diff --git a/packages/compiler/src/frontend/lowering/class-callbacks.ts b/packages/compiler/src/frontend/lowering/class-callbacks.ts
index b51158022..0bda0068e 100644
--- a/packages/compiler/src/frontend/lowering/class-callbacks.ts
+++ b/packages/compiler/src/frontend/lowering/class-callbacks.ts
@@ -164,6 +164,7 @@ export function classCallbackCall(
   fallback: IrExpr,
   loc: SrcLoc,
   calleeName = name,
+  original?: { originalId: () => string | undefined; directCall: () => IrExpr },
 ): IrExpr {
   const bag = lowerer.declareHiddenLocal("%callbackBag", DYN);
   const present = lowerer.declareHiddenLocal("%callbackPresent", BOOL);
@@ -235,6 +236,44 @@ export function classCallbackCall(
     answer =
       fallback.type.kind === "void" ? result : lowerer.coerceToExpected(result, fallback.type);
   }
+  // Read after observedSlot built the prototype data and its originals.
+  const originalId = observed ? original?.originalId() : undefined;
+  if (original && originalId !== undefined) {
+    // The slot still holds the method's original value: calling it is
+    // calling the method, so the call stays native (no boxed arguments,
+    // dynamic call or checked result). A replacement takes the dynamic call.
+    const unchanged: IrExpr = {
+      kind: "libCall",
+      fn: "dyn.sameValue",
+      args: [varRef(callback.id, DYN, loc), varRef(originalId, DYN, loc)],
+      type: BOOL,
+      loc,
+    };
+    answer =
+      fallback.type.kind === "void"
+        ? {
+            kind: "ternary",
+            cond: unchanged,
+            then: {
+              kind: "seqExpr",
+              stmts: [{ kind: "exprStmt", expr: original.directCall(), loc }],
+              result: dynUndefinedExpr(loc),
+              type: DYN,
+              loc,
+            },
+            else_: call,
+            type: DYN,
+            loc,
+          }
+        : {
+            kind: "ternary",
+            cond: unchanged,
+            then: original.directCall(),
+            else_: answer,
+            type: answer.type,
+            loc,
+          };
+  }
   return {
     kind: "seqExpr",
     stmts: [
diff --git a/packages/compiler/src/frontend/lowering/class-prototypes.ts b/packages/compiler/src/frontend/lowering/class-prototypes.ts
index 2a05cf977..3ecdd84aa 100644
--- a/packages/compiler/src/frontend/lowering/class-prototypes.ts
+++ b/packages/compiler/src/frontend/lowering/class-prototypes.ts
@@ -207,12 +207,21 @@ export function classPrototypeData(
       // promise-returning signature) gets no data slot.
       const boxedValue = lowerer.coerceToExpected(compiled, DYN);
       if (boxedValue.type.kind !== "dyn") continue;
+      const originalId = `%g.${info.def.prototypeDataHelper}.original.${method}`;
+      lowerer.globalsList.push({
+        id: originalId,
+        name: `${info.def.jsName ?? info.def.name}.prototype.${method}.original`,
+        type: DYN,
+        mutable: true,
+      });
+      info.prototypeOriginals ??= new Map();
+      info.prototypeOriginals.set(method, originalId);
       const descriptor: IrExpr = {
         kind: "dynObjLit",
         fields: [
           {
             key: { kind: "strLit", value: "value", type: STRING, loc },
-            value: boxedValue,
+            value: varRef(originalId, DYN, loc),
           },
           {
             key: { kind: "strLit", value: "writable", type: STRING, loc },
@@ -227,6 +236,8 @@ export function classPrototypeData(
         loc,
       };
       const init = helper.body[0]!;
+      if (init.kind === "if")
+        init.then.push({ kind: "assign", localId: originalId, value: boxedValue, loc });
       if (init.kind === "if")
         init.then.push({
           kind: "exprStmt",
diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 14d94ad24..493d979f5 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -10195,6 +10195,20 @@ export function lowerObjectMethodCall(
           type: found.sig.ret,
           loc,
         };
+    // The declaring class's original value, built with the observed slot.
+    // An override below (a virtual call) keeps the dynamic call: a slot
+    // reassigned to an ancestor's method must not reach the override. So do
+    // methods declared in unchecked JavaScript, whose results are inferred.
