diff --git a/aai_cli/agent/voices.py b/aai_cli/agent/voices.py
index f53bb20..b084eca 100644
--- a/aai_cli/agent/voices.py
+++ b/aai_cli/agent/voices.py
@@ -63,12 +63,11 @@ class Voice:
 
 def format_voice_list() -> str:
     """Human-readable voice IDs for --list-voices, grouped by language."""
-    groups = dict.fromkeys(voice.language for voice in VOICES)
-    blocks: list[str] = []
-    for language in groups:
-        names = "\n".join(f"  {voice.name}" for voice in VOICES if voice.language == language)
-        blocks.append(f"{language}:\n{names}")
-    return "\n\n".join(blocks)
+    languages = dict.fromkeys(voice.language for voice in VOICES)
+    return choices.render_grouped(
+        (language, [voice.name for voice in VOICES if voice.language == language])
+        for language in languages
+    )
 
 
 def complete_voice(incomplete: str) -> list[str]:
diff --git a/aai_cli/agent_cascade/voices.py b/aai_cli/agent_cascade/voices.py
index b6abb6e..3023b38 100644
--- a/aai_cli/agent_cascade/voices.py
+++ b/aai_cli/agent_cascade/voices.py
@@ -35,10 +35,7 @@ def complete_voice(incomplete: str) -> list[str]:
 
 def format_voice_list() -> str:
     """Human-readable voice ids for ``--list-voices``, grouped by language."""
-    blocks: list[str] = []
-    for code, label in _LANGUAGE_LABELS.items():
-        names = [name for name in VOICE_NAMES if tts_voices.VOICE_LANGUAGES[name] == code]
-        if names:
-            listing = "\n".join(f"  {name}" for name in names)
-            blocks.append(f"{label}:\n{listing}")
-    return "\n\n".join(blocks)
+    return choices.render_grouped(
+        (label, [name for name in VOICE_NAMES if tts_voices.VOICE_LANGUAGES[name] == code])
+        for code, label in _LANGUAGE_LABELS.items()
+    )
diff --git a/aai_cli/app/agent_shared.py b/aai_cli/app/agent_shared.py
index 188ea75..86ad1ec 100644
--- a/aai_cli/app/agent_shared.py
+++ b/aai_cli/app/agent_shared.py
@@ -7,9 +7,19 @@
 
 from __future__ import annotations
 
+from collections.abc import Container
 from pathlib import Path
 
-from aai_cli.core.errors import CLIError
+from aai_cli.core.errors import CLIError, UsageError
+
+
+def validate_voice(voice: str, valid_names: Container[str], *, command: str) -> None:
+    """Reject an unknown ``--voice`` with a usage error pointing at ``--list-voices``."""
+    if voice not in valid_names:
+        raise UsageError(
+            f"Unknown voice {voice!r}.",
+            suggestion=f"Run 'assembly {command} --list-voices' to see the options.",
+        )
 
 
 def resolve_system_prompt(system_prompt: str, system_prompt_file: Path | None) -> str:
diff --git a/aai_cli/commands/agent/_exec.py b/aai_cli/commands/agent/_exec.py
index 814154c..74fbcfc 100644
--- a/aai_cli/commands/agent/_exec.py
+++ b/aai_cli/commands/agent/_exec.py
@@ -21,6 +21,7 @@
 from aai_cli.agent.session import AgentRunConfig, run_session
 from aai_cli.agent.voices import VOICE_NAMES
 from aai_cli.app.agent_shared import resolve_system_prompt as _resolve_system_prompt
+from aai_cli.app.agent_shared import validate_voice
 from aai_cli.app.context import AppState
 from aai_cli.core import choices, client, errors, signals
 from aai_cli.core.errors import UsageError
@@ -94,11 +95,7 @@ def _print_show_code(opts: AgentOptions, system_prompt_text: str) -> None:
 def run_agent(opts: AgentOptions, state: AppState, *, json_mode: bool) -> None:
     """Execute one `assembly agent` conversation from already-parsed flags."""
     text_mode, json_mode = resolve_output_modes(opts.output_field, json_mode=json_mode)
-    if opts.voice not in VOICE_NAMES:
-        raise UsageError(
-            f"Unknown voice {opts.voice!r}.",
-            suggestion="Run 'assembly agent --list-voices' to see the options.",
-        )
+    validate_voice(opts.voice, VOICE_NAMES, command="agent")
     system_prompt_text = _resolve_system_prompt(opts.system_prompt, opts.system_prompt_file)
 
