diff --git a/nanobot/agent/tools/shell.py b/nanobot/agent/tools/shell.py
index 02dc3ec3..f6d37f14 100644
--- a/nanobot/agent/tools/shell.py
+++ b/nanobot/agent/tools/shell.py
@@ -89,7 +89,11 @@ class _PreparedCommand:
             maximum=600,
         ),
         shell=StringSchema(
-            "Optional shell binary to launch. Unix: sh, bash, zsh. Windows: powershell, pwsh, cmd.",
+            (
+                "Optional shell binary to launch. Windows: powershell, pwsh, cmd."
+                if _IS_WINDOWS
+                else "Optional shell binary to launch. Unix: sh, bash, zsh."
+            ),
             nullable=True,
         ),
         login=BooleanSchema(
@@ -227,6 +231,13 @@ class ExecTool(Tool):
 
     @property
     def description(self) -> str:
+        platform_note = (
+            "On Windows, use PowerShell syntax by default; pass shell='cmd' "
+            "only for cmd-specific commands. "
+            if _IS_WINDOWS
+            else "On Unix, commands run through bash by default; pass shell='sh' "
+            "or shell='zsh' when needed. "
+        )
         return (
             "Execute a shell command and return its output. "
             "Use this for tests, builds, package commands, git commands, and "
@@ -234,8 +245,7 @@ class ExecTool(Tool):
             "inspection and apply_patch/write_file/edit_file for file changes "
             "instead of cat, shell find/grep, echo, or sed. "
             "Use -y or --yes flags to avoid interactive prompts. "
-            "On Windows, use PowerShell syntax by default; pass shell='cmd' "
-            "only for cmd-specific commands. "
+            f"{platform_note}"
             "For long-running or interactive commands, pass yield_time_ms; "
             "if the command keeps running, exec returns a session_id that can "
             "be polled or written to with write_stdin. Output is truncated at "
diff --git a/tests/tools/test_tool_descriptions.py b/tests/tools/test_tool_descriptions.py
index bb7665e4..9ae2fc58 100644
--- a/tests/tools/test_tool_descriptions.py
+++ b/tests/tools/test_tool_descriptions.py
@@ -1,3 +1,6 @@
+import sys
+from unittest.mock import patch
+
 from nanobot.agent.tools.apply_patch import ApplyPatchTool
 from nanobot.agent.tools.exec_session import ListExecSessionsTool, WriteStdinTool
 from nanobot.agent.tools.filesystem import EditFileTool, ReadFileTool, WriteFileTool
@@ -44,3 +47,26 @@ def test_coding_tool_descriptions_steer_discovery_and_shell_usage() -> None:
     assert "do not use this to start new commands" in write_stdin
     assert "wait_for" in write_stdin
     assert "recover a session_id" in list_sessions
+
+
+def test_exec_tool_shell_guidance_matches_platform() -> None:
+    with patch("nanobot.agent.tools.shell._IS_WINDOWS", False):
+        unix_description = ExecTool().description.lower()
+    assert "on unix" in unix_description
+    assert "powershell" not in unix_description
+    assert "cmd-specific" not in unix_description
+
+    with patch("nanobot.agent.tools.shell._IS_WINDOWS", True):
+        windows_description = ExecTool().description.lower()
+    assert "powershell syntax" in windows_description
+    assert "shell='cmd'" in windows_description
+
+    shell_parameter = ExecTool().parameters["properties"]["shell"]["description"].lower()
+    if sys.platform == "win32":
+        assert "powershell" in shell_parameter
+        assert "cmd" in shell_parameter
+        assert "unix" not in shell_parameter
+    else:
+        assert "unix" in shell_parameter
+        assert "powershell" not in shell_parameter
+        assert "cmd" not in shell_parameter