diff --git a/CHANGELOG.md b/CHANGELOG.md
index f2f65733..5b34d835 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,16 @@ This file is the canonical source for unreleased changes and published release n
 
 - Add in-progress work under `## [Unreleased]
 
+## [Unreleased]
+
+### Fixed
+
+- The `bash` tool now runs the model's command under bash on every sandbox. It ran under `sh`, which is dash on Debian and Ubuntu, so bash syntax such as arrays, `[[ ]]`, `{1..3}`, here-strings and `source` worked on a Mac but failed on Linux and in Debian-based containers, and broke the tool's own output handling, so the model got an error pointing into CodeWork's script rather than its command. Where bash is not on `PATH`, the command runs under `sh`, and a failure says so: `Command exited with code 2 (ran under sh: bash is not on PATH in this sandbox)`.
+
+### Internal
+
+- Added E2E tests that run bash-only syntax through the `bash` tool on the host, just-bash, Daytona and Vercel sandboxes and check they print the same output, and that a host without bash runs the command under `sh` and says so when it fails.
+
 ## [@codeworksh/cli@0.3.0] - 2026-10-13
 
 ### Added
diff --git a/packages/harness/src/plugin/builtin/tool/bash.ts b/packages/harness/src/plugin/builtin/tool/bash.ts
index a8fbc4e0..139baf32 100644
--- a/packages/harness/src/plugin/builtin/tool/bash.ts
+++ b/packages/harness/src/plugin/builtin/tool/bash.ts
@@ -53,6 +53,8 @@ const BashSuccess = Schema.Struct({ ...outputFields, exitCode: Schema.Finite });
 class BashFailed extends Schema.TaggedError<BashFailed>()("BashFailed", {
 	...outputFields,
 	exitCode: Schema.Finite,
+	/** The shell that ran the command, when the sandbox reported it. */
+	shell: Schema.optional(Schema.Literals(["bash", "sh"])),
 }) {}
 
 /** Deadline exceeded — expected, model-visible. Carries the partial output produced so far. */
@@ -71,7 +73,7 @@ const withStatus = (failure: BashFailureError): string => {
 			? `Command timed out after ${failure.timeoutSeconds} seconds`
 			: failure.exitCode === -1
 				? "Command terminated without an exit code"
-				: `Command exited with code ${failure.exitCode}`;
+				: `Command exited with code ${failure.exitCode}${failure.shell === "sh" ? " (ran under sh: bash is not on PATH in this sandbox)" : ""}`;
 	return failure.output ? `${failure.output.replace(/\n$/, "")}\n\n${status}` : status;
 };
 
@@ -134,6 +136,8 @@ const present = (snap: OutputSnapshot, lastLineBytes: number, saved: string | un
 
 const utf8 = new TextEncoder();
 
+const shellOf = (report: Output.Report | undefined) => (report !== undefined ? { shell: report.shell } : {});
+
 /**
  * A command cut off by its deadline never reached the wrapper's own cleanup: kill
  * what is left of it and remove its side files, keeping the output file only when
@@ -195,7 +199,7 @@ const runBuffered = (
 		acc.finish();
 		const presented = present(acc.snapshot(), acc.getLastLineBytes(), window?.kept === true ? file : undefined);
 		if (result.exitCode !== 0) {
-			return yield* new BashFailed({ exitCode: result.exitCode, ...presented });
+			return yield* new BashFailed({ exitCode: result.exitCode, ...presented, ...shellOf(window) });
 		}
 		return { exitCode: result.exitCode, ...presented } satisfies typeof BashSuccess.Type;
 	});
@@ -275,7 +279,7 @@ const runStreaming = (
 		const code = yield* Ref.get(exitCode);
 		// No Exit event (e.g. killed before reporting one) → treat as a failure.
 		if (code === null || code !== 0) {
-			return yield* new BashFailed({ exitCode: code ?? -1, ...presented });
+			return yield* new BashFailed({ exitCode: code ?? -1, ...presented, ...shellOf(report) });
 		}
 		return { exitCode: code, ...presented } satisfies typeof BashSuccess.Type;
 	});
diff --git a/packages/harness/src/tool/output.ts b/packages/harness/src/tool/output.ts
index 3ce95c5a..33c98126 100644
--- a/packages/harness/src/tool/output.ts
+++ b/packages/harness/src/tool/output.ts
@@ -49,10 +49,15 @@ const script = (
 ) =>
 	[
 		`f=${quote(file)}`,
+		// The command stays a quoted string, so syntax `sh` lacks cannot break the
+		// wrapper: it runs under bash where the sandbox has one, else under `sh`.
+		`c=${quote(command)}`,
+		"if command -v bash >/dev/null 2>&1; then s=bash; else s=sh; fi",
 		// A function whose body is a subshell: `exit` in the command ends only it,
-		// and the command is written once for both branches below.
+		// and the command is written once for both branches below. No `exec`:
+		// just-bash carries on past it.
 		"run() (",
-		command,
+		'\tif [ "$s" = bash ]; then bash -c "$c"; else eval "$c"; fi',
 		")",
 		// The side files are created the same exclusive way as the output file.
 		// `umask` may be missing where modes mean nothing (an in-process shell over a virtual filesystem).
@@ -83,21 +88,24 @@ const script = (
 		"run 2>&1",
 	].join("\n");
 
