diff --git a/server/src/atlas/district_mix.rs b/server/src/atlas/district_mix.rs
index c07a42064..4b8e1bf15 100644
--- a/server/src/atlas/district_mix.rs
+++ b/server/src/atlas/district_mix.rs
@@ -12,7 +12,7 @@
 //! **Determinism (D-010, D-194):** Integer weights throughout. No f32 in the
 //! district count computation. Seed-driven noise uses seeded RNG.
 
-use crate::seed::AtlasRng;
+use crate::seed::{SeedChain, SeedDomain};
 use crate::simulation::generator::{DistrictType, PoliticalArchetype};
 
 // ---------------------------------------------------------------------------
@@ -172,13 +172,14 @@ pub struct DistrictMix {
 /// `total_districts` is the number of districts to allocate. A good default is
 /// `max(4, population_tier * 2)`.
 ///
-/// `seed` is the city-level RNG seed (D-010 determinism).
+/// `chain` is this city's position in the deterministic seed tree; the district
+/// allocation draws from its `Layer4Quarter` sub-stream (D-224, D-010).
 pub fn compute_district_mix(
     population: i64,
     economic_role: &str,
     archetype: &PoliticalArchetype,
     total_districts: u32,
-    seed: u64,
+    chain: SeedChain,
 ) -> DistrictMix {
     let tier = population_tier(population);
     let (transit_min, commercial_min, residential_min) = tier_guarantees(tier);
@@ -197,7 +198,7 @@ pub fn compute_district_mix(
     // We avoid f32 by using integer weighted random selection.
     let weight_sum: u32 = weights.iter().sum();
     let mut counts: [u32; 9] = [0; 9];
-    let mut lcg = AtlasRng::new(seed.wrapping_add(1));
+    let mut lcg = chain.derive(SeedDomain::Layer4Quarter, 0).atlas_rng();
 
     for _ in 0..total_districts {
         let mut pick = lcg.next_u32() % weight_sum;
@@ -262,7 +263,7 @@ mod tests {
             "manufacturing",
             &PoliticalArchetype::Industrial,
             8,
-            42,
+            SeedChain::root(42),
         );
         // total may exceed requested due to guarantees
         assert!(mix.total >= 8, "district count should be >= requested");
@@ -277,7 +278,7 @@ mod tests {
             "service_mixed",
             &PoliticalArchetype::Pioneer,
             6,
-            7,
+            SeedChain::root(7),
         );
         let transit = mix
             .districts
@@ -312,14 +313,14 @@ mod tests {
             "financial",
             &PoliticalArchetype::Commission,
             6,
-            99,
+            SeedChain::root(99),
         );
         let mix2 = compute_district_mix(
             5_000_000,
             "financial",
             &PoliticalArchetype::Commission,
             6,
-            99,
+            SeedChain::root(99),
         );
         assert_eq!(mix1, mix2, "same inputs must produce identical output");
     }
@@ -331,10 +332,15 @@ mod tests {
             "financial",
             &PoliticalArchetype::Corporate,
             8,
-            42,
+            SeedChain::root(42),
+        );
+        let mix_pioneer = compute_district_mix(
+            5_000_000,
+            "financial",
+            &PoliticalArchetype::Pioneer,
+            8,
+            SeedChain::root(42),
         );
-        let mix_pioneer =
-            compute_district_mix(5_000_000, "financial", &PoliticalArchetype::Pioneer, 8, 42);
         // Should differ in at least one district type count.
         assert_ne!(
             mix_corp.districts, mix_pioneer.districts,
@@ -344,7 +350,13 @@ mod tests {
 
     #[test]
     fn military_archetype_has_administrative() {
-        let mix = compute_district_mix(2_000_000, "military", &PoliticalArchetype::Military, 8, 10);
+        let mix = compute_district_mix(
+            2_000_000,
+            "military",
+            &PoliticalArchetype::Military,
+            8,
+            SeedChain::root(10),
+        );
         let admin = mix
             .districts
             .iter()
@@ -358,13 +370,18 @@ mod tests {
 
