Measure fan-out use on the fs read tools

• Status: Todo
• Kind: Chore
• Authors: jp
• Date: 2026-10-06
• Implements: 117
• Label: domain=mcp
• Label: domain=tooling

RFD 117 adds fan_out but enables it for no tool. Its open question "Does the assistant use it?" is answered by turning it on for the read-only filesystem tools in this repository's own configuration and watching what models do.

Proposal

1. Set fan_out = true on fs_read_file, fs_grep_files, and fs_list_files in .jp/mcp/tools/fs/*.toml. These are run = "allow" readers, so fan-out adds no prompts. #1183 already has the three config changes; the rest of that PR targets the abandoned provider-side design and should not be merged as is.
2. Use them for a week and record, per call to those tools, how many operations it carried. A distribution that stays at one operation per call means the envelope costs tokens and buys nothing, and the batching sentence or the schema needs another look before fan-out is enabled anywhere else.
3. Note the result in RFD 117's "Does the assistant use it?" risk.

Split out of #1182 review (comment 4195052824): the measurement is a use of the feature, not part of building it, so it does not hold back RFD 117's Implemented status.