diff --git a/src/plyngent/tools/chat/ask.py b/src/plyngent/tools/chat/ask.py
index aff2ff7..81ecbf4 100644
--- a/src/plyngent/tools/chat/ask.py
+++ b/src/plyngent/tools/chat/ask.py
@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from plyngent.agent import ToolTag, tool
 from plyngent.prompting import NonInteractiveError, ask_async
+from plyngent.tools.chat.shape import first_error, string_error
 
 
 @tool(name="ask_user_line", tags=ToolTag.LOCAL | ToolTag.READ_ONLY)
@@ -12,6 +13,12 @@ async def ask_user(question: str, default: str = "") -> str:
     or any input that is not a fixed menu. Optional ``default`` is used if the
     user submits empty input (and in non-interactive mode when provided).
     """
+    shape_error = first_error(
+        string_error("ask_user_line", "question", question),
+        string_error("ask_user_line", "default", default),
+    )
+    if shape_error is not None:
+        return f"error: {shape_error}"
     try:
         return await ask_async(question, default=default or None)
     except NonInteractiveError as exc:
diff --git a/src/plyngent/tools/chat/choose.py b/src/plyngent/tools/chat/choose.py
index 2fa71cd..0feb703 100644
--- a/src/plyngent/tools/chat/choose.py
+++ b/src/plyngent/tools/chat/choose.py
@@ -4,6 +4,7 @@ from typing import cast
 
 from plyngent.agent import ToolTag, tool
 from plyngent.prompting import ChoiceOption, NonInteractiveError, choose_async
+from plyngent.tools.chat.shape import first_error, list_error, string_error
 
 
 def parse_options(options: list[object]) -> list[ChoiceOption]:
@@ -51,6 +52,19 @@ async def choose_user(
     When ``allow_custom`` is true (default), free-text answers are accepted.
     Returns the chosen option value (or custom text).
     """
+    shape_error = first_error(
+        string_error("ask_user_choice", "question", question),
+        list_error(
+            "ask_user_choice",
+            "options",
+            options,
+            items="strings or objects with a label",
+            example='["alpha", "beta"]',
+        ),
+        string_error("ask_user_choice", "default", default),
+    )
+    if shape_error is not None:
+        return f"error: {shape_error}"
     try:
         parsed = parse_options([cast("object", o) for o in options])
     except (TypeError, ValueError) as exc:
diff --git a/src/plyngent/tools/chat/form.py b/src/plyngent/tools/chat/form.py
index 79c53c9..63026b3 100644
--- a/src/plyngent/tools/chat/form.py
+++ b/src/plyngent/tools/chat/form.py
@@ -1,21 +1,28 @@
 from __future__ import annotations
 
 import json
-from typing import cast
+from typing import TYPE_CHECKING, cast
 
 from plyngent.agent import ToolTag, tool
 from plyngent.prompting import FormField, NonInteractiveError, form_async
 from plyngent.tools.chat.choose import parse_options
+from plyngent.tools.chat.shape import first_error, list_error, string_error
+
+if TYPE_CHECKING:
+    from collections.abc import Sequence
 
