diff --git a/lib/quickbeam.ex b/lib/quickbeam.ex
index 8271c0fc3..6d8c4315f 100644
--- a/lib/quickbeam.ex
+++ b/lib/quickbeam.ex
@@ -41,7 +41,11 @@ defmodule QuickBEAM do
       automatically bundled — imports are resolved from the filesystem and
       `node_modules/`, then compiled into a single script via OXC.
     * `:memory_limit` — maximum JS heap in bytes (default: 256 MB)
-    * `:max_stack_size` — maximum JS call stack in bytes (default: 4 MB)
+    * `:max_stack_size` — maximum JS call stack in bytes (default: 8 MB)
+    * `:wasm_stack_size` — WASM operand stack in bytes for guests started via the JS
+      `WebAssembly.instantiate` path (default: 65536). Distinct from `:max_stack_size`
+      (the JS call stack); raise it for guests whose deep init overflows the 64 KB default.
+    * `:wasm_heap_size` — WASM auxiliary heap in bytes for the same path (default: 65536)
     * `:max_convert_depth` — maximum nesting depth for JS→BEAM value conversion (default: 32)
     * `:max_convert_nodes` — maximum total nodes for JS→BEAM value conversion (default: 10,000)
 
@@ -83,6 +87,10 @@ defmodule QuickBEAM do
 
     * `:memory_limit` — maximum JS heap in bytes (default: 256 MB)
     * `:max_stack_size` — maximum JS call stack in bytes (default: 8 MB)
+    * `:wasm_stack_size` — WASM operand stack in bytes for guests started via the JS
+      `WebAssembly.instantiate` path (default: 65536). Distinct from `:max_stack_size`
+      (the JS call stack); raise it for guests whose deep init overflows the 64 KB default.
+    * `:wasm_heap_size` — WASM auxiliary heap in bytes for the same path (default: 65536)
     * `:max_convert_depth` — maximum nesting depth for JS→BEAM value conversion (default: 32)
     * `:max_convert_nodes` — maximum total nodes for JS→BEAM value conversion (default: 10,000)
   """
diff --git a/lib/quickbeam/context_pool.ex b/lib/quickbeam/context_pool.ex
index 52c2b1d33..ed5e71f30 100644
--- a/lib/quickbeam/context_pool.ex
+++ b/lib/quickbeam/context_pool.ex
@@ -22,6 +22,10 @@ defmodule QuickBEAM.ContextPool do
     * `:size` — number of runtime threads (default: `System.schedulers_online()`)
     * `:memory_limit` — maximum JS heap per thread in bytes (default: 256 MB)
     * `:max_stack_size` — maximum JS call stack in bytes (default: 8 MB)
+    * `:wasm_stack_size` — WASM operand stack in bytes for guests started via the JS
+      `WebAssembly.instantiate` path (default: 65536). Distinct from `:max_stack_size`
+      (the JS call stack); raise it for guests whose deep init overflows the 64 KB default.
+    * `:wasm_heap_size` — WASM auxiliary heap in bytes for the same path (default: 65536)
     * `:max_convert_depth` — maximum nesting depth for JS→BEAM value conversion (default: 32)
     * `:max_convert_nodes` — maximum total nodes for JS→BEAM value conversion (default: 10,000)
   """
@@ -46,7 +50,14 @@ defmodule QuickBEAM.ContextPool do
 
     nif_opts =
       opts
-      |> Keyword.take([:memory_limit, :max_stack_size, :max_convert_depth, :max_convert_nodes])
+      |> Keyword.take([
+        :memory_limit,
+        :max_stack_size,
+        :wasm_stack_size,
+        :wasm_heap_size,
+        :max_convert_depth,
+        :max_convert_nodes
+      ])
       |> Map.new()
 
