diff --git a/docs/research/ISSUE_419_SHELL_PATH_RETROSPECTIVE_2026-09-07.md b/docs/research/ISSUE_419_SHELL_PATH_RETROSPECTIVE_2026-09-07.md
new file mode 100644
index 00000000..a0340ce0
--- /dev/null
+++ b/docs/research/ISSUE_419_SHELL_PATH_RETROSPECTIVE_2026-09-07.md
@@ -0,0 +1,45 @@
+# Issue #419：headless Session 的 shellPath 继承复盘
+
+- 状态：在源码与回归测试边界验证
+- 创建 / 验证：2026-09-07
+- 基线：`c8f2c13`（OpenPI 0.6.0，Pi 0.85.1）
+- 修复边界：`1bbd243` 与 PR [#423](https://github.com/openpi-dev/openpi/pull/423)
+- Issue：[#419](https://github.com/openpi-dev/openpi/issues/419)
+- Supersedes：无
+
+## 已验证事实
+
+Pi 的 `AgentSession` 会从 `SettingsManager.getShellPath()` 构造内置
+`bash` 工具。OpenPI 的 `file-mutation-display` 扩展在 `session_start` 中
+重新注册 `createBashToolDefinition(ctx.cwd)`，但没有传入 `shellPath`。
+这个处理在 TUI 中用于替换展示器，却也在 `print` 模式执行；Direct
+Subagent 和 Workflow 子 Session 通过该模式绑定扩展，因此会丢失 Pi 的
+shell 设置。Windows 没有 WSL 时，裸 `bash` 可能被系统目录中的 WSL
+launcher stub 截获。
+
+在修复前的最小替身中，子 Session 的设置将 `shellPath` 指向不存在的文件，
+绑定扩展后调用 `bash` 仍成功执行 `printf`，证明调用的是裸 shell。修复后，
+同一调用返回 Pi 的 `Custom shell path not found` 错误，说明配置仍由原生
+工具读取。
+
+## 实施与证据
+
+修复让 `file-mutation-display` 只在 `ctx.mode === "tui"` 时修改工具注册和
+TUI 展开状态；`print`、RPC、JSON Session 保留 Pi 原生定义。TUI 测试改为
+显式使用 `tui` 模式，新增子 Session 测试覆盖扩展启动后的 shellPath 边界。
+
+本地验证结果：
+
+- `npx --yes bun@1.3.14 run test`：1428 通过、1 跳过、0 失败；Vitest 30 通过；
+- `npx --yes bun@1.3.14 run lint`：通过；
+- `npx --yes bun@1.3.14 run typecheck`：通过；
+- 子 Session 与展示扩展定向测试：20 通过；
+- 变更文件格式检查与 `git diff --check`：通过。
+
+## 影响与限制
+
+该改动不改变 TUI 的可见展示、模型工具 schema、Session 文件或设置持久化。
+Headless Session 重新使用 Pi 的 shell 解析和生命周期。当前环境不是 Windows，
+因此没有宣称真实 Windows WSL stub 的手工验收；回归测试使用跨平台的“不存在
+shell 路径”证据验证覆盖边界。Windows 原生命令编码和 Git Bash 安装发现仍由
+Pi 0.85.1 负责。
diff --git a/docs/research/README.md b/docs/research/README.md
index 9a19db98..fbbc8f07 100644
--- a/docs/research/README.md
+++ b/docs/research/README.md
@@ -4,6 +4,8 @@ Research records preserve sourced investigation and distinguish observations, in
 
 ## Validated investigations
 
+- [`ISSUE_419_SHELL_PATH_RETROSPECTIVE_2026-09-07.md`](ISSUE_419_SHELL_PATH_RETROSPECTIVE_2026-09-07.md) — headless 子 Session 被展示扩展覆盖 bash 定义、shellPath 丢失的根因与回归证据 ([#419](https://github.com/openpi-dev/openpi/issues/419), [PR #423](https://github.com/openpi-dev/openpi/pull/423))。
+
 - [`CURSOR_CONTEXT_AND_TRANSPORT_2026-09-07.md`](CURSOR_CONTEXT_AND_TRANSPORT_2026-09-07.md) — Cursor protocol context boundaries, linear frame accumulation and cwd-sensitive transcript caching ([#431](https://github.com/openpi-dev/openpi/issues/431)).
 
