A session wrote records into the main checkout's working tree on main while another live session (abcd-0d) was working in the same checkout, because nothing enforces the rule that a concurrent session works in its own worktree. On 2026-10-24 this session ran capture, intent, decide --adr and go generate from the main checkout: an issue record, an intent draft, an ADR and two regenerated index regions landed in the shared tree, the regenerated indexes folding in a peer's untracked draft. Nothing refused or warned. capture, intent and decide write into whichever checkout they run in; the guard judges shell commands and questions, not where records are written; and the CONCURRENCY rules domain, which carries the rule, recalls only on prompts about worktrees, peers or sessions, so a prompt saying 'file the decision record' never injected it. The agent also skipped the required scan (session listing and abcd peers) before mutating. The rule held only as prose.

Frontmatter

.abcd/work/issues/resolved/iss-2610240859590333-a-session-wrote-records-into-the-main.md
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mentions4

• dcn-26102719225024802026-10-27 — Majors ruling for the next cut, product thinker, 27 October 2026, on abcd-78's recommendations for the 17 hand-captured majors since v0.15.1. Fix before the cut: the eight recommended fixes (iss-2610251555094324, iss-2610241537197997, iss-2610241414124940, iss-2610241911052191, iss-2610241240133009, iss-2610241133284600, iss-2610241239492159, iss-2610241519088123) and five recommended for deferral that the product thinker chose to fix instead: the short install command in the docs (iss-2610240827243654), a worktree per session (iss-2610240859590333), clearing lapsed holds everywhere (iss-2610241107191205), the reflect criteria (iss-2610241431221889) and the measured home-folder receipt (iss-2610241551259104). Deferred one cycle: iss-2610241228359748, iss-2610240932215918, iss-2610241613191748. Amended and closed: iss-2610241427397332 (itd-3's obsolete criteria). The 69 consistency-pass majors are ruled on branch decisions/interview-2026-10-27.
• iss-2608230847432285Per-agent worktrees do not isolate a session whose shell cwd reverts to the shared checkout, so the mitigation iss-213 recommended has a hole. This refines iss-213, which recorded several agents sharing one git worktree silently invalidating a verification result, and whose recommended direction was to give each agent its own worktree so the whole class disappears. That direction is now in force via the AGENTS.md Concurrent sessions section, and on 2026-08-23 three concurrent sessions demonstrated it does not hold. A session's shell cwd can be silently reset from its worktree back to the primary working directory, and the notice arrives on the tool result AFTER the command that caused it, so the contamination lands on the NEXT command. It fails toward the shared tree, which is the wrong direction: two sessions wrote into the main checkout while believing they were in their own worktree. One appended to two Go test files, the other filed a capture and edited three record files, and both discovered it only when a third session read git status in the main checkout and asked who owned the diffs. Nobody lost work, because the convention that a diff you did not make is a peer's work held and the owners were asked rather than the files committed. The isolation property did not hold; the coordination convention compensated for it. Two details generalise beyond this instance. First, the failure is silent in BOTH directions: nothing warns the writer, and nothing would have warned a committer using git add -A, because the misplaced files are indistinguishable from that session's own in git status. What stood between this and a bad commit was one session committing with explicit paths and another happening to run git status for an unrelated reason. Detection was not mechanical in any of the three cases. Second, this is the write-side twin of iss-213 rather than a restatement of it: iss-213's dangerous member was a verification result that described no tree in particular, on the read side, while this is work landing in a tree whose HEAD another session is preparing to move. Same root cause, that the checkout is the unit of isolation and nothing enforces which checkout a session is in, on opposite sides of the read/write boundary. The mechanical mitigation is to address the tree explicitly on every git invocation, git -C <abs-path>, and to use absolute paths for file writes, rather than relying on a persisted cd: a cd is a session-global mutation with no scope and no expiry, which is the wrong shape for the mechanism that is supposed to provide isolation. AGENTS.md states that the checkout is the unit of isolation without saying what makes a session stay in its checkout, and that gap is what let three sessions make the same mistake in one day. Decide the routing: an AGENTS.md line under Concurrent sessions is the cheapest rung and matches how the sequential-id caveat was handled, while the durable form is whatever makes a session's tree unambiguous rather than remembered.
• iss-2610280051498465A file an agent writes with the host's own edit tools into a main checkout two or more live sessions share is not refused. On 2026-10-28 the record writers (capture, intent, decide, spec close and the index generator) began refusing a shared main checkout through a per-session occupancy marker (iss-2610240859590333), but an agent's direct Write or Edit of a record file, or of any tracked file, goes through the host's file tools, which the shell guard never sees. The 2026-10-24 incident used the verbs, so the verbs were closed first; the same mistake made with the file tools lands the same way.
• itd-2610240850336695The person grants their sessions what they may do alone, until a time