diff --git a/packages/compiler/src/frontend/lowering/builtins/child-process-methods.ts b/packages/compiler/src/frontend/lowering/builtins/child-process-methods.ts
index c1a6ea89b..2c93f41e6 100644
--- a/packages/compiler/src/frontend/lowering/builtins/child-process-methods.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/child-process-methods.ts
@@ -824,6 +824,32 @@ function childStreamEncodedAt(
   return before;
 }
 
+/** Whether the program calls `setEncoding` on the stream `receiver`
+ * names: a call whose receiver has the same access path, anywhere in the
+ * file. Its 'data' chunks can then be strings. */
+export function childStreamSetsEncoding(lowerer: Lowerer, receiver: ts.Expression): boolean {
+  const key = accessPathOf(lowerer, receiver);
+  if (key === null) return false;
+  let found = false;
+  const visit = (node: ts.Node): void => {
+    if (found) return;
+    if (
+      ts.isCallExpression(node) &&
+      ts.isPropertyAccessExpression(node.expression) &&
+      node.expression.name.text === "setEncoding"
+    ) {
+      const path = accessPathOf(lowerer, node.expression.expression);
+      if (path !== null && path.root === key.root && path.path === key.path) {
+        found = true;
+        return;
+      }
+    }
+    ts.forEachChild(node, visit);
+  };
+  visit(ts.getSourceFileOfNode(receiver));
+  return found;
+}
+
 /** An inline 'exit'/'close' listener may declare `(code: number, signal:
  * string)`: TypeScript accepts it through ChildProcess's catch-all `on`
  * overload, while Node passes `number | null` and `Signals | null`. Those
@@ -850,7 +876,7 @@ function narrowedExitListenerParams(lowerer: Lowerer, listener: ts.Expression):
 
 /** Lowers an inline listener with the given parameters bound as
  * checked-dynamic values typed `unknown` for the body. */
-function lowerWithCheckedParams(
+export function lowerWithCheckedParams(
   lowerer: Lowerer,
   listener: ts.Expression,
   symbols: readonly ts.Symbol[],
diff --git a/packages/compiler/src/frontend/lowering/handle-properties.ts b/packages/compiler/src/frontend/lowering/handle-properties.ts
index ba098aab5..a3eefbc57 100644
--- a/packages/compiler/src/frontend/lowering/handle-properties.ts
+++ b/packages/compiler/src/frontend/lowering/handle-properties.ts
@@ -16,6 +16,10 @@ import { DYN, STRING, VOID } from "../../ir/ir.js";
 import { everyStmt } from "../../ir/traverse.js";
 import { tryLowerExpression } from "./expressions/try-lower-expression.js";
 import { ENTRY_NAME, type Lowerer } from "./lowerer.js";
+import {
+  childStreamSetsEncoding,
+  lowerWithCheckedParams,
+} from "./builtins/child-process-methods.js";
 
 /** `child.name = value` in a JavaScript file over a typed child: the write
  * lands in the child's property table through its checked-dynamic box. */
@@ -235,12 +239,50 @@ export function lowerChildPropertyCall(
     recv,
     method: access.name.text,
     calleeName: ts.getTextOfNode(access),
-    args: call.arguments.nodes.map((arg) => lowerer.lowerExprExpecting(arg, DYN)),
+    args: call.arguments.nodes.map((arg, i) =>
+      i === 1 && dataListenerOfEncodedStream(lowerer, call, access)
+        ? lowerer.coerceToExpected(lowerUntypedChunkListener(lowerer, arg), DYN)
+        : lowerer.lowerExprExpecting(arg, DYN),
+    ),
     type: DYN,
     loc,
   };
 }
 