     #[test]
     fn all_district_types_can_appear() {
-        // With enough districts and a balanced role, every type should appear at least once.
+        // Reachability check: every type has a clamped weight of at least 1
+        // (compute_district_mix `.max(1)`), so all are reachable. A large draw
+        // count makes every one appear regardless of the RNG sequence — a small
+        // count can miss a low-weight type purely by luck, which is sequence- and
+        // therefore seed-dependent (this is not a realistic city size).
+        let requested: u32 = 500;
         let mix = compute_district_mix(
             50_000_000,
             "service_mixed",
             &PoliticalArchetype::Pioneer,
-            50,
-            0,
+            requested,
+            SeedChain::root(0),
         );
         for dt in &DIST_COLS {
             let present = mix
@@ -373,8 +390,7 @@ mod tests {
                 .any(|d| std::mem::discriminant(d) == std::mem::discriminant(dt));
             assert!(
                 present,
-                "DistrictType {:?} never appeared in 50-district mix",
-                dt
+                "DistrictType {dt:?} never appeared in {requested}-district mix"
             );
         }
     }
diff --git a/server/src/atlas/skeleton_gen.rs b/server/src/atlas/skeleton_gen.rs
index 61601a0bb..c3277ba40 100644
--- a/server/src/atlas/skeleton_gen.rs
+++ b/server/src/atlas/skeleton_gen.rs
@@ -20,7 +20,7 @@
 
 use crate::atlas::block_irregularity::block_irregularity;
 use crate::atlas::district_mix::{compute_district_mix, population_tier};
-use crate::seed::AtlasRng;
+use crate::seed::{SeedChain, SeedDomain};
 use crate::simulation::generator::{
     BlockPlacement, BlockSkeleton, CityGenerationContext, ComplexityTier, DistrictId,
     DistrictLayoutMode, DistrictSkeleton, DistrictType, MultiBlockReservation, PoliticalArchetype,
@@ -39,14 +39,14 @@ use crate::simulation::generator::{
 /// - `economic_role`: Economic role string (one of the 10 canonical values).
 /// - `district_id`: Content-addressable identifier for this district.
 /// - `founding_age_years`: Years since founding — controls block irregularity.
-/// - `seed`: Deterministic seed for this district (derived from master seed via SeedChain).
+/// - `chain`: This district's position in the deterministic seed tree (D-224 SeedChain).
 pub fn generate_skeleton(
     context: &CityGenerationContext,
     population: i64,
     economic_role: &str,
     district_id: DistrictId,
     founding_age_years: u32,
-    seed: u64,
+    chain: SeedChain,
 ) -> DistrictSkeleton {
     // ── 1. SettingType ────────────────────────────────────────────────────
     // Pass through the surrounding_biome from context — it already encodes
@@ -59,7 +59,7 @@ pub fn generate_skeleton(
 
     // ── 3. DistrictLayoutMode ─────────────────────────────────────────────
     let irregularity = block_irregularity(founding_age_years, &context.political_archetype);
-    let layout_mode = derive_layout_mode(&context.political_archetype, irregularity, seed);
+    let layout_mode = derive_layout_mode(&context.political_archetype, irregularity, chain);
 
     // ── 4. District mix → block grid ─────────────────────────────────────
     // A single district occupies a 4×4 block grid = 16 blocks.
@@ -69,11 +69,11 @@ pub fn generate_skeleton(
         economic_role,
         &context.political_archetype,
         total_blocks,
-        seed,
+        chain,
     );
 
     // ── 5. Multi-block reservations ───────────────────────────────────────
-    let reservations = derive_reservations(tier, economic_role, seed);
+    let reservations = derive_reservations(tier, economic_role, chain);
 
     // Build the reservation lookup: block position → reservation id.
     let mut block_reservation: [[Option<ReservationId>; 4]; 4] = [[None, None, None, None]; 4];
@@ -105,7 +105,7 @@ pub fn generate_skeleton(
 
     DistrictSkeleton {
         district_id,
-        seed,
+        seed: chain.seed(),
         district_type: district_type_from_mix(&primary_district_type),
         context: String::new(), // stub — DistrictContext = String
         world_tier: context.world_tier.clone(),
@@ -223,7 +223,7 @@ fn derive_complexity(world_tier: &WorldTier, pop_tier: u8, population: i64) -> C
 fn derive_layout_mode(
     archetype: &PoliticalArchetype,
     irregularity: f32,
-    seed: u64,
+    chain: SeedChain,
 ) -> DistrictLayoutMode {
     match archetype {
         PoliticalArchetype::Commission
@@ -233,7 +233,7 @@ fn derive_layout_mode(
 
