diff --git a/src/lecode/agent/tools/bash.py b/src/lecode/agent/tools/bash.py
index 31bd319..5030eb1 100644
--- a/src/lecode/agent/tools/bash.py
+++ b/src/lecode/agent/tools/bash.py
@@ -3,7 +3,8 @@
 Commands are first passed through ``rtk rewrite``, which swaps supported
 commands for their token-optimized rtk proxies (``git status`` → ``rtk git
 status``); the original command runs unchanged when rtk has no equivalent
-(fail-open). Runs via ``/bin/sh -c`` with stderr merged into stdout. Output
+(fail-open). Runs via the user's shell (``$SHELL``, falling back to
+``/bin/sh``) with ``-c``, stderr merged into stdout. Output
 over the cap is truncated head/tail and the full text is saved to
 ``<config_dir>/overflow/<uuid>.log`` with a pointer line.
 """
@@ -23,6 +24,7 @@
 from lecode.agent.tools.base import Tool, ToolContext, ToolResult
 from lecode.extras.proc import ProcResult
 from lecode.extras.rtk import rewrite_command
+from lecode.extras.shell import user_shell
 
 DEFAULT_TIMEOUT_S = 120.0
 MAX_TIMEOUT_S = 600.0
@@ -30,7 +32,7 @@
 
 
 def _kill_tree(proc: asyncio.subprocess.Process) -> None:
-    """Kill the process *group* — ``sh -c`` children must not survive."""
+    """Kill the process *group* — shell children must not survive."""
     import os
     import signal
 
@@ -48,18 +50,21 @@ async def _run_shell(
     idle_timeout: float,
     max_bytes: int,
     *,
+    shell: str | None = None,
     on_chunk: Callable[[bytes], None] | None = None,
     proc_slot: dict[str, Any] | None = None,
 ) -> tuple[bytes, int, bool, bool]:
     """Run a shell command; returns (output, exit_code, timed_out, idle_killed).
 
-    ``on_chunk`` (background tasks) receives each chunk as it arrives; the
-    retained buffer is then capped at ``max_bytes`` (the tail), since the
-    caller streams the full output elsewhere. ``proc_slot`` receives the
-    spawned process under ``"proc"`` so the caller can signal it.
+    ``shell`` overrides the executable; the default is the user's shell
+    (:func:`lecode.extras.shell.user_shell`). ``on_chunk`` (background tasks)
+    receives each chunk as it arrives; the retained buffer is then capped at
+    ``max_bytes`` (the tail), since the caller streams the full output
+    elsewhere. ``proc_slot`` receives the spawned process under ``"proc"`` so
+    the caller can signal it.
     """
     proc = await asyncio.create_subprocess_exec(
-        "/bin/sh",
+        shell or user_shell(),
         "-c",
         command,
         cwd=cwd,
@@ -137,11 +142,12 @@ def _save_overflow(output: BinaryIO) -> tuple[Path, str, str]:
 