+    const declarerFile = found.declarer.decl ? ts.getSourceFileOfNode(found.declarer.decl) : null;
+    const original =
+      virtual ||
+      !declarerFile ||
+      (isJsSourceFile(declarerFile) && !lowerer.checkedJsFile(declarerFile))
+        ? undefined
+        : {
+            originalId: (): string | undefined => found.declarer.prototypeOriginals?.get(method),
+            directCall: (): IrExpr => fallback,
+          };
     const result = classCallbackCall(
       lowerer,
       value,
@@ -10203,6 +10217,7 @@ export function lowerObjectMethodCall(
       fallback,
       loc,
       ts.getTextOfNode(access),
+      original,
     );
     return {
       kind: "seqExpr",
diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts
index 6dd0124bd..29afbb4fe 100644
--- a/packages/compiler/src/frontend/lowering/lower-classes.ts
+++ b/packages/compiler/src/frontend/lowering/lower-classes.ts
@@ -478,6 +478,10 @@ export interface ClassInfo {
   prototypeMethodValues?: Map<string, IrExpr>;
   prototypeReflectionReady?: true;
   prototypeObservedMethodsReady?: true;
+  /** The globals holding each observed method's original prototype value
+   * (set when the prototype data is built), by method name. A typed call
+   * whose slot still holds that value calls the method natively. */
+  prototypeOriginals?: Map<string, string>;
   prototypeReflectionRefusal?: string;
   localClass?: {
     context: FnCtx | null;
diff --git a/packages/compiler/test/observed-method-guard.test.ts b/packages/compiler/test/observed-method-guard.test.ts
new file mode 100644
index 000000000..5964c482b
--- /dev/null
+++ b/packages/compiler/test/observed-method-guard.test.ts
@@ -0,0 +1,77 @@
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, expect, test } from "vitest";
+import { compile, deserializeModule, validateModule } from "../src/index.js";
+
+const dirs: string[] = [];
+
+afterEach(async () => {
+  await Promise.all(dirs.splice(0).map((dir) => rm(dir, { recursive: true, force: true })));
+});
+
+type IrRecord = Record<string, unknown>;
+
+function recordsOf(value: unknown, out: IrRecord[] = []): IrRecord[] {
+  if (value === null || typeof value !== "object") return out;
+  if (Array.isArray(value)) {
+    for (const item of value) recordsOf(item, out);
+    return out;
+  }
+  const record = value as IrRecord;
+  out.push(record);
+  for (const child of Object.values(record)) recordsOf(child, out);
+  return out;
+}
+
+// An untyped computed write puts every method name in the census, so typed
+// calls read the observable slot. While the slot holds the declared method,
+// the call is the native method call, not a dynamic call of the slot.
+test("typed calls of an observed method call it natively while its slot is unchanged", async () => {
+  const dir = await mkdtemp(join(tmpdir(), "scriptc-observed-guard-"));
+  dirs.push(dir);
+  const entry = join(dir, "main.ts");
+  const outDir = join(dir, ".scriptc");
+  const outPath = join(outDir, "main.ir.json");
+  await writeFile(
+    join(dir, "untyped.js"),
+    "export function put(target, key, value) { target[key] = value; }\n",
+  );
+  await writeFile(
+    entry,
+    [
+      'import { put } from "./untyped.js";',
+      "class Repo {",
+      "  items: number[] = [];",
+      "  total(): number { let s = 0; for (const n of this.items) s += n; return s; }",
+      "}",
+      "const repo = new Repo();",
+      "repo.items.push(1, 2);",
+      "put({}, String(Math.random()), 1);",
+      "export function sum(r: Repo): number { return r.total(); }",
+      "console.log(sum(repo));",
+      "",
+    ].join("\n"),
+  );
+  const result = await compile(entry, { outDir, outPath, outputKind: "ir" });
+  if (!result.ok)
+    throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
+  const module = deserializeModule(await readFile(outPath, "utf8"));
+  expect(validateModule(module)).toEqual([]);
+  const sum = module.functions.find((fn) => fn.name.endsWith("sum"));
+  expect(sum).toBeDefined();
+  const records = recordsOf(sum);
+  const guards = records.filter(
+    (record) =>
+      record.kind === "ternary" &&
+      (record.cond as IrRecord | undefined)?.kind === "libCall" &&
+      (record.cond as IrRecord).fn === "dyn.sameValue",
+  );
+  expect(guards).toHaveLength(1);
+  const then = guards[0]!.then as IrRecord;
+  expect(then.kind).toBe("call");
+  expect(String(then.callee)).toMatch(/Repo\.total$/);
+  expect(module.globals?.some((global) => global.name === "Repo.prototype.total.original")).toBe(
+    true,
+  );
+});
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts.json
new file mode 100644
index 000000000..58cdacb62
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts.json
@@ -0,0 +1,7 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7365-class-methods-untyped-computed-writes/untyped.js",
+  "<repo>/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts b/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts
new file mode 100644
index 000000000..f7449f1af
--- /dev/null
+++ b/tests/corpus/7365-class-methods-untyped-computed-writes/main.ts
@@ -0,0 +1,59 @@
+// Typed classes next to untyped computed writes. Calls through a typed
+// receiver keep the method while its prototype slot holds the declared
+// method, and observe a replacement made by untyped code (on the prototype
+// or as an own property of one instance) exactly as Node does.
