diff --git a/src/simulator/api-gdext/src/lair.rs b/src/simulator/api-gdext/src/lair.rs
new file mode 100644
index 00000000..95cd19ec
--- /dev/null
+++ b/src/simulator/api-gdext/src/lair.rs
@@ -0,0 +1,310 @@
+//! p3-10a — `GdLair` GDExtension bridge over `mc_combat::lair::resolve_assault`.
+//!
+//! Presentation-only wrapper (Rail 3): GDScript reads the authored JSON
+//! (attacker stack, lair defenders, the tier loot table from
+//! `public/resources/lairs/loot/tier_NN.json`), passes the strings in, and the
+//! bridge parses → `LairAssaultParams` → `mc_combat::resolve_assault` → a
+//! Godot `Dictionary` describing the outcome. The bridge does NO IO of its own;
+//! file reads stay on the GDScript side, exactly mirroring `GdLootRoller`.
+//!
+//! The combat math, posture bonus, and loot rolling all live in `mc-combat`
+//! (`resolve_assault`, `LAIR_DEFENDER_POSTURE_BONUS`) — the source of truth.
+//! This file only marshals data across the FFI boundary.
+//!
+//! The Assault / Siege / Raid UI mode picker is a godot-ui follow-up (a small
+//! modal that calls `assault()` for the Assault branch and shows Siege/Raid
+//! disabled until p3-10b/p3-10c land their dispatcher branches). It is NOT
+//! built here — this objective owns only the Rust bridge.
+
+use godot::prelude::*;
+
+use mc_combat::{
+    resolve_assault, AssaultOutcome, LairAssaultParams, LairDefender, LootEntry, StackUnit,
+};
+use mc_combat::resolver::UnitStats;
+use mc_core::lair::LairId;
+
+/// One attacker/defender combatant as authored on the GDScript side: a stable
+/// `entity_id` (for reproducible loot seed mixing) plus the combat stat-line.
+/// Wire shape: `{ "entity_id": u32, "stats": { hp, max_hp, attack, ... } }`.
+#[derive(serde::Deserialize)]
+struct CombatantDoc {
+    entity_id: u32,
+    stats: UnitStats,
+}
+
+/// The tier loot file shape (`tier_NN.json`): only `loot_table` is consumed
+/// here; the other fields (`tier`, `id`, `description`) are documentation.
+#[derive(serde::Deserialize)]
+struct LootTierDoc {
+    #[serde(default)]
+    loot_table: Vec<LootEntry>,
+}
+
+/// Stateless GDExtension wrapper around the lair assault resolver.
+#[derive(GodotClass)]
+#[class(base=RefCounted)]
+pub struct GdLair {
+    base: Base<RefCounted>,
+}
+
+#[godot_api]
+impl IRefCounted for GdLair {
+    fn init(base: Base<RefCounted>) -> Self {
+        Self { base }
+    }
+}
+
+#[godot_api]
+impl GdLair {
+    /// Resolve an Assault against a wild lair.
+    ///
+    /// Inputs (all JSON strings except the scalars):
+    /// - `attackers_json`: array of `{entity_id, stats}` — the attacking stack
+    ///   in engagement priority order.
+    /// - `defenders_json`: array of `{entity_id, stats}` — the lair defenders
+    ///   (typically built from `mc_combat::wild_combat_stats(tier, size, diet)`
+    ///   on the GDScript side, or a future `GdWildStats` helper).
+    /// - `loot_tier_json`: the full `tier_NN.json` document; the bridge pulls
+    ///   `loot_table` out of it (empty table → no loot, never an error).
+    /// - `lair_id`: content id for the outcome payload (UI / event log).
+    /// - `lair_tier`: 1..=10.
+    /// - `turn_seed`: per-turn deterministic seed (GDScript mixes from `turn`).
+    /// - `defender_terrain_bonus`: extra terrain defense on top of the built-in
+    ///   `LAIR_DEFENDER_POSTURE_BONUS` (e.g. `0.25` for forest; `0.0` on plain).
+    ///
+    /// Returns a `Dictionary`:
+    /// - `outcome`: `"cleared" | "repulsed" | "withdrawn"`.
+    /// - on `cleared`: `loot` (Array of `{resource, amount}`),
+    ///   `stack_survivors` (Array of `{entity_id, hp, max_hp, attack, defense}`).
+    /// - on `repulsed`: `surviving_defenders` (same survivor shape).
+    /// - on a JSON parse error: `{ outcome: "error", message: String }`.
