diff --git a/docs/src/generated/node-v24-compatibility-meta.json b/docs/src/generated/node-v24-compatibility-meta.json
index cbbe881f97..223a3027ba 100644
--- a/docs/src/generated/node-v24-compatibility-meta.json
+++ b/docs/src/generated/node-v24-compatibility-meta.json
@@ -2,6 +2,6 @@
   "schemaVersion": 3,
   "nodeVersion": "24.15.0",
   "nodeCommit": "848430679556aed0bd073f2bc263331ad84fa119",
-  "artifactVersion": "ae934a592466ef1e67c1",
+  "artifactVersion": "cb524f2eb043e40eb9f4",
   "rowCount": 3662
 }
diff --git a/docs/src/generated/node-v24-compatibility.json b/docs/src/generated/node-v24-compatibility.json
index 1391cf23b7..1553483063 100644
--- a/docs/src/generated/node-v24-compatibility.json
+++ b/docs/src/generated/node-v24-compatibility.json
@@ -746,9 +746,9 @@
       "apiEntries": 24,
       "static": {
         "supported": 0,
-        "partial": 10,
+        "partial": 11,
         "refused": 0,
-        "not-implemented": 14,
+        "not-implemented": 13,
         "by-design": 0,
         "unreviewed": 0,
         "not-applicable": 0
@@ -61038,9 +61038,9 @@
         "inherited": true
       },
       "static": {
-        "status": "not-implemented",
-        "detail": "No static compiler lowering is registered for this API yet.",
-        "verification": "registry-gap"
+        "status": "partial",
+        "detail": "Implemented for the call shapes accepted by the compiler lowering.",
+        "verification": "test-backed"
       },
       "dynamic": {
         "status": "partial",
diff --git a/internal/compatibility/generated/node-v24-backlog.json b/internal/compatibility/generated/node-v24-backlog.json
index e73d033de3..e16c10848d 100644
--- a/internal/compatibility/generated/node-v24-backlog.json
+++ b/internal/compatibility/generated/node-v24-backlog.json
@@ -8,8 +8,8 @@
     "tiers": {
       "static": {
         "replace-refusal": 42,
-        "verify-gap": 2418,
-        "audit-partial": 746,
+        "verify-gap": 2417,
+        "audit-partial": 747,
         "classify": 2,
         "implement": 80
       },
@@ -66294,12 +66294,14 @@
       "priority": "high",
       "tiers": {
         "static": {
-          "status": "not-implemented",
-          "action": "verify-gap",
-          "verification": "registry-gap",
-          "confidence": "low",
-          "source": "compiler-unmatched:os",
-          "tests": []
+          "status": "partial",
+          "action": "audit-partial",
+          "verification": "test-backed",
+          "confidence": "high",
+          "source": "surface-manifest:node-builtin.os.availableParallelism",
+          "tests": [
+            "tests/corpus/4598-os-available-parallelism.ts"
+          ]
         },
         "dynamic": {
           "status": "partial",
diff --git a/internal/compatibility/generated/node-v24-internal.json b/internal/compatibility/generated/node-v24-internal.json
index faa5855d9b..4a4faed267 100644
--- a/internal/compatibility/generated/node-v24-internal.json
+++ b/internal/compatibility/generated/node-v24-internal.json
@@ -819,9 +819,9 @@
       "apiEntries": 24,
       "static": {
         "supported": 0,
-        "partial": 10,
+        "partial": 11,
         "refused": 0,
-        "not-implemented": 14,
+        "not-implemented": 13,
         "by-design": 0,
         "unreviewed": 0,
         "not-applicable": 0
@@ -83033,7 +83033,8 @@
         "evidence": "derived:descendants",
         "tests": [
           "tests/corpus/1480-os-network-interfaces.ts",
-          "tests/corpus/2917-builtin-callable-widths.ts"