+import { patch, tally, unpatch } from "./untyped.js";
+
+class Repository {
+  private items = new Map<string, number>();
+  add(name: string, cents: number): number {
+    this.items.set(name, cents);
+    return this.items.size;
+  }
+  list(): string[] {
+    return [...this.items.keys()];
+  }
+  total(): number {
+    let sum = 0;
+    for (const cents of this.items.values()) sum += cents;
+    return sum;
+  }
+}
+
+const words: string[] = ["list", "add", "list", "total"];
+console.log(JSON.stringify(tally(words)));
+
+const repo = new Repository();
+repo.add("keyboard", 12900);
+repo.add("mouse", 4900);
+let sum = 0;
+for (let i = 0; i < 1000; i++) sum += repo.total();
+console.log(repo.list().join(","), sum);
+
+// untyped code replaces the method on the prototype
+const proto: unknown = Object.getPrototypeOf(repo);
+patch(proto, "total", function (this: Repository): number {
+  return -1;
+});
+console.log(repo.total(), new Repository().total());
+unpatch(proto, "total");
+try {
+  repo.total();
+} catch (error) {
+  console.log("after delete:", error instanceof TypeError);
+}
+
+// an own replacement on one instance shadows the prototype for that instance only
+const other = new Repository();
+other.add("cable", 900);
+patch(other, "list", () => ["shadowed"]);
+console.log(other.list().join(","), repo.list().join(","));
+unpatch(other, "list");
+console.log(other.list().join(","));
+
+// restoring the original value through the prototype brings back the method
+const saved: unknown = (proto as Record<string, unknown>)["list"];
+patch(proto, "list", () => ["patched"]);
+console.log(repo.list().join(","));
+patch(proto, "list", saved);
+console.log(repo.list().join(","));
diff --git a/tests/corpus/7365-class-methods-untyped-computed-writes/untyped.js b/tests/corpus/7365-class-methods-untyped-computed-writes/untyped.js
new file mode 100644
index 000000000..3c0d20189
--- /dev/null
+++ b/tests/corpus/7365-class-methods-untyped-computed-writes/untyped.js
@@ -0,0 +1,15 @@
+// Untyped helpers with computed writes whose keys TypeScript cannot know.
+// They put every class method name in play for the method census.
+export function tally(words) {
+  const counts = {};
+  for (const word of words) counts[word] = (counts[word] || 0) + 1;
+  return counts;
+}
+
+export function patch(target, name, impl) {
+  target[name] = impl;
+}
+
+export function unpatch(target, name) {
+  delete target[name];
+}
diff --git a/tests/harness/corpus-costs/7365-class-methods-untyped-computed-writes/main.ts.json b/tests/harness/corpus-costs/7365-class-methods-untyped-computed-writes/main.ts.json
new file mode 100644
index 000000000..0347bbd22
--- /dev/null
+++ b/tests/harness/corpus-costs/7365-class-methods-untyped-computed-writes/main.ts.json
@@ -0,0 +1 @@
+{ "plain": 3000, "san": 3400 }