diff --git a/packages/compiler/src/frontend/lowering/call-signatures.ts b/packages/compiler/src/frontend/lowering/call-signatures.ts
index 45329d077..c37eba707 100644
--- a/packages/compiler/src/frontend/lowering/call-signatures.ts
+++ b/packages/compiler/src/frontend/lowering/call-signatures.ts
@@ -630,6 +630,41 @@ export function checkDefaultParamBodyType(
   }
 }
 
+/** Whether a lambda's OWN signature maps to its completed ABI position for
+ * position — the contract its reads carry (mapType spells `x?: T` as the
+ * `T | undefined` slot). Two positions can escape the mapped-TYPE
+ * comparison in requireExactArityValue: a parameter the checker spells
+ * `any` holds the checked-dynamic value that mapType leaves unmapped when
+ * its spelling is context-free, and a *contextually* typed lambda's own
+ * signature need not be the slot's (a comparator with a surplus optional
+ * parameter under a two-parameter slot). */
+function lambdaSpellsCompletedAbi(
+  lowerer: Lowerer,
+  lambda: ts.ArrowFunction | ts.FunctionExpression,
+  funcType: IrType,
+): boolean {
+  if (funcType.kind !== "func" || funcType.rest === true) return false;
+  const sigs = lowerer.checker.getCallSignatures(lowerer.typeOf(lambda));
+  if (sigs.length !== 1) return false;
+  const sig = sigs[0]!;
+  const params = sig.getParameters();
+  if (params.length !== funcType.params.length) return false;
+  for (let i = 0; i < params.length; i++) {
+    const pTs = lowerer.checker.getTypeOfSymbol(params[i]!);
+    const slot = funcType.params[i]!;
+    if ((pTs.flags & ts.TypeFlags.Any) !== 0) {
+      if (slot.kind !== "dyn") return false;
+      continue;
+    }
+    const mappedParam = lowerer.mapTypeOf(pTs);
+    if (!mappedParam || !typeEquals(mappedParam, slot)) return false;
+  }
+  const retTs = lowerer.checker.getReturnTypeOfSignature(sig);
+  if ((retTs.flags & ts.TypeFlags.Any) !== 0) return funcType.ret.kind === "dyn";
+  const mappedRet = lowerer.mapTypeOf(retTs);
+  return mappedRet !== null && typeEquals(mappedRet, funcType.ret);
+}
+
 /** DECISION (docs/ir.md): function VALUES carry one completed native ABI.
  * Optional/default parameters appear as literal `T | undefined` slots;
  * typed rest parameters appear as one trailing typed-array slot plus the
@@ -675,6 +710,20 @@ export function requireExactArityValue(
       ? lowerer.mapTypeOf(lowerer.typeOf(contextual))
       : null;
   if (mapped && typeEquals(mapped, funcType)) return;
+  // A lambda whose OWN signature already spells the completed ABI is a
+  // value: every read of it types it that way, and its slot converts it
+  // (the surplus-parameter and undefined-arm adapters). The mapped-type
+  // comparison above misses two such lambdas — one whose `any` parameter
+  // mapType leaves unmapped (`(a: any, b: any, loose?: boolean) => number`
+  // beside a two-parameter slot), and one whose signature is wider than
+  // the contextual slot it is written under.
+  if (
+    contextual !== null &&
+    (ts.isArrowFunction(contextual) || ts.isFunctionExpression(contextual)) &&
+    lambdaSpellsCompletedAbi(lowerer, contextual, funcType)
+  ) {
+    return;
+  }
   // An 'any' slot (`C.prototype.m = function (a, b) {…}` through an
   // untyped receiver), or a slot whose type maps to a checked-dynamic
   // value (a member of a literal lowered with a generic function's type
diff --git a/packages/compiler/src/frontend/lowering/coercions/function-adapters.ts b/packages/compiler/src/frontend/lowering/coercions/function-adapters.ts
index 44bca35e1..50366eeb6 100644
--- a/packages/compiler/src/frontend/lowering/coercions/function-adapters.ts
+++ b/packages/compiler/src/frontend/lowering/coercions/function-adapters.ts
@@ -114,10 +114,22 @@ export function coercibleValue(lowerer: Lowerer, src: IrType, dst: IrType): bool
   return false;
 }
 
+/** Whether an omitted parameter position completes with the native
+ * undefined value: a checked-dynamic slot takes undefined as-is, and an
+ * undefined-armed union (`x?: T`, `x: T = e`) is the completed ABI
+ * spelling. */
+export function omittableTrailingParam(lowerer: Lowerer, type: IrType): boolean {
+  if (type.kind === "dyn" || typeEquals(type, UNDEFINED_T)) return true;
+  return type.kind === "union" && lowerer.armTag(type.unionId, UNDEFINED_T) >= 0;
+}
+
 /** Whether all parameters and the result adapt without a trap-only path.
  * Rest signatures are excluded, except a typed-rest source in a slot with
  * fixed parameters (the rest packing adapter); extra destination
- * parameters are ignored. */
+ * parameters are ignored. A source with MORE parameters than the slot is
+ * clean when every extra trailing one is omittable — the adapter passes
+ * that absent value explicitly (funcCoerceAdapter's omittedParams), the
+ * same way a call through the slot would. */
 export function cleanFuncAdaptable(
   lowerer: Lowerer,
   src: IrType & { kind: "func" },
@@ -125,8 +137,10 @@ export function cleanFuncAdaptable(
 ): boolean {
   if (restPackingAdapterFits(lowerer, src, dst)) return true;
   if (src.rest === true || dst.rest === true) return false;
-  if (src.params.length > dst.params.length) return false;
-  for (let i = 0; i < src.params.length; i++) {
+  for (let i = dst.params.length; i < src.params.length; i++) {
+    if (!omittableTrailingParam(lowerer, src.params[i]!)) return false;
+  }
+  for (let i = 0; i < src.params.length && i < dst.params.length; i++) {
     if (!lowerer.coercibleValue(dst.params[i]!, src.params[i]!)) return false;
   }
   if (dst.ret.kind === "void") return true;
diff --git a/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts b/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
index 71d3d0876..1224bce3e 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
@@ -70,6 +70,7 @@ import {
 import { dynamicReceiverThrows } from "./dynamic-receivers.js";
 import { defaultAfterUndefined, lowerStaticallyUndefinedArgument } from "../optional-arguments.js";
 import { checkedFilterNarrowing } from "./filter-predicates.js";
+import { omittableTrailingParam } from "../coercions/function-adapters.js";
 
