diff --git a/packages/compiler/src/frontend/lowering/assertion-members.ts b/packages/compiler/src/frontend/lowering/assertion-members.ts
new file mode 100644
index 000000000..d61c379d6
--- /dev/null
+++ b/packages/compiler/src/frontend/lowering/assertion-members.ts
@@ -0,0 +1,168 @@
+/* Members a type assertion attaches to record values. */
+import * as ts from "../ts7/adapter.js";
+import type { Lowerer } from "./lowerer.js";
+import { type IrType, typeEquals } from "../../ir/ir.js";
+import { hostsAll } from "./expressions/hosted-narrowing.js";
+
+/** `route as Route & { order: number; internal: boolean }` followed by
+ * `view.internal = true` adds a property to the very object `route`
+ * names: Node has no layouts, so every later read of `route` (its JSON,
+ * its keys, a spread) sees the member. Native records are closed structs,
+ * and the asserted intersection maps to a different struct, so neither a
+ * conversion (a copy that severs the write from `route`) nor an erasure
+ * (a struct without the slot) is faithful.
+ *
+ * This pass finds such assertions before any body lowers and gives each
+ * record arm of the operand's type the asserted members it does not
+ * declare, as optional fields. Values the program builds hold them absent,
+ * which is invisible to every key surface (`in`, Object.keys, JSON,
+ * inspect, spread). The asserted type then refines the stored arms
+ * instead of naming new ones (hosted-narrowing.ts), so the assertion keeps
+ * the value and its writes land in the stored object.
+ *
+ * Only the plain form qualifies: one intersection whose constituents are
+ * the operand's own type and type literals of required property
+ * signatures, over plain data types (object types, aliases and interfaces;
+ * not classes or the standard library's types). Arms that are tuples or
+ * carry an index signature, members whose type does not map, and members
+ * two assertions type differently are left alone: those assertions keep
+ * their exact-shape diagnostics. Assertions whose operand already declares
+ * every member are skipped before any type maps, so programs without the
+ * form intern exactly the types they did before. */
+export function extendAssertedRecordMembers(
+  lowerer: Lowerer,
+  files: readonly ts.SourceFile[],
+): void {
+  const additions = new Map<string, Map<string, IrType>>();
+  const conflicts = new Set<string>();
+  const consider = (node: ts.AsExpression | ts.TypeAssertion): void => {
+    let typeNode: ts.TypeNode = node.type;
+    while (ts.isParenthesizedTypeNode(typeNode)) typeNode = typeNode.type;
+    if (!ts.isIntersectionTypeNode(typeNode)) return;
+    const parts = typeNode.types.nodes;
+    const literals = parts.filter((p) => ts.isTypeLiteralNode(p));
+    const bases = parts.filter((p) => !ts.isTypeLiteralNode(p));
+    if (literals.length === 0 || bases.length !== 1) return;
+    const memberNodes: { name: string; type: ts.TypeNode }[] = [];
+    for (const literal of literals) {
+      if (!ts.isTypeLiteralNode(literal)) return;
+      for (const member of literal.members.nodes) {
+        // A required member: the form that writes through the view. An
+        // optional one only reads, and keeps its exact-shape fence.
+        if (!ts.isPropertySignatureDeclaration(member) || !member.type || member.questionToken)
+          return;
+        const name = ts.isIdentifier(member.name)
+          ? member.name.text
+          : ts.isStringLiteral(member.name)
+            ? member.name.text
+            : null;
+        if (name === null || name.startsWith("%")) return;
+        memberNodes.push({ name, type: member.type });
+      }
+    }
+    // Checker-only gate first: an assertion every constituent of whose
+    // operand type already declares the members (`e as Expr & { kind:
+    // "mul" }`, the refinement idiom) attaches nothing, and mapping its
+    // types here would intern them out of the usual order.
+    const operandTs = lowerer.typeOf(node.expression);
+    const constituents = operandTs.isUnionType() ? ts.constituentTypes(operandTs) : [operandTs];
+    const declares = (t: ts.Type, name: string): boolean =>
+      lowerer.checker.getPropertyOfType(t, name) !== undefined;
+    if (constituents.every((t) => memberNodes.every((m) => declares(t, m.name)))) return;
+    // Plain data types only: object literal types, aliases and interfaces
+    // (not classes, and not the standard library's types).