+          "tests/corpus/2917-builtin-callable-widths.ts",
+          "tests/corpus/4598-os-available-parallelism.ts"
         ]
       },
       "dynamic": {
@@ -83279,8 +83280,11 @@
         "inherited": true
       },
       "static": {
-        "status": "not-implemented",
-        "evidence": "compiler-unmatched:os"
+        "status": "partial",
+        "evidence": "surface-manifest:node-builtin.os.availableParallelism",
+        "tests": [
+          "tests/corpus/4598-os-available-parallelism.ts"
+        ]
       },
       "dynamic": {
         "status": "partial",
diff --git a/internal/compatibility/static-support.json b/internal/compatibility/static-support.json
index 80cdeb2f98..3f5ecefff8 100644
--- a/internal/compatibility/static-support.json
+++ b/internal/compatibility/static-support.json
@@ -80,6 +80,7 @@
     "node-builtin.module": ["tests/corpus/1598-cjs-builtin-require/main.js"],
     "node-builtin.net": ["tests/corpus/2500-net-autosel-timeout.ts", "tests/corpus/2596-net-arg-ladders.cjs"],
     "node-builtin.os": ["tests/corpus/1480-os-network-interfaces.ts", "tests/corpus/2917-builtin-callable-widths.ts"],
+    "node-builtin.os.availableParallelism": ["tests/corpus/4598-os-available-parallelism.ts"],
     "node-builtin.path": ["tests/corpus/957-builtins-namespace.ts", "tests/corpus/2915-path-function-values.ts", "tests/corpus/2916-path-function-values-cjs.ts", "tests/corpus/2917-builtin-callable-widths.ts", "tests/corpus/2918-builtin-callable-widths-cjs.ts"],
     "node-builtin.perf_hooks": ["tests/corpus/957-builtins-namespace.ts"],
     "node-builtin.process": ["tests/corpus/990-process-basics.ts", "tests/corpus/2314-process-introspection.ts"],
diff --git a/packages/compiler/ambient/scriptc-node-fallback.d.ts b/packages/compiler/ambient/scriptc-node-fallback.d.ts
index 7e4590be56..b3933ef5e6 100644
--- a/packages/compiler/ambient/scriptc-node-fallback.d.ts
+++ b/packages/compiler/ambient/scriptc-node-fallback.d.ts
@@ -1688,6 +1688,7 @@ declare module "os" {
   export function type(): string;
   /* Total system memory in bytes (sysctl hw.memsize / sysconf). */
   export function totalmem(): number;
+  export function availableParallelism(): number;
   /* The passwd-entry snapshot (uv_os_get_passwd): shell is `string |
    * null` to match @types/node (POSIX always answers the string arm);
    * homedir is pw_dir — NOT os.homedir()'s $HOME-first cascade. The type
diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index 4d367c1eb1..07df1406f7 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -151,6 +151,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "os.homedir": "scr_os_homedir",
   "os.type": "scr_os_type",
   "os.totalmem": "scr_os_totalmem",
+  "os.availableParallelism": "scr_os_available_parallelism",
   "os.release": "scr_os_release",
   "os.userName": "scr_os_user_name",
   "os.userShell": "scr_os_user_shell",
diff --git a/packages/compiler/src/frontend/lowering/surfaces.ts b/packages/compiler/src/frontend/lowering/surfaces.ts
index 8080b0fed8..91ef6c4b5e 100644
--- a/packages/compiler/src/frontend/lowering/surfaces.ts
+++ b/packages/compiler/src/frontend/lowering/surfaces.ts
@@ -1311,6 +1311,14 @@ export const BUILTIN_MODULE_FNS: Record<
     type: { fn: "os.type", params: [], result: STRING, valueParams: [] },
     // Total physical memory in bytes — same predating-pair story.
     totalmem: { fn: "os.totalmem", params: [], result: F64, valueParams: [] },
+    // uv_available_parallelism: the CPUs this process may use (affinity,
+    // then cgroup quota on Linux), at least 1.
