diff --git a/src/plyngent/tools/process/run_command.py b/src/plyngent/tools/process/run_command.py
index 8b34cd8..814dfbb 100644
--- a/src/plyngent/tools/process/run_command.py
+++ b/src/plyngent/tools/process/run_command.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from plyngent.agent import ToolTag, tool
-from plyngent.tools.workspace import WorkspaceError
+from plyngent.tools.workspace import WorkspaceError, argv_shape_error
 
 from .command_exec import (
     DEFAULT_TIMEOUT_SECONDS,
@@ -20,6 +20,9 @@ async def run_command(
     env: dict[str, str] | None = None,
 ) -> str:
     """Run ``command`` (argv, no shell) under the workspace; capture stdout/stderr."""
+    shape_error = argv_shape_error(command)
+    if shape_error is not None:
+        return f"error: `run_command` {shape_error}"
     try:
         result = await execute_argv(
             command,
diff --git a/src/plyngent/tools/workspace.py b/src/plyngent/tools/workspace.py
index 6dfa469..d4fb0b0 100644
--- a/src/plyngent/tools/workspace.py
+++ b/src/plyngent/tools/workspace.py
@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, cast
 
 if TYPE_CHECKING:
     from collections.abc import Callable, Sequence
@@ -268,6 +268,23 @@ def resolve_path(path: str | Path) -> Path:
     return resolved
 
 
+def argv_shape_error(argv: object) -> str | None:
+    """Return a model-facing error when ``argv`` is not a non-empty argv list.
+
+    Models occasionally pass a shell-style string or a JSON-encoded array as a
+    string; duck-typing would otherwise let it reach ``create_subprocess_exec``
+    and splat into single characters. Declared as ``object`` so the isinstance
+    checks are not flagged as unnecessary.
+    """
+    if isinstance(argv, str):
+        return 'only accepts an array of args, not a string; pass e.g. ["ls", "-la"]'
+    if not isinstance(argv, list):
+        return f"only accepts an array of args, not a {type(argv).__name__}"
+    if not all(isinstance(part, str) for part in cast("list[object]", argv)):
+        return "only accepts an array of string args"
+    return None
+
+
 def check_command_allowed(argv: list[str]) -> None:
     """Raise if argv is empty or the executable basename is denylisted.
 
@@ -275,6 +292,10 @@ def check_command_allowed(argv: list[str]) -> None:
     installed: the human is asked (with a timeout; default deny). Session grants
     skip re-prompting for the same basename. Independent of YOLO soft-confirm.
     """
+    shape_error = argv_shape_error(argv)
+    if shape_error is not None:
+        msg = f"command {shape_error}"
+        raise WorkspaceError(msg)
     if not argv:
         msg = "command argv must not be empty"
         raise WorkspaceError(msg)
diff --git a/tests/test_tools/test_process.py b/tests/test_tools/test_process.py
index 9dcda69..4154622 100644
--- a/tests/test_tools/test_process.py
+++ b/tests/test_tools/test_process.py
@@ -48,6 +48,36 @@ async def test_run_command_echo(workspace: object) -> None:
     assert "hi" in out
 
 
+async def test_run_command_string_command_error(workspace: object) -> None:
+    del workspace
+    out = await call_async(run_command, "ls -la")
+    assert "error:" in out
+    assert "only accepts an array" in out
+    assert "executable not found" not in out
+
+
+async def test_run_command_stringified_json_array_error(workspace: object) -> None:
+    del workspace
+    out = await call_async(run_command, '["ls", "-la"]')
+    assert "error:" in out
+    assert "only accepts an array" in out
+    assert "executable not found" not in out
+
+
+async def test_run_command_non_string_args_error(workspace: object) -> None:
+    del workspace
+    out = await call_async(run_command, ["ls", 1])
+    assert "error:" in out
+    assert "array of string args" in out
+
+
+async def test_open_pty_string_command_error(workspace: object) -> None:
+    del workspace
+    out = await call_async(open_pty, "echo hi")
+    assert "error:" in out
+    assert "only accepts an array" in out
+
+
 async def test_run_command_batch_serial(workspace: object) -> None:
     del workspace
     out = await call_async(