+    const dataType = (t: ts.Type): boolean => {
+      const symbol = t.getAliasSymbol() ?? t.getSymbol();
+      return (
+        !!symbol &&
+        (symbol.flags & ts.SymbolFlags.Class) === 0 &&
+        symbol.declarations.length > 0 &&
+        symbol.declarations.every(
+          (d) =>
+            !lowerer.isStdlibFile(d.getSourceFile()) &&
+            (ts.isTypeLiteralNode(d) ||
+              ts.isInterfaceDeclaration(d) ||
+              ts.isTypeAliasDeclaration(d)),
+        )
+      );
+    };
+    if (!constituents.every(dataType)) return;
+    const operand = lowerer.mapTypeOf(operandTs);
+    const base = lowerer.mapTypeOf(lowerer.checker.getTypeFromTypeNode(bases[0]!));
+    if (!operand || !base || !typeEquals(operand, base)) return;
+    const arms =
+      operand.kind === "union" ? (lowerer.unions.get(operand.unionId)?.arms ?? []) : [operand];
+    if (arms.length === 0) return;
+    const shapeIds: string[] = [];
+    for (const arm of arms) {
+      if (arm.kind !== "record") return;
+      const shape = lowerer.shapes.get(arm.shapeId);
+      if (!shape || shape.tuple || shape.indexValue) return;
+      shapeIds.push(arm.shapeId);
+    }
+    const members: { name: string; type: IrType }[] = [];
+    for (const { name, type } of memberNodes) {
+      if (shapeIds.every((id) => lowerer.shapes.get(id)!.fields.some((f) => f.name === name)))
+        continue;
+      const mapped = lowerer.mapTypeOf(lowerer.checker.getTypeFromTypeNode(type));
+      if (!mapped || mapped.kind === "void") return;
+      const optional = lowerer.withUndefinedArmOf(mapped);
+      if (!optional) return;
+      members.push({ name, type: optional });
+    }
+    for (const shapeId of shapeIds) {
+      const shape = lowerer.shapes.get(shapeId)!;
+      for (const member of members) {
+        if (shape.fields.some((f) => f.name === member.name)) continue;
+        let added = additions.get(shapeId);
+        if (!added) additions.set(shapeId, (added = new Map()));
+        const prior = added.get(member.name);
+        if (prior && !typeEquals(prior, member.type)) conflicts.add(`${shapeId}:${member.name}`);
+        else added.set(member.name, member.type);
+      }
+    }
+  };
+  const visit = (node: ts.Node): void => {
+    if (ts.isAsExpression(node) || ts.isTypeAssertion(node)) consider(node);
+    ts.forEachChild(node, visit);
+  };
+  for (const sf of files) {
+    if (sf.isDeclarationFile) continue;
+    // Cheap text gate: the form needs an intersection with a type literal.
+    if (!/&\s*\{/.test(sf.text)) continue;
+    visit(sf);
+  }
+  for (const [shapeId, added] of additions) {
+    const extra = [...added]
+      .filter(([name]) => !conflicts.has(`${shapeId}:${name}`))
+      .map(([name, type]) => ({ name, type }));
+    lowerer.shapes.extendShape(shapeId, extra);
+  }
+  if (additions.size > 0) keepStoredTypeOfAssertedGlobals(lowerer, files);
+}
+
+/** A module-level `const view = value as T & { member: X }` binding keeps
+ * the stored type of `value` once the members are attached, exactly as a
+ * local binding does (lowerVarDecl): the assertion keeps the object, and
+ * writes through the binding must reach it. Globals took their checker
+ * (or annotated) type during collection, before the members existed; an
+ * annotation the stored type hosts gives way to it the same way. */
+function keepStoredTypeOfAssertedGlobals(lowerer: Lowerer, files: readonly ts.SourceFile[]): void {
+  for (const sf of files) {
+    if (sf.isDeclarationFile) continue;
+    for (const statement of sf.statements.nodes) {
+      if (!ts.isVariableStatement(statement)) continue;
+      if ((statement.declarationList.flags & ts.NodeFlags.Const) === 0) continue;
+      for (const decl of statement.declarationList.declarations.nodes) {
+        if (!ts.isIdentifier(decl.name) || !decl.initializer) continue;
+        let init: ts.Expression = decl.initializer;
+        while (ts.isParenthesizedExpression(init)) init = init.expression;
+        if (!ts.isAsExpression(init) && !ts.isTypeAssertion(init)) continue;
+        const global = lowerer.globalOf(decl.name);
+        const stored = lowerer.mapTypeOf(lowerer.typeOf(init.expression));
+        if (!global || !stored || (stored.kind !== "union" && stored.kind !== "record")) continue;
+        if (hostsAll(lowerer, stored, global.type)) global.type = stored;
+      }
+    }
+  }
+}
diff --git a/packages/compiler/src/frontend/lowering/expressions/hosted-narrowing.ts b/packages/compiler/src/frontend/lowering/expressions/hosted-narrowing.ts
new file mode 100644
index 000000000..87a7ba38e
--- /dev/null
+++ b/packages/compiler/src/frontend/lowering/expressions/hosted-narrowing.ts
@@ -0,0 +1,157 @@
+/* Narrowings that refine record arms without naming a stored arm. */
+import type { Lowerer } from "../lowerer.js";
+import { type IrExpr, type IrType, type SrcLoc, typeEquals } from "../../../ir/ir.js";
+
+/** A checker narrowing can refine the arms of a union of records with an
+ * intersection: a predicate `r is Route & { path: string }`, or the
+ * `"path" in r` narrowing, which intersects arms that do not declare the
+ * name with `Record<"path", unknown>`. Each narrowed arm then maps to a
+ * new record shape that is not an arm of the stored union, although the
+ * value never changed layout. A stored record arm HOSTS a narrowed record
+ * when both name the same members, each narrowed member refined to a
+ * subset of the stored slot type (or to `unknown`); a member an assertion
+ * attached to the stored shape (assertion-members.ts) may be missing from
+ * the narrowed one. A required member the stored arm does not declare
+ * cannot exist on its values: native records are closed, so `in` already
+ * answers false there and the narrowed arm is uninhabited.
+ *
+ * Only unions of two or more record arms without a literal discriminant
+ * qualify: discriminated unions narrow by their discriminant, and a
+ * record with unit arms keeps its own predicate-strengthening reads.