-/** The sandbox's own account of the output: its raw size, and whether the file was kept. */
+/** The sandbox's own account of the output: its raw size, whether the file was kept, and the shell that ran it. */
 export interface Report {
 	/** Bytes and newlines in the whole output. */
 	readonly bytes: number;
 	readonly newlines: number;
 	/** Whether the sandbox kept the output file — decided on raw bytes, which the harness only sees decoded. */
 	readonly kept: boolean;
+	/** `sh` when the sandbox has no bash. */
+	readonly shell: "bash" | "sh";
 }
 
-const header = new RegExp(`^${HEADER} (\\d+) (\\d+) ([01])(?: (\\d+))?$`, "m");
+const header = new RegExp(`^${HEADER} (\\d+) (\\d+) ([01]) (bash|sh)(?: (\\d+))?$`, "m");
 
 const parse = (match: RegExpExecArray): Report => ({
 	bytes: Number(match[1]),
 	newlines: Number(match[2]),
 	kept: match[3] === "1",
+	shell: match[4] === "sh" ? "sh" : "bash",
 });
 
 /**
@@ -107,7 +115,7 @@ const parse = (match: RegExpExecArray): Report => ({
  */
 export const streaming = (command: string, file: string, limits: Limits) =>
 	script(command, file, limits, (source) => `${source} | tee -a "$f"`, [
-		`printf '${HEADER} %s %s %s\\n' "$b" "$n" "$keep" >&2`,
+		`printf '${HEADER} %s %s %s %s\\n' "$b" "$n" "$keep" "$s" >&2`,
 	]);
 
 /** Read {@link streaming}'s stderr: the report, if it got that far, and anything else the wrapper said. */
@@ -134,7 +142,11 @@ export const buffered = (command: string, file: string, limits: Limits) => {
 		// The redirect sits on the whole group, not on `cat`: an in-process shell
 		// (just-bash) re-encodes text that a command reading a pipe writes to a file.
 		(source) => `{ ${source} | cat; } >> "$f"`,
-		[`w=$(${tail} | wc -c | awk '{ print $1 }')`, `printf '${HEADER} %s %s %s %s\\n' "$b" "$n" "$keep" "$w"`, tail],
+		[
+			`w=$(${tail} | wc -c | awk '{ print $1 }')`,
+			`printf '${HEADER} %s %s %s %s %s\\n' "$b" "$n" "$keep" "$s" "$w"`,
+			tail,
+		],
 	);
 };
 
@@ -149,8 +161,8 @@ export interface Window extends Report {
 export const window = (stdout: string): Window | undefined => {
 	const newline = stdout.indexOf("\n");
 	const match = header.exec(newline === -1 ? stdout : stdout.slice(0, newline));
-	if (match === null || match[4] === undefined) return undefined;
-	return { ...parse(match), text: stdout.slice(newline + 1), textBytes: Number(match[4]) };
+	if (match === null || match[5] === undefined) return undefined;
+	return { ...parse(match), text: stdout.slice(newline + 1), textBytes: Number(match[5]) };
 };
 