+    #[func]
+    #[allow(clippy::too_many_arguments)]
+    fn assault(
+        attackers_json: GString,
+        defenders_json: GString,
+        loot_tier_json: GString,
+        lair_id: GString,
+        lair_tier: i64,
+        turn_seed: i64,
+        defender_terrain_bonus: f64,
+    ) -> Dictionary {
+        let params = match build_params(
+            &attackers_json.to_string(),
+            &defenders_json.to_string(),
+            &loot_tier_json.to_string(),
+            &lair_id.to_string(),
+            lair_tier as i32,
+            turn_seed.max(0) as u64,
+            defender_terrain_bonus as f32,
+        ) {
+            Ok(p) => p,
+            Err(msg) => return error_dict(&msg),
+        };
+
+        outcome_to_dict(resolve_assault(params))
+    }
+}
+
+/// Pure (Godot-free) parse + assembly of `LairAssaultParams` from the wire
+/// JSON. Split out from the `#[func]` so the load-bearing marshalling logic is
+/// unit-testable without a live Godot runtime (the `Dictionary`/`Array`
+/// conversions in `outcome_to_dict` stay GUT-covered).
+#[allow(clippy::too_many_arguments)]
+fn build_params(
+    attackers_json: &str,
+    defenders_json: &str,
+    loot_tier_json: &str,
+    lair_id: &str,
+    lair_tier: i32,
+    turn_seed: u64,
+    defender_terrain_bonus: f32,
+) -> Result<LairAssaultParams, String> {
+    let attackers = parse_combatants(attackers_json, "attackers")?
+        .into_iter()
+        .map(|c| StackUnit { entity_id: c.entity_id, stats: c.stats })
+        .collect();
+    let defenders = parse_combatants(defenders_json, "defenders")?
+        .into_iter()
+        .map(|c| LairDefender { entity_id: c.entity_id, stats: c.stats })
+        .collect();
+    let loot_table = serde_json::from_str::<LootTierDoc>(loot_tier_json)
+        .map_err(|e| format!("loot_tier parse error: {e}"))?
+        .loot_table;
+
+    Ok(LairAssaultParams {
+        lair_id: LairId::new(lair_id),
+        lair_tier,
+        turn_seed,
+        attackers,
+        defenders,
+        loot_table,
+        defender_terrain_bonus,
+    })
+}
+
+/// Parse a combatant array, tagging the error with which side failed.
+fn parse_combatants(raw: &str, side: &str) -> Result<Vec<CombatantDoc>, String> {
+    serde_json::from_str::<Vec<CombatantDoc>>(raw)
+        .map_err(|e| format!("{side} parse error: {e}"))
+}
+
+fn error_dict(message: &str) -> Dictionary {
+    godot_error!("GdLair::assault: {message}");
+    let mut d = Dictionary::new();
+    d.set("outcome", "error");
+    d.set("message", GString::from(message));
+    d
+}
+
+/// Survivor row shape shared by `stack_survivors` and `surviving_defenders`.
+fn stack_survivor_dict(entity_id: u32, s: &UnitStats) -> Dictionary {
+    let mut d = Dictionary::new();
+    d.set("entity_id", entity_id as i64);
+    d.set("hp", s.hp as i64);
+    d.set("max_hp", s.max_hp as i64);
+    d.set("attack", s.attack as i64);
+    d.set("defense", s.defense as i64);
+    d
+}
+
+fn outcome_to_dict(outcome: AssaultOutcome) -> Dictionary {
+    let mut d = Dictionary::new();
+    match outcome {
+        AssaultOutcome::Cleared { loot, stack_survivors } => {
+            d.set("outcome", "cleared");
+            let mut loot_arr = Array::<Dictionary>::new();
+            for drop in loot {
+                let mut row = Dictionary::new();
+                row.set("resource", GString::from(drop.resource));
+                row.set("amount", drop.amount as i64);
+                loot_arr.push(&row);
+            }
+            d.set("loot", loot_arr);
+            let mut surv = Array::<Dictionary>::new();
+            for u in stack_survivors {
+                surv.push(&stack_survivor_dict(u.entity_id, &u.stats));
+            }
+            d.set("stack_survivors", surv);
+        }
+        AssaultOutcome::Repulsed { surviving_defenders } => {
+            d.set("outcome", "repulsed");
+            let mut defs = Array::<Dictionary>::new();
+            for u in surviving_defenders {
+                defs.push(&stack_survivor_dict(u.entity_id, &u.stats));
+            }
+            d.set("surviving_defenders", defs);
+        }
+        AssaultOutcome::Withdrawn => {
+            d.set("outcome", "withdrawn");
+        }
+    }
+    d
+}
+
+#[cfg(test)]
+mod tests {
+    //! Godot's runtime is unavailable during `cargo test`, so these cover the
+    //! pure marshalling half (`build_params`) end-to-end into the resolver.