+ *
+ * Returns the stored arms that can hold a value of the narrowed type, or
+ * null when the union does not qualify, when some narrowed arm is neither
+ * a stored arm nor a record, or when no stored arm is excluded (there is
+ * nothing to narrow). */
+export function hostedArmTags(
+  lowerer: Lowerer,
+  fromId: string,
+  narrowed: IrType,
+): Set<number> | null {
+  const from = lowerer.unions.get(fromId);
+  if (!from || !recordUnion(lowerer, fromId)) return null;
+  const narrowedArms =
+    narrowed.kind === "union" ? lowerer.unions.get(narrowed.unionId)?.arms : [narrowed];
+  if (!narrowedArms || narrowedArms.length === 0) return null;
+  const allowed = new Set<number>();
+  let refined = false;
+  for (const arm of narrowedArms) {
+    const direct = lowerer.armTag(fromId, arm);
+    if (direct >= 0) {
+      allowed.add(direct);
+      continue;
+    }
+    if (arm.kind !== "record") return null;
+    refined = true;
+    from.arms.forEach((stored, tag) => {
+      if (stored.kind === "record" && recordHosts(lowerer, stored.shapeId, arm.shapeId)) {
+        allowed.add(tag);
+      }
+    });
+  }
+  if (!refined || allowed.size === 0 || allowed.size === from.arms.length) return null;
+  return allowed;
+}
+
+/** A union of at least two record arms without a literal discriminant. */
+function recordUnion(lowerer: Lowerer, unionId: string): boolean {
+  const def = lowerer.unions.get(unionId);
+  if (!def || def.discriminant) return false;
+  return def.arms.filter((arm) => arm.kind === "record").length >= 2;
+}
+
+/** Whether every value of `narrowed` (a checker refinement of a value
+ * stored as `storage`) lives in the stored layout: each narrowed arm is a
+ * stored arm or a record a stored record arm hosts (see hostedArmTags),
+ * and at least one needs the hosting. A union storage must qualify as in
+ * hostedArmTags; a single record only when an assertion attached members
+ * to it. A binding initialized by such an assertion keeps the stored type,
+ * so its member writes reach the asserted object. */
+export function hostsAll(lowerer: Lowerer, storage: IrType, narrowed: IrType): boolean {
+  if (typeEquals(storage, narrowed)) return false;
+  let storedArms: readonly IrType[];
+  if (storage.kind === "union") {
+    if (!recordUnion(lowerer, storage.unionId)) return false;
+    storedArms = lowerer.unions.get(storage.unionId)!.arms;
+  } else if (storage.kind === "record" && lowerer.shapes.attachedNames(storage.shapeId).length) {
+    storedArms = [storage];
+  } else {
+    return false;
+  }
+  const narrowedArms =
+    narrowed.kind === "union" ? lowerer.unions.get(narrowed.unionId)?.arms : [narrowed];
+  if (!narrowedArms || narrowedArms.length === 0) return false;
+  let refined = false;
+  for (const arm of narrowedArms) {
+    if (storedArms.some((stored) => typeEquals(stored, arm))) continue;
+    if (arm.kind !== "record") return false;
+    const hosted = storedArms.some(
+      (stored) => stored.kind === "record" && recordHosts(lowerer, stored.shapeId, arm.shapeId),
+    );
+    if (!hosted) return false;
+    refined = true;
+  }
+  return refined;
+}
+
+function recordHosts(lowerer: Lowerer, storedId: string, narrowedId: string): boolean {
+  const stored = lowerer.shapes.get(storedId);
+  const narrowed = lowerer.shapes.get(narrowedId);
+  if (!stored || !narrowed || stored.tuple || narrowed.tuple) return false;
+  if (stored.indexValue || narrowed.indexValue) return false;
+  const attached = lowerer.shapes.attachedNames(storedId);
+  for (const sf of stored.fields) {
+    if (!narrowed.fields.some((nf) => nf.name === sf.name) && !attached.includes(sf.name))
+      return false;
+  }
+  for (const nf of narrowed.fields) {
+    const sf = stored.fields.find((f) => f.name === nf.name);
+    if (!sf || !slotRefines(lowerer, nf.type, sf.type)) return false;
+  }
+  return true;
+}
+
+/** Whether a stored slot of type `stored` can be read as `narrowed`: the
+ * same type, `unknown`, or a subset of the stored union's arms (a read
+ * site narrows from the stored slot exactly as it does for a predicate
+ * strengthening one record). */
+function slotRefines(lowerer: Lowerer, narrowed: IrType, stored: IrType): boolean {
+  if (typeEquals(narrowed, stored) || narrowed.kind === "dyn") return true;
+  if (stored.kind !== "union") return false;
+  if (narrowed.kind !== "union") return lowerer.armTag(stored.unionId, narrowed) >= 0;
+  const arms = lowerer.unions.get(narrowed.unionId)?.arms;
+  return !!arms && arms.every((arm) => lowerer.armTag(stored.unionId, arm) >= 0);
+}
+
+/** The stored view of a hosted narrowing (see hostedArmTags): one stored
+ * record arm extracted with the checked helper, or the sub-union of the
+ * hosting arms re-tagged with the excluded arms as traps. Both keep the
+ * value's stored layout, so member reads and writes reach the same
+ * object. Null when the narrowing is not a hosted one. */
+export function hostedStorageView(
+  lowerer: Lowerer,
+  value: IrExpr,
+  narrowed: IrType,
+  loc: SrcLoc,
+): IrExpr | null {
+  if (value.type.kind !== "union") return null;
+  const fromId = value.type.unionId;
+  const allowed = hostedArmTags(lowerer, fromId, narrowed);
+  if (!allowed) return null;
+  const from = lowerer.unions.get(fromId)!;
+  if (allowed.size === 1) {
+    const arm = from.arms[[...allowed][0]!]!;
+    const helper = lowerer.narrowedArmHelper(fromId, arm, loc);
+    return helper ? lowerer.checkedNarrowCall(helper, value, arm, loc) : null;
+  }
+  const arms = from.arms.filter((_, tag) => allowed.has(tag));
+  const subId = lowerer.unions.transform(from, arms);
+  const trappable = new Set<number>();
+  from.arms.forEach((_, tag) => {
+    if (!allowed.has(tag)) trappable.add(tag);
+  });
+  const helper = lowerer.unionRetagHelper(fromId, subId, loc, trappable);
+  if (!helper) return null;
+  const type: IrType = { kind: "union", unionId: subId };
+  return { kind: "call", callee: helper, args: [value], type, loc };
+}
diff --git a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
index ca68d34da..b54da8180 100644
--- a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
@@ -29,6 +29,7 @@ import { isOptionalProcessStreamProperty } from "../builtins/process.js";
 import { literalValues } from "../../literal-values.js";
 import { literalUnionArm } from "../../union-discriminants.js";
 import { checkedArmExtraction } from "../coercions/unions.js";
+import { hostedStorageView, hostsAll } from "./hosted-narrowing.js";
 import { nodeThrowExpr, varRef } from "../../../ir/build.js";
 import { InternalCompilerError } from "../../../errors.js";
 import {
@@ -659,6 +660,12 @@ export function maybeNarrow(lowerer: Lowerer, expr: IrExpr, node: ts.Node): IrEx
       };
     }
   }
+  // A narrowing that refines record arms with an intersection keeps the
+  // stored layout: view the value as the stored arms that can hold it.
