diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index ce58d520d..d54c3e4fe 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Windows bash-tool child processes defaulting interpreter pipe I/O to the ANSI codepage by adding UTF-8 encoding defaults when the inherited environment is unset ([#2701](https://github.com/can1357/oh-my-pi/issues/2701)).
+
 ## [16.0.1] - 2026-06-15
 
 ### Breaking Changes
diff --git a/packages/coding-agent/src/exec/bash-executor.ts b/packages/coding-agent/src/exec/bash-executor.ts
index f781289a6..81a5afc82 100644
--- a/packages/coding-agent/src/exec/bash-executor.ts
+++ b/packages/coding-agent/src/exec/bash-executor.ts
@@ -11,7 +11,7 @@ import { Settings, type ShellMinimizerSettings } from "../config/settings";
 import { OutputSink } from "../session/streaming-output";
 import { resolveOutputMaxColumns, resolveOutputSinkHeadBytes } from "../tools/output-meta";
 import { getOrCreateSnapshot } from "../utils/shell-snapshot";
-import { NON_INTERACTIVE_ENV } from "./non-interactive-env";
+import { buildNonInteractiveEnv } from "./non-interactive-env";
 
 export interface BashExecutorOptions {
 	cwd?: string;
@@ -184,7 +184,7 @@ export async function executeBash(command: string, options?: BashExecutorOptions
 	const minimizer = buildMinimizerOptions(settings.getGroup("shellMinimizer"));
 
 	const commandCwd = await resolveShellCwd(options?.cwd);
-	const commandEnv = options?.env ? { ...NON_INTERACTIVE_ENV, ...options.env } : NON_INTERACTIVE_ENV;
+	const commandEnv = buildNonInteractiveEnv(options?.env);
 
 	// Apply command prefix if configured
 	const prefixedCommand = prefix ? `${prefix} ${command}` : command;
diff --git a/packages/coding-agent/src/exec/non-interactive-env.ts b/packages/coding-agent/src/exec/non-interactive-env.ts
index 8befc7260..94bb61343 100644
--- a/packages/coding-agent/src/exec/non-interactive-env.ts
+++ b/packages/coding-agent/src/exec/non-interactive-env.ts
@@ -46,3 +46,45 @@ export const NON_INTERACTIVE_ENV: Readonly<Record<string, string>> = {
 	COMPOSER_NO_INTERACTION: "1",
 	CLOUDSDK_CORE_DISABLE_PROMPTS: "1",
 };
+
+const WINDOWS_UTF8_ENV_DEFAULTS: Readonly<Record<string, string>> = {
+	PYTHONIOENCODING: "utf-8",
+	PYTHONUTF8: "1",
+	LANG: "C.UTF-8",
+	LC_ALL: "C.UTF-8",
+};
+
+function hasEnvValue(
+	env: Record<string, string | undefined> | undefined,
+	key: string,
+	platform: NodeJS.Platform,
+): boolean {
+	if (!env) return false;
+	if (platform !== "win32") return env[key] !== undefined;
+
+	for (const [existingKey, value] of Object.entries(env)) {
+		if (value !== undefined && existingKey.toLowerCase() === key.toLowerCase()) {
+			return true;
+		}
+	}
+	return false;
+}
+
+/** Builds the per-command environment for non-interactive child processes. */
+export function buildNonInteractiveEnv(
+	overrides?: Record<string, string>,
+	baseEnv: Record<string, string | undefined> = Bun.env,
+	platform: NodeJS.Platform = process.platform,
+): Record<string, string> {
+	if (platform !== "win32") {
+		return overrides ? { ...NON_INTERACTIVE_ENV, ...overrides } : NON_INTERACTIVE_ENV;
+	}
+
+	const env: Record<string, string> = { ...NON_INTERACTIVE_ENV };
+	for (const [key, value] of Object.entries(WINDOWS_UTF8_ENV_DEFAULTS)) {
+		if (!hasEnvValue(baseEnv, key, platform) && !hasEnvValue(overrides, key, platform)) {
+			env[key] = value;
+		}
+	}
+	return overrides ? { ...env, ...overrides } : env;
+}
diff --git a/packages/coding-agent/test/non-interactive-env.test.ts b/packages/coding-agent/test/non-interactive-env.test.ts
new file mode 100644
index 000000000..29e1a4366
--- /dev/null
+++ b/packages/coding-agent/test/non-interactive-env.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, it } from "bun:test";
+import { buildNonInteractiveEnv } from "@oh-my-pi/pi-coding-agent/exec/non-interactive-env";
+
+describe("buildNonInteractiveEnv", () => {
+	it("defaults Windows child-process encoding to UTF-8 when inherited env is unset", () => {
+		const env = buildNonInteractiveEnv(undefined, {}, "win32");
+
+		expect(env.PYTHONIOENCODING).toBe("utf-8");
+		expect(env.PYTHONUTF8).toBe("1");
+		expect(env.LANG).toBe("C.UTF-8");
+		expect(env.LC_ALL).toBe("C.UTF-8");
+	});
+
+	it("preserves inherited and per-command Windows encoding overrides", () => {
+		const env = buildNonInteractiveEnv(
+			{ PYTHONUTF8: "0", LC_ALL: "en_US.UTF-8" },
+			{ pythonioencoding: "cp1252", LANG: "de_DE.UTF-8" },
+			"win32",
+		);
+
+		expect(env.PYTHONIOENCODING).toBeUndefined();
+		expect(env.PYTHONUTF8).toBe("0");
+		expect(env.LANG).toBeUndefined();
+		expect(env.LC_ALL).toBe("en_US.UTF-8");
+	});
+
+	it("does not force UTF-8 encoding defaults on non-Windows platforms", () => {
+		const env = buildNonInteractiveEnv(undefined, {}, "linux");
+
+		expect(env.PYTHONIOENCODING).toBeUndefined();
+		expect(env.PYTHONUTF8).toBeUndefined();
+		expect(env.LANG).toBeUndefined();
+		expect(env.LC_ALL).toBeUndefined();
+	});
+});