+    availableParallelism: {
+      fn: "os.availableParallelism",
+      params: [],
+      result: F64,
+      valueParams: [],
+    },
     // Entirely special-cased (lowerOsNetworkInterfacesCall): the result is
     // the call site's mapped Dict<NetworkInterfaceInfo[]> shape, verified
     // structurally there — this entry only routes the dispatch.
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index 9e367aacf5..245157851c 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -432,6 +432,8 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
   "os.type": { argTypes: [], result: STRING },
   /** os.totalmem(): total physical memory in bytes. Never throws. */
   "os.totalmem": { argTypes: [], result: F64 },
+  /** os.availableParallelism(): CPUs the process may use (libuv's answer), >= 1. Never throws. */
+  "os.availableParallelism": { argTypes: [], result: F64 },
   /** umask(2): arg < 0 reads without setting (umask has no read-only form
    * — set 0, restore); otherwise sets and answers the PREVIOUS mask.
    * Never throws. */
diff --git a/packages/compiler/surface-manifest.json b/packages/compiler/surface-manifest.json
index 00a1908f5c..531a171974 100644
--- a/packages/compiler/surface-manifest.json
+++ b/packages/compiler/surface-manifest.json
@@ -1334,6 +1334,12 @@
       "status": "static",
       "note": "constant value read"
     },
+    {
+      "id": "node-builtin.os.availableParallelism",
+      "kind": "node-builtin",
+      "name": "os.availableParallelism",
+      "status": "static"
+    },
     {
       "id": "node-builtin.os.homedir",
       "kind": "node-builtin",
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index 06376bc1b2..35d3bc0b43 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -11409,12 +11409,6 @@
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/typed-array-nan-coded-reads.ts": {
-   "order": [
-    "<repo>/tests/corpus/typed-array-nan-coded-reads.ts"
-   ],
-   "diags": []
-  },
   "<repo>/tests/corpus/typed-array-unknown.ts": {
    "order": [
     "<repo>/tests/corpus/typed-array-unknown.ts"
@@ -14536,6 +14530,12 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/native-worker-arena-reuse.ts": {
+   "order": [
+    "<repo>/tests/corpus/native-worker-arena-reuse.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/native-worker-buffer-consumers.ts": {
    "order": [
     "<repo>/tests/corpus/native-worker-buffer-consumers.ts"
@@ -15743,83 +15743,83 @@
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4598-live-array-ref-element-access.ts": {
+  "<repo>/tests/corpus/4596-tdz-nullable-forward-capture.ts": {
    "order": [
-    "<repo>/tests/corpus/4598-live-array-ref-element-access.ts"
+    "<repo>/tests/corpus/4596-tdz-nullable-forward-capture.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/native-worker-arena-reuse.ts": {
+  "<repo>/tests/corpus/4598-map-set-clear-reuse.ts": {
    "order": [
-    "<repo>/tests/corpus/native-worker-arena-reuse.ts"
+    "<repo>/tests/corpus/4598-map-set-clear-reuse.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4596-tdz-nullable-forward-capture.ts": {
+  "<repo>/tests/corpus/missing-read-lambda-results.ts": {
    "order": [
-    "<repo>/tests/corpus/4596-tdz-nullable-forward-capture.ts"
+    "<repo>/tests/corpus/missing-read-lambda-results.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4620-optional-chain-borrowed-receivers.ts": {
+  "<repo>/tests/corpus/missing-read-narrowed-storage.ts": {
