
+                }
+            }
+        }
+    }
+
     /// Stage 7 — after `load_from_json` succeeds, GDScript calls this
     /// to ask: "are all the controllers this save references currently
     /// registered with `mc_player_api`?" Returns an empty `GString` on
diff --git a/src/simulator/api-gdext/src/mod_host.rs b/src/simulator/api-gdext/src/mod_host.rs
new file mode 100644
index 00000000..9f080a12
--- /dev/null
+++ b/src/simulator/api-gdext/src/mod_host.rs
@@ -0,0 +1,46 @@
+//! Process-wide singleton `Arc<WasmHost>` for Stage 5d mod loading.
+//!
+//! `mc_mod_host::WasmHost::new()` spawns a 1 Hz daemon thread that drives
+//! `wasmtime::Engine::increment_epoch` so per-call deadlines fire on
+//! schedule. The thread is intentionally a process-lifetime leak — fine
+//! for the one-shot singleton this module exposes, but constructing
+//! multiple hosts would accumulate threads. We therefore stash one host
+//! behind a [`OnceLock`] and hand out `Arc` clones.
+//!
+//! Failures during initial construction (e.g. wasmtime can't realise the
+//! configured flag set on this build) propagate to the caller; on
+//! success every subsequent call returns the cached host without
+//! re-initialising the engine or the ticker.
+//!
+//! Wired into the gdext bridge by `GdGameState::register_mod_from_path`.
+
+use std::sync::{Arc, OnceLock};
+
+use mc_mod_host::{ModError, WasmHost};
+
+static SHARED_HOST: OnceLock<Arc<WasmHost>> = OnceLock::new();
+
+/// Returns the process-wide [`WasmHost`], lazily constructing it on the
+/// first call.
+///
+/// # Errors
+/// Propagates [`ModError::EngineConfig`] from the first
+/// [`WasmHost::new`] call. Subsequent calls cannot fail — once the host
+/// is cached they hand out clones unconditionally. The cache is *not*
+/// populated on the failing first call, so a second attempt will retry
+/// construction (useful when a transient resource constraint blocked
+/// the daemon-thread spawn).
+pub(crate) fn shared_host() -> Result<Arc<WasmHost>, ModError> {
+    if let Some(existing) = SHARED_HOST.get() {
+        return Ok(Arc::clone(existing));
+    }
+    let host = Arc::new(WasmHost::new()?);
+    // `set` returns Err if another thread populated the cell between our
+    // `get` and `set`. In that race we discard our freshly-built host
+    // (its daemon thread leaks for the process lifetime, but the cell
+    // races are limited to engine boot) and return the winner.
+    match SHARED_HOST.set(Arc::clone(&host)) {
+        Ok(()) => Ok(host),
+        Err(_) => Ok(Arc::clone(SHARED_HOST.get().expect("just set"))),
+    }
+}