diff --git a/lib/js/.gitignore b/lib/js/.gitignore
index cb52cc17..3c4d1c7a 100644
--- a/lib/js/.gitignore
+++ b/lib/js/.gitignore
@@ -1 +1,2 @@
 test262-upstream/
+.harness-cache/
diff --git a/lib/js/test262-runner.py b/lib/js/test262-runner.py
index 0fa15c97..49dbe999 100644
--- a/lib/js/test262-runner.py
+++ b/lib/js/test262-runner.py
@@ -55,6 +55,10 @@ HARNESS_DIR = UPSTREAM / "harness"
 DEFAULT_PER_TEST_TIMEOUT_S = 5.0
 DEFAULT_BATCH_TIMEOUT_S = 120
 
+# Cache dir for precomputed SX source of harness JS (one file per Python run).
+# Written once in main(), read via (load ...) by every worker session.
+HARNESS_CACHE_DIR = REPO / "lib" / "js" / ".harness-cache"
+
 
 # ---------------------------------------------------------------------------
 # Harness stub — replaces assert.js + sta.js with something our parser handles.
@@ -588,6 +592,175 @@ def load_test(path: Path):
     )
 
 
+# ---------------------------------------------------------------------------
+# Harness cache — transpile HARNESS_STUB once, write SX to disk.
+# Every worker then loads the cached .sx (a few ms) instead of re-running
+# js-tokenize + js-parse + js-transpile (15+ s).
+# ---------------------------------------------------------------------------
+
+# Remembered across the Python process. None until we've run the precompute.
+_HARNESS_CACHE_PATH: "Path | None" = None
+# Per-filename include cache: maps 'compareArray.js' -> Path of cached .sx.
+_EXTRA_HARNESS_CACHE: dict = {}
+
+
+def _harness_cache_rel_path() -> "str | None":
+    if _HARNESS_CACHE_PATH is None:
+        return None
+    try:
+        return _HARNESS_CACHE_PATH.relative_to(REPO).as_posix()
+    except ValueError:
+        return str(_HARNESS_CACHE_PATH)
+
+
+def _precompute_sx(js_source: str, timeout_s: float = 120.0) -> str:
+    """Run one throwaway sx_server to turn a chunk of JS into the SX text that
+    js-eval would have evaluated. Returns the raw SX source (no outer quotes).
+    """
+    proc = subprocess.Popen(
+        [str(SX_SERVER)],
+        stdin=subprocess.PIPE,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.DEVNULL,
+        cwd=str(REPO),
+        bufsize=0,
+    )
+    fd = proc.stdout.fileno()
+    os.set_blocking(fd, False)
+
+    buf = [b""]
+
+    def readline(timeout: float):
+        deadline = time.monotonic() + timeout
+        while True:
+            nl = buf[0].find(b"\n")
+            if nl >= 0:
+                line = buf[0][: nl + 1]
+                buf[0] = buf[0][nl + 1 :]
+                return line
+            remaining = deadline - time.monotonic()
+            if remaining <= 0:
+                raise TimeoutError("precompute readline timeout")
+            rlist, _, _ = select.select([fd], [], [], remaining)
+            if not rlist:
+                raise TimeoutError("precompute readline timeout")
+            try:
+                chunk = os.read(fd, 65536)
+            except (BlockingIOError, InterruptedError):
+                continue
+            if not chunk:
+                return None
+            buf[0] += chunk
+
+    def run(epoch: int, cmd: str, to: float = 60.0):
+        proc.stdin.write(f"(epoch {epoch})\n{cmd}\n".encode("utf-8"))
+        proc.stdin.flush()
+        deadline = time.monotonic() + to
+        while time.monotonic() < deadline:
+            line = readline(deadline - time.monotonic())
+            if line is None:
+                raise RuntimeError("precompute: sx_server closed stdout")
+            m = RX_OK_INLINE.match(line.decode("utf-8", "replace"))
+            if m and int(m.group(1)) == epoch:
+                return "ok", m.group(2)
+            m = RX_OK_LEN.match(line.decode("utf-8", "replace"))
+            if m and int(m.group(1)) == epoch:
+                val = readline(deadline - time.monotonic())
+                return "ok", (val or b"").decode("utf-8", "replace").rstrip("\n")
+            m = RX_ERR.match(line.decode("utf-8", "replace"))
+            if m and int(m.group(1)) == epoch:
+                return "error", m.group(2)
+        raise TimeoutError(f"precompute epoch {epoch}")
+
+    try:
+        # Wait for ready
+        deadline = time.monotonic() + 15.0
+        while time.monotonic() < deadline:
+            line = readline(deadline - time.monotonic())
+            if line is None:
+                raise RuntimeError("precompute: sx_server closed before ready")
+            if b"(ready)" in line:
+                break
