diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index fae7e8a1..b46a0e7b 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -1,4 +1,23 @@
-# Current Task — Phase 21.7（Normalization & Unification: Methodize Static Boxes）
+# Current Task — Phase 21.8（Numeric Core Integration & Builder Support）
+
+Update (2025-11-14 — 21.8 kickoff: MatI64/IntArrayCore builder integration)
+- Context:
+  - 21.5: AotPrep/CollectionsHot v1 + microbench整備まで完了（linidx/maplin ≒ C=100%）。arraymap/matmul は次フェーズ送り。
+  - 21.6: NyRT IntArrayCore + Hako IntArrayCore/MatI64/matmul_core スケルトン実装まで完了（builder 経路未対応）。
+- Current goal:
+  - Hakorune selfhost chain（Stage‑B → MirBuilder → ny‑llvmc(crate)）に IntArrayCore/MatI64 を統合し、`matmul_core` ベンチを EXE ラインで実行できるようにする。
+  - 実装は Claude Code 担当、このホストは仕様・構造・診断の整理に専念。
+
+Planned tasks (for Claude Code)
+1) Fix MatI64 visibility in Stage‑B / MirBuilder
+   - Reproduce provider error: `[mirbuilder/parse/error] undefined variable: MatI64` from `env.mirbuilder.emit` when compiling a small test using `using nyash.core.numeric.matrix_i64 as MatI64`.
+   - Wire `nyash.core.numeric.matrix_i64` / `nyash.core.numeric.intarray` modules into the resolver/prelude so that Stage‑B/MirBuilder can see MatI64 and IntArrayCore like other core boxes.
+2) Make `tools/hakorune_emit_mir.sh` emit MIR(JSON) for `matmul_core`
+   - With `HAKO_APPLY_AOT_PREP=1 NYASH_AOT_COLLECTIONS_HOT=1 NYASH_LLVM_FAST=1 NYASH_MIR_LOOP_HOIST=1 NYASH_JSON_ONLY=1`, emit MIR(JSON) for the `matmul_core` case in microbench and ensure no undefined‑variable errors.
+3) Finish `matmul_core` bench EXE path
+   - Confirm `tools/perf/microbench.sh --case matmul_core --backend llvm --exe --runs 1 --n 64` builds EXE and runs, logging ratio vs the matching C implementation.
+4) Keep defaults stable
+   - No behaviour changes for existing code/benches; IntArrayCore/MatI64 integration is additive and behind explicit use.
 
 Update (2025-11-14 — CollectionsHot rewrite expansion, waiting for Claude Code)
 - Status: pending (waiting on Claude Code to land rewrite coverage improvements)
diff --git a/crates/nyash_kernel/src/plugin/intarray.rs b/crates/nyash_kernel/src/plugin/intarray.rs
new file mode 100644
index 00000000..4c535d1d
--- /dev/null
+++ b/crates/nyash_kernel/src/plugin/intarray.rs
@@ -0,0 +1,159 @@
+// IntArrayCore helpers for AOT/VM bridge (handle-based, ring1 numeric core)
+// API (Hako-facing via externcall):
+// - nyash.intarray.new_h(len)   -> handle (IntArrayCore)
+// - nyash.intarray.len_h(h)     -> i64
+// - nyash.intarray.get_hi(h,i)  -> i64
+// - nyash.intarray.set_hii(h,i,v) -> i64 (0=ok, non-zero=error)
+
+use nyash_rust::{
+    box_trait::{BoxCore, NyashBox, StringBox},
+    boxes::basic::BoolBox,
+    runtime::host_handles as handles,
+};
+use std::any::Any;
+use std::sync::RwLock;
+
+/// Minimal numeric core: contiguous i64 buffer + length.