 /**
diff --git a/packages/harness/test/__artifacts__/tools.bash.syntax.json b/packages/harness/test/__artifacts__/tools.bash.syntax.json
new file mode 100644
index 00000000..e0495155
--- /dev/null
+++ b/packages/harness/test/__artifacts__/tools.bash.syntax.json
@@ -0,0 +1,9 @@
+{
+  "status": "completed",
+  "text": "array b
+1 2 3
+here-string
+source x=1
+shell bash
+",
+}
\ No newline at end of file
diff --git a/packages/harness/test/fixtures/bash.spec.ts b/packages/harness/test/fixtures/bash.spec.ts
index 54908d33..2c578eae 100644
--- a/packages/harness/test/fixtures/bash.spec.ts
+++ b/packages/harness/test/fixtures/bash.spec.ts
@@ -11,7 +11,7 @@ import type { SandboxDriver } from "../../src/sandbox/driver.ts";
 import { SandboxIO } from "../../src/sandbox/io.ts";
 import { bashPlugin } from "../../src/plugin/builtin/tool/bash.ts";
 import { defaultPromptPlugin } from "../../src/plugin/builtin/prompt/default.ts";
-import { pendingCall } from "../tools.fixture.ts";
+import { BASH_SYNTAX, pendingCall } from "../tools.fixture.ts";
 import { toolTurn } from "./llm.ts";
 import { remoteSuite } from "./live.ts";
 import { withSettings } from "./settings.ts";
@@ -112,6 +112,20 @@ export const bashPluginSpec = (options: {
 			180_000,
 		);
 
+		it(
+			"runs bash-only syntax under bash, as every local backend does",
+			() =>
+				withSettings(async (input) => {
+					const { call } = await exchange(input, BASH_SYNTAX);
+					if (call.status !== "completed" && call.status !== "error") throw new Error("bash was not settled");
+					const text = call.result.content.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("");
+					await expect({ status: call.status, text }).toMatchFileSnapshot(
+						"./__artifacts__/tools.bash.syntax.json",
+					);
+				}),
+			180_000,
+		);
+
 		it(
 			"truncates real output and preserves the entire spill file",
 			() =>
diff --git a/packages/harness/test/tools.fixture.ts b/packages/harness/test/tools.fixture.ts
index 0d1823f9..b5f50c29 100644
--- a/packages/harness/test/tools.fixture.ts
+++ b/packages/harness/test/tools.fixture.ts
@@ -12,3 +12,12 @@ export const pendingCall = (
 	status: "pending",
 	time: { start: 1, end: 1 },
 });
+
+/** Syntax that `sh` (dash, ash) rejects: every backend must run it under bash. */
+export const BASH_SYNTAX = [
+	'arr=(a b c); [[ ${#arr[@]} -eq 3 ]] && echo "array ${arr[1]}"',
+	"echo {1..3}",
+	"cat <<< here-string",
+	`printf 'x=1\\n' > bash-syntax.env && source bash-syntax.env && echo "source x=$x"`,
+	'echo "shell ${BASH_VERSION:+bash}"',
+].join("; ");
diff --git a/packages/harness/test/tools.output.test.ts b/packages/harness/test/tools.output.test.ts
index 8a321500..90ef5f59 100644
--- a/packages/harness/test/tools.output.test.ts
+++ b/packages/harness/test/tools.output.test.ts
@@ -1,5 +1,6 @@
 import { Effect, Fiber, Layer, Stream } from "effect";
 import type { ChildProcessSpawner } from "effect/process";
+import { existsSync } from "node:fs";
 import fs from "node:fs/promises";
 import { basename } from "node:path";
 import { describe, expect, it } from "vite-plus/test";
@@ -14,7 +15,7 @@ import * as Executor from "../src/tool/executor.ts";
 import { fromSandboxShell, ToolShell } from "../src/tool/shell.ts";
 import * as Tool from "../src/tool/tool.ts";
 import { tmpdir } from "./fixtures/tempdir.ts";
-import { pendingCall } from "./tools.fixture.ts";
+import { BASH_SYNTAX, pendingCall } from "./tools.fixture.ts";
 
 /**
  * The bash tool end to end over two real backends — the host shell, which
@@ -33,6 +34,8 @@ import { pendingCall } from "./tools.fixture.ts";
  *   - a backend that cannot cut a command short leaves it running after the deadline
  *   - a backend that hands text rather than bytes reports a file the sandbox removed,
  *     or numbers the shown lines wrongly
+ *   - bash-only syntax fails where `sh` is not bash, or breaks the wrapper itself
+ *   - a sandbox without bash fails the command without saying it ran under `sh`
  */
 
 interface Details {
@@ -172,6 +175,23 @@ describe.each([
 		if (backend !== "justbash") await fs.rm(result.details.fullOutputPath!);
 	});
 