 - [`CHILD_ACQUISITION_AND_PROGRESS_2026-09-07.md`](CHILD_ACQUISITION_AND_PROGRESS_2026-09-07.md) — cancelled child/worktree acquisitions and stale progress evidence under output pressure ([#428](https://github.com/openpi-dev/openpi/issues/428)).
diff --git a/extensions/file-mutation-display/index.ts b/extensions/file-mutation-display/index.ts
index 32766051..fd885834 100644
--- a/extensions/file-mutation-display/index.ts
+++ b/extensions/file-mutation-display/index.ts
@@ -27,15 +27,19 @@ function compact<TParams extends TSchema, TDetails, TState>(
 export default function fileMutationDisplay(pi: ExtensionAPI) {
   pi.on("session_start", (_event, ctx) => {
     const display = loadSetupConfig().ui;
-    if (ctx.mode === "tui") {
-      // Ctrl+O remains a temporary override. A new/reloaded session starts from
-      // the persisted defaults instead of inheriting an old expanded toggle.
-      ctx.ui.setToolsExpanded(
-        display.subagentResultDisplay === "full" &&
-          display.bashToolDisplay === "full" &&
-          display.fileMutationDisplay === "full",
-      );
-    }
+    // This extension changes only the interactive TUI projection. Headless
+    // sessions must keep Pi's native definitions, especially bash: replacing
+    // it here would drop the SettingsManager-provided shellPath and can make
+    // Windows resolve the WSL System32 stub instead of the configured shell.
+    if (ctx.mode !== "tui") return;
+
+    // Ctrl+O remains a temporary override. A new/reloaded session starts from
+    // the persisted defaults instead of inheriting an old expanded toggle.
+    ctx.ui.setToolsExpanded(
+      display.subagentResultDisplay === "full" &&
+        display.bashToolDisplay === "full" &&
+        display.fileMutationDisplay === "full",
+    );
 
     pi.registerTool(
       compact(
diff --git a/tests/extensions/file-mutation-display/index.test.ts b/tests/extensions/file-mutation-display/index.test.ts
index 64bbdf3d..030783fa 100644
--- a/tests/extensions/file-mutation-display/index.test.ts
+++ b/tests/extensions/file-mutation-display/index.test.ts
@@ -54,7 +54,7 @@ async function withSession(
     sessionManager: SessionManager.inMemory(cwd),
   });
   try {
-    await session.bindExtensions({ mode: "print" });
+    await session.bindExtensions({ mode: "tui" });
     await run(session, cwd);
   } finally {
     session.dispose();
diff --git a/tests/extensions/shared/child-session.test.ts b/tests/extensions/shared/child-session.test.ts
index 8084efd9..ff49e6c6 100644
--- a/tests/extensions/shared/child-session.test.ts
+++ b/tests/extensions/shared/child-session.test.ts
@@ -19,6 +19,7 @@ import {
   DefaultPackageManager,
   DefaultResourceLoader,
   defineTool,
+  type ExtensionContext,
   ProjectTrustStore,
   SessionManager,
   type SessionShutdownEvent,
@@ -260,6 +261,60 @@ test("child resources remove only verified parent-only OpenPI extensions", async
   });
 });
 
+test("headless children preserve Pi shellPath through display extension startup", async () => {
+  await withTempDir(async (directory) => {
+    const cwd = path.join(directory, "project");
+    const agentDir = path.join(directory, "agent");
+    const repoRoot = fileURLToPath(new URL("../../..", import.meta.url));
+    const missingShell = path.join(directory, "missing-shell");
+    await mkdir(cwd, { recursive: true });
+    await mkdir(agentDir, { recursive: true });
+    await writeFile(
+      path.join(agentDir, "settings.json"),
+      JSON.stringify({ shellPath: missingShell, packages: [repoRoot] }),
+    );
+
+    const { loader, settingsManager } = await createChildResources({
+      cwd,
+      agentDir,
+      projectTrusted: true,
+    });
+    const { session } = await createAgentSession({
+      cwd,
+      agentDir,
+      resourceLoader: loader,
+      settingsManager,
+      sessionManager: SessionManager.inMemory(cwd),
+      ...childToolPolicy(["bash"]),
+    });
+
+    try {
+      await bindChildSessionExtensions(session, ["bash"]);
+      const bash = session.getToolDefinition("bash");
+      assert.ok(bash);
+      const context = {
+        cwd,
+        sessionManager: {
+          getSessionId: () => "shell-path",
+          getSessionFile: () => undefined,
+        },
+      } as unknown as ExtensionContext;
+      await assert.rejects(
+        bash.execute(
+          "shell-path",
+          { command: "printf should-not-run" },
+          undefined,
+          undefined,
+          context,
+        ),
+        /Custom shell path not found/,
+      );
+    } finally {
+      await shutdownAndDisposeChildSession(session);
+    }
+  });
+});
+
 test("production child binding skips foreign Workflow artifacts", async () => {
   await withTempDir(async (directory) => {
     const cwd = path.join(directory, "project");