diff --git a/crates/core/src/client/message_handlers_v1.rs b/crates/core/src/client/message_handlers_v1.rs
index 0ab284b2da1..039f4e4900c 100644
--- a/crates/core/src/client/message_handlers_v1.rs
+++ b/crates/core/src/client/message_handlers_v1.rs
@@ -49,7 +49,7 @@ pub async fn handle(client: &ClientConnection, message: DataMessage, timer: Inst
             res.map_err(|e| {
                 (
                     Some(reducer),
-                    mod_info.module_def.reducer_full(&**reducer).map(|(id, _)| id),
+                    mod_info.module_def.reducer_by_name(&**reducer).map(|(id, _)| id),
                     e.into(),
                 )
             })
diff --git a/crates/core/src/host/module_host.rs b/crates/core/src/host/module_host.rs
index 553c7ff685c..4a7f6448d7b 100644
--- a/crates/core/src/host/module_host.rs
+++ b/crates/core/src/host/module_host.rs
@@ -2163,7 +2163,7 @@ impl ModuleHost {
         let (reducer_id, reducer_def) = self
             .info
             .module_def
-            .reducer_full(reducer_name)
+            .reducer_by_name(reducer_name)
             .ok_or(ReducerCallError::NoSuchReducer)?;
         if let Some(lifecycle) = reducer_def.lifecycle {
             return Err(ReducerCallError::LifecycleReducer(lifecycle));
diff --git a/crates/core/src/host/scheduler.rs b/crates/core/src/host/scheduler.rs
index 0e68e7c792e..d21bfca96f5 100644
--- a/crates/core/src/host/scheduler.rs
+++ b/crates/core/src/host/scheduler.rs
@@ -686,7 +686,7 @@ fn function_to_call_params(
 
     // Find the function and deserialize the arguments.
     let module = &module.module_def;
-    let (id, args) = if let Some((id, def)) = module.reducer_full(name) {
+    let (id, args) = if let Some((id, def)) = module.reducer_by_name(name) {
         let args = args.into_tuple_for_def(module, def).map_err(InvalidReducerArguments)?;
         (FunctionId::Reducer(id), args)
     } else if let Some((id, def)) = module.procedure_full(name) {
diff --git a/crates/core/src/host/wasm_common/module_host_actor.rs b/crates/core/src/host/wasm_common/module_host_actor.rs
index b269bbb92e4..ee74691cfca 100644
--- a/crates/core/src/host/wasm_common/module_host_actor.rs
+++ b/crates/core/src/host/wasm_common/module_host_actor.rs
@@ -1453,7 +1453,7 @@ impl AllVmMetrics {
         let def = &info.module_def;
         let reducers = def.reducer_ids_and_defs();
         let num_reducers = reducers.len() as u32;
-        let reducers = reducers.map(|(_, def)| def.name());
+        let reducers = reducers.into_iter().map(|(_, def)| def.name());
 
         // These are the views:
         let views = def.views().map(|def| def.name());
diff --git a/crates/schema/src/def.rs b/crates/schema/src/def.rs
index 9892500ad0c..e472062a6ed 100644
--- a/crates/schema/src/def.rs
+++ b/crates/schema/src/def.rs
@@ -205,9 +205,30 @@ impl ModuleDef {
         self.reducers.values()
     }
 
-    /// Returns an iterator over all reducer ids and definitions.
-    pub fn reducer_ids_and_defs(&self) -> impl ExactSizeIterator<Item = (ReducerId, &ReducerDef)> {
-        self.reducers.values().enumerate().map(|(idx, def)| (idx.into(), def))
+    /// Returns all reducer ids and definitions in depth-first mount order.
+    ///
+    /// IDs are assigned as follows: consumer's own reducers first (0..N), then each
+    /// mounted submodule's reducers in the order they appear in `mounts`, recursively.
+    pub fn reducer_ids_and_defs(&self) -> Vec<(ReducerId, &ReducerDef)> {
+        let mut out = Vec::with_capacity(self.reducer_count());
+        self.collect_reducers(0, &mut out);
+        out
+    }
+
+    /// Total reducer count including all mounted submodules (depth-first sum).
+    pub fn reducer_count(&self) -> usize {
+        self.reducers.len() + self.mounts.values().map(|m| m.reducer_count()).sum::<usize>()
+    }
+
+    fn collect_reducers<'a>(&'a self, offset: usize, out: &mut Vec<(ReducerId, &'a ReducerDef)>) {
+        for (i, def) in self.reducers.values().enumerate() {
+            out.push(((offset + i).into(), def));
+        }
+        let mut child_offset = offset + self.reducers.len();
+        for mount in self.mounts.values() {
+            mount.collect_reducers(child_offset, out);
+            child_offset += mount.reducer_count();
+        }
     }
 
     /// The procedures of the module definition.
@@ -304,14 +325,49 @@ impl ModuleDef {
         self.reducers.get_full(name).map(|(idx, _, def)| (idx.into(), def))
     }
 