+  if (narrowed?.kind === "union") {
+    const hosted = hostedStorageView(lowerer, expr, narrowed, expr.loc);
+    if (hosted) return hosted;
+  }
   // A checker type narrowed to a UNIT arm (the `=== undefined` branch)
   // also stays union-typed: a unit arm has no payload to extract, and
   // nothing useful reads such a value anyway. (Standalone undefined maps
@@ -668,6 +675,14 @@ export function maybeNarrow(lowerer: Lowerer, expr: IrExpr, node: ts.Node): IrEx
     return expr;
   }
   const tag = lowerer.armTag(expr.type.unionId, narrowed);
+  if (
+    tag < 0 &&
+    narrowed.kind === "record" &&
+    !lowerer.unions.get(expr.type.unionId)?.discriminant
+  ) {
+    const hosted = hostedStorageView(lowerer, expr, narrowed, expr.loc);
+    if (hosted) return hosted;
+  }
   if (tag < 0) {
     // The checker may refine fields inside a discriminated record too.
     // That creates a different mapped shape, but does not change the
@@ -926,6 +941,10 @@ export function runtimeOptionalReceiverRead(
       loc,
     );
     if (!helper) {
+      // Intersection-refined record arms read through their stored
+      // layout (the predicate `r is Route & { path: string }`).
+      const hosted = hostedStorageView(lowerer, varRef(local.id, local.type, loc), narrowed, loc);
+      if (hosted) return hosted;
       lowerer.unsupported(
         "SC1090",
         expr,
@@ -946,6 +965,10 @@ export function runtimeOptionalReceiverRead(
     // path below; mixed storage also needs to validate its concrete class.
     if (view && (lowerer.unions.get(local.type.unionId)?.arms.length ?? 0) > 2) return view;
   }
+  if (narrowed.kind === "record" && lowerer.armTag(local.type.unionId, narrowed) < 0) {
+    const hosted = hostedStorageView(lowerer, varRef(local.id, local.type, loc), narrowed, loc);
+    if (hosted) return hosted;
+  }
   const undefTag = lowerer.armTag(local.type.unionId, UNDEFINED_T);
   const def = lowerer.unions.get(local.type.unionId);
   if (!def || undefTag < 0)
@@ -1300,6 +1323,12 @@ export function lowerAsExpression(
     ) {
       return lowerer.coerceInto(expr, inner, target);
     }
+    // `job as Job & { startedAt: number }` on a record: the asserted
+    // record refines the stored one, so the assertion keeps the object
+    // (a width copy would sever writes through the asserted view).
+    if (inner.type.kind === "record" && target && hostsAll(lowerer, inner.type, target)) {
+      return inner;
+    }
     // Static assertions normally erase, but record layouts are
     // monomorphic: a consumer selected from the asserted shape must see
     // that shape physically. Reuse the ordinary slot coercion so plain
@@ -1326,6 +1355,12 @@ export function lowerAsExpression(
           return lowerer.checkedNarrowCall(helper, inner, target, locOf(expr));
         }
       }
+      // An intersection refining the stored record arms keeps the value
+      // (hosted-narrowing.ts): the stored arms that can hold it.
+      if (target && (target.kind === "union" || target.kind === "record")) {
+        const hosted = hostedStorageView(lowerer, inner, target, locOf(expr));
+        if (hosted) return hosted;
+      }
     }
     // `node.parent as Sub | undefined`: a union target whose class arm is
     // narrower than the stored class gets the same checked class view as
diff --git a/packages/compiler/src/frontend/lowering/expressions/relational.ts b/packages/compiler/src/frontend/lowering/expressions/relational.ts
index 619c0fe37..4aa62534e 100644
--- a/packages/compiler/src/frontend/lowering/expressions/relational.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/relational.ts
@@ -38,7 +38,7 @@ import { lowerCryptoKeyInstanceof } from "../builtins/webcrypto.js";
 import { lowerNetAddressInstanceof } from "../builtins/net-address.js";
 import { fenceEarlyNsMemberRef, nsMemberIdentOf } from "../lower-namespaces.js";
 import { maybePromiseInstanceOf } from "../maybe-promise.js";
-import { isSafeToDiscard } from "./evaluation-safety.js";
+import { isSafeToDiscard, isSafeToRepeat } from "./evaluation-safety.js";
 import { type ClassInfo, storedClassValueType } from "../lower-classes.js";
 import { classInstanceOf } from "../class-dynamic-dispatch.js";
 import { InternalCompilerError } from "../../../errors.js";
@@ -1429,13 +1429,14 @@ export function lowerInExpression(
   // arm answers membership STATICALLY (a declared non-optional field is
   // always present, an undeclared name never is), so the whole test
   // collapses to runtime TAG tests — tsc's own narrowing then types the
-  // branches (the discriminating-`in` idiom). Optional (undefined-armed)
-  // fields and tuple/index-signature arms answer per VALUE, not per arm
-  // — those unions keep the fence below.