+        # Load JS kernel
+        run(1, '(load "lib/r7rs.sx")')
+        run(2, '(load "lib/js/lexer.sx")')
+        run(3, '(load "lib/js/parser.sx")')
+        run(4, '(load "lib/js/transpile.sx")')
+        # Transpile to SX source via inspect
+        inner = js_source.replace("\\", "\\\\").replace('"', '\\"')
+        inner = inner.replace("\n", "\\n").replace("\r", "\\r").replace("\t", "\\t")
+        outer = inner.replace("\\", "\\\\").replace('"', '\\"')
+        cmd = f'(eval "(inspect (js-transpile (js-parse (js-tokenize \\"{outer}\\"))))")'
+        kind, payload = run(5, cmd, timeout_s)
+        if kind != "ok":
+            raise RuntimeError(f"precompute error: {payload[:200]}")
+        # payload is an SX string-literal — peel one layer of quoting.
+        import json as _json
+        if payload.startswith('"') and payload.endswith('"'):
+            return _json.loads(payload)
+        return payload
+    finally:
+        try:
+            proc.stdin.close()
+        except Exception:
+            pass
+        try:
+            proc.terminate()
+            proc.wait(timeout=3)
+        except Exception:
+            try:
+                proc.kill()
+            except Exception:
+                pass
+
+
+def _harness_fingerprint() -> str:
+    import hashlib
+    # Include the exact runtime/transpile source hash so a change to the
+    # transpiler invalidates the cache automatically.
+    h = hashlib.sha256()
+    h.update(HARNESS_STUB.encode("utf-8"))
+    for p in ("lib/js/lexer.sx", "lib/js/parser.sx", "lib/js/transpile.sx"):
+        try:
+            h.update((REPO / p).read_bytes())
+        except Exception:
+            pass
+    return h.hexdigest()[:16]
+
+
+def precompute_harness_cache() -> Path:
+    """Populate _HARNESS_CACHE_PATH by transpiling HARNESS_STUB once and
+    writing it to disk. Every worker session then does (load <path>) instead.
+
+    Reuses a prior cache file from a previous `python3 test262-runner.py`
+    run when the fingerprint (harness text + transpiler source hash) still
+    matches — that covers the common case of re-running scoreboards back-to-back
+    without touching transpile.sx.
+    """
+    global _HARNESS_CACHE_PATH
+    HARNESS_CACHE_DIR.mkdir(parents=True, exist_ok=True)
+    fp = _harness_fingerprint()
+    dst = HARNESS_CACHE_DIR / f"stub.{fp}.sx"
+    stable = HARNESS_CACHE_DIR / "stub.sx"
+    if dst.exists() and dst.stat().st_size > 0:
+        # Expose both the canonical and fingerprinted names — sessions load
+        # the canonical one.
+        stable.write_bytes(dst.read_bytes())
+        _HARNESS_CACHE_PATH = stable
+        print(f"harness cache: reused {dst.name} ({dst.stat().st_size} bytes)",
+              file=sys.stderr)
+        return stable
+    t0 = time.monotonic()
+    sx = _precompute_sx(HARNESS_STUB)
+    dst.write_text(sx, encoding="utf-8")
+    stable.write_text(sx, encoding="utf-8")
+    _HARNESS_CACHE_PATH = stable
+    dt = time.monotonic() - t0
+    print(f"harness cache: {len(HARNESS_STUB)} JS chars → {len(sx)} SX chars "
+          f"at {stable.relative_to(REPO)} (fp={fp}, {dt:.2f}s)", file=sys.stderr)
+    return stable
+
+
 # ---------------------------------------------------------------------------
 # Long-lived server session
 # ---------------------------------------------------------------------------
@@ -625,13 +798,18 @@ class ServerSession:
         self._run_and_collect(3, '(load "lib/js/parser.sx")', timeout=60.0)
         self._run_and_collect(4, '(load "lib/js/transpile.sx")', timeout=60.0)
         self._run_and_collect(5, '(load "lib/js/runtime.sx")', timeout=60.0)
-        # Preload the stub harness as one big js-eval
-        stub_escaped = sx_escape_for_nested_eval(HARNESS_STUB)
-        self._run_and_collect(
-            6,
-            f'(eval "(js-eval \\"{stub_escaped}\\")")',
-            timeout=60.0,
-        )
+        # Preload the stub harness — use precomputed SX cache when available
+        # (huge win: ~15s js-eval HARNESS_STUB → ~0s load precomputed .sx).
+        cache_rel = _harness_cache_rel_path()
+        if cache_rel is not None:
+            self._run_and_collect(6, f'(load "{cache_rel}")', timeout=60.0)
+        else:
+            stub_escaped = sx_escape_for_nested_eval(HARNESS_STUB)
+            self._run_and_collect(
+                6,
+                f'(eval "(js-eval \\"{stub_escaped}\\")")',
+                timeout=60.0,
+            )
 