+/// This box is intended for internal numeric kernels (matmul_core 等) 専用で、
+/// 一般APIは .hako 側のラッパー（MatI64 等）から利用する。
+#[derive(Debug)]
+pub struct IntArrayCore {
+    base: nyash_rust::box_trait::BoxBase,
+    data: RwLock<Vec<i64>>,
+}
+
+impl IntArrayCore {
+    pub fn new(len: i64) -> Self {
+        let n = if len <= 0 { 0 } else { len as usize };
+        IntArrayCore {
+            base: nyash_rust::box_trait::BoxBase::new(),
+            data: RwLock::new(vec![0; n]),
+        }
+    }
+
+    pub fn len_i64(&self) -> i64 {
+        self.data.read().unwrap().len() as i64
+    }
+
+    pub fn get_i64(&self, idx: i64) -> Option<i64> {
+        if idx < 0 {
+            return None;
+        }
+        let i = idx as usize;
+        let guard = self.data.read().unwrap();
+        guard.get(i).copied()
+    }
+
+    pub fn set_i64(&self, idx: i64, v: i64) -> bool {
+        if idx < 0 {
+            return false;
+        }
+        let i = idx as usize;
+        let mut guard = self.data.write().unwrap();
+        if i >= guard.len() {
+            return false;
+        }
+        guard[i] = v;
+        true
+    }
+}
+
+impl BoxCore for IntArrayCore {
+    fn box_id(&self) -> u64 {
+        self.base.id
+    }
+
+    fn parent_type_id(&self) -> Option<std::any::TypeId> {
+        self.base.parent_type_id
+    }
+
+    fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "IntArrayCore(len={})", self.data.len())
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+}
+
+impl NyashBox for IntArrayCore {
+    fn to_string_box(&self) -> StringBox {
+        StringBox::new(&format!("IntArrayCore(len={})", self.data.len()))
+    }
+
+    fn equals(&self, other: &dyn NyashBox) -> BoolBox {
+        if let Some(o) = other.as_any().downcast_ref::<IntArrayCore>() {
+            BoolBox::new(self.data == o.data)
+        } else {
+            BoolBox::new(false)
+        }
+    }
+
+    fn clone_box(&self) -> Box<dyn NyashBox> {
+        Box::new(IntArrayCore {
+            base: self.base.clone(),
+            data: RwLock::new(self.data.read().unwrap().clone()),
+        })
+    }
+
+    fn share_box(&self) -> Box<dyn NyashBox> {
+        // Identity semantics are not required here; clone is fine.
+        self.clone_box()
+    }
+}
+
+// --- Extern API (handle-based) ---
+
+fn get_core(handle: i64) -> Option<std::sync::Arc<dyn NyashBox>> {
+    if handle <= 0 {
+        return None;
+    }
+    handles::get(handle as u64)
+}
+
+#[export_name = \"nyash.intarray.new_h\"]
+pub extern \"C\" fn nyash_intarray_new_h(len: i64) -> i64 {
+    let core = IntArrayCore::new(len);
+    let arc: std::sync::Arc<dyn NyashBox> = std::sync::Arc::new(core);
+    let h = handles::to_handle_arc(arc) as i64;
+    if std::env::var(\"NYASH_CLI_VERBOSE\").ok().as_deref() == Some(\"1\") {
+        eprintln!(\"[INTARRAY] new_h(len={}) -> handle={}\", len, h);
+    }
+    h
+}
+
+#[export_name = \"nyash.intarray.len_h\"]
+pub extern \"C\" fn nyash_intarray_len_h(handle: i64) -> i64 {
+    if let Some(obj) = get_core(handle) {
+        if let Some(core) = obj.as_any().downcast_ref::<IntArrayCore>() {
