`revert` can leave a config swapped and delete every backup

[tony]

Summary

scripts/mcp_swap.py revert can finish with exit 0, print restored from … for every entry, delete all backups, and still leave the config holding the swap. It happens whenever two swap-state entries point at the same file: the older snapshot already contains the newer swap, so restoring it last re-applies what was just undone. Because the backups are then removed, the pre-swap config is unrecoverable.

Two ways to reach it, both from the documented flag surface:

• Claude's user and project scopes both write ~/.claude.json, so revert --scope unwinds one layer at a time in whatever order they are invoked.
• Any two CLIs whose config paths resolve to one file — for example ~/.cursor/mcp.json symlinked to ~/.gemini/config/mcp_config.json, a normal dotfiles arrangement.

Reproduction

Claude's two scopes, reverted oldest-last. Everything runs under a throwaway HOME:

SB=$(mktemp -d); mkdir -p "$SB/.config"

printf '{\n  "mcpServers": {\n    "other": {"command": "x", "args": ["y"]}\n  }\n}\n' > "$SB/.claude.json"

cp "$SB/.claude.json" "$SB/pristine.json"

run() { HOME="$SB" XDG_CONFIG_HOME="$SB/.config" XDG_STATE_HOME="$SB/.local/state" uv run scripts/mcp_swap.py "$@"; }

run use-local --repo "$PWD" --cli claude --scope project --no-preflight

run use-local --repo "$PWD" --cli claude --scope user --no-preflight

run revert --cli claude --scope project

run revert --cli claude --scope user

diff "$SB/pristine.json" "$SB/.claude.json"; ls "$SB"/.claude.json.bak.* 2>/dev/null || echo NONE

The second trigger needs no --scope. Point two CLIs at one file and revert them together:

ln -s "$SB/.cursor/mcp.json" "$SB/.gemini/config/mcp_config.json"

run use-local --repo "$PWD" --cli cursor --server alpha --no-preflight

run use-local --repo "$PWD" --cli agy --server beta --no-preflight

run revert --cli cursor --cli agy

Expected

revert unwinds every selected entry newest-first by seq_no, so the oldest snapshot — the only one that predates all the swaps — is restored last. A revert that cannot reach that state should refuse and keep the backups rather than report success.

Actual

Both reverts print restored from … and exit 0. ~/.claude.json still carries the libtmux entry under projects.<repo>.mcpServers, and both .bak.mcp-swap-* files are gone. The shared-file variant leaves the first CLI's server name behind in the same way.

Ordering is also not deterministic without --cli: targets is built from a set comprehension, so which CLI unwinds first depends on string-hash randomization.

Environment

Versions

• libtmux-mcp 0.1.0a20 (v0.1.0a20)
• Python 3.14.6
• uv 0.12.1
• Linux 6.18.33.2 (WSL2)

Evidence

Claude two-scope revert, run against v0.1.0a20

$ run revert --cli claude --scope project
[claude:project] restored from <sandbox>/.claude.json.bak.mcp-swap-20260809203355-project
rc=0
$ run revert --cli claude --scope user
[claude:user] restored from <sandbox>/.claude.json.bak.mcp-swap-20260809203355-user
rc=0
$ diff pristine.json .claude.json
CORRUPT — swap still present: "libtmux"
$ ls .claude.json.bak.*
NONE — all backups deleted

Two CLIs sharing one file via symlink

$ rg -o '"(cursor|agy):user"|"target_path": "[^"]*"' state.json
"cursor:user"
"target_path": "$HOME/.cursor/mcp.json"
"agy:user"
"target_path": "$HOME/.cursor/mcp.json"
$ run revert --cli cursor --cli agy
(2 "restored from" lines, rc=0)
$ diff pristine.json .cursor/mcp.json
CORRUPT — leftover: "alpha"
$ ls .cursor/*.bak.*
NONE

Proposal

SwapEntry.seq_no already documents itself as "the primary LIFO sort key for cmd_revert", and the sort honouring it is correct — it is just applied in the wrong place. In _cmd_revert the entries are filtered per CLI and per scope before being sorted, so seq_no only ever orders entries within one CLI.

Sort the whole selected set by seq_no descending once, then iterate, rather than sorting inside the per-CLI loop. That fixes the shared-file case directly and makes the iteration order deterministic without needing to sort targets separately.

That leaves the scoped case, where the user has asked to unwind only part of a stack. Restoring an older snapshot while a newer one for the same target_path is still outstanding cannot be done safely from a whole-file backup. revert --scope should refuse when a higher-seq_no entry shares its target_path, naming the entry that has to go first.

The module docstring's claim that "Both Claude scopes can coexist with independent backups" should go with it: the backups are stacked snapshots of one file, not independent.

References

• _cmd_revert per-CLI filter and sort at v0.1.0a20
• targets built from a set comprehension at v0.1.0a20
• SwapEntry.seq_no, which documents the global LIFO contract
• tests/test_mcp_swap.py::test_claude_full_revert_unwinds_both_scopes_in_lifo_order covers the bare revert path; neither the scoped sequence nor the shared-file case is covered