         PoliticalArchetype::Pioneer | PoliticalArchetype::Industrial => {
             // Organic: generate per-block offsets and rotations seeded from district seed.
-            let placements = organic_placements(irregularity, seed);
+            let placements = organic_placements(irregularity, chain);
             DistrictLayoutMode::Organic { placements }
         }
     }
@@ -243,9 +243,9 @@ fn derive_layout_mode(
 ///
 /// Uses a seeded LCG; offset range controlled by `irregularity` (0.05–1.0)
 /// scaled to the ±16 sim tile maximum from `block_irregularity::max_offset_sim_tiles`.
-fn organic_placements(irregularity: f32, seed: u64) -> [[BlockPlacement; 4]; 4] {
+fn organic_placements(irregularity: f32, chain: SeedChain) -> [[BlockPlacement; 4]; 4] {
     let max_offset = (irregularity * 16.0) as i16;
-    let mut lcg = AtlasRng::new(seed.wrapping_add(0x9e37_79b9_7f4a_7c15));
+    let mut lcg = chain.derive(SeedDomain::Block, 0).atlas_rng();
 
     // Build the 2D array using a flat closure to keep things readable.
     let mut flat: [BlockPlacement; 16] = core::array::from_fn(|_| BlockPlacement {
@@ -362,7 +362,7 @@ fn density_for_zoning(zoning: &ZoningType) -> u8 {
 fn derive_reservations(
     pop_tier: u8,
     economic_role: &str,
-    _seed: u64,
+    _chain: SeedChain,
 ) -> Vec<MultiBlockReservation> {
     let mut out = Vec::new();
 
@@ -429,14 +429,14 @@ mod tests {
     fn setting_station_passthrough() {
         let mut ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);
         ctx.surrounding_biome = SettingType::Station;
-        let sk = generate_skeleton(&ctx, 500_000, "institutional", 1, 200, 42);
+        let sk = generate_skeleton(&ctx, 500_000, "institutional", 1, 200, SeedChain::root(42));
         assert!(matches!(sk.setting, SettingType::Station));
     }
 
     #[test]
     fn setting_urban_for_city_on_planet() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);
-        let sk = generate_skeleton(&ctx, 500_000, "financial", 1, 200, 42);
+        let sk = generate_skeleton(&ctx, 500_000, "financial", 1, 200, SeedChain::root(42));
         assert!(matches!(sk.setting, SettingType::Urban));
     }
 
@@ -444,14 +444,14 @@ mod tests {
     fn complexity_epicenter_high_pop_is_full() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Epicenter);
         // 10M pop → pop_tier = 1 → Full on Epicenter
-        let sk = generate_skeleton(&ctx, 10_000_000, "financial", 1, 200, 42);
+        let sk = generate_skeleton(&ctx, 10_000_000, "financial", 1, 200, SeedChain::root(42));
         assert_eq!(sk.complexity, ComplexityTier::Full);
     }
 
     #[test]
     fn complexity_waypoint_tiny_pop_is_empty() {
         let ctx = make_context(PoliticalArchetype::Pioneer, WorldTier::Waypoint);
-        let sk = generate_skeleton(&ctx, 1_000, "residential", 1, 50, 42);
+        let sk = generate_skeleton(&ctx, 1_000, "residential", 1, 50, SeedChain::root(42));
         assert_eq!(sk.complexity, ComplexityTier::Empty);
     }
 
@@ -459,7 +459,7 @@ mod tests {
     fn complexity_backwater_low_pop_is_moderate() {
         let ctx = make_context(PoliticalArchetype::Pioneer, WorldTier::Backwater);
         // 50_000 pop → pop_tier = 0 → Moderate on Backwater (D-218: not budget-capped)
-        let sk = generate_skeleton(&ctx, 50_000, "residential", 1, 50, 42);
+        let sk = generate_skeleton(&ctx, 50_000, "residential", 1, 50, SeedChain::root(42));
         assert_eq!(sk.complexity, ComplexityTier::Moderate);
     }
 
@@ -467,7 +467,7 @@ mod tests {
     fn complexity_backwater_high_pop_is_full() {
         let ctx = make_context(PoliticalArchetype::Pioneer, WorldTier::Backwater);
         // 10M pop → pop_tier = 1 → Full on Backwater (D-218: not budget-capped)
-        let sk = generate_skeleton(&ctx, 10_000_000, "residential", 1, 50, 42);
+        let sk = generate_skeleton(&ctx, 10_000_000, "residential", 1, 50, SeedChain::root(42));
         assert_eq!(sk.complexity, ComplexityTier::Full);
     }
 