    "order": [
-    "<repo>/tests/corpus/4620-optional-chain-borrowed-receivers.ts"
+    "<repo>/tests/corpus/missing-read-narrowed-storage.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4621-edge-preserving-callees/main.ts": {
+  "<repo>/tests/corpus/missing-read-number-conversions.ts": {
    "order": [
-    "<repo>/tests/corpus/4621-edge-preserving-callees/compare.ts",
-    "<repo>/tests/corpus/4621-edge-preserving-callees/items.ts",
-    "<repo>/tests/corpus/4621-edge-preserving-callees/main.ts"
+    "<repo>/tests/corpus/missing-read-number-conversions.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/native-worker-arena-reuse.ts": {
+  "<repo>/tests/corpus/typed-array-nan-coded-reads.ts": {
    "order": [
-    "<repo>/tests/corpus/native-worker-arena-reuse.ts"
+    "<repo>/tests/corpus/typed-array-nan-coded-reads.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4598-map-set-clear-reuse.ts": {
+  "<repo>/tests/corpus/typed-array-present-reads.ts": {
    "order": [
-    "<repo>/tests/corpus/4598-map-set-clear-reuse.ts"
+    "<repo>/tests/corpus/typed-array-present-reads.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/missing-read-lambda-results.ts": {
+  "<repo>/tests/corpus/4601-cycle-holders.ts": {
    "order": [
-    "<repo>/tests/corpus/missing-read-lambda-results.ts"
+    "<repo>/tests/corpus/4601-cycle-holders.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/missing-read-narrowed-storage.ts": {
+  "<repo>/tests/corpus/4602-constructor-escape-order.ts": {
    "order": [
-    "<repo>/tests/corpus/missing-read-narrowed-storage.ts"
+    "<repo>/tests/corpus/4602-constructor-escape-order.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/missing-read-number-conversions.ts": {
+  "<repo>/tests/corpus/4598-os-available-parallelism.ts": {
    "order": [
-    "<repo>/tests/corpus/missing-read-number-conversions.ts"
+    "<repo>/tests/corpus/4598-os-available-parallelism.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/typed-array-present-reads.ts": {
+  "<repo>/tests/corpus/4598-live-array-ref-element-access.ts": {
    "order": [
-    "<repo>/tests/corpus/typed-array-present-reads.ts"
+    "<repo>/tests/corpus/4598-live-array-ref-element-access.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4601-cycle-holders.ts": {
+  "<repo>/tests/corpus/4620-optional-chain-borrowed-receivers.ts": {
    "order": [
-    "<repo>/tests/corpus/4601-cycle-holders.ts"
+    "<repo>/tests/corpus/4620-optional-chain-borrowed-receivers.ts"
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/4602-constructor-escape-order.ts": {
+  "<repo>/tests/corpus/4621-edge-preserving-callees/main.ts": {
    "order": [
-    "<repo>/tests/corpus/4602-constructor-escape-order.ts"
+    "<repo>/tests/corpus/4621-edge-preserving-callees/compare.ts",
+    "<repo>/tests/corpus/4621-edge-preserving-callees/items.ts",
+    "<repo>/tests/corpus/4621-edge-preserving-callees/main.ts"
    ],
    "diags": []
   },
diff --git a/packages/runtime/src/scr_lib.c b/packages/runtime/src/scr_lib.c
index e259f6f7e1..14ba35d213 100644
--- a/packages/runtime/src/scr_lib.c
+++ b/packages/runtime/src/scr_lib.c
@@ -123,6 +123,11 @@ static ScrStr *scr_fs_win_utf8(const WCHAR *text, size_t length) {
 #elif defined(__linux__)
 #include <netpacket/packet.h>
 #endif
+#if defined(__linux__)
+#include <sched.h> /* sched_getaffinity, CPU_COUNT (os.availableParallelism) */
+#elif defined(__APPLE__)