+  // branches (the discriminating-`in` idiom). An arm declaring the key
+  // as an optional (undefined-armed) slot answers per VALUE instead:
+  // its tag test joins that payload's presence test (see
+  // perValueUnionIn). Tuple/index-signature arms keep the fence below.
   if (recv.type.kind === "union") {
     const unionId = recv.type.unionId;
     const arms = lowerer.unions.get(unionId)?.arms ?? [];
-    const answers: { tag: number; has: boolean }[] = [];
+    const answers: { tag: number; has: boolean | "slot"; arm: IrType }[] = [];
     let staticAnswers = arms.length > 0;
     for (const arm of arms) {
       const tag = lowerer.armTag(unionId, arm);
@@ -1452,14 +1453,17 @@ export function lowerInExpression(
         f.type.kind === "union" &&
         lowerer.armTag(f.type.unionId, UNDEFINED_T) >= 0
       ) {
-        staticAnswers = false; // an optional slot: presence is per-value
-        break;
+        answers.push({ tag, has: "slot", arm }); // presence is per-value
+        continue;
       }
       // Accessor properties are own properties to `in` (Node answers
       // true without invoking the getter) — either slot present makes
       // the name a member.
       const acc = shape.fields.some((x) => x.name === `%get:${key}` || x.name === `%set:${key}`);
-      answers.push({ tag, has: f !== undefined || acc || inherited });
+      answers.push({ tag, has: f !== undefined || acc || inherited, arm });
+    }
+    if (staticAnswers && answers.some((a) => a.has === "slot")) {
+      return perValueUnionIn(lowerer, recv, unionId, key, answers, loc);
     }
     if (staticAnswers) {
       const pureRecv =
@@ -1611,6 +1615,75 @@ export function lowerInExpression(
   );
 }
 
+/** `"k" in u` over a union of fixed record shapes where some arm declares
+ * `k` as an optional slot. Arms without the name answer false and arms
+ * with an always-present member answer true, both by tag; an optional
+ * slot answers its payload's own-property presence (an explicit
+ * undefined is present, an omitted or deleted property is not), read
+ * through the checked arm extraction the tag test has just proven. The
+ * receiver evaluates once: a non-repeatable one is stabilized in a
+ * hidden local first. */
+function perValueUnionIn(
+  lowerer: Lowerer,
+  recv: IrExpr,
+  unionId: string,
+  key: string,
+  answers: readonly { tag: number; has: boolean | "slot"; arm: IrType }[],
+  loc: SrcLoc,
+): IrExpr {
+  const recvType = recv.type;
+  const stable = isSafeToRepeat(recv) ? null : lowerer.declareHiddenLocal("%inRecv", recvType);
+  const ref = (): IrExpr => (stable ? varRef(stable.id, recvType, loc) : recv);
+  const isTag = (tag: number): IrExpr => ({
+    kind: "unionIsTag",
+    unionId,
+    tag,
+    negated: false,
+    value: ref(),
+    type: BOOL,
+    loc,
+  });
+  let out: IrExpr | null = null;
+  for (const a of answers) {
+    if (a.has === false) continue;
+    let test = isTag(a.tag);
+    if (a.has === "slot" && a.arm.kind === "record") {
+      test = {
+        kind: "logical",
+        op: "&&",
+        left: test,
+        right: {
+          kind: "recordHas",
+          obj: {
+            kind: "unionNarrow",
+            unionId,
+            tag: a.tag,
+            value: ref(),
+            type: a.arm,
+            loc,
+          },
+          shapeId: a.arm.shapeId,
+          field: key,
+          type: BOOL,
+          loc,
+        },
+        type: BOOL,
+        loc,
+      };
+    }
+    out = out ? { kind: "logical", op: "||", left: out, right: test, type: BOOL, loc } : test;
+  }
+  const result: IrExpr = out ?? { kind: "boolLit", value: false, type: BOOL, loc };
+  if (!stable) return result;
+  return {
+    kind: "seqExpr",
+    stmts: [{ kind: "varDecl", localId: stable.id, init: recv, loc }],
+    result,
+    type: BOOL,
+    loc,
+  };
+}
+
 /** The runtime-key `in` (see lowerInExpression): `k in r` where k is a
  * runtime string and r an index-signature record — an interned
  * `%rec.haskey.<n>(k, r)` walks the declared names (a string-equality
diff --git a/packages/compiler/src/frontend/lowering/key-order-witness.ts b/packages/compiler/src/frontend/lowering/key-order-witness.ts
index ba8001792..437a29563 100644
--- a/packages/compiler/src/frontend/lowering/key-order-witness.ts
+++ b/packages/compiler/src/frontend/lowering/key-order-witness.ts
@@ -224,12 +224,30 @@ export function applyKeyOrderWitnesses(lowerer: Lowerer, fns: readonly IrFunctio
   const loops: number[] = [];
   let loopIds = 0;
   const loopKey = (): string => loops.join("/");
+  /** Bindings initialized by another binding's reference (`const view =
+   * value`, the snapshot a union-receiver write takes), so writes through
+   * either count as writes through the one object. */
+  const aliases = new Map<string, { target: string; fn: string }>();
+  const bindingOf = (obj: IrExpr): string | null => {
+    let o = obj;