@@ -475,28 +475,28 @@ mod tests {
     fn complexity_passage_low_pop_is_minimal() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Passage);
         // 50_000 pop → pop_tier = 0 → Minimal on Passage (transit stop, budget-capped)
-        let sk = generate_skeleton(&ctx, 50_000, "transit_hub", 1, 100, 42);
+        let sk = generate_skeleton(&ctx, 50_000, "transit_hub", 1, 100, SeedChain::root(42));
         assert_eq!(sk.complexity, ComplexityTier::Minimal);
     }
 
     #[test]
     fn layout_commission_is_grid() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);
-        let sk = generate_skeleton(&ctx, 500_000, "institutional", 1, 100, 99);
+        let sk = generate_skeleton(&ctx, 500_000, "institutional", 1, 100, SeedChain::root(99));
         assert!(matches!(sk.layout_mode, DistrictLayoutMode::Grid));
     }
 
     #[test]
     fn layout_pioneer_is_organic() {
         let ctx = make_context(PoliticalArchetype::Pioneer, WorldTier::Regional);
-        let sk = generate_skeleton(&ctx, 500_000, "residential", 1, 400, 99);
+        let sk = generate_skeleton(&ctx, 500_000, "residential", 1, 400, SeedChain::root(99));
         assert!(matches!(sk.layout_mode, DistrictLayoutMode::Organic { .. }));
     }
 
     #[test]
     fn block_grid_is_fully_populated() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);
-        let sk = generate_skeleton(&ctx, 500_000, "financial", 1, 200, 42);
+        let sk = generate_skeleton(&ctx, 500_000, "financial", 1, 200, SeedChain::root(42));
         // All 16 blocks must have valid positions.
         for row in 0..4 {
             for col in 0..4 {
@@ -511,7 +511,7 @@ mod tests {
     fn no_reservations_for_small_city() {
         let ctx = make_context(PoliticalArchetype::Pioneer, WorldTier::Backwater);
         // pop_tier 0, not transit_hub → no reservations.
-        let sk = generate_skeleton(&ctx, 80_000, "residential", 1, 50, 42);
+        let sk = generate_skeleton(&ctx, 80_000, "residential", 1, 50, SeedChain::root(42));
         assert!(sk.reservations.is_empty());
     }
 
@@ -519,7 +519,7 @@ mod tests {
     fn park_reservation_for_large_city() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Epicenter);
         // 100M pop → pop_tier 2 → park reservation.
-        let sk = generate_skeleton(&ctx, 100_000_000, "financial", 1, 300, 42);
+        let sk = generate_skeleton(&ctx, 100_000_000, "financial", 1, 300, SeedChain::root(42));
         let has_park = sk
             .reservations
             .iter()
@@ -531,7 +531,7 @@ mod tests {
     fn transit_terminal_for_transit_hub_role() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);
         // pop_tier 0 but transit_hub → terminal reservation.
-        let sk = generate_skeleton(&ctx, 80_000, "transit_hub", 1, 200, 42);
+        let sk = generate_skeleton(&ctx, 80_000, "transit_hub", 1, 200, SeedChain::root(42));
         let has_terminal = sk
             .reservations
             .iter()
@@ -543,7 +543,7 @@ mod tests {
     fn reserved_blocks_linked_in_grid() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Epicenter);
         // 100M pop → park at (1,2),(1,3),(2,2),(2,3) with reservation id 1.
-        let sk = generate_skeleton(&ctx, 100_000_000, "financial", 1, 300, 42);
+        let sk = generate_skeleton(&ctx, 100_000_000, "financial", 1, 300, SeedChain::root(42));
         // All park blocks must reference the park reservation (id=1).
         for &(row, col) in &[(1u8, 2u8), (1, 3), (2, 2), (2, 3)] {
             let b = &sk.blocks[row as usize][col as usize];
@@ -557,8 +557,22 @@ mod tests {
     #[test]
     fn determinism_same_seed_same_output() {
         let ctx = make_context(PoliticalArchetype::Industrial, WorldTier::Regional);
-        let sk1 = generate_skeleton(&ctx, 2_000_000, "manufacturing", 77, 250, 12345);
-        let sk2 = generate_skeleton(&ctx, 2_000_000, "manufacturing", 77, 250, 12345);
+        let sk1 = generate_skeleton(
+            &ctx,
+            2_000_000,
+            "manufacturing",
+            77,
+            250,
+            SeedChain::root(12345),
+        );
+        let sk2 = generate_skeleton(
+            &ctx,
+            2_000_000,
+            "manufacturing",
+            77,
+            250,
+            SeedChain::root(12345),
+        );
         // Compare block grid zoning and positions.
         for row in 0..4 {
             for col in 0..4 {