+    //! `outcome_to_dict` (Godot `Dictionary`/`Array`) is GUT-covered at the
+    //! phase gate. The wire JSON here mirrors what `world_map_combat.gd` will
+    //! pass: `[{entity_id, stats:{...}}]` stacks + a `tier_NN.json` loot doc.
+    use super::*;
+
+    fn strong_attacker() -> &'static str {
+        r#"[{"entity_id":1,"stats":{"hp":80,"max_hp":80,"attack":40,"defense":10,"ranged_attack":0,"range":0,"movement":2}}]"#
+    }
+    fn weak_attacker() -> &'static str {
+        r#"[{"entity_id":1,"stats":{"hp":5,"max_hp":5,"attack":1,"defense":0,"ranged_attack":0,"range":0,"movement":2}}]"#
+    }
+    fn lone_defender() -> &'static str {
+        r#"[{"entity_id":99,"stats":{"hp":20,"max_hp":20,"attack":6,"defense":2,"ranged_attack":0,"range":0,"movement":1}}]"#
+    }
+    /// A genuinely tough defender, built from the real `wild_combat_stats`
+    /// (T8 huge carnivore — the same fixture `mc-combat`'s own
+    /// `test_assault_repulses_weak_attacker` uses), serialized to the wire
+    /// shape the bridge parses. Guarantees a repulse against a weak stack.
+    fn tough_defender_json() -> String {
+        let stats = mc_combat::wild_combat_stats(8, "huge", "carnivore");
+        format!(
+            r#"[{{"entity_id":99,"stats":{}}}]"#,
+            serde_json::to_string(&stats).unwrap()
+        )
+    }
+    // tier_01.json shape — flint is the guaranteed (chance 1.0) drop.
+    fn tier1_loot() -> &'static str {
+        r#"{"tier":1,"id":"lair_loot_tier_1","loot_table":[{"resource":"flint","amount":2,"chance":1.0},{"resource":"hides","amount":1,"chance":0.6}]}"#
+    }
+
+    #[test]
+    fn build_params_assembles_from_wire_json() {
+        let p = build_params(
+            strong_attacker(), lone_defender(), tier1_loot(),
+            "test_lair", 1, 42, 0.25,
+        )
+        .expect("params build");
+        assert_eq!(p.lair_id.as_str(), "test_lair");
+        assert_eq!(p.lair_tier, 1);
+        assert_eq!(p.attackers.len(), 1);
+        assert_eq!(p.attackers[0].entity_id, 1);
+        assert_eq!(p.attackers[0].stats.attack, 40);
+        assert_eq!(p.defenders.len(), 1);
+        assert_eq!(p.loot_table.len(), 2);
+        assert!((p.defender_terrain_bonus - 0.25).abs() < f32::EPSILON);
+    }
+
+    #[test]
+    fn strong_stack_clears_lair_and_drops_guaranteed_loot() {
+        let p = build_params(
+            strong_attacker(), lone_defender(), tier1_loot(),
+            "test_lair", 1, 7, 0.0,
+        )
+        .unwrap();
+        match resolve_assault(p) {
+            AssaultOutcome::Cleared { loot, stack_survivors } => {
+                assert!(
+                    loot.iter().any(|d| d.resource == "flint" && d.amount == 2),
+                    "the guaranteed (chance 1.0) flint drop must be present: {loot:?}",
+                );
+                assert!(!stack_survivors.is_empty(), "strong attacker survives the clear");
+            }
+            other => panic!("expected Cleared, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn weak_stack_is_repulsed() {
+        let p = build_params(
+            weak_attacker(), &tough_defender_json(), tier1_loot(),
+            "test_lair", 8, 7, 0.0,
+        )
+        .unwrap();
+        match resolve_assault(p) {
+            AssaultOutcome::Repulsed { surviving_defenders } => {
+                assert!(!surviving_defenders.is_empty(), "defender survives a repulse");
+            }
+            other => panic!("expected Repulsed, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn empty_stack_withdraws() {
+        let p = build_params("[]", lone_defender(), tier1_loot(), "test_lair", 1, 7, 0.0).unwrap();
+        assert_eq!(resolve_assault(p), AssaultOutcome::Withdrawn);
+    }
+
+    #[test]
+    fn malformed_attacker_json_is_an_error_not_a_panic() {
+        let err = build_params("not json", lone_defender(), tier1_loot(), "x", 1, 0, 0.0)
+            .expect_err("malformed attacker JSON must error");
+        assert!(err.starts_with("attackers parse error"), "got: {err}");
+    }
+
+    #[test]
+    fn malformed_loot_json_is_an_error() {
+        let err = build_params(strong_attacker(), lone_defender(), "{", "x", 1, 0, 0.0)
+            .expect_err("malformed loot JSON must error");
+        assert!(err.starts_with("loot_tier parse error"), "got: {err}");
+    }
+}