+    while (o.kind === "unionNarrow" || o.kind === "downcast") o = o.value;
+    if (o.kind !== "varRef") return null;
+    let id = o.localId;
+    const seen = new Set<string>();
+    for (;;) {
+      const alias = aliases.get(id);
+      if (!alias || alias.fn !== fnName || inits.get(id)?.count !== 1 || seen.has(id)) return id;
+      seen.add(id);
+      id = alias.target;
+    }
+  };
   const noteInit = (id: string, init: IrExpr): void => {
     const previous = inits.get(id);
     if (previous !== undefined) {
       previous.count++;
       return;
     }
+    if (init.kind === "varRef") aliases.set(id, { target: init.localId, fn: fnName });
     const fresh =
       init.kind === "recordLit" ||
       init.kind === "recordClone" ||
@@ -315,7 +333,7 @@ export function applyKeyOrderWitnesses(lowerer: Lowerer, fns: readonly IrFunctio
           field: st.field,
           kind: st.value.kind === "fieldAbsent" ? "delete" : "write",
           fn: fnName,
-          binding: st.obj.kind === "varRef" ? st.obj.localId : null,
+          binding: bindingOf(st.obj),
           loop: loopKey(),
           pos,
         });
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index e530a57f7..0e4f3c24c 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -1,3 +1,4 @@
+import { hostsAll } from "./expressions/hosted-narrowing.js";
 import {
   matchAllRowsReadByElements,
   storedMatchAllRowsReadByElements,
@@ -5481,6 +5482,23 @@ export function lowerVarDecl(
       settledType = init.type;
     }
   }
+  // `const view = route as Route & { order: number }`: the asserted type
+  // refines the record arms the value is stored in (the assertion keeps
+  // that value, see hosted-narrowing.ts). The binding keeps the stored
+  // type, so writes through it reach the asserted object. An annotation
+  // the stored type hosts (`const view: Route & { order: number } = ...`)
+  // keeps it too: coercing into the annotated layout would copy the
+  // value and read members the writes have not created yet.
+  if (
+    !isLet &&
+    decl.initializer &&
+    isAssertionInitializer(decl.initializer) &&
+    (settledType.kind === "union" || settledType.kind === "record") &&
+    (init.type.kind === "union" || init.type.kind === "record") &&
+    hostsAll(lowerer, init.type, settledType)
+  ) {
+    settledType = init.type;
+  }
   // An exact class behind a generic interface keeps its native
   // representation so generic calls can specialize against its body.
   if (
@@ -11212,3 +11230,9 @@ export function stdlibBufferVarAlias(lowerer: Lowerer, decl: ts.VariableDeclarat
     requireEarlyReference(lowerer.program, decl) === null
   );
 }
+
+function isAssertionInitializer(node: ts.Expression): boolean {
+  let e = node;
+  while (ts.isParenthesizedExpression(e)) e = e.expression;
+  return ts.isAsExpression(e) || ts.isTypeAssertion(e);
+}
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 7144e574e..4a8bee1fa 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -1,3 +1,4 @@
+import { extendAssertedRecordMembers } from "./assertion-members.js";
 import { jsDynamicDeclarations } from "./js-dynamic-objects.js";
 import {
   lifetimeRegionType,
@@ -6781,6 +6782,10 @@ export class Lowerer {
     }
     this.collectProgram(parts);
     this.speculativeTypeQueries(() => {
+      extendAssertedRecordMembers(
+        this,
+        parts.map((fp) => fp.sf),
+      );
       this.analyzeRuntimeOptionalArrayReads(parts);
       this.analyzeDateJsonResults(parts);
       this.analyzeGuardedRecordReadGlobals(parts);
@@ -7188,6 +7193,10 @@ export class Lowerer {
     };
     this.collectProgram(parts);
     this.speculativeTypeQueries(() => {
+      extendAssertedRecordMembers(
+        this,
+        parts.map((fp) => fp.sf),
+      );
       this.analyzeRuntimeOptionalArrayReads(parts);
       this.analyzeDateJsonResults(parts);
       this.analyzeGuardedRecordReadGlobals(parts);
diff --git a/packages/compiler/src/frontend/type-mapper.ts b/packages/compiler/src/frontend/type-mapper.ts
index ea706992a..22b4c4e7a 100644
--- a/packages/compiler/src/frontend/type-mapper.ts
+++ b/packages/compiler/src/frontend/type-mapper.ts
@@ -325,7 +325,7 @@ export class ShapeRegistry {
     ) {
       return;
     }
-    shape.declaredOrder = declaredOrder;
+    shape.declaredOrder = this.withExtendedNames(shape.id, declaredOrder);
     this.declaredOrderPriority.set(shape.id, this.currentDeclaredOrderPriority);
   }
 
@@ -341,7 +341,7 @@ export class ShapeRegistry {
       }
       const shape = this.byId.get(shapeId);
       if (!shape) continue;
-      shape.declaredOrder = best.order;
+      shape.declaredOrder = this.withExtendedNames(shapeId, best.order);
       this.declaredOrderPriority.set(shapeId, best.priority);
     }
     for (const finalize of this.derivedDeclaredOrderFinalizers) finalize();
@@ -374,7 +374,7 @@ export class ShapeRegistry {
       // still obeys first-writer-wins.