     threads =
diff --git a/lib/quickbeam/context_types.zig b/lib/quickbeam/context_types.zig
index d22555acf..fb8f4c4aa 100644
--- a/lib/quickbeam/context_types.zig
+++ b/lib/quickbeam/context_types.zig
@@ -141,6 +141,12 @@ pub const PoolData = struct {
     thread: ?std.Thread,
     memory_limit: usize = 256 * 1024 * 1024,
     max_stack_size: usize = 8 * 1024 * 1024,
+    // WASM operand stack / heap for the JS `WebAssembly.instantiate` path
+    // (distinct from `max_stack_size`, the JS call stack). Default mirrors the
+    // WASM NIF path; raised via the pool `:wasm_stack_size` opt. Copied into
+    // each context's RuntimeData at create time.
+    wasm_stack_size: u32 = 65_536,
+    wasm_heap_size: u32 = 65_536,
     max_convert_depth: u32 = 32,
     max_convert_nodes: u32 = 10_000,
     shutting_down: std.atomic.Value(bool) = std.atomic.Value(bool).init(false),
diff --git a/lib/quickbeam/context_worker.zig b/lib/quickbeam/context_worker.zig
index 57454cd23..c18ae159b 100644
--- a/lib/quickbeam/context_worker.zig
+++ b/lib/quickbeam/context_worker.zig
@@ -209,6 +209,8 @@ fn handle_create_context(
         .thread = null,
         .max_convert_depth = pd.max_convert_depth,
         .max_convert_nodes = pd.max_convert_nodes,
+        .wasm_stack_size = pd.wasm_stack_size,
+        .wasm_heap_size = pd.wasm_heap_size,
     };
     entry.owner_pid = p.owner_pid;
     entry.id = p.context_id;
diff --git a/lib/quickbeam/quickbeam.zig b/lib/quickbeam/quickbeam.zig
index fb2b32f5b..b0bd813cd 100644
--- a/lib/quickbeam/quickbeam.zig
+++ b/lib/quickbeam/quickbeam.zig
@@ -85,6 +85,18 @@ pub fn start_runtime(owner_pid: beam.pid, opts: beam.term) !RuntimeResource {
     if (get_map_uint(env, opts.v, "max_stack_size")) |v| {
         data.max_stack_size = v;
     }
+    if (get_map_uint(env, opts.v, "wasm_stack_size")) |v| {
+        data.wasm_stack_size = std.math.cast(u32, v) orelse {
+            gpa.destroy(data);
+            return error.WasmStackSizeTooLarge;
+        };
+    }
+    if (get_map_uint(env, opts.v, "wasm_heap_size")) |v| {
+        data.wasm_heap_size = std.math.cast(u32, v) orelse {
+            gpa.destroy(data);
+            return error.WasmHeapSizeTooLarge;
+        };
+    }
     if (get_map_uint(env, opts.v, "max_convert_depth")) |v| {
         data.max_convert_depth = @intCast(v);
     }
@@ -573,6 +585,18 @@ pub fn pool_start(opts: beam.term) !PoolResource {
     if (get_map_uint(env, opts.v, "max_stack_size")) |v| {
         data.max_stack_size = v;
     }
+    if (get_map_uint(env, opts.v, "wasm_stack_size")) |v| {
+        data.wasm_stack_size = std.math.cast(u32, v) orelse {
+            gpa.destroy(data);
+            return error.WasmStackSizeTooLarge;
+        };
+    }
+    if (get_map_uint(env, opts.v, "wasm_heap_size")) |v| {
+        data.wasm_heap_size = std.math.cast(u32, v) orelse {
+            gpa.destroy(data);
+            return error.WasmHeapSizeTooLarge;
+        };
+    }
     if (get_map_uint(env, opts.v, "max_convert_depth")) |v| {
         data.max_convert_depth = @intCast(v);
     }
diff --git a/lib/quickbeam/runtime.ex b/lib/quickbeam/runtime.ex
index 5ceb6cbfc..294c8838b 100644
--- a/lib/quickbeam/runtime.ex
+++ b/lib/quickbeam/runtime.ex
@@ -299,7 +299,14 @@ defmodule QuickBEAM.Runtime do
 
     nif_opts =
       opts
-      |> Keyword.take([:memory_limit, :max_stack_size, :max_convert_depth, :max_convert_nodes])
+      |> Keyword.take([
+        :memory_limit,
+        :max_stack_size,
+        :wasm_stack_size,
+        :wasm_heap_size,
+        :max_convert_depth,
+        :max_convert_nodes
+      ])
       |> Map.new()
 