+            return core.len_i64();
+        }
+    }
+    0
+}
+
+#[export_name = \"nyash.intarray.get_hi\"]
+pub extern \"C\" fn nyash_intarray_get_hi(handle: i64, idx: i64) -> i64 {
+    if let Some(obj) = get_core(handle) {
+        if let Some(core) = obj.as_any().downcast_ref::<IntArrayCore>() {
+            if let Some(v) = core.get_i64(idx) {
+                return v;
+            }
+        }
+    }
+    0
+}
+
+#[export_name = \"nyash.intarray.set_hii\"]
+pub extern \"C\" fn nyash_intarray_set_hii(handle: i64, idx: i64, val: i64) -> i64 {
+    if let Some(obj) = get_core(handle) {
+        if let Some(core) = obj.as_any().downcast_ref::<IntArrayCore>() {
+            return if core.set_i64(idx, val) { 0 } else { 1 };
+        }
+    }
+    1
+}
diff --git a/crates/nyash_kernel/src/plugin/mod.rs b/crates/nyash_kernel/src/plugin/mod.rs
index d382ed3e..12a405ab 100644
--- a/crates/nyash_kernel/src/plugin/mod.rs
+++ b/crates/nyash_kernel/src/plugin/mod.rs
@@ -6,6 +6,7 @@ pub mod instance;
 pub mod invoke;
 pub mod invoke_core;
 pub mod map;
+pub mod intarray;
 pub mod semantics;
 pub mod string;
 
@@ -17,5 +18,6 @@ pub use instance::*;
 pub use invoke::*;
 pub use invoke_core::*;
 pub use map::*;
+pub use intarray::*;
 pub use semantics::*;
 pub use string::*;
diff --git a/docs/development/roadmap/phases/phase-21.5-optimization/README.md b/docs/development/roadmap/phases/phase-21.5-optimization/README.md
index 928892c3..39273f25 100644
--- a/docs/development/roadmap/phases/phase-21.5-optimization/README.md
+++ b/docs/development/roadmap/phases/phase-21.5-optimization/README.md
@@ -4,7 +4,7 @@
 - .hako 側（AotPrep）で前処理最適化（構造のみ）を行い、LLVM/AOT に渡すIRを軽量にする。
 - 既定は挙動不変（opt‑in）。Return 純化ガードで安全性を担保。
 
-チェックリスト
+チェックリスト（21.5 時点の着地）
 - [x] パス分割（StrlenFold / LoopHoist / ConstDedup / CollectionsHot / BinopCSE）
 - [x] CollectionsHot（Array/Map）導入（既定OFF）
 - [x] Map key モード `NYASH_AOT_MAP_KEY_MODE={h|i64|hh|auto}`
@@ -17,6 +17,11 @@
 - [ ] Idempotence（置換済みタグで再実行時も不変）
 - [ ] `arraymap`/`matmul` ≤ 125%（C基準）
 
+メモ（21.5 クロージング）
+- linidx/maplin など「線形インデックス＋Array/Map」系は CollectionsHot + hoist/CSE で C≒100% 近辺まで到達。
+- arraymap は Array/Map 部分の externcall 化は進んだものの、文字列キー生成（toString/`\"k\"+idx`）と hash パスが支配的なため、C の単純 int[] とは根本的に前提が異なる状態で終了。
+- matmul は CollectionsHot 自体は単体では効いているが、行列積そのものが ArrayBox ベースであり、Core 数値箱不在のまま 80% 目標には届かず。これは 21.6 以降の「Core 数値箱＋行列箱」導入で扱う。
+
 トグル
 - `NYASH_MIR_LOOP_HOIST=1` … StrlenFold/LoopHoist/ConstDedup/BinopCSE を有効化
 - `NYASH_AOT_COLLECTIONS_HOT=1` … CollectionsHot（Array/Map）
diff --git a/docs/development/roadmap/phases/phase-21.6/core_numeric_boxes.md b/docs/development/roadmap/phases/phase-21.6/core_numeric_boxes.md
new file mode 100644
index 00000000..c3a188c4
--- /dev/null
+++ b/docs/development/roadmap/phases/phase-21.6/core_numeric_boxes.md
@@ -0,0 +1,90 @@
+# Phase 21.6 — Core Numeric Boxes (Draft)
+
+Status: proposal (to refine at 21.6 kickoff)
+
+## Goal
+
+Provide explicit, low‑level numeric boxes that:
+
+- Give Nyash a “fair” core for int/f64 benchmarks against C.
+- Stay compatible with the existing ArrayBox API (no breaking changes).