+	it("runs bash-only syntax under bash", async () => {
+		const { dir, layer } = await mount();
+		await using _ = dir;
+		const result = await run(layer, BASH_SYNTAX);
+		await expect({ status: result.status, text: result.text }).toMatchFileSnapshot(
+			"./__artifacts__/tools.bash.syntax.json",
+		);
+	});
+
+	it("keeps a syntax error inside the command", async () => {
+		const { dir, layer } = await mount();
+		await using _ = dir;
+		const result = await run(layer, "echo before; if then");
+		expect(result.details).toMatchObject({ _tag: "BashFailed", exitCode: 2, shell: "bash" });
+		expect(result.text).toMatch(/\n\nCommand exited with code 2$/);
+	});
+
 	it("tells the model when there is no output, and why a silent command failed", async () => {
 		const { dir, layer } = await mount();
 		await using _ = dir;
@@ -291,6 +311,43 @@ describe.each([
 	}
 });
 
+// A host without bash: the tools the wrapper needs, and nothing else.
+const withoutBash = async (dir: string) => {
+	const bin = `${dir}/bin`;
+	await fs.mkdir(bin);
+	for (const tool of ["awk", "cat", "mkdir", "ps", "rm", "tail", "tee", "wc"]) {
+		const found = (process.env.PATH ?? "")
+			.split(":")
+			.map((path) => `${path}/${tool}`)
+			.find((path) => existsSync(path));
+		if (found === undefined) throw new Error(`${tool} not on PATH`);
+		await fs.symlink(found, `${bin}/${tool}`);
+	}
+	return HostExe.layer({ shell: "/bin/sh", env: { PATH: bin } });
+};
+
+describe.each([
+	["streams", (shell: Layer.Layer<Shell, never, ChildProcessSpawner.ChildProcessSpawner>) => shell],
+	[
+		"buffers",
+		(shell: Layer.Layer<Shell, never, ChildProcessSpawner.ChildProcessSpawner>) =>
+			Layer.effect(
+				Shell,
+				Effect.map(Shell, (inner) => Shell.of({ exec: inner.exec, execArgv: inner.execArgv })),
+			).pipe(Layer.provide(shell)),
+	],
+] as const)("bash tool on a host without bash (%s)", (_mode, backend) => {
+	it("runs the command under sh, and says so when it fails", async () => {
+		await using dir = await tmpdir();
+		const layer = host(dir.path, backend(await withoutBash(dir.path)));
+		const ok = await run(layer, "echo ok");
+		expect(ok).toMatchObject({ status: "completed", text: "ok\n" });
+		const failed = await run(layer, "echo no; exit 3");
+		expect(failed.details).toMatchObject({ _tag: "BashFailed", exitCode: 3, shell: "sh" });
+		expect(failed.text).toBe("no\n\nCommand exited with code 3 (ran under sh: bash is not on PATH in this sandbox)");
+	});
+});
+
 describe("ToolShell over the host sandbox shell", () => {
 	it("force-kills a command that ignores SIGTERM at its deadline", async () => {
 		await using dir = await tmpdir();
diff --git a/sites/www/content/docs/sandboxes/local.mdx b/sites/www/content/docs/sandboxes/local.mdx
index dced2005..d9a1d761 100644
--- a/sites/www/content/docs/sandboxes/local.mdx
+++ b/sites/www/content/docs/sandboxes/local.mdx
@@ -5,7 +5,7 @@ description: The default sandbox. Your folder and your real shell.
 
 ## How It Works
 
-`local` is the default driver. It binds the agent directly to your machine: file tools use your real filesystem, starting in the current directory or in `--cwd`, and `bash` runs real commands through `sh` with the environment CodeWork started with.
+`local` is the default driver. It binds the agent directly to your machine: file tools use your real filesystem, starting in the current directory or in `--cwd`, and `bash` runs real commands through bash (`sh` when bash is not on `PATH`) with the environment CodeWork started with.
 
 Anything you have installed, such as `git`, `node`, or your test runner, the agent can use. There's nothing to provision, so it's the fastest sandbox, and the only one that works on your repository without copying anything.