     resource = QuickBEAM.Native.start_runtime(self(), nif_opts)
diff --git a/lib/quickbeam/types.zig b/lib/quickbeam/types.zig
index 4efcb9f94..6fa05f47f 100644
--- a/lib/quickbeam/types.zig
+++ b/lib/quickbeam/types.zig
@@ -24,6 +24,11 @@ pub const RuntimeData = struct {
     thread: ?std.Thread,
     memory_limit: usize = 256 * 1024 * 1024,
     max_stack_size: usize = 8 * 1024 * 1024,
+    // WASM operand stack / heap for the JS `WebAssembly.instantiate` path
+    // (distinct from `max_stack_size`, the JS call stack). Default mirrors the
+    // WASM NIF path; raised via the runtime `:wasm_stack_size` opt.
+    wasm_stack_size: u32 = 65_536,
+    wasm_heap_size: u32 = 65_536,
     max_convert_depth: u32 = 32,
     max_convert_nodes: u32 = 10_000,
     sync_slots_mutex: std.Thread.Mutex = .{},
diff --git a/lib/quickbeam/wasm.ex b/lib/quickbeam/wasm.ex
index f0fad3992..30c1da203 100644
--- a/lib/quickbeam/wasm.ex
+++ b/lib/quickbeam/wasm.ex
@@ -33,6 +33,15 @@ defmodule QuickBEAM.WASM do
     * `:name` — GenServer name registration
     * `:stack_size` — execution stack in bytes (default: 65536)
     * `:heap_size` — auxiliary heap in bytes (default: 65536)
+
+  > #### JS `WebAssembly` path {: .info}
+  >
+  > These `:stack_size`/`:heap_size` options apply to this native NIF path. Guests
+  > started from JavaScript via `WebAssembly.instantiate` instead take their WASM
+  > operand stack / heap from the owning runtime's `:wasm_stack_size` /
+  > `:wasm_heap_size` options (see `QuickBEAM.Runtime` / `QuickBEAM.ContextPool`),
+  > which also default to 65536. Raise those for guests (e.g. Go `GOOS=js`) whose
+  > deep initialization overflows the 64 KB default.
   """
 
   @type instance :: GenServer.server()
diff --git a/lib/quickbeam/wasm_js.zig b/lib/quickbeam/wasm_js.zig
index d09cc3c14..d0a3bebaf 100644
--- a/lib/quickbeam/wasm_js.zig
+++ b/lib/quickbeam/wasm_js.zig
@@ -17,6 +17,13 @@ const InstanceEntry = struct {
 const ContextState = struct {
     next_instance_id: u64 = 1,
     max_reductions: i64 = 0,
+    // WASM operand stack / auxiliary heap for instances started via the JS
+    // `WebAssembly.instantiate` path. Distinct from the JS call stack
+    // (`max_stack_size`). Default mirrors the WASM NIF path; a consumer raises
+    // it (via the runtime/pool `:wasm_stack_size` opt) for guests whose deep
+    // init would otherwise overflow the 64 KB default.
+    wasm_stack_size: u32 = 65_536,
+    wasm_heap_size: u32 = 65_536,
     instances: std.AutoHashMapUnmanaged(u64, InstanceEntry) = .{},
 
     fn deinit(self: *ContextState) void {
@@ -46,18 +53,20 @@ fn context_key(ctx: *qjs.JSContext) usize {
     return @intFromPtr(ctx);
 }
 
-fn ensure_context_state(ctx: *qjs.JSContext, max_reductions: i64) ?*ContextState {
+fn ensure_context_state(ctx: *qjs.JSContext, max_reductions: i64, wasm_stack_size: u32, wasm_heap_size: u32) ?*ContextState {
     states_mutex.lock();
     defer states_mutex.unlock();
 
     const key = context_key(ctx);
     if (states.get(key)) |state| {
         state.max_reductions = max_reductions;
+        state.wasm_stack_size = wasm_stack_size;
+        state.wasm_heap_size = wasm_heap_size;
         return state;
     }
 
     const state = gpa.create(ContextState) catch return null;
-    state.* = .{ .max_reductions = max_reductions };
+    state.* = .{ .max_reductions = max_reductions, .wasm_stack_size = wasm_stack_size, .wasm_heap_size = wasm_heap_size };
     states.put(gpa, key, state) catch {
         gpa.destroy(state);
         return null;
@@ -82,8 +91,8 @@ pub fn destroy_context(ctx: *qjs.JSContext) void {
     }
 }
 
-pub fn install(ctx: *qjs.JSContext, global: qjs.JSValue, max_reductions: i64) void {
-    _ = ensure_context_state(ctx, max_reductions) orelse return;
+pub fn install(ctx: *qjs.JSContext, global: qjs.JSValue, max_reductions: i64, wasm_stack_size: u32, wasm_heap_size: u32) void {
+    _ = ensure_context_state(ctx, max_reductions, wasm_stack_size, wasm_heap_size) orelse return;
 
     _ = qjs.JS_SetPropertyStr(ctx, global, "__qb_wasm_start", qjs.JS_NewCFunction(ctx, &wasm_start_impl, "__qb_wasm_start", 3));
     _ = qjs.JS_SetPropertyStr(ctx, global, "__qb_wasm_call", qjs.JS_NewCFunction(ctx, &wasm_call_impl, "__qb_wasm_call", 3));
@@ -327,7 +336,7 @@ fn wasm_start_impl(
         wasm_host_imports.PreparedImports.empty();
     errdefer prepared_imports.deinit();
 