+- Can be used both explicitly in `.hako` and (later) as conservative AotPrep targets.
+
+This phase focuses on design + minimal implementation; aggressive auto‑rewrites stay behind opt‑in flags.
+
+## Scope (21.6)
+
+- Design and add **IntArrayCore** numeric core (NyRT + Hako wrapper):
+  - NyRT: `IntArrayCore` box（Rust）with internal layout `Vec<i64>`（contiguous, row‑major semantics）。
+  - Hako: `IntArrayCoreBox` in `nyash.core.numeric.intarray`, wrapping NyRT via externcall:
+    - `static new(len: i64) -> IntArrayCoreBox` → `nyash.intarray.new_h`
+    - `length(self) -> i64` → `nyash.intarray.len_h`
+    - `get_unchecked(self, idx: i64) -> i64` → `nyash.intarray.get_hi`
+    - `set_unchecked(self, idx: i64, v: i64)` → `nyash.intarray.set_hii`
+  - Semantics: i64‑only、固定長（構造変更なし）。境界チェックは NyRT 側（Fail‑Fast）に限定し、Hako 側は数値カーネル専用の薄いラッパーに留める。
+
+- Design and add **MatI64** (matrix box) on top of IntArrayCore:
+  - Internal layout: `rows: i64`, `cols: i64`, `stride: i64`, `core: IntArrayCoreBox`.
+  - Minimal API:
+    - `new(rows: i64, cols: i64) -> MatI64`
+    - `rows(self) -> i64`, `cols(self) -> i64`
+    - `at(self, r: i64, c: i64) -> i64`
+    - `set(self, r: i64, c: i64, v: i64)`
+  - Provide one reference implementation:
+    - `MatOps.matmul_naive(a: MatI64, b: MatI64) -> MatI64` (O(n³), clear structure, not tuned).
+
+- Bench alignment:
+  - Add `matmul_core` benchmark:
+    - Nyash: MatI64 + IntArrayCore implementation.
+    - C: struct `{ int64_t *ptr; int64_t rows; int64_t cols; int64_t stride; }` + helper `get/set`.
+  - Keep existing `matmul` (ArrayBox vs raw `int*`) as “language‑level” benchmark.
+
+Out of scope for 21.6:
+
+- Auto‑rewrite from `ArrayBox` → `IntArrayCore` / `MatI64` in AotPrep (only sketched, not default).
+- SIMD / blocked matmul / cache‑tuned kernels (can be separate optimization phases).
+- f64/complex variants (only type skeletons, if any).
+
+## Design Notes
+
+- **Layering**
+  - Core: IntArrayCore (and future F64ArrayCore) are “muscle” boxes: minimal, numeric‑only. NyRT では IntArrayCore（Rust）、Hako では IntArrayCoreBox として露出。
+  - Matrix: MatI64 expresses 2D shape and indexing; it owns an IntArrayCoreBox.
+  - High‑level: ArrayBox / MapBox / existing user APIs remain unchanged.
+
+- **Hako ABI vs Nyash implementation**
+  - IntArrayCore lives as a NyRT box (C/Rust implementation) exposed via Hako ABI (`nyash.intarray.*`).
+  - IntArrayCoreBox, MatI64 and MatOps are written in Nyash, calling IntArrayCore via externcall while exposing boxcall APIs to user code.
+  - This keeps heavy lifting in NyRT while keeping the 2D semantics in `.hako`.
+
+- **Fair C comparison**
+  - For `matmul_core`, C should mirror IntArrayCore/MatI64:
+    - Same struct layout (ptr + len / rows + cols + stride).
+    - Same naive O(n³) algorithm.
+  - This separates:
+    - “Nyash vs C as languages” → existing `matmul` (ArrayBox vs `int*`).
+    - “Core numeric kernel parity” → new `matmul_core` (IntArrayCore vs equivalent C).
+
+## AotPrep / Future Work (21.6+)
+
+Not for default in 21.6, but to keep in mind:
+
+- Add conservative patterns in Collections/AotPrep to detect:
+  - `ArrayBox<i64>` with:
+    - Fixed length.