     def stop(self) -> None:
         if self.proc is not None:
@@ -964,6 +1142,14 @@ def main(argv):
         all_paths = all_paths[: args.limit]
     print(f"Discovered {len(all_paths)} test files.", file=sys.stderr)
 
+    # Precompute harness cache once per run. Workers (forked) inherit module
+    # globals, so the cache path is visible to every session.start() call.
+    try:
+        precompute_harness_cache()
+    except Exception as e:
+        print(f"harness cache precompute failed ({e}); falling back to js-eval per session",
+              file=sys.stderr)
+
     tests = []
     results = []
     per_cat_count = defaultdict(int)
diff --git a/lib/js/test262-scoreboard.json b/lib/js/test262-scoreboard.json
index f794ef92..fc9e0008 100644
--- a/lib/js/test262-scoreboard.json
+++ b/lib/js/test262-scoreboard.json
@@ -1,12 +1,12 @@
 {
   "totals": {
-    "pass": 118,
-    "fail": 160,
+    "pass": 115,
+    "fail": 174,
     "skip": 1597,
-    "timeout": 22,
+    "timeout": 11,
     "total": 1897,
     "runnable": 300,
-    "pass_rate": 39.3
+    "pass_rate": 38.3
   },
   "categories": [
     {
@@ -35,46 +35,42 @@
     {
       "category": "built-ins/Number",
       "total": 340,
-      "pass": 48,
-      "fail": 45,
+      "pass": 44,
+      "fail": 52,
       "skip": 240,
-      "timeout": 7,
-      "pass_rate": 48.0,
+      "timeout": 4,
+      "pass_rate": 44.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          42
+          52
         ],
         [
           "Timeout",
-          7
-        ],
-        [
-          "ReferenceError (undefined symbol)",
-          3
+          4
         ]
       ]
     },
     {
       "category": "built-ins/String",
       "total": 1223,
-      "pass": 30,
-      "fail": 56,
+      "pass": 31,
+      "fail": 63,
       "skip": 1123,
-      "timeout": 14,
-      "pass_rate": 30.0,
+      "timeout": 6,
+      "pass_rate": 31.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          43
+          53
         ],
         [
           "Timeout",
-          14
+          6
         ],
         [
           "ReferenceError (undefined symbol)",
-          7
+          2
         ],
         [
           "Unhandled: Not callable: \\\\\\",
@@ -100,19 +96,19 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      108
+      128
     ],
     [
       "TypeError: not a function",
-      36
+      37
     ],
     [
       "Timeout",
-      22
+      11
     ],
     [
       "ReferenceError (undefined symbol)",
-      10
+      2
     ],
     [
       "Unhandled: Not callable: \\\\\\",
@@ -129,9 +125,13 @@
     [
       "Unhandled: Not callable: {:__proto__ {}} (kont=5 frames)\\",
       1
+    ],
+    [
+      "Unhandled: js-transpile-binop: unsupported op: >>>\\",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 429.3,
+  "elapsed_seconds": 288.0,
   "workers": 1
 }
\ No newline at end of file
diff --git a/lib/js/test262-scoreboard.md b/lib/js/test262-scoreboard.md
index 3869618c..09c381d4 100644
--- a/lib/js/test262-scoreboard.md
+++ b/lib/js/test262-scoreboard.md
@@ -1,36 +1,37 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 429.3s
+Wall time: 288.0s
 
-**Total:** 118/300 runnable passed (39.3%). Raw: pass=118 fail=160 skip=1597 timeout=22 total=1897.
+**Total:** 115/300 runnable passed (38.3%). Raw: pass=115 fail=174 skip=1597 timeout=11 total=1897.
 
