diff --git a/packages/compiler/src/frontend/lowering/containers/array-concat.ts b/packages/compiler/src/frontend/lowering/containers/array-concat.ts
index a7ce0a106..e3164689e 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-concat.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-concat.ts
@@ -3,6 +3,7 @@ import { identityPreservingWidening } from "../coercions/identity.js";
 import { countedFor, numLit, varRef } from "../../../ir/build.js";
 import {
   BOOL,
+  DYN,
   F64,
   arrayOf,
   typeEquals,
@@ -18,15 +19,18 @@ import type { Lowerer } from "../lowerer.js";
 
 /** Copy into fresh storage after every argument has evaluated. Array
  * arguments retain their own element ABI; only copied values are widened.
- * Missing slots stay holes and present undefined stays present. */
+ * Missing slots stay holes and present undefined stays present. A shape
+ * entry is an array argument's element type, null for an element value, or
+ * "dyn" for a checked-dynamic argument, which JS spreads when it is an
+ * array at run time (IsConcatSpreadable) and appends otherwise. */
 export function arrayConcatHelper(
   lowerer: Lowerer,
   elem: IrType,
-  shape: (IrType | null)[],
+  shape: (IrType | null | "dyn")[],
   node: ts.CallExpression,
   loc: SrcLoc,
 ): string {
-  const key = `concat:${typeKey(elem)}:${shape.map((type) => (type ? `a:${typeKey(type)}` : "e")).join(";")}`;
+  const key = `concat:${typeKey(elem)}:${shape.map((type) => (type === "dyn" ? "d" : type ? `a:${typeKey(type)}` : "e")).join(";")}`;
   const existing = lowerer.arrHofHelpers.get(key);
   if (existing) return existing;
   const name = keyedHelperName(lowerer, "%arr.concat", key);
@@ -36,7 +40,7 @@ export function arrayConcatHelper(
     ...shape.map((type, i) => ({
       localId: `x.${i}`,
       name: `x${i}`,
-      type: type ? arrayOf(type) : elem,
+      type: type === "dyn" ? DYN : type ? arrayOf(type) : elem,
     })),
   ];
   const locals: IrLocal[] = params.map((p) => ({
@@ -146,6 +150,43 @@ export function arrayConcatHelper(
       },
     ];
   };
+  // A checked-dynamic argument: an array spreads its elements, converted
+  // to the element type; any other value is appended as one element.
+  const appendDyn = (source: IrExpr, site: ts.Node): IrStmt[] => [
+    {
+      kind: "if",
+      cond: { kind: "dynTest", test: "array", value: source, type: BOOL, loc },
+      then: [
+        {
+          kind: "exprStmt",
+          expr: {
+            kind: "arrIntrinsic",
+            method: "concatSpread",
+            receiver: out,
+            args: [lowerer.coerceInto(site, source, arrT)],
+            type: F64,
+            loc,
+          },
+          loc,
+        },
+      ],
+      else_: [
+        {
+          kind: "exprStmt",
+          expr: {
+            kind: "arrIntrinsic",
+            method: "push",
+            receiver: out,
+            args: [lowerer.coerceInto(site, source, elem)],
+            type: F64,
+            loc,
+          },
+          loc,
+        },
+      ],
+      loc,
+    },
+  ];
   const body: IrStmt[] = [
     {
       kind: "varDecl",
@@ -155,11 +196,17 @@ export function arrayConcatHelper(
     },
     ...append(varRef("a.0", arrT, loc), elem, node.expression),
     ...shape.flatMap((type, i) =>
-      append(varRef(`x.${i}`, type ? arrayOf(type) : elem, loc), type, node.arguments.nodes[i]!),
+      type === "dyn"
+        ? appendDyn(varRef(`x.${i}`, DYN, loc), node.arguments.nodes[i]!)
+        : append(
+            varRef(`x.${i}`, type ? arrayOf(type) : elem, loc),
+            type,
+            node.arguments.nodes[i]!,
+          ),
     ),
     { kind: "return", value: out, loc },
   ];
-  if (shape.some((type) => type && !typeEquals(type, elem))) {
+  if (shape.some((type) => type && type !== "dyn" && !typeEquals(type, elem))) {
     for (const id of ["i.0", "offset.0", "count.0"])
       locals.push({ id, name: id, type: F64, mutable: true });
   }
diff --git a/packages/compiler/src/frontend/lowering/containers/array-methods.ts b/packages/compiler/src/frontend/lowering/containers/array-methods.ts
index 4ef203edd..7f0cd6396 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-methods.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-methods.ts
@@ -924,7 +924,7 @@ export function lowerArrayMethodCall(
     // an array-of-arrays receiver given a bare inner array, where TS
     // types it as an element but JS would SPREAD it — is fenced.
     const receiver = lowerer.lowerExpr(access.expression);
-    const shape: (IrType | null)[] = [];
+    const shape: (IrType | null | "dyn")[] = [];
     const args: IrExpr[] = [];
     for (const argNode of call.arguments.nodes) {
       if (ts.isSpreadElement(argNode)) {
@@ -958,6 +958,16 @@ export function lowerArrayMethodCall(
         args.push(arg);
         continue;
       }
+      // A checked-dynamic value: whether JS spreads it is decided at run
+      // time (an array spreads one level, anything else is one element).
