fix(tools): percent-decode filename in mcp_resource_to_file

[Zawwarsami16]

What's happening

mcp_resource_to_file in src/anthropic/lib/tools/mcp.py extracts a filename from the MCP resource's URI using urlparse(uri_str).path. urlparse() does not percent-decode the path component, so a resource URI like file:///docs/my%20notes.txt was returning a filename of "my%20notes.txt" instead of "my notes.txt". The same affects any non-ASCII filename — e.g. file:///%E6%97%A5%E8%A8%98.txt was returning the literal percent-encoded string instead of 日記.txt.

Downstream file-upload paths (the function's whole purpose is to feed files.upload()) generally don't recognize percent-encoded names, so the file ends up with a wrong-looking name on the user's account.

There's also a related smaller issue: when the URI ends with a / (so there is no last path segment), the previous code returned "" rather than None. The function's return-type annotation already declares tuple[str | None, bytes, str | None], so callers expect None as the "no filename" signal — getting "" instead is a footgun that downstream code can treat as a real (empty) filename.

The fix

path = urlparse(uri_str).path
last_segment = path.rsplit("/", 1)[-1] if path else ""
name = unquote(last_segment) if last_segment else None

That's it. urllib.parse.unquote handles both ASCII percent-encoding (%20 → space) and percent-encoded UTF-8 multi-byte sequences. The empty-string branch correctly collapses to None.

Tests

Three new tests in tests/lib/tools/test_mcp_tool.py::TestMCPResourceToFile:

• test_percent_encoded_filename_is_decoded — file:///docs/my%20notes.txt → "my notes.txt"
• test_percent_encoded_unicode_filename_is_decoded — file:///%E6%97%A5%E8%A8%98.txt → "日記.txt"
• test_uri_with_trailing_slash_yields_no_filename — file:///some/dir/ → None

The existing tests (which already cover plain ASCII filenames like doc.txt and img.png) keep passing, so this is purely additive on the behavior side.

$ uv run pytest tests/lib/tools/test_mcp_tool.py
..........................................                              [100%]
42 passed in 1.51s

No production code paths outside mcp_resource_to_file were touched.