 
 class BashTool(Tool):
-    def __init__(self) -> None:
+    def __init__(self, *, shell: str | None = None) -> None:
+        self._shell = shell or user_shell()
         super().__init__(
             name="bash",
             description=(
-                "Run a shell command (/bin/sh -c). Supported commands are "
+                f"Run a shell command ({self._shell} -c). Supported commands are "
                 "rewritten to token-optimized rtk proxies when available; "
                 "output is truncated head/tail (full output saved to a file)."
             ),
@@ -204,7 +210,13 @@ async def run(self, args: dict, ctx: ToolContext) -> ToolResult:
         try:
             with tempfile.SpooledTemporaryFile(max_size=MAX_OUTPUT_BYTES) as output:
                 _, exit_code, timed_out, idle_killed = await _run_shell(
-                    command, ctx.cwd, timeout, idle, MAX_OUTPUT_BYTES, on_chunk=output.write
+                    command,
+                    ctx.cwd,
+                    timeout,
+                    idle,
+                    MAX_OUTPUT_BYTES,
+                    shell=self._shell,
+                    on_chunk=output.write,
                 )
                 truncated = output.tell() > MAX_OUTPUT_BYTES
                 output.seek(0)
diff --git a/src/lecode/extras/shell.py b/src/lecode/extras/shell.py
new file mode 100644
index 0000000..1c03a67
--- /dev/null
+++ b/src/lecode/extras/shell.py
@@ -0,0 +1,21 @@
+"""Detect the user's shell for command execution."""
+
+from __future__ import annotations
+
+import os
+import shutil
+
+#: Fallback when $SHELL is unset or does not resolve to an executable.
+FALLBACK_SHELL = "/bin/sh"
+
+
+def user_shell() -> str:
+    """The user's shell: ``$SHELL`` resolved, else ``/bin/sh``.
+
+    ``shutil.which`` accepts absolute paths and bare names alike; an unset,
+    empty, or non-executable ``$SHELL`` falls back to ``/bin/sh`` — the
+    previously hardcoded behavior.
+    """
+    shell = os.environ.get("SHELL", "")
+    resolved = shutil.which(shell) if shell else None
+    return resolved or FALLBACK_SHELL
diff --git a/src/lecode/tui/app.py b/src/lecode/tui/app.py
index cf312ae..dd32fb6 100644
--- a/src/lecode/tui/app.py
+++ b/src/lecode/tui/app.py
@@ -68,6 +68,7 @@
 from lecode.extras.loop_mode import run_plan_loop
 from lecode.extras.mcp_client import MCP_EXTRA, attach_mcp
 from lecode.extras.proc import run_proc
+from lecode.extras.shell import user_shell
 from lecode.extras.status_signals import START, STOP, StatusEmitter
 from lecode.extras.subagents import (
     SubagentError,
@@ -1691,7 +1692,9 @@ async def _run_shell(self, cmd: str, *, share_with_llm: bool, steer: bool) -> No
         self._activity("running shell")
         self._shell_task = asyncio.current_task()
         try:
-            result = await run_proc(["bash", "-c", cmd], cwd=self._cwd, timeout=SHELL_TIMEOUT_S)
+            result = await run_proc(
+                [user_shell(), "-c", cmd], cwd=self._cwd, timeout=SHELL_TIMEOUT_S
+            )
         except asyncio.CancelledError:
             self._feed.info("shell command cancelled")
             return
diff --git a/tests/test_shell.py b/tests/test_shell.py
new file mode 100644
index 0000000..a92e5bc
--- /dev/null
+++ b/tests/test_shell.py
@@ -0,0 +1,51 @@
+"""Tests for user shell detection (extras/shell.py)."""
+
+from __future__ import annotations
+
+import os
+import stat
+
+from lecode.extras.shell import FALLBACK_SHELL, user_shell
+
+
+def _make_executable(path, body="#!/bin/sh\nexit 0\n"):
+    path.write_text(body)
+    path.chmod(path.stat().st_mode | stat.S_IXUSR)
+    return path
+
+
+def test_falls_back_to_sh_when_shell_unset(monkeypatch):
+    monkeypatch.delenv("SHELL", raising=False)
+    assert user_shell() == FALLBACK_SHELL
+
+
+def test_falls_back_to_sh_when_shell_empty(monkeypatch):
+    monkeypatch.setenv("SHELL", "")
+    assert user_shell() == FALLBACK_SHELL
+
+
+def test_falls_back_when_shell_path_missing(monkeypatch, tmp_path):
+    monkeypatch.setenv("SHELL", str(tmp_path / "nope"))
+    assert user_shell() == FALLBACK_SHELL
+
+
+def test_falls_back_when_shell_not_executable(monkeypatch, tmp_path):
+    plain = tmp_path / "not-a-shell"
+    plain.write_text("text\n")
+    monkeypatch.setenv("SHELL", str(plain))
+    assert user_shell() == FALLBACK_SHELL
+
+
+def test_absolute_shell_path_is_used(monkeypatch, tmp_path):
+    shell = _make_executable(tmp_path / "myshell")
+    monkeypatch.setenv("SHELL", str(shell))
+    assert user_shell() == str(shell)
+
+
+def test_bare_shell_name_resolves_on_path(monkeypatch, tmp_path):
+    bin_dir = tmp_path / "bin"
+    bin_dir.mkdir()
+    shell = _make_executable(bin_dir / "myshell")
+    monkeypatch.setenv("PATH", f"{bin_dir}{os.pathsep}{os.environ.get('PATH', '')}")
+    monkeypatch.setenv("SHELL", "myshell")
+    assert user_shell() == str(shell)
diff --git a/tests/test_tool_bash.py b/tests/test_tool_bash.py
index b310a8e..f25e8d5 100644
--- a/tests/test_tool_bash.py
+++ b/tests/test_tool_bash.py
@@ -26,6 +26,22 @@ async def test_echo(tool_ctx):
     assert not proc.timed_out
 
 
+async def test_uses_detected_user_shell(tool_ctx, monkeypatch):
+    """Commands run through $SHELL -c — here /bin/echo, which prints its argv."""
+    monkeypatch.setenv("SHELL", "/bin/echo")
+    result = await bash.make_tool().run({"command": "hello-shell"}, tool_ctx)
+    assert result.content.startswith("-c hello-shell")
+
+
+def test_description_names_the_detected_shell(tmp_path, monkeypatch):
+    """The model sees the real shell so it can adapt its syntax."""
+    shell = tmp_path / "mysh"
+    shell.write_text("#!/bin/sh\n")
+    shell.chmod(shell.stat().st_mode | stat.S_IXUSR)
+    monkeypatch.setenv("SHELL", str(shell))
+    assert f"{shell} -c" in bash.make_tool().description
+
+
 async def test_nonzero_exit_reported(tool_ctx):
     result = await bash.make_tool().run({"command": "exit 7"}, tool_ctx)
     assert result.is_error
diff --git a/tests/test_tui_app.py b/tests/test_tui_app.py
index d6f5af7..c5c7a1a 100644
--- a/tests/test_tui_app.py
+++ b/tests/test_tui_app.py
@@ -1010,6 +1010,14 @@ async def test_bang_runs_shell_without_llm(tmp_path, monkeypatch):
     assert provider.requests == []
 
 
+async def test_bang_uses_the_detected_user_shell(tmp_path, monkeypatch):
+    """!cmd runs through $SHELL -c — here /bin/echo, which prints its argv."""
+    monkeypatch.setenv("SHELL", "/bin/echo")
+    app, _, out = make_app(tmp_path, monkeypatch, [])
+    await app._submit("!echo-marker")
+    assert "-c echo-marker" in out.getvalue()
+
+
 async def test_double_bang_feeds_output_to_llm(tmp_path, monkeypatch):
     app, provider, out = make_app(tmp_path, monkeypatch, [{"text": "noted"}])
     await app._submit("!!echo from-shell")