+const LISTENER_METHODS: ReadonlySet<string> = new Set([
+  "on",
+  "once",
+  "addListener",
+  "prependListener",
+  "prependOnceListener",
+]);
+
+/** `stream.on("data", (chunk) => ...)` with an unannotated chunk, on a
+ * stream the program calls `setEncoding` on. The declarations type the
+ * chunk as a Buffer, but after `setEncoding` Node passes strings. */
+function dataListenerOfEncodedStream(
+  lowerer: Lowerer,
+  call: ts.CallExpression,
+  access: ts.PropertyAccessExpression,
+): boolean {
+  const [event, listener] = call.arguments.nodes;
+  if (!LISTENER_METHODS.has(access.name.text) || event === undefined || listener === undefined)
+    return false;
+  if (!ts.isStringLiteralLike(event) || event.text !== "data") return false;
+  if (!ts.isArrowFunction(listener) && !ts.isFunctionExpression(listener)) return false;
+  const chunk = listener.parameters.nodes[0];
+  if (chunk === undefined || chunk.type !== undefined || !ts.isIdentifier(chunk.name)) return false;
+  return childStreamSetsEncoding(lowerer, access.expression);
+}
+
+/** The listener with its chunk parameter bound as a checked-dynamic value,
+ * which holds a string or a Buffer as Node passes it. */
+function lowerUntypedChunkListener(lowerer: Lowerer, listener: ts.Expression): IrExpr {
+  const fn = listener as ts.ArrowFunction | ts.FunctionExpression;
+  const symbol = lowerer.checker.getSymbolAtLocation(fn.parameters.nodes[0]!.name);
+  return symbol ? lowerWithCheckedParams(lowerer, listener, [symbol]) : lowerer.lowerExpr(listener);
+}
+
 /** `child.name` in JavaScript over an untyped receiver holding a child, or
  * for a name the program adds to children. */
 export function lowerChildPropertyRead(
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/child-stream-set-encoding-untyped-listener.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/child-stream-set-encoding-untyped-listener.cjs.json
new file mode 100644
index 000000000..3256267c4
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/child-stream-set-encoding-untyped-listener.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/child-stream-set-encoding-untyped-listener.cjs"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/child-stream-set-encoding-untyped-listener.cjs b/tests/corpus/child-stream-set-encoding-untyped-listener.cjs
new file mode 100644
index 000000000..db1d91393
--- /dev/null
+++ b/tests/corpus/child-stream-set-encoding-untyped-listener.cjs
@@ -0,0 +1,37 @@
+'use strict';
+// An unannotated 'data' listener on a child stream that setEncoding
+// switched to strings receives strings, as in Node's
+// test-child-process-default-options: the declarations type the chunk as
+// a Buffer. A stream without setEncoding keeps delivering Buffers, and a
+// listener added before the setEncoding call also receives strings.
+const { spawn } = require('child_process');
+
+process.env.HELLO = 'WORLD';
+const child = spawn('/usr/bin/env', [], {});
+let response = '';
+const kinds = new Set();
+child.stdout.setEncoding('utf8');
+child.stdout.on('data', function (chunk) {
+  kinds.add(typeof chunk);
+  response += chunk;
+});
+
+const raw = spawn('printf', ['raw bytes']);
+let rawText = '';
+raw.stdout.on('data', (chunk) => {
+  kinds.add(Buffer.isBuffer(chunk) ? 'buffer' : typeof chunk);
+  rawText += chunk.toString('utf8', 0, chunk.length);
+});
+
+const early = spawn('printf', ['early listener']);
+let earlyText = '';
+early.stdout.on('data', (chunk) => {
+  earlyText += chunk;
+  kinds.add(`early ${typeof chunk}`);
+});
+early.stdout.setEncoding('utf8');
+
+process.on('exit', () => {
+  console.log(response.includes('HELLO=WORLD'), JSON.stringify(rawText), JSON.stringify(earlyText));
+  console.log([...kinds].sort().join(' '));
+});
diff --git a/tests/harness/corpus-costs/child-stream-set-encoding-untyped-listener.cjs.json b/tests/harness/corpus-costs/child-stream-set-encoding-untyped-listener.cjs.json
new file mode 100644
index 000000000..652ae8ea4
--- /dev/null
+++ b/tests/harness/corpus-costs/child-stream-set-encoding-untyped-listener.cjs.json
@@ -0,0 +1 @@
+{ "plain": 6521, "san": 6908 }