     if opts.show_code:
diff --git a/aai_cli/commands/agent_cascade/_exec.py b/aai_cli/commands/agent_cascade/_exec.py
index 022affd..0b97e23 100644
--- a/aai_cli/commands/agent_cascade/_exec.py
+++ b/aai_cli/commands/agent_cascade/_exec.py
@@ -21,6 +21,7 @@
 from aai_cli.agent_cascade import engine, voices
 from aai_cli.agent_cascade.config import DEFAULT_MAX_HISTORY, CascadeConfig
 from aai_cli.app.agent_shared import resolve_system_prompt as _resolve_system_prompt
+from aai_cli.app.agent_shared import validate_voice
 from aai_cli.app.context import AppState
 from aai_cli.core import choices, client, config_builder, errors, llm, signals
 from aai_cli.core.errors import UsageError
@@ -168,11 +169,7 @@ def _print_show_code(opts: AgentCascadeOptions, system_prompt_text: str) -> None
 def run_agent_cascade(opts: AgentCascadeOptions, state: AppState, *, json_mode: bool) -> None:
     """Execute one `assembly agent-cascade` cascade from already-parsed flags."""
     text_mode, json_mode = resolve_output_modes(opts.output_field, json_mode=json_mode)
-    if opts.voice not in voices.VOICE_NAMES:
-        raise UsageError(
-            f"Unknown voice {opts.voice!r}.",
-            suggestion="Run 'assembly agent-cascade --list-voices' to see the options.",
-        )
+    validate_voice(opts.voice, voices.VOICE_NAMES, command="agent-cascade")
     # Streaming TTS has no production host, so the whole cascade is sandbox-only.
     tts_session.require_available("agent-cascade")
     system_prompt_text = _resolve_system_prompt(opts.system_prompt, opts.system_prompt_file)
diff --git a/aai_cli/core/choices.py b/aai_cli/core/choices.py
index fe3d4da..ed843d6 100644
--- a/aai_cli/core/choices.py
+++ b/aai_cli/core/choices.py
@@ -9,6 +9,21 @@ def complete_prefix(options: Iterable[str], incomplete: str) -> list[str]:
     return [option for option in options if option.startswith(incomplete)]
 
 
+def render_grouped(groups: Iterable[tuple[str, Iterable[str]]]) -> str:
+    """Render labelled groups of names as an indented, blank-line-separated block list.
+
+    Each non-empty group renders as a ``label:`` header followed by its names
+    indented two spaces, one per line; groups are separated by a blank line and
+    empty groups are skipped. Backs the voice commands' grouped ``--list-voices``.
+    """
+    blocks: list[str] = []
+    for label, names in groups:
+        listing = "\n".join(f"  {name}" for name in names)
+        if listing:
+            blocks.append(f"{label}:\n{listing}")
+    return "\n\n".join(blocks)
+
+
 # CLI-owned closed value sets for ``-o/--output``. ``StrEnum`` members *are* their
 # string values: Typer renders them as choices in ``--help`` (e.g.
 # [text|id|status|utterances|srt|json]), validates input with a clean listing error,
diff --git a/tests/test_choices.py b/tests/test_choices.py
new file mode 100644
index 0000000..1c5518b
--- /dev/null
+++ b/tests/test_choices.py
@@ -0,0 +1,29 @@
+"""Tests for the shared choice/listing helpers in `aai_cli.core.choices`."""
+
+from __future__ import annotations
+
+from aai_cli.core import choices
+
+
+def test_complete_prefix_filters_by_prefix():
+    assert choices.complete_prefix(["alpha", "alto", "beta"], "al") == ["alpha", "alto"]
+
+
+def test_render_grouped_headers_indentation_and_separator():
+    rendered = choices.render_grouped([("English", ["jane", "michael"]), ("Italian", ["giovanni"])])
+    # A blank line separates groups; each name is indented two spaces under its header.
+    assert rendered == "English:\n  jane\n  michael\n\nItalian:\n  giovanni"
+
+
+def test_render_grouped_skips_empty_groups():
+    # A label with no names contributes nothing — no dangling "French:" header and
+    # no extra blank-line separator around it.
+    rendered = choices.render_grouped(
+        [("English", ["jane"]), ("French", []), ("Italian", ["luca"])]
+    )
+    assert rendered == "English:\n  jane\n\nItalian:\n  luca"
+    assert "French" not in rendered
+
+
+def test_render_grouped_empty_input_is_empty_string():
+    assert choices.render_grouped([]) == ""