fix(memory): use split("\n") in insert() to match view()/str_replace()

[otiscuilei]

Who hits this

Anyone using the built-in filesystem memory tool (BetaLocalFilesystemMemoryTool / BetaAsyncLocalFilesystemMemoryTool) whose memory files contain any of the characters str.splitlines() treats as line boundaries but "\n" does not: carriage return, vertical tab, form feed (\x0c), file/group/record separators (\x1c-\x1e), NEL (\x85), U+2028, or U+2029. When the model issues an insert command against such a file, the insert lands at the wrong line and those characters are silently rewritten to \n.

Root cause

Every memory command reads and splits file content the same way except insert. view() (line 451) and str_replace() (line 526) split on "\n", but both the sync and async insert() used content.splitlines(). splitlines() breaks on ~8 extra Unicode/control boundaries in addition to \n. As a result insert():

• counted and located lines differently from what view() reports to the model, so an insert_line chosen from the numbers view() showed landed at the wrong offset (text spliced into the middle of what the model saw as a single line), and
• rejoined the lines with "\n", discarding the boundary characters that were part of the file's content.

Fix

Change content.splitlines() to content.split("\n") in both insert() methods, making line indexing consistent with view() and str_replace() and preserving the file's characters. No behavior change for ordinary \n-delimited files.

Before / after

from anthropic.lib.tools._beta_builtin_memory_tool import BetaLocalFilesystemMemoryTool
from anthropic.types.beta import (
    BetaMemoryTool20250818CreateCommand,
    BetaMemoryTool20250818InsertCommand,
)

tool = BetaLocalFilesystemMemoryTool(base_path="/tmp/mem-demo")

# A file whose content contains a form feed (\x0c). view() reports it as ONE line.
tool.create(BetaMemoryTool20250818CreateCommand(
    command="create", file_text="alpha beta\x0cgamma", path="/memories/f.txt"
))

# Insert after that single line.
tool.insert(BetaMemoryTool20250818InsertCommand(
    command="insert", path="/memories/f.txt", insert_line=1, insert_text="INSERTED"
))

print(repr(open("/tmp/mem-demo/memories/f.txt").read()))
# Before: 'alpha beta\nINSERTED\ngamma\n'   -> \x0c lost, INSERTED spliced into the middle
# After:  'alpha beta\x0cgamma\nINSERTED\n' -> \x0c preserved, INSERTED appended after the line

Tests

Adds one regression test (sync and async) in tests/lib/tools/memory_tools/test_filesystem.py that creates a file containing U+2028 and a form feed, asserts view() numbers it as a single line, runs insert(insert_line=1, ...), and asserts the boundary characters are preserved and the text is appended after the whole line. Fails on the current code, passes with the fix. Full memory-tool suite: 65 passed. ruff check / ruff format clean.