+    - No structural mutations after initialization.
+    - Access patterns of the form `base + i*cols + j` (or similar linear forms).
+  - Allow opt‑in rewrite from such patterns to IntArrayCore/MatI64 calls.
+
+- Keep all auto‑rewrites:
+  - Behind env toggles (e.g. `NYASH_AOT_INTARRAY_CORE=1`).
+  - Semantics‑preserving by construction; fall back to ArrayBox path when unsure.
+
+## Open Questions for 21.6 Kickoff
+
+- Exact module names:
+  - `nyash.core.intarray` / `nyash.core.matrix` vs `nyash.linalg.*`.
+- Bounds checking policy for IntArrayCore:
+  - Always on (fail‑fast) vs dev toggle for light checks in hot loops.
+- Interop:
+  - Whether MatI64 should expose its IntArrayCore (e.g. `as_core_row_major()`) for advanced users.
diff --git a/docs/development/roadmap/phases/phase-21.8/README.md b/docs/development/roadmap/phases/phase-21.8/README.md
new file mode 100644
index 00000000..40e30a36
--- /dev/null
+++ b/docs/development/roadmap/phases/phase-21.8/README.md
@@ -0,0 +1,85 @@
+# Phase 21.8 — Numeric Core Integration & Builder Support
+
+Status: proposal (to hand off to Claude Code)
+
+## Goal
+
+Integrate the new numeric core boxes (IntArrayCore + MatI64) into the Hakorune selfhost chain so that:
+
+- Stage‑B → MirBuilder → ny‑llvmc(crate) can emit MIR(JSON) and EXE for code that uses:
+  - `using nyash.core.numeric.intarray as IntArrayCore`
+  - `using nyash.core.numeric.matrix_i64 as MatI64`
+- The `matmul_core` microbench (MatI64 + IntArrayCore) runs end‑to‑end in EXE mode and can be compared fairly against a matching C implementation.
+
+21.6 provides the core boxes; 21.8 focuses on wiring them into the builder/runtime chain without changing default behaviour for other code.
+
+## Scope (21.8, this host)
+
+- Stage‑B / MirBuilder:
+  - Ensure `MatI64` and `IntArrayCore` are recognized as valid boxes when referenced via:
+    - `using nyash.core.numeric.matrix_i64 as MatI64`
+    - `using nyash.core.numeric.intarray as IntArrayCore`
+  - Fix the current provider‑emit failure:
+    - Error today: `[mirbuilder/parse/error] undefined variable: MatI64` during `env.mirbuilder.emit`.
+    - Diagnose and adjust Stage‑B / MirBuilder so that static box references (`MatI64.new`, `A.mul_naive`) compile in the same way as other boxes.
+
+- AotPrep / emit pipeline:
+  - Keep AotPrep unchanged for now; the goal is to make `tools/hakorune_emit_mir.sh` succeed on `matmul_core` sources without special‑casing.
+  - Ensure `tools/hakorune_emit_mir.sh` with:
+    - `HAKO_APPLY_AOT_PREP=1 NYASH_AOT_COLLECTIONS_HOT=1 NYASH_LLVM_FAST=1 NYASH_MIR_LOOP_HOIST=1`
+    - can emit valid MIR(JSON) for MatI64/IntArrayCore code.
+
+- Microbench integration:
+  - Finish wiring `matmul_core` in `tools/perf/microbench.sh`:
+    - Hako side: MatI64/IntArrayCore based O(n³) matmul (`MatI64.mul_naive`).
+    - C side: `MatI64Core { int64_t *ptr; rows; cols; stride; }` with identical algorithm.
+  - Accept that performance may still be far from the 80% target; 21.8 focuses on **structural integration and parity**, not tuning.
+
+Out of scope:
+
+- New optimizations inside AotPrep / CollectionsHot.
+- SIMD/blocked matmul kernels (to be handled in a later optimization phase).
+- f64/complex matrix variants.