+      if (
+        (argIr === null || argIr.kind === "dyn") &&
+        (probedArg ?? tryLowerExpression(lowerer, argNode))?.type.kind === "dyn"
+      ) {
+        shape.push("dyn");
+        args.push(lowerer.lowerExpr(argNode));
+        continue;
+      }
       // An element value — union elements wrap exactly like a push.
       shape.push(null);
       args.push(lowerer.lowerExprExpecting(argNode, elem));
diff --git a/packages/compiler/src/frontend/lowering/expressions/binary.ts b/packages/compiler/src/frontend/lowering/expressions/binary.ts
index 00a23caf5..8d6181a6e 100644
--- a/packages/compiler/src/frontend/lowering/expressions/binary.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/binary.ts
@@ -2280,6 +2280,33 @@ export function lowerStrictEquality(
   negated: boolean,
   loc: SrcLoc,
 ): IrExpr {
+  // JavaScript: a checked-dynamic name the checker's flow analysis narrowed
+  // (`let c; ... (c = s[i]) === " "`) is compared unchecked. Strict
+  // equality never throws, and the flow type is the checker's inference
+  // (an indexed read past the end is undefined whatever it says).
+  const flowNarrowed = (n: ts.Node, e: IrExpr): IrExpr | null =>
+    ts.isIdentifier(n) &&
+    e.kind === "dynCheck" &&
+    e.value.type.kind === "dyn" &&
+    isJsSourceFile(ts.getSourceFileOfNode(n))
+      ? e.value
+      : null;
+  const scalar = (e: IrExpr): boolean =>
+    e.type.kind === "string" || e.type.kind === "f64" || e.type.kind === "bool";
+  {
+    const l = flowNarrowed(leftNode, left);
+    const r = flowNarrowed(rightNode, right);
+    if ((l !== null && (scalar(right) || r !== null)) || (r !== null && scalar(left))) {
+      const cmp: IrExpr = {
+        kind: "dynScalarEq",
+        left: l ?? lowerer.coerceInto(leftNode, left, DYN),
+        right: r ?? lowerer.coerceInto(rightNode, right, DYN),
+        type: BOOL,
+        loc,
+      };
+      return negated ? { kind: "unary", op: "!", operand: cmp, type: BOOL, loc } : cmp;
+    }
+  }
   const plainBothNum = left.type.kind === "f64" && right.type.kind === "f64";
   const bothStr = left.type.kind === "string" && right.type.kind === "string";
   const indexed = tryLowerIndexedComparison(lowerer, left, right, negated, loc);
diff --git a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
index 01cab9901..3bedb7acb 100644
--- a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
@@ -513,6 +513,27 @@ export function maybeNarrow(lowerer: Lowerer, expr: IrExpr, node: ts.Node): IrEx
         )
       )
         return expr;
+      // A `let c;` binding without a type takes its flow types from what
+      // the code assigns, and the checker's types for those values can omit
+      // undefined (`c = s[i]` past the end): reads keep the actual value.
+      if (
+        symbol &&
+        symbol.declarations.some(
+          (decl) =>
+            ts.isVariableDeclaration(decl) &&
+            decl.initializer === undefined &&
+            decl.type === undefined &&
+            ts.isIdentifier(decl.name) &&
+            decl.parent?.parent !== undefined &&
+            ts.isVariableStatement(decl.parent.parent) &&
+            !/@type\b/.test(
+              ts
+                .getSourceFileOfNode(decl)
+                .text.slice(decl.parent?.parent?.pos ?? decl.pos, ts.tokenStartOfNode(decl)),
+            ),
+        )
+      )
+        return expr;
       if (
         symbol &&
         symbol.declarations.some(
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 96d185b44..7feb2d351 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -5367,6 +5367,14 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
           invalid = true;
           return "stop";
         }
+        // An index read's checker type omits the undefined a read past the
+        // end (or of an absent key) answers.