       if (shape?.declaredOrder !== originalOrder) return;
       const next = order();
-      if (next !== undefined) shape.declaredOrder = next;
+      if (next !== undefined) shape.declaredOrder = this.withExtendedNames(shape.id, next);
     };
   }
 
@@ -482,6 +482,53 @@ export class ShapeRegistry {
   get(shapeId: string): IrRecordShape | undefined {
     return this.byId.get(shapeId);
   }
+
+  /** Names extendShape added to a shape, which every key-order table the
+   * shape adopts lists after its declared names. */
+  private readonly extendedNames = new Map<string, string[]>();
+
+  /** The members extendShape attached to a shape (empty for most). */
+  attachedNames(shapeId: string): readonly string[] {
+    return this.extendedNames.get(shapeId) ?? [];
+  }
+
+  private withExtendedNames(shapeId: string, order: string[]): string[] {
+    const extra = this.extendedNames.get(shapeId);
+    if (!extra) return order;
+    const missing = extra.filter((name) => !order.includes(name));
+    return missing.length === 0 ? order : [...order, ...missing];
+  }
+
+  /** Adds optional fields to an interned shape in place: members a type
+   * assertion attaches to the shape's values (`r as R & { tag: string }`
+   * followed by `view.tag = ...`; see assertion-members.ts). Every value
+   * of the shape holds them absent until a write through such a view, so
+   * the shape keeps its id, both structural keys resolve to it, and its
+   * key-order table lists the new names after the declared ones. Runs
+   * before any body lowers; existing layouts change, so the revision
+   * advances like a closing recursive placeholder. */
+  extendShape(shapeId: string, extra: readonly { name: string; type: IrType }[]): void {
+    const shape = this.byId.get(shapeId);
+    if (!shape || shape.tuple || extra.length === 0) return;
+    const fresh = extra.filter((f) => !shape.fields.some((x) => x.name === f.name));
+    if (fresh.length === 0) return;
+    shape.fields = [...shape.fields, ...fresh].sort((a, b) =>
+      a.name < b.name ? -1 : a.name > b.name ? 1 : 0,
+    );
+    this.extendedNames.set(shapeId, [
+      ...(this.extendedNames.get(shapeId) ?? []),
+      ...fresh.map((f) => f.name),
+    ]);
+    if (shape.declaredOrder)
+      shape.declaredOrder = this.withExtendedNames(shapeId, shape.declaredOrder);
+    const candidates = this.declaredOrderCandidates.get(shapeId);
+    if (candidates) {
+      for (const c of candidates) c.order = this.withExtendedNames(shapeId, c.order);
+    }
+    this.revision++;
+    const key = this.keyOf(shape.fields, false, shape.indexValue);
+    if (!this.byKey.has(key)) this.byKey.set(key, shapeId);
+  }
 }
 
 /** The frontend's union interner — mirrors ShapeRegistry. A union's
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6460-in-union-optional-slots.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6460-in-union-optional-slots.ts.json
new file mode 100644
index 000000000..8491507eb
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6460-in-union-optional-slots.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6460-in-union-optional-slots.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6461-predicate-intersection-narrowing.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6461-predicate-intersection-narrowing.ts.json
new file mode 100644
index 000000000..3ce078006
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6461-predicate-intersection-narrowing.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6461-predicate-intersection-narrowing.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6462-assertion-attached-members.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6462-assertion-attached-members.ts.json
new file mode 100644
index 000000000..36e333373
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6462-assertion-attached-members.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6462-assertion-attached-members.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6460-in-union-optional-slots.ts b/tests/corpus/6460-in-union-optional-slots.ts
new file mode 100644
index 000000000..f381217b9
--- /dev/null
+++ b/tests/corpus/6460-in-union-optional-slots.ts
@@ -0,0 +1,53 @@
+// `"k" in u` over a union of records where some arms declare `k` as an
+// optional slot: those arms answer by the value's own property presence
+// (an explicit undefined is present, an omitted or deleted key is not),
+// the other arms by their tag.
+type Cond = { type: string; value?: string };
+type HandleRoute = { handle: string; src?: string; dest?: string; status?: number };
+type SourceRoute = {
+  src: string;
+  dest?: string;
+  headers?: { [key: string]: string };
+  middlewarePath?: string;
+  continue?: boolean;
+  status?: number;
+  has?: Cond[];
+};
+type Route = HandleRoute | SourceRoute;
+
+const routes: Route[] = [
+  { handle: "filesystem" },
+  { src: "/a", middlewarePath: "/mw" },
+  { src: "/b", dest: "/c", status: undefined },
+  { handle: "miss", src: "/x", dest: undefined },
+  { src: "/d", has: [{ type: "header" }] },
+];
+
+for (const r of routes) {
+  console.log("handle" in r, "middlewarePath" in r, "dest" in r, "status" in r, "has" in r, "toString" in r);
+}
+
+// A deleted optional key leaves the value; a later write brings it back.
+const second = routes[1]!;
+if ("middlewarePath" in second) {
+  delete second.middlewarePath;
+}
+console.log("middlewarePath" in second, JSON.stringify(second));
+if (!("handle" in second)) {
+  second.middlewarePath = "/again";
+}
+console.log("middlewarePath" in second, JSON.stringify(second));
+
+// The receiver evaluates once, even when it is a call.
+let calls = 0;
+function pick(i: number): Route {
+  calls++;
+  return routes[i]!;
+}
+console.log("dest" in pick(2), "dest" in pick(0), "src" in pick(3), calls);
+
+// `in` as a narrowing test inside a predicate and a filter callback.
+function hasMiddleware(route: Route): boolean {
+  return "middlewarePath" in route && typeof route.middlewarePath === "string";
+}
+console.log(routes.filter(hasMiddleware).length, routes.filter((r) => "status" in r).length);
diff --git a/tests/corpus/6461-predicate-intersection-narrowing.ts b/tests/corpus/6461-predicate-intersection-narrowing.ts
new file mode 100644
index 000000000..7e8ff9d8a
--- /dev/null
+++ b/tests/corpus/6461-predicate-intersection-narrowing.ts
@@ -0,0 +1,70 @@
+// A predicate `r is Route & { path: string }` refines the record arms of
+// a union without naming a stored arm: arms that do not declare `path`
+// cannot hold such a value, the others are read through their own
+// layout. The narrowed value can be read in place, in templates, in
+// object literals and from closures.