+
+## Tasks for implementation (Claude Code)
+
+1) **Fix MatI64 visibility in Stage‑B / MirBuilder**
+   - Reproduce the current failure:
+     - Use a small `.hako` like:
+       - `using nyash.core.numeric.matrix_i64 as MatI64`
+       - `static box Main { method main(args) { local n = 4; local A = MatI64.new(n,n); return A.at(0,0); } }`
+     - Confirm `env.mirbuilder.emit` reports `undefined variable: MatI64`.
+   - Investigate how modules from `nyash.toml` (`"nyash.core.numeric.matrix_i64" = "lang/src/runtime/numeric/mat_i64_box.hako"`) are made visible to Stage‑B and MirBuilder.
+   - Adjust the resolver / module prelude so that `MatI64` (and `IntArrayCore`) are treated like other core boxes:
+     - Either via explicit prelude inclusion,
+     - Or via module registry entries consumed by the builder.
+
+2) **Ensure `tools/hakorune_emit_mir.sh` can emit MIR(JSON) for matmul_core**
+   - Once MatI64 is visible, run:
+     - `HAKO_APPLY_AOT_PREP=1 NYASH_AOT_COLLECTIONS_HOT=1 NYASH_LLVM_FAST=1 NYASH_MIR_LOOP_HOIST=1 NYASH_JSON_ONLY=1 tools/hakorune_emit_mir.sh <matmul_core.hako> tmp/matmul_core.json`
+   - Acceptance:
+     - No `undefined variable: MatI64` / `IntArrayCore` errors.
+     - `tmp/matmul_core.json` is valid MIR(JSON) (same schema as existing matmul case).
+
+3) **Finish `matmul_core` microbench**
+   - Use the existing skeleton in `tools/perf/microbench.sh` (`case matmul_core`):
+     - Confirm Hako side compiles and runs under `--backend vm`.
+     - Confirm EXE path works:
+       - `NYASH_SKIP_TOML_ENV=1 NYASH_LLVM_SKIP_BUILD=1 tools/perf/microbench.sh --case matmul_core --backend llvm --exe --runs 1 --n 64`
+   - Update `benchmarks/README.md`:
+     - Add `matmul_core` row with a short description:
+       - “MatI64/IntArrayCore vs MatI64Core C struct (ptr+rows+cols+stride)”
+     - Record initial ratios (even if far from 80%).
+
+4) **Keep existing behaviour stable**
+   - No changes to default user behaviour, env toggles, or existing benches beyond adding `matmul_core`.
+   - Ensure quick/profile smokes (where applicable) remain green with numeric core present.
+
+## Notes
+
+- 21.6 already introduced:
+  - NyRT `IntArrayCore` (Vec<i64> + RwLock) and handle‑based externs (`nyash.intarray.*`).
+  - Hako wrappers `IntArrayCore` and `MatI64` in `lang/src/runtime/numeric/`.
+  - `nyash.toml` module aliases for `nyash.core.numeric.intarray` and `nyash.core.numeric.matrix_i64`.
+- 21.8 is about wiring these into the builder/emit chain so that Hakorune can compile and benchmark numeric core code end‑to‑end.
+
diff --git a/hako.toml b/hako.toml
new file mode 100644
index 00000000..2cc3b60c
--- /dev/null
+++ b/hako.toml
@@ -0,0 +1,4 @@
+[env]
+# This file mirrors nyash.toml for Hakorune/Hako tools.
+# Primary runtime config remains nyash.toml; hako.toml exists as a dev-facing alias.
+
diff --git a/lang/src/runtime/numeric/intarray_core_box.hako b/lang/src/runtime/numeric/intarray_core_box.hako
new file mode 100644
index 00000000..32414bfa
--- /dev/null
+++ b/lang/src/runtime/numeric/intarray_core_box.hako
@@ -0,0 +1,28 @@
+// IntArrayCoreBox — thin Hako wrapper over NyRT IntArrayCore (handle-based)
+// This provides a box-level API for numeric kernels while delegating storage
+// to nyash.intarray.* extern calls in NyRT (ring1).