+#include <sys/sysctl.h> /* sysctlbyname (os.availableParallelism) */
+#endif
 
 #define scr_sys_mkdir(p, m) mkdir((p), (m))
 
@@ -1376,6 +1381,13 @@ ScrStr *scr_os_type(void) {
   return scr_str_new("Windows_NT", 10);
 }
 
+double scr_os_available_parallelism(void) {
+  /* uv_available_parallelism on Windows: GetSystemInfo's processor count. */
+  SYSTEM_INFO info;
+  GetSystemInfo(&info);
+  return info.dwNumberOfProcessors < 1 ? 1 : (double)info.dwNumberOfProcessors;
+}
+
 double scr_os_totalmem(void) {
   MEMORYSTATUSEX ms;
   memset(&ms, 0, sizeof ms);
@@ -1413,6 +1425,8 @@ ScrStr *scr_os_user_homedir(void) { return scr_os_homedir(); }
 ScrStr *scr_os_release(void) { return scr_str_new("", 0); }
 ScrStr *scr_os_type(void) { return scr_str_new("WASI", 4); }
 double scr_os_totalmem(void) { return 0; }
+/* One thread of execution, as the dynamic island's os answers. */
+double scr_os_available_parallelism(void) { return 1; }
 /* The guest temp namespace is stable across hosts. `scriptc run` preopens
  * the host's /tmp at this path; other WASI hosts can provide the same
  * capability without leaking a host-specific TMPDIR into the module. */
@@ -1486,6 +1500,98 @@ ScrStr *scr_os_type(void) {
   return scr_str_new(u.sysname, strlen(u.sysname));
 }
 
+#ifdef __linux__
+/* libuv's uv__slurp: one read of at most cap - 1 bytes of path into buf
+ * (NUL-terminated); false when the file cannot be opened or read. A single
+ * read, like libuv, so a long /proc/self/cgroup is cut where Node cuts it. */
+static bool scr_os_slurp(const char *path, char *buf, size_t cap) {
+  int fd = open(path, O_RDONLY | O_CLOEXEC);
+  if (fd < 0) return false;
+  ssize_t n;
+  do
+    n = read(fd, buf, cap - 1);
+  while (n < 0 && errno == EINTR);
+  close(fd);
+  if (n < 0) return false;
+  buf[n] = '\0';
+  return true;
+}
+
+/* libuv's uv__get_constrained_cpu (1.51): the CPU quota of the process's
+ * cgroup in whole CPUs. For cgroup v2 it walks cpu.max from the process's
+ * cgroup up to the root; like libuv, the last level with a numeric limit
+ * (the outermost) decides. Returns false when there is no answer. */
+static bool scr_os_cgroup_cpu_quota(long long *quota) {
+  char cgroup[1024];
+  if (!scr_os_slurp("/proc/self/cgroup", cgroup, sizeof cgroup)) return false;
+  if (strncmp(cgroup, "0::/", 4) == 0) {
+    static const char mount[] = "/sys/fs/cgroup";
+    const char *trimmed = cgroup + 4;
+    int size = (int)strcspn(trimmed, "\n");
+    char path[256], file[256], buf[1024], value[16];
+    snprintf(path, sizeof path, "%s/%.*s/cgroup.controllers", mount, size, trimmed);
+    if (!scr_os_slurp(path, buf, sizeof buf)) return false;
+    snprintf(path, sizeof path, "%s/%.*s", mount, size, trimmed);
+    while (strncmp(path, mount, sizeof mount - 1) == 0) {
+      long long limit, period;
+      snprintf(file, sizeof file, "%s/cpu.max", path);
+      if (scr_os_slurp(file, value, sizeof value) && strncmp(value, "max", 3) != 0 &&
+          sscanf(value, "%lld %lld", &limit, &period) == 2 && period != 0)
+        *quota = limit / period;
+      char *slash = strrchr(path, '/');
+      if (slash == NULL || strcmp(path, mount) == 0) break;
+      *slash = '\0';
+    }
+    return true;
+  }
+  char *cpu = strstr(cgroup, ":cpu,");
+  if (cpu == NULL) return false;
+  cpu += sizeof ":cpu," - 1;
+  int size = (int)strcspn(cpu, "\n");
+  char path[256], buf[64];
+  long long per_period, period;