 ## Top failure modes
 
-- **108x** Test262Error (assertion failed)
-- **36x** TypeError: not a function
-- **22x** Timeout
-- **10x** ReferenceError (undefined symbol)
+- **128x** Test262Error (assertion failed)
+- **37x** TypeError: not a function
+- **11x** Timeout
+- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
 - **2x** Unhandled: Not callable: {:__proto__ {}} (kont=6 frames)\
 - **1x** SyntaxError (parse/unsupported syntax)
 - **1x** Unhandled: Not callable: {:__proto__ {}} (kont=5 frames)\
+- **1x** Unhandled: js-transpile-binop: unsupported op: >>>\
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 30 | 56 | 1123 | 14 | 1223 | 30.0% |
+| built-ins/String | 31 | 63 | 1123 | 6 | 1223 | 31.0% |
 | built-ins/Math | 40 | 59 | 227 | 1 | 327 | 40.0% |
-| built-ins/Number | 48 | 45 | 240 | 7 | 340 | 48.0% |
+| built-ins/Number | 44 | 52 | 240 | 4 | 340 | 44.0% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (30/100 — 30.0%)
+### built-ins/String  (31/100 — 31.0%)
 
-- **43x** Test262Error (assertion failed)
-- **14x** Timeout
-- **7x** ReferenceError (undefined symbol)
+- **53x** Test262Error (assertion failed)
+- **6x** Timeout
+- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
 - **2x** Unhandled: Not callable: {:__proto__ {}} (kont=6 frames)\
 
@@ -40,8 +41,7 @@ Wall time: 429.3s
 - **23x** Test262Error (assertion failed)
 - **1x** Timeout
 
-### built-ins/Number  (48/100 — 48.0%)
+### built-ins/Number  (44/100 — 44.0%)
 
-- **42x** Test262Error (assertion failed)
-- **7x** Timeout
-- **3x** ReferenceError (undefined symbol)
+- **52x** Test262Error (assertion failed)
+- **4x** Timeout
diff --git a/lib/js/transpile.sx b/lib/js/transpile.sx
index 42c9dd4d..fbbb452b 100644
--- a/lib/js/transpile.sx
+++ b/lib/js/transpile.sx
@@ -23,7 +23,46 @@
 
 ;; ── tiny helpers ──────────────────────────────────────────────────
 
-(define js-sym (fn (name) (make-symbol name)))
+(define js-has-dollar? (fn (name) (js-has-dollar-loop? name 0 (len name))))
+(define
+  js-has-dollar-loop?
+  (fn
+    (s i n)
+    (cond
+      ((>= i n) false)
+      ((= (char-at s i) "$") true)
+      (else (js-has-dollar-loop? s (+ i 1) n)))))
+
+(define
+  js-mangle-ident
+  (fn
+    (name)
+    (if
+      (js-has-dollar? name)
+      (js-mangle-ident-loop name 0 (len name) "")
+      name)))
+
+;; ── main dispatcher ───────────────────────────────────────────────
+
+(define
+  js-mangle-ident-loop
+  (fn
+    (s i n acc)
+    (cond
+      ((>= i n) acc)
+      ((= (char-at s i) "$")
+        (js-mangle-ident-loop s (+ i 1) n (str acc "_js_dollar_")))
+      (else (js-mangle-ident-loop s (+ i 1) n (str acc (char-at s i)))))))
+
+;; ── Identifier lookup ─────────────────────────────────────────────
+
+;; `undefined` in JS is really a global binding. If the parser emits
+;; (js-undef) we handle that above. A bare `undefined` ident also maps
+;; to the same sentinel.
+(define js-sym (fn (name) (make-symbol (js-mangle-ident name))))
+
+;; ── Unary ops ─────────────────────────────────────────────────────
+
 (define
   js-tag?
   (fn
@@ -34,9 +73,11 @@
       (= (type-of (first ast)) "symbol")
       (= (symbol-name (first ast)) tag))))
 
+;; ── Binary ops ────────────────────────────────────────────────────
+
 (define js-ast-tag (fn (ast) (symbol-name (first ast))))
 
-;; ── main dispatcher ───────────────────────────────────────────────
+;; ── Member / index ────────────────────────────────────────────────
 
 (define
   js-transpile
@@ -146,11 +187,6 @@
       (else
         (error (str "js-transpile: unexpected value type: " (type-of ast)))))))
 