+
+static box IntArrayCore {
+  init { handle, len }
+
+  static new(len) {
+    local h = externcall "nyash.intarray.new_h"(len)
+    local b = new IntArrayCore()
+    b.handle = h
+    b.len = len
+    return b
+  }
+
+  length() {
+    return externcall "nyash.intarray.len_h"(me.handle)
+  }
+
+  get_unchecked(idx) {
+    return externcall "nyash.intarray.get_hi"(me.handle, idx)
+  }
+
+  set_unchecked(idx, v) {
+    externcall "nyash.intarray.set_hii"(me.handle, idx, v)
+    return null
+  }
+}
diff --git a/lang/src/runtime/numeric/mat_i64_box.hako b/lang/src/runtime/numeric/mat_i64_box.hako
new file mode 100644
index 00000000..053c43a0
--- /dev/null
+++ b/lang/src/runtime/numeric/mat_i64_box.hako
@@ -0,0 +1,65 @@
+// MatI64 — simple i64 matrix box built on top of IntArrayCore.
+// Internal layout: rows, cols, stride, core (IntArrayCore).
+
+using nyash.core.numeric.intarray as IntArrayCore
+
+static box MatI64 {
+  init { rows, cols, stride, core }
+
+  static new(rows, cols) {
+    local total = rows * cols
+    local core = IntArrayCore.new(total)
+    local m = new MatI64()
+    m.rows = rows
+    m.cols = cols
+    m.stride = cols
+    m.core = core
+    return m
+  }
+
+  rowsCount() {
+    return me.rows
+  }
+
+  colsCount() {
+    return me.cols
+  }
+
+  at(r, c) {
+    local idx = r * me.stride + c
+    return me.core.get_unchecked(idx)
+  }
+
+  set(r, c, v) {
+    local idx = r * me.stride + c
+    me.core.set_unchecked(idx, v)
+    return null
+  }
+
+  // Naive O(n^3) matmul: this * b
+  mul_naive(b) {
+    local n = me.rows
+    local mcols = me.cols
+    local bcols = b.cols
+    // assume shapes are compatible and square for now (Phase 21.6 draft)
+    local out = MatI64.new(n, bcols)
+    local i = 0
+    loop(i < n) {
+      local k = 0
+      loop(k < mcols) {
+        local aik = me.at(i, k)
+        local j = 0
+        loop(j < bcols) {
+          local idx = i * out.stride + j
+          local v = out.core.get_unchecked(idx) + aik * b.at(k, j)
+          out.core.set_unchecked(idx, v)
+          j = j + 1
+        }
+        k = k + 1
+      }
+      i = i + 1
+    }
+    return out
+  }
+}
+
diff --git a/nyash.toml b/nyash.toml
index 1278c437..79fe4134 100644
--- a/nyash.toml
+++ b/nyash.toml
@@ -279,6 +279,10 @@ path = "lang/src/shared/common/string_helpers.hako"
 
