Problem

Found during exploratory error testing of the CLI. Running assembly transcribe "" (empty-string source) silently entered batch mode and recursively walked the working directory, queuing every audio file it found anywhere underneath:

$ assembly transcribe ""
 Source                        Status   ...
 .venv/lib/python3.12/site-…   queued
 tests/e2e/fixtures/fox.wav    queued
 ...   (every audio file under cwd, recursively)

Root cause

In expand_sources (aai_cli/transcribe_batch.py), an empty string fell through the source is None guard. Path("") resolves to ., so path.is_dir() was True and the source was treated as "batch this directory" → rglob("*") over the entire tree. With a valid API key this would upload arbitrary local audio (a real cost/privacy footgun).

Fix

Treat an empty source like a missing one (not source) so it stays on the single-source path, where it already reads correctly as:

$ assembly transcribe ""
Error: Provide an audio path or URL.
Suggestion: Or pass --sample to use the hosted demo file.

A real directory argument (assembly transcribe .) still batches as before — only the empty string is redirected.

Tests

• Extended test_non_batch_sources_return_none to cover "".
• Added test_empty_source_argument_does_not_batch_the_working_directory, a CLI-level regression test that monkeypatches Path.rglob to fail loudly if an empty source ever reaches directory scanning again, and asserts exit code 2 + the correct message.

Full ./scripts/check.sh gate passes (incl. 100% patch coverage + mutation gate).

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