Fix UnicodeDecodeError when non-ASCII text falls past the sniffed charset prefix

[sesharif]

Problem

The charset is sniffed from the first 4k of a stream but then used to decode all of it, so a mostly-ASCII document whose first non-ASCII character appears beyond that window fails to convert.

This repo's own README reproduces it — its first em dash sits at byte 5030:

$ markitdown README.md
markitdown._exceptions.FileConversionException: File conversion failed after 1 attempts:
 - PlainTextConverter threw UnicodeDecodeError with message: 'ascii' codec can't decode byte 0xe2 in position 5030: ordinal not in range(128)

Fix

Two layers, because the bug has two halves.

Detection — widen an ASCII guess to UTF-8 in _get_stream_info_guesses. ASCII is a strict subset of UTF-8, so genuine ASCII decodes identically, while UTF-8 content beyond the sniffed prefix now decodes correctly. This layer matters beyond decoding: the charset is part of the StreamInfo identity used to decide which guesses are compatible, so it also affects which converters get tried.

Decoding — add decode_bytes() in a new _charset_utils module: try the declared charset, re-detect over the full data if that fails, and fall back to a lossy decode as a last resort. A charset can also arrive already declared — from a Content-Type header, or from the caller — in which case it is trusted without being sniffed and can be wrong in the same way.

Wired into the three converters that decode a whole stream with such a charset: plain text, CSV, and ipynb. The HTML-family converters (_html_converter, _bing_serp_converter, _wikipedia_converter, _youtube_converter) pass the charset to BeautifulSoup as from_encoding, which recovers on its own, so they are left alone.

_normalize_charset now delegates to the shared helper rather than duplicating it.

Tests

New tests/test_charset.py (16 tests) covering the sniffed-prefix regression, a too-narrow declared charset through each of the three converters, and the decode_bytes fallback chain. Verified they catch the bug: with the fix reverted, 4 fail; restored, all pass.

Full suite goes from 335 to 351 passing, with no new failures.

One expectation changed

Two vectors in _test_vectors.py asserted charset="ascii" for test.json and test_notebook.ipynb. Both fixtures are pure ASCII, so UTF-8 decodes them byte-identically; the expectations are updated to match the widened guess. Flagging it explicitly since it is an assertion change rather than a code fix — happy to take a different approach if you would rather the widening not be observable in _get_stream_info_guesses output.

🤖 Generated with Claude Code

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