feat(mcp): expose fallow impact as a read-only MCP tool#804
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Add an "impact" tool wrapping "fallow impact --format json" so AI agents can read the local value report (surfacing, trend, gate containment, and v1.5 resolved/suppressed attribution) the same way they query check_health and audit. Impact was the only first-class CLI surface with no MCP tool. The tool is read-only and root-only: it runs no analysis and reads only .fallow/impact.json, so the config / no_cache / threads knobs the analysis tools expose are omitted as inert. The mutating enable / disable lifecycle is intentionally not exposed; on an enabled:false report the agent should recommend the user run "fallow impact enable", not toggle it. Annotated read_only_hint=true, idempotent_hint=true, open_world_hint=false (a single closed-world local file read, like fallow_explain).
Bump the fallow-mcp tool count to 21 and add an impact entry in .claude/rules/mcp-server.md, add a CHANGELOG [Unreleased] note, and re-bless scripts/agent-files.sha256 for the rules-file edit (CI drift gate).
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Summary
impactMCP tool (the 21st) wrappingfallow impact --format json, so AI agents can read the local value report (current surfacing, trend since the last recorded run, pre-commit gate containment, and v1.5 resolved/suppressed attribution) the same way they callcheck_healthoraudit. Impact was the only first-class CLI surface with no MCP tool.root-only params:impactruns no analysis and reads only.fallow/impact.json, so theconfig/no_cache/threadsknobs the analysis tools expose are deliberately omitted as inert.enable/disablelifecycle is not exposed. On a never-enabled project the report is a populated{"enabled": false, ...}object (never{}), so an agent branches onenabledthenrecord_countand recommends the user runfallow impact enablerather than toggling it.read_only_hint=true,idempotent_hint=true,open_world_hint=false(a single closed-world local file read, likefallow_explain).schema_version(they ship under unchangedschema_version"1").Test plan
cargo test -p fallow-mcp: all pass, including 8 new impact tests (arg building, empty-root drop, param deserialization with stray-field tolerance, registration + count 21, annotations, schema-property inclusion/exclusion).cargo build --workspaceclean (post-rebase onto current main).cargo clippy -p fallow-mcp --all-targets -- -D warningsclean.cargo fmt --all -- --checkclean.cargo doc -p fallow-mcp --no-deps --document-private-itemsclean.fallow impact --format jsonon a never-enabled project returns the populated{"enabled": false, "record_count": 0, ...}report (exit 0, deterministic); afterimpact enableit returnsenabled: true, record_count: 0..claude/rules/mcp-server.md(count 20 to 21 + tool entry) andCHANGELOG.mdupdated; companion repos (fallow-docs, fallow-skills) updated.