diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index 42f9f843..6e170d61 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -1123,23 +1123,28 @@
             (if
               (>= esplit 0)
               (let
-                ((mant (js-string-slice trimmed 0 esplit))
-                  (expstr
-                    (js-string-slice
-                      trimmed
-                      (+ esplit 1)
-                      (len trimmed))))
-                (let
-                  ((m (js-parse-decimal mant 0 0 1 false 0))
-                    (e
-                      (js-parse-decimal
-                        expstr
-                        0
-                        0
-                        1
-                        false
-                        0)))
-                  (* m (pow 10 e))))
+                ((parsed (string->number trimmed)))
+                (if
+                  (= parsed nil)
+                  (let
+                    ((mant (js-string-slice trimmed 0 esplit))
+                      (expstr
+                        (js-string-slice
+                          trimmed
+                          (+ esplit 1)
+                          (len trimmed))))
+                    (let
+                      ((m (js-parse-decimal mant 0 0 1 false 0))
+                        (e
+                          (js-parse-decimal
+                            expstr
+                            0
+                            0
+                            1
+                            false
+                            0)))
+                      (* m (pow 10 e))))
+                  parsed))
               (js-parse-decimal
                 trimmed
                 0
diff --git a/lib/js/test262-scoreboard.json b/lib/js/test262-scoreboard.json
index 753f9b70..f4cdbc0c 100644
--- a/lib/js/test262-scoreboard.json
+++ b/lib/js/test262-scoreboard.json
@@ -56,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 273.6,
+  "elapsed_seconds": 152.5,
   "workers": 1
 }
\ No newline at end of file
diff --git a/lib/js/test262-scoreboard.md b/lib/js/test262-scoreboard.md
index 385613d2..802eeb95 100644
--- a/lib/js/test262-scoreboard.md
+++ b/lib/js/test262-scoreboard.md
@@ -1,7 +1,7 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 273.6s
+Wall time: 152.5s
 
 **Total:** 78/99 runnable passed (78.8%). Raw: pass=78 fail=15 skip=1 timeout=6 total=100.
 
diff --git a/plans/js-on-sx.md b/plans/js-on-sx.md
index cc919152..91cc916b 100644
--- a/plans/js-on-sx.md
+++ b/plans/js-on-sx.md
@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-07 — **`js-num-from-string` uses SX `string->number` for exponent-form numbers.** Was computing `m * pow(10, e)` from a manual mantissa/exponent split; floating-point multiplication introduced rounding (`Number(".12345e-3") - 0.00012345 == 2.7e-20`). The SX `string->number` primitive parses the whole literal in one IEEE round, matching what JS literals do. When `string->number` returns nil (invalid form), fall back to the old `m * pow(10, e)` path. built-ins/Number: 42/50 → 43/50. conformance.sh: 148/148.
+
 - 2026-05-07 — **Constructors (`Object`/`Array`/`Number`/`String`/`Boolean`) carry `__proto__ = Function.prototype`.** Per spec, the constructors are functions and inherit from `Function.prototype`, so `Function.prototype.foo = 1; Array.foo === 1`. Previously the constructor dicts had no `__proto__`, so they only saw `Object.prototype` via the recent fallback — `Function.prototype` mutations were invisible. Added a `(begin (dict-set! ...))` post-init at the end of `runtime.sx` after the constructors are defined. Combined with the existing Object.prototype fallback, the proto chain now terminates correctly for the constructor → `Function.prototype` → `Object.prototype` walk. built-ins/Number: 41/50 → 42/50, built-ins/String: 75/99 → 78/99, built-ins/Array: 12/45 → 13/45. conformance.sh: 148/148.
 
 - 2026-05-07 — **`js-neg` preserves IEEE-754 negative zero.** `-0` was returning `0` (rational integer) because `js-neg` did `(- 0 (js-to-number a))`, which loses sign-of-zero in any arithmetic implementation that follows IEEE 754. Per JS spec, `-0` and `1/-0 === -Infinity` must be observable. Switched to `(* -1 (exact->inexact (js-to-number a)))` so the result is always a float and `-0.0` is preserved. Fixes `Math.asinh(-0)` and other `-0`-sensitive tests; `1/(-0) === -Infinity` now works. built-ins/Math: 41/45 → 42/45. conformance.sh: 148/148.