+type HandleRoute = { handle: string; src?: string; dest?: string };
+type SourceRoute = { src: string; dest?: string; path?: string; status?: number };
+type Route = HandleRoute | SourceRoute;
+
+function isPathRoute(route: Route): route is Route & { path: string } {
+  return "path" in route && typeof route.path === "string";
+}
+
+type Output = { path: string; index: number };
+
+function collect(result: { names: Set<string>; routes: Route[] }): Map<string, Output> {
+  const out = new Map<string, Output>();
+  for (const route of result.routes) {
+    if (!isPathRoute(route)) continue;
+    let name = route.path;
+    if (!result.names.has(name)) name = name.replace(/^\//, "");
+    if (!result.names.has(name)) {
+      throw new Error(`Mapping ${route.path} not found`);
+    }
+    out.set(name, { path: route.path, index: out.get(name)?.index ?? out.size });
+  }
+  return out;
+}
+
+const routes: Route[] = [
+  { handle: "filesystem" },
+  { src: "/a", path: "/one" },
+  { src: "/b" },
+  { src: "/c", path: "two" },
+  { src: "/d", path: "/one", status: 308 },
+];
+console.log([...collect({ names: new Set(["one", "two"]), routes })]);
+try {
+  collect({ names: new Set(), routes: [{ src: "/z", path: "/missing" }] });
+} catch (e) {
+  console.log((e as Error).message);
+}
+
+// Closures capture the narrowed binding.
+const readers: (() => string)[] = [];
+for (const route of routes) {
+  if (!isPathRoute(route)) continue;
+  readers.push(() => `${route.src} -> ${route.path}`);
+}
+console.log(readers.map((read) => read()));
+
+// Three arms, two of which can hold the refinement: the narrowed value is
+// either of them, and their shared members still read.
+type A = { kind: string; a: number; tag?: string };
+type B = { kind: string; b: boolean; tag?: string };
+type C = { kind: string; c: string };
+type Item = A | B | C;
+function isTagged(item: Item): item is Item & { tag: string } {
+  return "tag" in item && item.tag !== undefined;
+}
+const items: Item[] = [
+  { kind: "a", a: 1, tag: "x" },
+  { kind: "b", b: true },
+  { kind: "c", c: "plain" },
+  { kind: "b", b: false, tag: "y" },
+];
+for (const item of items) {
+  if (isTagged(item)) console.log(item.kind, item.tag.toUpperCase());
+  else console.log(item.kind, "untagged");
+}
diff --git a/tests/corpus/6462-assertion-attached-members.ts b/tests/corpus/6462-assertion-attached-members.ts
new file mode 100644
index 000000000..8a2be31b7
--- /dev/null
+++ b/tests/corpus/6462-assertion-attached-members.ts
@@ -0,0 +1,58 @@
+// `route as Route & { order: number; internal: boolean }` followed by
+// writes through the asserted view adds the members to the object the
+// original binding names: they appear wherever the route is observed
+// later, in the order the writes created them.
+type HandleRoute = { handle: string; src?: string; dest?: string; status?: number };
+type SourceRoute = { src: string; dest?: string; path?: string; status?: number };
+type Route = HandleRoute | SourceRoute;
+
+function markInternal(input: Route[], out: Route[], order: number): void {
+  for (const route of input) {
+    const marked = route as Route & { internal: boolean; order: number };
+    marked.internal = true;
+    marked.order = order;
+    out.unshift(route);
+  }
+}
+
+const out: Route[] = [{ handle: "filesystem" }, { src: "/plain", dest: "/x" }];
+markInternal([{ src: "/a" }, { handle: "miss" }, { src: "/b", status: 404 }], out, 3);
+console.log(JSON.stringify(out));
+console.log(out);
+console.log(out.map((r) => ("internal" in (r as Route & { internal?: boolean }) ? "marked" : "plain")));
+console.log(out.map((r) => ({ ...r })));
+
+// Values the assertion never reached keep no trace of the members.
+const fresh: Route = { src: "/new" };
+console.log(JSON.stringify(fresh), Object.keys(fresh));
+
+// A single record shape works the same way, and the asserted view reads
+// back what it wrote.
+type Job = { id: string; retries?: number };
+const job: Job = { id: "j1" };
+const tracked = job as Job & { startedAt: number };
+tracked.startedAt = 42;
+console.log(tracked.startedAt + 1, JSON.stringify(job), Object.keys(job));
+
+// An annotated binding of the asserted type keeps the object as well:
+// the members appear on it once written, not before.
+function stamp(job: Job): Job & { finishedAt: number } {
+  const done: Job & { finishedAt: number } = job as Job & { finishedAt: number };
+  done.finishedAt = 7;
+  return done;
+}
+const second: Job = { id: "j2", retries: 1 };
+console.log(stamp(second).finishedAt, JSON.stringify(second), Object.keys(second));
+
+const third: Job = { id: "j3" };
+const queued: Job & { queuedAt: number } = third as Job & { queuedAt: number };
+queued.queuedAt = 9;
+console.log(queued.queuedAt, JSON.stringify(third), Object.keys(third));
+
+function markAnnotated(route: Route): void {
+  const marked: Route & { internal: boolean } = route as Route & { internal: boolean };
+  marked.internal = false;
+}
+const annotatedRoute: Route = { handle: "hit" };
+markAnnotated(annotatedRoute);
+console.log(JSON.stringify(annotatedRoute));