-    /// Look up a reducer by its id.
+    /// Look up a reducer by its wire name, resolving qualified names like `"myauth/verify_token"`.
+    ///
+    /// A plain name searches the consumer's own reducers. A slash-qualified name routes to
+    /// the matching mount and recurses. Nesting is supported: `"auth/baz/cleanup"`.
+    /// Returns the depth-first `ReducerId` and the `ReducerDef`.
+    pub fn reducer_by_name(&self, name: &str) -> Option<(ReducerId, &ReducerDef)> {
+        match name.split_once('/') {
+            None => self.reducer_full(name),
+            Some((namespace, rest)) => {
+                let mut offset = self.reducers.len();
+                for (ns, mount) in &self.mounts {
+                    if ns == namespace {
+                        let (inner_id, def) = mount.reducer_by_name(rest)?;
+                        return Some(((offset + inner_id.idx()).into(), def));
+                    }
+                    offset += mount.reducer_count();
+                }
+                None
+            }
+        }
+    }
+
+    /// Look up a reducer by its depth-first id.
     pub fn reducer_by_id(&self, id: ReducerId) -> &ReducerDef {
-        &self.reducers[id.idx()]
+        self.get_reducer_by_id(id)
+            .unwrap_or_else(|| panic!("reducer id {id:?} out of range"))
     }
 
-    /// Look up a reducer by its id.
+    /// Look up a reducer by its depth-first id, returning `None` if it doesn't exist.
     pub fn get_reducer_by_id(&self, id: ReducerId) -> Option<&ReducerDef> {
-        self.reducers.get_index(id.idx()).map(|(_, def)| def)
+        let idx = id.idx();
+        if idx < self.reducers.len() {
+            return self.reducers.get_index(idx).map(|(_, def)| def);
+        }
+        let mut offset = self.reducers.len();
+        for mount in self.mounts.values() {
+            let count = mount.reducer_count();
+            if idx < offset + count {
+                return mount.get_reducer_by_id(ReducerId::from(idx - offset));
+            }
+            offset += count;
+        }
+        None
     }
 
     /// Look up a view by its id, and whether it is anonymous.
@@ -2053,4 +2109,78 @@ mod tests {
             .count()
             == 2))
     }
+
+    #[test]
+    fn mounted_reducer_ids_are_depth_first() {
+        use spacetimedb_lib::db::raw_def::v10::{
+            RawModuleDefV10Builder, RawModuleDefV10Section, RawModuleMountV10,
+        };
+
+        // baz library: 1 reducer
+        let mut baz_builder = RawModuleDefV10Builder::new();
+        baz_builder.add_reducer("baz_reduce", ProductType::unit());
+
+        // auth library: 1 own reducer, mounts baz
+        let mut auth_builder = RawModuleDefV10Builder::new();
+        auth_builder.add_reducer("auth_verify", ProductType::unit());
+        let mut auth_raw = auth_builder.finish();
+        auth_raw.sections.push(RawModuleDefV10Section::Mounts(vec![RawModuleMountV10 {
+            namespace: "baz".to_string(),
+            module: baz_builder.finish(),
+        }]));
+
+        // consumer: 2 own reducers, mounts auth
+        let mut consumer_builder = RawModuleDefV10Builder::new();
+        consumer_builder.add_reducer("consumer_a", ProductType::unit());
+        consumer_builder.add_reducer("consumer_b", ProductType::unit());
+        let mut consumer_raw = consumer_builder.finish();
+        consumer_raw.sections.push(RawModuleDefV10Section::Mounts(vec![RawModuleMountV10 {
+            namespace: "auth".to_string(),
+            module: auth_raw,
+        }]));
+
+        let def: ModuleDef = consumer_raw.try_into().expect("valid module");
+
+        // Total count: 2 consumer + 1 auth + 1 baz
+        assert_eq!(def.reducer_count(), 4);
+
+        // Depth-first order: consumer_a=0, consumer_b=1, auth_verify=2, baz_reduce=3
+        let ids_and_defs = def.reducer_ids_and_defs();
+        assert_eq!(ids_and_defs.len(), 4);
+        assert_eq!(ids_and_defs[0].0, ReducerId(0));
+        assert_eq!(&*ids_and_defs[0].1.name, "consumer_a");
+        assert_eq!(ids_and_defs[1].0, ReducerId(1));
+        assert_eq!(&*ids_and_defs[1].1.name, "consumer_b");
+        assert_eq!(ids_and_defs[2].0, ReducerId(2));
+        assert_eq!(&*ids_and_defs[2].1.name, "auth_verify");
+        assert_eq!(ids_and_defs[3].0, ReducerId(3));
+        assert_eq!(&*ids_and_defs[3].1.name, "baz_reduce");
+
+        // get_reducer_by_id resolves mounted reducer IDs correctly
+        assert_eq!(&*def.reducer_by_id(ReducerId(2)).name, "auth_verify");
+        assert_eq!(&*def.reducer_by_id(ReducerId(3)).name, "baz_reduce");
+        assert!(def.get_reducer_by_id(ReducerId(4)).is_none());
+
+        // reducer_by_name routes plain names to own reducers
+        let (id, rdef) = def.reducer_by_name("consumer_a").expect("plain name resolves");
+        assert_eq!(id, ReducerId(0));
+        assert_eq!(&*rdef.name, "consumer_a");
+
+        // reducer_by_name routes qualified names to mounted reducers
+        let (id, rdef) = def.reducer_by_name("auth/auth_verify").expect("qualified name resolves");
+        assert_eq!(id, ReducerId(2));
+        assert_eq!(&*rdef.name, "auth_verify");
+
+        // reducer_by_name routes deeply nested qualified names
+        let (id, rdef) = def
+            .reducer_by_name("auth/baz/baz_reduce")
+            .expect("nested qualified name resolves");
+        assert_eq!(id, ReducerId(3));
+        assert_eq!(&*rdef.name, "baz_reduce");
+
+        // Non-existent names return None
+        assert!(def.reducer_by_name("auth/nonexistent").is_none());
+        assert!(def.reducer_by_name("nonexistent").is_none());
+        assert!(def.reducer_by_name("nonamespace/auth_verify").is_none());
+    }
 }