 /** `a.map(fn)` / `a.filter(fn)` / `a.forEach(fn)` desugar to a direct
  * call of a synthetic module function — one per method + element/result
@@ -2657,11 +2658,15 @@ export function lowerArraySortCall(
   let fnArg =
     undefinedArg ?? lowerOptionalElementComparator(lowerer, argNode, unwrappedArg, elem, arrT);
   // Reusable JS comparators often accept checked values. Adapt the array's
-  // element ABI just as other array callbacks do before sorting.
+  // element ABI just as other array callbacks do before sorting. A
+  // comparator declaring MORE parameters than sort passes (`(a, b, loose?)`,
+  // the imported semver shape) receives the surplus ones as undefined — the
+  // completion an ordinary two-argument call performs — so it adapts to the
+  // (a, b) slot like any other wider signature.
   if (
     fnArg.type.kind === "func" &&
-    fnArg.type.params.length <= 2 &&
-    fnArg.type.params.every((param) => lowerer.coercibleValue(elem, param))
+    fnArg.type.params.slice(2).every((param) => omittableTrailingParam(lowerer, param)) &&
+    fnArg.type.params.slice(0, 2).every((param) => lowerer.coercibleValue(elem, param))
   ) {
     // Arithmetic on inferred JS values retains a checked result because
     // it can produce BigInt. The sort callback still requires a number.
@@ -2670,7 +2675,7 @@ export function lowerArraySortCall(
         ? F64
         : fnArg.type.ret;
     const expected = funcOf(
-      fnArg.type.params.map(() => elem),
+      fnArg.type.params.slice(0, 2).map(() => elem),
       result,
     );
     if (!typeEquals(fnArg.type, expected)) fnArg = lowerer.coerceToExpected(fnArg, expected);
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8529-sort-comparator-surplus-params.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8529-sort-comparator-surplus-params.ts.json
new file mode 100644
index 000000000..56abd06eb
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8529-sort-comparator-surplus-params.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8529-sort-comparator-surplus-params.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/8529-sort-comparator-surplus-params.ts b/tests/corpus/8529-sort-comparator-surplus-params.ts
new file mode 100644
index 000000000..9b0d0054b
--- /dev/null
+++ b/tests/corpus/8529-sort-comparator-surplus-params.ts
@@ -0,0 +1,42 @@
+// A comparator may declare more parameters than `sort` passes: the surplus
+// positions receive the native undefined value, exactly as in a two-argument
+// call. The imported `(a, b, loose) => …` comparators and lambdas with an
+// optional or defaulted trailing parameter take this shape, and a lambda
+// whose own signature already spells its completed ABI is a value wherever
+// its slot converts.
+
+type Comparer = (a: string, b: string, loose?: boolean) => number;
+
+const seen: string[] = [];
+const cmp: Comparer = (a, b, loose) => {
+  seen.push(loose === undefined ? "absent" : String(loose));
+  return a < b ? -1 : a > b ? 1 : 0;
+};
+
+const versions = ["1.9.0", "1.10.0", "1.2.0"];
+console.log([...versions].sort(cmp).join(","));
+console.log(seen.length > 0 && seen.every((entry) => entry === "absent"));
+console.log(versions.join(","));
+
+// `toSorted` adapts the same way and leaves the receiver untouched.
+console.log(versions.toSorted(cmp).join(","), versions.join(","));
+
+// Direct calls through the value complete the same parameters.
+console.log(cmp("a", "b"), cmp("b", "a"), cmp("a", "a", true));
+
+// An inline comparator with an optional surplus parameter sorts by number.
+const nums = [3, 1, 2];
+const descending = (a: number, b: number, dir?: boolean): number =>
+  dir === undefined ? b - a : a - b;
+console.log(nums.toSorted(descending).join(","));
+console.log([...nums].sort((a, b, dir?: boolean) => (dir ? a - b : b - a)).join(","));
+console.log(nums.join(","));
+
+// A defaulted surplus parameter keeps its default when sort omits it.
+const withDefault = (a: string, b: string, scale = 1): number =>
+  (a < b ? -1 : a > b ? 1 : 0) * scale;
+console.log(["b", "a", "c"].sort(withDefault).join(""));
+console.log(withDefault("a", "b"), withDefault("a", "b", 3));
+
+// A one-parameter comparator is still a prefix of the slot.
+console.log(["d", "c", "a"].sort((left) => left.charCodeAt(0) - 97).join(","));
diff --git a/tests/harness/corpus-costs/8529-sort-comparator-surplus-params.ts.json b/tests/harness/corpus-costs/8529-sort-comparator-surplus-params.ts.json
new file mode 100644
index 000000000..6c25b05fd
--- /dev/null
+++ b/tests/harness/corpus-costs/8529-sort-comparator-surplus-params.ts.json
@@ -0,0 +1 @@
+{ "plain": 3100, "san": 3700 }