diff --git a/strix/agents/factory.py b/strix/agents/factory.py
index 2b7de85e..977d01f1 100644
--- a/strix/agents/factory.py
+++ b/strix/agents/factory.py
@@ -209,6 +209,13 @@ def _wrap_exec_command(tool: FunctionTool) -> FunctionTool:
     invoke_tool = tool.on_invoke_tool
 
     async def invoke(ctx: Any, raw_input: str) -> Any:
+        try:
+            parsed = json.loads(raw_input)
+        except (json.JSONDecodeError, TypeError):
+            parsed = None
+        if isinstance(parsed, dict) and "shell" not in parsed:
+            parsed["shell"] = "bash"
+            raw_input = json.dumps(parsed)
         try:
             return await invoke_tool(ctx, raw_input)
         except ValidationError as exc:
diff --git a/tests/test_agent_factory_shell.py b/tests/test_agent_factory_shell.py
new file mode 100644
index 00000000..6de93219
--- /dev/null
+++ b/tests/test_agent_factory_shell.py
@@ -0,0 +1,51 @@
+"""Tests for the shell tool adapters in the agent factory."""
+
+from __future__ import annotations
+
+import json
+from typing import Any, cast
+
+import pytest
+from agents.tool import FunctionTool
+
+from strix.agents import factory
+
+
+def _capturing_exec_tool(captured: dict[str, str]) -> FunctionTool:
+    async def invoke(_ctx: Any, raw_input: str) -> str:
+        captured["raw_input"] = raw_input
+        return "ok"
+
+    return FunctionTool(
+        name="exec_command",
+        description="test tool",
+        params_json_schema={"type": "object", "properties": {}},
+        on_invoke_tool=invoke,
+    )
+
+
+@pytest.mark.asyncio
+async def test_wrap_exec_command_defaults_shell_to_bash() -> None:
+    captured: dict[str, str] = {}
+    wrapped = factory._wrap_exec_command(_capturing_exec_tool(captured))
+
+    result = await wrapped.on_invoke_tool(cast("Any", None), json.dumps({"cmd": "source /tmp/env"}))
+
+    assert result == "ok"
+    assert json.loads(captured["raw_input"]) == {
+        "cmd": "source /tmp/env",
+        "shell": "bash",
+    }
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("shell", ["/bin/zsh", ""])
+async def test_wrap_exec_command_preserves_explicit_shell(shell: str) -> None:
+    captured: dict[str, str] = {}
+    wrapped = factory._wrap_exec_command(_capturing_exec_tool(captured))
+
+    await wrapped.on_invoke_tool(
+        cast("Any", None), json.dumps({"cmd": "echo test", "shell": shell})
+    )
+
+    assert json.loads(captured["raw_input"])["shell"] == shell