+        let right = node.right;
+        while (ts.isParenthesizedExpression(right)) right = right.expression;
+        if (ts.isElementAccessExpression(right)) {
+          invalid = true;
+          return "stop";
+        }
         const mapped = lowerer.mapTypeOf(lowerer.typeOf(node.right));
         if (!mapped || mapped.kind === "dyn" || mapped.kind === "void" || isUnitType(mapped)) {
           invalid = true;
diff --git a/tests/fixtures/npm/cases/concat-dynamic-args/main.ts b/tests/fixtures/npm/cases/concat-dynamic-args/main.ts
new file mode 100644
index 000000000..35d6e0268
--- /dev/null
+++ b/tests/fixtures/npm/cases/concat-dynamic-args/main.ts
@@ -0,0 +1,3 @@
+// concat of checked-dynamic values decides spreading at run time.
+import { run } from "concat-dynamic";
+console.log(run(JSON.stringify({ text: "desc", list: ["a", "b"] })));
diff --git a/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.d.ts b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.d.ts
new file mode 100644
index 000000000..615876884
--- /dev/null
+++ b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.d.ts
@@ -0,0 +1 @@
+export declare function run(raw: string): string;
diff --git a/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.js b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.js
new file mode 100644
index 000000000..e2bc8066f
--- /dev/null
+++ b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/index.js
@@ -0,0 +1,12 @@
+'use strict';
+// Array.prototype.concat spreads an argument that is an array at run time and
+// appends any other value, whatever the static type says.
+function run(raw) {
+  const parsed = JSON.parse(raw);
+  let output = ['Usage:', ''];
+  output = output.concat([parsed.text, '']);
+  output = output.concat(parsed.list);
+  output = output.concat(parsed.text);
+  return output.join('|') + ' ' + output.length;
+}
+module.exports = { run };
diff --git a/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/package.json b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/package.json
new file mode 100644
index 000000000..afd15f22b
--- /dev/null
+++ b/tests/fixtures/npm/cases/concat-dynamic-args/node_modules/concat-dynamic/package.json
@@ -0,0 +1 @@
+{ "name": "concat-dynamic", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/fixtures/npm/cases/concat-dynamic-args/package.json b/tests/fixtures/npm/cases/concat-dynamic-args/package.json
new file mode 100644
index 000000000..55b029ac8
--- /dev/null
+++ b/tests/fixtures/npm/cases/concat-dynamic-args/package.json
@@ -0,0 +1 @@
+{ "name": "concat-dynamic-args-case", "private": true, "type": "module" }
diff --git a/tests/fixtures/npm/cases/evolving-let-index/main.ts b/tests/fixtures/npm/cases/evolving-let-index/main.ts
new file mode 100644
index 000000000..f34197343
--- /dev/null
+++ b/tests/fixtures/npm/cases/evolving-let-index/main.ts
@@ -0,0 +1,4 @@
+// Reads of a `let` binding whose checker type comes from string index reads
+// see undefined past the end.
+import { countUntilBracket, skipSpaces } from "evolving-let";
+console.log(skipSpaces("ab  ", 2), skipSpaces("a b", 1), countUntilBracket("abc"), countUntilBracket("ab]c"));
diff --git a/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.d.ts b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.d.ts
new file mode 100644
index 000000000..51158faab
--- /dev/null
+++ b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.d.ts
@@ -0,0 +1 @@
+export declare function skipSpaces(str: string, ptr: number): number; export declare function countUntilBracket(str: string): number;
diff --git a/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.js b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.js
new file mode 100644
index 000000000..37cbcac70
--- /dev/null
+++ b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/index.js
@@ -0,0 +1,24 @@
+'use strict';
+// `let c;` takes what an index read past the end answers: undefined.
+/**
+ * @param {string} str
+ * @param {number} ptr
+ * @returns {number}
+ */
+function skipSpaces(str, ptr) {
+  let c;
+  while ((c = str[ptr]) === ' ' || c === '\t') ptr++;
+  return ptr;
+}
+/**
+ * @param {string} str
+ * @returns {number}
+ */
+function countUntilBracket(str) {
+  let c;
+  let ptr = 0;
+  let n = 0;
+  while ((c = str[ptr++]) !== ']' && c) n++;
+  return n;
+}
+module.exports = { skipSpaces, countUntilBracket };
diff --git a/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/package.json b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/package.json
new file mode 100644
index 000000000..02f3315c7
--- /dev/null
+++ b/tests/fixtures/npm/cases/evolving-let-index/node_modules/evolving-let/package.json
@@ -0,0 +1 @@
+{ "name": "evolving-let", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/fixtures/npm/cases/evolving-let-index/package.json b/tests/fixtures/npm/cases/evolving-let-index/package.json
new file mode 100644
index 000000000..ec244a62b
--- /dev/null
+++ b/tests/fixtures/npm/cases/evolving-let-index/package.json
@@ -0,0 +1 @@
+{ "name": "evolving-let-index-case", "private": true, "type": "module" }