 # Temporary alias keys removed (Phase‑20.33 TTL reached). Use `selfhost.shared.*` above.
 
+# Numeric core boxes (Phase 21.6)
+"nyash.core.numeric.intarray" = "lang/src/runtime/numeric/intarray_core_box.hako"
+"nyash.core.numeric.matrix_i64" = "lang/src/runtime/numeric/mat_i64_box.hako"
+
 # v2 Plugin libraries (loader reads these for TypeBox ABI)
 [libraries]
 [libraries."libnyash_filebox_plugin.so"]
diff --git a/tools/perf/microbench.sh b/tools/perf/microbench.sh
index eeb371a7..90f23d22 100644
--- a/tools/perf/microbench.sh
+++ b/tools/perf/microbench.sh
@@ -5,7 +5,7 @@ SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
 BIN="$ROOT/target/release/hakorune"
 
-usage() { echo "Usage: $0 --case {loop|strlen|box|branch|call|stringchain|arraymap|chip8|kilo|sieve|matmul|linidx|maplin} [--n N] [--runs R] [--backend {llvm|vm}] [--exe] [--budget-ms B]"; }
+usage() { echo "Usage: $0 --case {loop|strlen|box|branch|call|stringchain|arraymap|chip8|kilo|sieve|matmul|matmul_core|linidx|maplin} [--n N] [--runs R] [--backend {llvm|vm}] [--exe] [--budget-ms B]"; }
 
 CASE="loop"; N=5000000; RUNS=5; BACKEND="llvm"; EXE_MODE=0; BUDGET_MS=0
 while [[ $# -gt 0 ]]; do
@@ -562,6 +562,91 @@ C
       rm -f "$TMP_CHECK_JSON" 2>/dev/null || true
     fi
     ;;
+  matmul_core)
+    # Core numeric matmul using MatI64 + IntArrayCore
+    # Use smaller default N to keep runtime reasonable
+    if [[ "$EXE_MODE" = "1" && "$N" = "5000000" ]]; then
+      N=256
+    fi
+    HAKO_FILE=$(mktemp_hako)
+    cat >"$HAKO_FILE" <<HAKO
+using nyash.core.numeric.matrix_i64 as MatI64
+
+static box Main { method main(args) {
+  local n = ${N}
+  // Initialize A, B, C as n x n matrices
+  local A = MatI64.new(n, n)
+  local B = MatI64.new(n, n)
+  local C = MatI64.new(n, n)
+  local i = 0
+  loop(i < n) {
+    local j = 0
+    loop(j < n) {
+      local idx = i*n + j
+      A.set(i, j, idx % 97)
+      B.set(i, j, (idx * 3) % 101)
+      C.set(i, j, 0)
+      j = j + 1
+    }
+    i = i + 1
+  }
+  // Naive matmul via MatI64.mul_naive
+  local out = A.mul_naive(B)
+  return out.at(n-1, n-1)
+} }
+HAKO
+    C_FILE=$(mktemp_c)
+    cat >"$C_FILE" <<'C'
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct {
+  int64_t *ptr;
+  int64_t rows;
+  int64_t cols;
+  int64_t stride;
+} MatI64Core;
+
+static inline int64_t mat_get(MatI64Core *m, int64_t r, int64_t c) {
+  return m->ptr[r * m->stride + c];
+}
+
+static inline void mat_set(MatI64Core *m, int64_t r, int64_t c, int64_t v) {
+  m->ptr[r * m->stride + c] = v;
+}
+
+int main() {
+  int64_t n = N_PLACEHOLDER;
+  int64_t total = n * n;
+  MatI64Core A, B, C;
+  A.rows = B.rows = C.rows = n;
+  A.cols = B.cols = C.cols = n;
+  A.stride = B.stride = C.stride = n;
+  A.ptr = (int64_t*)malloc(sizeof(int64_t)*total);
+  B.ptr = (int64_t*)malloc(sizeof(int64_t)*total);
+  C.ptr = (int64_t*)malloc(sizeof(int64_t)*total);
+  for (int64_t idx = 0; idx < total; idx++) {
+    A.ptr[idx] = idx % 97;
+    B.ptr[idx] = (idx * 3) % 101;
+    C.ptr[idx] = 0;
+  }
+  for (int64_t i = 0; i < n; i++) {
+    for (int64_t k = 0; k < n; k++) {
+      int64_t aik = mat_get(&A, i, k);
+      for (int64_t j = 0; j < n; j++) {
+        int64_t idx = i * C.stride + j;
+        int64_t v = C.ptr[idx] + aik * mat_get(&B, k, j);
+        C.ptr[idx] = v;
+      }
+    }
+  }
+  int64_t r = mat_get(&C, n-1, n-1);
+  free(A.ptr); free(B.ptr); free(C.ptr);
+  return (int)(r & 0xFF);
+}
+C
+    sed -i "s/N_PLACEHOLDER/${N}/" "$C_FILE"
+    ;;
   linidx)
     # Linear index pattern: idx = i*cols + j
     # Derive rows/cols from N to keep runtime stable