 
-def parse_fields(fields: list[dict[str, object]]) -> list[FormField]:
+def parse_fields(fields: Sequence[object]) -> list[FormField]:
     """Normalize a list of field dicts into ``FormField`` list."""
     if not fields:
         msg = "fields must be a non-empty list"
         raise ValueError(msg)
     out: list[FormField] = []
     for item_obj in fields:
-        raw_map = {str(key): value for key, value in item_obj.items()}
+        if not isinstance(item_obj, dict):
+            msg = "each field must be an object with name/prompt"
+            raise TypeError(msg)
+        raw_map = {str(key): value for key, value in cast("dict[object, object]", item_obj).items()}
         name = raw_map.get("name")
         prompt = raw_map.get("prompt")
         if not isinstance(name, str) or not name:
@@ -61,6 +68,18 @@ async def form_user(
     optional ``allow_custom`` (default true).
     When ``confirm_submit`` is true, the human reviews a summary before submit.
     """
+    shape_error = first_error(
+        string_error("ask_user_form", "title", title),
+        list_error(
+            "ask_user_form",
+            "fields",
+            fields,
+            items="objects",
+            example='[{"name": "port", "prompt": "Which port?"}]',
+        ),
+    )
+    if shape_error is not None:
+        return f"error: {shape_error}"
     try:
         parsed = parse_fields(fields)
     except (TypeError, ValueError) as exc:
diff --git a/src/plyngent/tools/chat/shape.py b/src/plyngent/tools/chat/shape.py
new file mode 100644
index 0000000..b3b4d52
--- /dev/null
+++ b/src/plyngent/tools/chat/shape.py
@@ -0,0 +1,56 @@
+"""Shape checks for ask-family tool arguments.
+
+Tool arguments arrive as decoded JSON, so a parameter declared ``list[str]`` can
+still be a plain string at runtime. Duck-typing then misleads instead of failing:
+``ask_user_choice(options="yes/no")`` iterates the characters and shows a menu of
+letters, and a string ``fields`` dies inside the form parser with an
+``AttributeError``. Each ask-family tool checks its container arguments up front
+and answers with a model-facing error that also shows the expected shape (same
+idea as :func:`plyngent.tools.workspace.argv_shape_error`).
+
+Every check takes ``object``, so the isinstance calls stay meaningful and the
+tools themselves stay free of unnecessary-isinstance warnings.
+"""
+
+from __future__ import annotations
+
+_SHAPES: dict[type, str] = {
+    str: "a string",
+    list: "an array",
+    dict: "an object",
+    bool: "a boolean",
+    int: "a number",
+    float: "a number",
+    type(None): "null",
+}
+
+
+def shape_of(value: object) -> str:
+    """Model-facing name for a decoded JSON value (``a string``, ``an array``…)."""
+    return _SHAPES.get(type(value), type(value).__name__)
+
+
+def first_error(*errors: str | None) -> str | None:
+    """First non-``None`` error (``None`` when every shape was acceptable)."""
+    return next((error for error in errors if error), None)
+
+
+def string_error(tool: str, field: str, value: object) -> str | None:
+    """Error when *value* must be a string but is not; ``None`` when it is."""
+    if isinstance(value, str):
+        return None
+    return f"`{tool}` expects `{field}` as a string, got {shape_of(value)}"
+
+
+def list_error(tool: str, field: str, value: object, *, items: str, example: str) -> str | None:
+    """Error when *value* must be a JSON array of *items* but is not; else ``None``.
+
+    Only the container is checked: item-level validation stays in the tool's own
+    parser (``parse_options`` / ``parse_fields``), which already reports what is
+    wrong with a bad entry.
+    """
+    if isinstance(value, str):
+        return f"`{tool}` expects `{field}` as a JSON array of {items}, not a string; pass e.g. {example}"
+    if not isinstance(value, list):
+        return f"`{tool}` expects `{field}` as a JSON array of {items}, got {shape_of(value)}; pass e.g. {example}"
+    return None
diff --git a/tests/test_tools/test_chat_tools.py b/tests/test_tools/test_chat_tools.py
index 4601bf3..0c15651 100644
--- a/tests/test_tools/test_chat_tools.py
+++ b/tests/test_tools/test_chat_tools.py
@@ -192,3 +192,85 @@ def test_wait_prompt_colored_on_tty(monkeypatch) -> None:
     prompt = wait_module._wait_prompt(5, reason="tests")
     assert "\x1b[36m" in prompt  # cyan status
     assert "\x1b[33m" in prompt  # yellow input prompt
+
+
+def test_shape_of_names_json_types() -> None:
+    from plyngent.tools.chat.shape import first_error, shape_of
+
+    assert shape_of("x") == "a string"
+    assert shape_of([]) == "an array"
+    assert shape_of({}) == "an object"
+    assert shape_of(True) == "a boolean"  # bool is not reported as a number
+    assert shape_of(3) == "a number"
+    assert shape_of(3.5) == "a number"
+    assert shape_of(None) == "null"
+    assert first_error(None, "boom", "later") == "boom"
+    assert first_error(None, None) is None
+
+
+async def test_ask_user_line_rejects_non_string_args() -> None:
+    """Mis-typed args get explained instead of reaching the human as garbage."""
+    backend = ScriptedBackend(["unused"])
+    with temporary_backend(backend):
+        registry = ToolRegistry([ask_user])
+        bad_question = await registry.execute("ask_user_line", json.dumps({"question": ["a", "b"]}))
+        bad_default = await registry.execute(
+            "ask_user_line",
+            json.dumps({"question": "Which?", "default": 5}),
+        )
+    assert bad_question.startswith("error:")
+    assert "`question`" in bad_question
+    assert "got an array" in bad_question
+    assert bad_default.startswith("error:")
+    assert "`default`" in bad_default
+    assert "got a number" in bad_default
+    assert backend.lines == ["unused"]  # the prompt never happened
+
+
+async def test_choose_user_rejects_string_options() -> None:
+    """``options="alpha, beta"`` used to build a menu of single characters."""
+    backend = ScriptedBackend(["1"])
+    with temporary_backend(backend):
+        registry = ToolRegistry([choose_user])
+        out = await registry.execute(
+            "ask_user_choice",
+            json.dumps({"question": "Pick", "options": "alpha, beta"}),
+        )
+    assert out.startswith("error:")
+    assert "`options`" in out
+    assert "not a string" in out
+    assert '["alpha", "beta"]' in out
+    assert backend.lines == ["1"]
+
+
+async def test_choose_user_rejects_object_options() -> None:
+    registry = ToolRegistry([choose_user])
+    out = await registry.execute(
+        "ask_user_choice",
+        json.dumps({"question": "Pick", "options": {"alpha": 1}}),
+    )
+    assert out.startswith("error:")
+    assert "got an object" in out
+
+
+async def test_form_user_rejects_string_fields() -> None:
+    backend = ScriptedBackend(["ncbm"], confirms=[True])
+    with temporary_backend(backend):
+        registry = ToolRegistry([form_user])
+        out = await registry.execute(
+            "ask_user_form",
+            json.dumps({"title": "Setup", "fields": '[{"name": "user"}]'}),
+        )
+    assert out.startswith("error:")
+    assert "`fields`" in out
+    assert "not a string" in out
+    assert backend.lines == ["ncbm"]
+
+
+async def test_form_user_rejects_non_object_field() -> None:
+    registry = ToolRegistry([form_user])
+    out = await registry.execute(
+        "ask_user_form",
+        json.dumps({"title": "Setup", "fields": ["user"]}),
+    )
+    assert out == "error: each field must be an object with name/prompt"