-    const managed = wasm_common.start_managed_instance(mod orelse return throw_error(ctx, "null module"), 65_536, 65_536, if (prepared_imports.registrations.len > 0) &prepared_imports else null, &err_buf) orelse return throw_error(ctx, std.mem.sliceTo(&err_buf, 0));
+    const managed = wasm_common.start_managed_instance(mod orelse return throw_error(ctx, "null module"), state.wasm_stack_size, state.wasm_heap_size, if (prepared_imports.registrations.len > 0) &prepared_imports else null, &err_buf) orelse return throw_error(ctx, std.mem.sliceTo(&err_buf, 0));
     const mod_nn = mod orelse return throw_error(ctx, "null module");
     errdefer managed.destroy();
 
diff --git a/lib/quickbeam/worker.zig b/lib/quickbeam/worker.zig
index 030f39921..c78c2823f 100644
--- a/lib/quickbeam/worker.zig
+++ b/lib/quickbeam/worker.zig
@@ -899,7 +899,7 @@ pub const WorkerState = struct {
 
         const global = qjs.JS_GetGlobalObject(self.ctx);
         defer qjs.JS_FreeValue(self.ctx, global);
-        wasm_js.install(self.ctx, global, self.max_reductions);
+        wasm_js.install(self.ctx, global, self.max_reductions, self.rd.wasm_stack_size, self.rd.wasm_heap_size);
     }
 };
 
diff --git a/test/wasm_test.exs b/test/wasm_test.exs
index b0f783f8f..cd4f52ff0 100644
--- a/test/wasm_test.exs
+++ b/test/wasm_test.exs
@@ -696,6 +696,61 @@ defmodule QuickBEAM.WASMTest do
         """)
     end
 
+    # exports `rec(n)`, which recurses `n` deep. WAMR keeps call frames on the
+    # operand-stack buffer sized by `stack_size`, so a deep call overflows a
+    # small stack but fits a larger one — used below to prove the JS
+    # `WebAssembly.instantiate` path honors the runtime's `:wasm_stack_size`.
+    @stack_deep_wasm """
+    new Uint8Array([
+      0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x06, 0x01, 0x60,
+      0x01, 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07, 0x07, 0x01, 0x03,
+      0x72, 0x65, 0x63, 0x00, 0x00, 0x0a, 0x15, 0x01, 0x13, 0x00, 0x20, 0x00,
+      0x45, 0x04, 0x40, 0x41, 0x00, 0x0f, 0x0b, 0x20, 0x00, 0x41, 0x01, 0x6b,
+      0x10, 0x00, 0x0f, 0x0b
+    ])
+    """
+
+    test "JS instantiate path overflows the default 64 KB WASM operand stack", %{rt: rt} do
+      {:error, err} =
+        QuickBEAM.eval(rt, """
+          const bytes = #{@stack_deep_wasm};
+          const {instance} = await WebAssembly.instantiate(bytes);
+          instance.exports.rec(10000);
+        """)
+
+      assert err.message =~ "stack"
+    end
+
+    test "JS instantiate path honors a raised :wasm_stack_size" do
+      {:ok, rt} = QuickBEAM.start(wasm_stack_size: 8 * 1024 * 1024)
+
+      assert {:ok, 0} =
+               QuickBEAM.eval(rt, """
+                 const bytes = #{@stack_deep_wasm};
+                 const {instance} = await WebAssembly.instantiate(bytes);
+                 instance.exports.rec(10000);
+               """)
+
+      QuickBEAM.stop(rt)
+    end
+
+    test "ContextPool propagates :wasm_stack_size to the pooled JS instantiate path" do
+      # Exercises the pool threading path (PoolData -> RuntimeData copy in
+      # context_worker), which the standalone QuickBEAM.start/1 test above does
+      # not cover. Without the copy the pooled context would silently keep 64 KB.
+      {:ok, pool} = QuickBEAM.ContextPool.start_link(size: 1, wasm_stack_size: 8 * 1024 * 1024)
+      {:ok, ctx} = QuickBEAM.Context.start_link(pool: pool)
+
+      assert {:ok, 0} =
+               QuickBEAM.Context.eval(ctx, """
+                 const bytes = #{@stack_deep_wasm};
+                 const {instance} = await WebAssembly.instantiate(bytes);
+                 instance.exports.rec(10000);
+               """)
+
+      QuickBEAM.Context.stop(ctx)
+    end
+
     test "WebAssembly.compile + instantiate", %{rt: rt} do
       {:ok, 300} =
         QuickBEAM.eval(rt, """