[Vasyl Smutok (vsmutok)]

Summary

nbformat allows a cell's source to be either a list of strings or a single string, and both forms validate. IpynbConverter iterated over source assuming it was a list of lines, so with the single-string form it walked the characters one at a time and line.startswith("# ") never matched — the notebook title was silently dropped while the body still converted correctly.

This normalizes string sources into a list of lines with splitlines(keepends=True) before the title scan. Line endings are preserved, so the generated markdown is byte-for-byte identical for both forms.

Before / after

# source: ["# My Title\n", "hello"]  -> title 'My Title'   (unchanged)
# source: "# My Title\nhello"        -> title None          (before)
# source: "# My Title\nhello"        -> title 'My Title'   (after)

Tests

Added two regression tests in packages/markitdown/tests/test_module_misc.py:

• test_ipynb_string_source_extracts_title — reproduces the issue.
• test_ipynb_string_and_list_sources_convert_identically — asserts both source forms produce the same markdown (markdown/code/raw cells, including a source without a trailing newline).

Both fail on main and pass with the fix. Full suite locally on Python 3.14:

1165 passed, 12 skipped

Fixes #2762

[Vasyl Smutok (vsmutok)]

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