[JacobStephens2]

Summary

A Custom process can name throwaway worktrees and run consecutive Steps that share a parallel group at the same time. Command Steps from the base stay in the same rules: every session or command Step edits its worktree, so a group cannot share one.

prepare
  refuse an unknown worktree, a missing name, or a shared worktree in a group
  create one throwaway checkout per worktree the Steps use
execute
  for each consecutive parallel group
    one Step: run it
    several: run them together, then record every result
    a required failure returns only after the group is recorded
  drop the checkouts (retention still keeps a dirty or uncertain one)

A process with no [worktrees] table still uses one shared throwaway worktree, and check still prints that plan.

Evidence

• Before: on issue-877, a process file cannot declare [worktrees.<name>] or parallel. Consecutive Steps always run one after another in one throwaway worktree.
  After: cargo test --test process -- --test-threads=8 — 26 passed, 1 ignored. The new tests cover overlap of three fake sessions, the judge prompt naming all three results, shared and unknown worktree refusals (including a command Step), a failed member finishing its siblings before the process fails, retention of a dirty failed worktree, and a repeated group name starting a new group. cargo clippy --all-targets -- -D warnings is clean. cd site && node generate-docs.mjs --check exited 0. cargo test --bin thirdshift prompts_page — 12 passed.

Merge Danger

Door: two-way

The change is experimental Custom process behaviour. Reverting the commit restores sequential Steps in one throwaway worktree. A file with no [worktrees] table keeps today's plan and execution.

Blast Radius: process

thirdshift process and thirdshift process check gain named worktrees, together: lines, and parallel execution. Ticket runs, Spec runs, and standalone skills are unchanged.

Seams

Tests go through the thirdshift binary in the tests/support Scenario (real Git, fake Harnesses). No new test seam.

Unaddressed findings

No Standards or Spec finding was skipped. Both axes reported none against origin/issue-877 (531dfc08).

Security (guidance-mode review of 531dfc08...HEAD, thirdshift-security-audit attack classes for resource exhaustion, data isolation, and AI/agent delegation): no introduced findings and none unaddressed. The process file is the local user's own config, so its commands and group sizes are same-principal input with no cross-boundary sink; worktree and step names are single_component-validated and worktree names never reach a filesystem path (tempdir checkouts); the parallel group name is only compared for grouping and never reaches a path, shell, or prompt; result files are per unique step name and recorded sequentially after threads join; each parallel Step runs in its own throwaway checkout, dropped at the end under the usual retention rules. No proof-of-concept demonstrated a boundary violation, so no fix was made here. No pre-existing findings were recorded.

Security review outcome

No unaddressed introduced findings.

Review coverage

site/docs/index.html was not read. It is generated from docs/guide.md by site/generate-docs.mjs, and cd site && node generate-docs.mjs --check exited 0 on this head. The Custom processes section was reviewed in docs/guide.md. No other changed file was left unread.

Closes #1383

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