Windows: tool calls fail when arguments contain single-backslash paths (unescaped in tool-call JSON)

Summary

On Windows, when the model emits a tool call whose arguments include an absolute path like E:\Folder\file.md, the backslashes go into the tool-call JSON unescaped. Sequences like \C or \A are invalid JSON escapes, so the entire arguments object fails to parse and the tool never runs.

Captured error (SendUserFile, 2026-08-16)

InputValidationError: SendUserFile was called with input that could not be
parsed as JSON.
You sent (first 181 of 181 bytes):
{"files": E:\Chat gpt Codex\Angels sword\Claude-Voice-Input-Bug-Report.md,
 "caption": "…", "status": "normal"}
Common causes: unescaped backslashes in file paths (use / or \\),
unescaped control characters, or truncated output. Retry with valid JSON.

Two defects visible in that emitted payload:

1. Unescaped backslashes — the path contains \C, \A, which are not valid JSON string escapes.
2. The path value wasn't wrapped as a quoted array element — "files": <bare path> instead of "files": ["E:/…"] — suggesting the argument serializer mishandles the whole value once a backslash path is involved.

Why it happens

JSON permits only a fixed set of escapes after a backslash (\", \\, \/, \b, \f, \n, \r, \t, \uXXXX). A raw Windows path such as E:\Chat\Angels contains invalid escapes, so a strict parser rejects the whole arguments object — not just the one field. Windows-only; POSIX forward-slash paths never trigger it.

Repro

1. On Windows, drive any flow where the model passes a single-backslash absolute path as a tool argument (file send/read/write/attach).
2. The tool call fails with "could not be parsed as JSON / unescaped backslashes."
3. The same call with forward slashes (E:/Folder/file.md) or doubled backslashes succeeds.

Occurs whenever a single-backslash path lands in tool arguments; appears intermittent only because the model often happens to use forward slashes.

Impact

• Tool call lost; at least one wasted turn per occurrence.
• The agent can misread the failure, retry incorrectly, or abandon the action.
• Disproportionately hits Windows users on paths that are perfectly valid on the OS.

Requested fix (serializer side — durable)

1. JSON-escape string argument values automatically when building tool-call payloads (backslashes → \\, quotes, control chars), rather than relying on the model to pre-sanitize paths.
2. Fail soft: on a parse failure caused by lone backslashes, attempt one automatic repair (escape them) and log it, instead of dropping the call.
3. Normalize Windows paths at the tool boundary (accept \, \\, and / interchangeably).

Environment

Windows 11 Pro 26200 · Claude Code desktop 1.30096.5.0 · observed 2026-08-16 on SendUserFile; defect is general to any tool receiving a Windows path argument.