-;; ── Identifier lookup ─────────────────────────────────────────────
-
-;; `undefined` in JS is really a global binding. If the parser emits
-;; (js-undef) we handle that above. A bare `undefined` ident also maps
-;; to the same sentinel.
 (define
   js-transpile-ident
   (fn
@@ -164,8 +200,10 @@
       ((= name "Function") (js-sym "js-function-global"))
       (else (js-sym name)))))
 
-;; ── Unary ops ─────────────────────────────────────────────────────
+;; ── Call ──────────────────────────────────────────────────────────
 
+;; JS `f(a, b, c)` → `(f a b c)` after transpile. Works for both
+;; identifier calls and computed callee (arrow fn, member access).
 (define
   js-transpile-unop
   (fn
@@ -196,7 +234,7 @@
             ((= op "void") (list (js-sym "quote") :js-undefined))
             (else (error (str "js-transpile-unop: unsupported op: " op)))))))))
 
-;; ── Binary ops ────────────────────────────────────────────────────
+;; ── Array literal ─────────────────────────────────────────────────
 
 (define
   js-transpile-binop
@@ -259,22 +297,25 @@
             (js-sym "_a"))))
       (else (error (str "js-transpile-binop: unsupported op: " op))))))
 
-;; ── Member / index ────────────────────────────────────────────────
+;; ── Object literal ────────────────────────────────────────────────
 
+;; Build a dict by `(dict)` + `dict-set!` inside a `let` that yields
+;; the dict as its final expression. This keeps keys in JS insertion
+;; order and allows computed values.
 (define
   js-transpile-member
   (fn (obj key) (list (js-sym "js-get-prop") (js-transpile obj) key)))
 
+;; ── Conditional ───────────────────────────────────────────────────
+
 (define
   js-transpile-index
   (fn
     (obj idx)
     (list (js-sym "js-get-prop") (js-transpile obj) (js-transpile idx))))
 
-;; ── Call ──────────────────────────────────────────────────────────
+;; ── Arrow function ────────────────────────────────────────────────
 
-;; JS `f(a, b, c)` → `(f a b c)` after transpile. Works for both
-;; identifier calls and computed callee (arrow fn, member access).
 (define
   js-transpile-call
   (fn
@@ -320,8 +361,11 @@
           (js-transpile callee)
           (js-transpile-args args))))))
 
-;; ── Array literal ─────────────────────────────────────────────────
+;; ── Assignment ────────────────────────────────────────────────────
 
+;; `a = b` on an ident → (set! a b).
+;; `a += b` on an ident → (set! a (js-add a b)).
+;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
 (define
   js-transpile-new
   (fn
@@ -331,11 +375,6 @@
       (js-transpile callee)
       (cons (js-sym "list") (map js-transpile args)))))
 
-;; ── Object literal ────────────────────────────────────────────────
-
-;; Build a dict by `(dict)` + `dict-set!` inside a `let` that yields
-;; the dict as its final expression. This keeps keys in JS insertion
-;; order and allows computed values.
 (define
   js-transpile-array
   (fn
@@ -354,8 +393,6 @@
           elts))
       (cons (js-sym "list") (map js-transpile elts)))))
 
-;; ── Conditional ───────────────────────────────────────────────────
-
 (define
   js-has-spread?
   (fn
@@ -365,8 +402,9 @@
       ((js-tag? (first lst) "js-spread") true)
       (else (js-has-spread? (rest lst))))))
 
-;; ── Arrow function ────────────────────────────────────────────────
+;; ── End-to-end entry points ───────────────────────────────────────
 
+;; Transpile + eval a single JS expression string.
 (define
   js-transpile-args
   (fn
@@ -385,11 +423,8 @@
           args))
       (cons (js-sym "list") (map js-transpile args)))))
 
-;; ── Assignment ────────────────────────────────────────────────────
-
-;; `a = b` on an ident → (set! a b).
-;; `a += b` on an ident → (set! a (js-add a b)).
-;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
+;; Transpile a JS expression string to SX source text (for inspection
+;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-transpile-object
   (fn
@@ -451,9 +486,6 @@
                 (append inits (list (js-transpile body))))))))
       (list (js-sym "fn") param-syms body-tr))))
 
-;; ── End-to-end entry points ───────────────────────────────────────
-
-;; Transpile + eval a single JS expression string.
 (define
   js-transpile-tpl
   (fn
@@ -465,8 +497,6 @@
       (else
         (cons (js-sym "js-template-concat") (js-transpile-tpl-parts parts))))))
 
-;; Transpile a JS expression string to SX source text (for inspection
-;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-transpile-tpl-parts
   (fn