+  snprintf(path, sizeof path, "/sys/fs/cgroup/%.*s/cpu.cfs_quota_us", size, cpu);
+  if (!scr_os_slurp(path, buf, sizeof buf) || sscanf(buf, "%lld", &per_period) != 1) return false;
+  snprintf(path, sizeof path, "/sys/fs/cgroup/%.*s/cpu.cfs_period_us", size, cpu);
+  if (!scr_os_slurp(path, buf, sizeof buf) || sscanf(buf, "%lld", &period) != 1 || period == 0)
+    return false;
+  *quota = per_period / period;
+  return true;
+}
+#endif
+
+double scr_os_available_parallelism(void) {
+  /* uv_available_parallelism (libuv 1.51, Node's os.availableParallelism):
+   * the CPUs this process may run on (Linux: its affinity mask, then any
+   * smaller cgroup CPU quota; Darwin: hw.activecpu, hw.logicalcpu, hw.ncpu),
+   * else the online processors; at least 1. */
+  long rc = -1;
+#if defined(__linux__)
+  cpu_set_t set;
+  memset(&set, 0, sizeof set);
+  if (sched_getaffinity(0, sizeof set, &set) == 0) rc = CPU_COUNT(&set);
+#elif defined(__APPLE__)
+  static const char *const mib[] = {"hw.activecpu", "hw.logicalcpu", "hw.ncpu"};
+  for (size_t i = 0; i < sizeof mib / sizeof mib[0]; i++) {
+    int nprocs = 0;
+    size_t len = sizeof nprocs;
+    if (sysctlbyname(mib[i], &nprocs, &len, NULL, 0) == 0 && len == sizeof nprocs && nprocs > 0) {
+      rc = nprocs;
+      break;
+    }
+  }
+#endif
+  if (rc < 0) rc = sysconf(_SC_NPROCESSORS_ONLN);
+#if defined(__linux__)
+  /* A quota below one CPU truncates to 0 and is ignored, not clamped to 1:
+   * libuv (1.51 through 1.52) applies only `quota > 0 && quota < rc`, so
+   * Node then answers the affinity count. */
+  long long quota = 0;
+  if (scr_os_cgroup_cpu_quota(&quota) && quota > 0 && quota < rc) rc = (long)quota;
+#endif
+  return rc < 1 ? 1 : (double)rc;
+}
+
 double scr_os_totalmem(void) {
   /* Total physical memory in bytes (sysconf pages × page size — Darwin
    * and Linux both answer _SC_PHYS_PAGES). */
diff --git a/packages/runtime/src/scr_runtime.h b/packages/runtime/src/scr_runtime.h
index 913f6e82dd..0649814e3e 100644
--- a/packages/runtime/src/scr_runtime.h
+++ b/packages/runtime/src/scr_runtime.h
@@ -4085,6 +4085,7 @@ ScrStr *scr_os_tmpdir(void);
 ScrStr *scr_os_release(void); /* uname(2) release — +1 fresh */
 ScrStr *scr_os_type(void);    /* uname(2) sysname — +1 fresh */
 double scr_os_totalmem(void); /* total physical memory, bytes */
+double scr_os_available_parallelism(void); /* uv_available_parallelism: CPUs this process may use, >= 1 */
 /* os.userInfo()'s field trio (pw_name / pw_shell / pw_dir — the passwd
  * home, not the $HOME cascade). +1 fresh; abort on lookup failure. */
 ScrStr *scr_os_user_name(void);
diff --git a/tests/corpus/4598-os-available-parallelism.ts b/tests/corpus/4598-os-available-parallelism.ts
new file mode 100644
index 0000000000..31a89a24c5
--- /dev/null
+++ b/tests/corpus/4598-os-available-parallelism.ts
@@ -0,0 +1,13 @@
+// os.availableParallelism() — differential against the host's own Node:
+// libuv's uv_available_parallelism (the CPU affinity mask, then a smaller
+// cgroup CPU quota on Linux; hw.activecpu on Darwin), so both processes
+// see the same count.
+import * as os from "node:os";
+import { availableParallelism } from "node:os";
+
+const n = os.availableParallelism();
+console.log(typeof n, Number.isInteger(n), n >= 1);
+console.log(n === availableParallelism());
+console.log(n);
+// A thread count derived from it, like a parallel tool's default.
+console.log(Math.max(1, Math.min(n, 64)) === n || n > 64);