Design: one helper marks every body the-loop posts

Phase 2 of 3 (bugfix → design → tasks). Derives from the approved bugfix spec.

Overview

The marker already exists (the_loop.authz.SELF_COMMENT_MARKER) and both trigger paths already check it (is_self_authored). What is missing is a producer-side counterpart to that consumer-side check, living next to it:

1. authz.py gains SELF_COMMENT_ATTRIBUTION (the visible line) and mark_self_authored(body) — the single, idempotent way to stamp a body the-loop wrote. It sits beside is_self_authored, so the writer and the reader of the marker are the same 30 lines of code and can never drift.
2. announce.py returns mark_self_authored(...) from announcement_body, which is the only comment body the CLI posts today.

Nothing else changes. The poller and the router already drop marker-carrying comments (poller.py:469, router.py:309), so AC5/AC6 are satisfied by the body changing — not by new routing logic.

1. authz.py — the producer half of the marker contract

SELF_COMMENT_ATTRIBUTION = "🤖 _the-loop, autonomous comment_"


def mark_self_authored(body: str) -> str:
    """``body`` stamped as the-loop's own: visible attribution + the marker.

    Idempotent — a body that already carries the marker is returned unchanged.
    Apply ONLY to text the-loop itself composed (never to payload-derived or
    another author's text): the marker asserts authorship, and the trigger paths
    silently drop whatever carries it.
    """
    if is_self_authored(body):
        return body
    return f"{body.rstrip()}\n\n{SELF_COMMENT_ATTRIBUTION}\n{SELF_COMMENT_MARKER}\n"

Design points:

• Placement. In authz.py, immediately after is_self_authored. The invariant "what we write is what we recognise" is then local and obvious; a helper in announce.py would make the next posting site re-derive it (the structural root cause the bugfix spec names). This is the minimalism ladder's reuse-what-exists rung: no new module, no new dependency, one function.
• Idempotence (AC3). Guarded by the same is_self_authored predicate the consumers use, so "already marked" means exactly what the router means by it.
• Ordering inside the body. Visible attribution first, invisible marker last, matching reference/collaboration.md ("appended after a blank line") and what the harness already emits. rstrip() normalises bodies that do or don't end in a newline, so there is exactly one blank line before the attribution.
• Signature. str -> str, pure, no I/O — trivially testable and impossible to fail in a way that could affect a dispatch.
• What it is not (AC4). It is not applied anywhere near payload data. The only call site is a body built from registry fields; the docstring states the rule so a future caller does not launder foreign text through it.

2. announce.py — mark the one body the CLI posts

announcement_body gains a single wrapping call:

    return mark_self_authored(
        f"🖥️ **the-loop** started an interactive session for `{ref}`.\n"
        ...
        f"{note}\n"
    )

The rendered comment is unchanged except for the two trailing lines (AC2). SessionAnnouncer._argv already posts whatever announcement_body returns, so no other change is needed; the module docstring gains a sentence recording that the body is self-marked and why.

3. Components and interfaces

Component	Change
the_loop.authz	new SELF_COMMENT_ATTRIBUTION, mark_self_authored; exported alongside the existing marker/predicate
the_loop.announce.announcement_body	wraps its return value in mark_self_authored
the_loop.poller.poller	none — already drops self-marked comments
the_loop.webhook.router	none — already drops self-marked events

4. Data model

None. The marker is a fixed literal already shipped (<!-- the-loop:agent-comment -->); SELF_COMMENT_MARKER must never change so older comments stay recognisable, and this change does not touch it. No config key, no registry field, no schema change.

5. Error handling

mark_self_authored is pure string work over a self-composed body: no failure mode beyond a TypeError on a non-str, which is a programming error, not a runtime condition. The announcement path keeps its best-effort posture unchanged — every gh failure remains a logged no-op that never affects the dispatch.

6. Testing strategy

Test	Covers
mark_self_authored appends attribution + marker; is_self_authored then true	AC1
applying it twice yields exactly one marker (byte-equal to one application)	AC3
announcement_body(session) is is_self_authored, and still contains the tmux target, harness, both attach commands and the retention note	AC2
poller integration: a work item whose thread contains the announcement forwards nothing to the session (Gherkin docstring + Requirement: link)	AC5, AC7
router: an issue_comment event carrying the announcement body is dropped before dispatch	AC6

The two path-level tests are the regressions that fail before the change: with an unmarked announcement the poller forwards it and the router dispatches it.

Existing coverage stays green: cli/tests/test_announce.py asserts on the body's content, and the appended lines do not disturb those assertions (they are in-checks, not equality).

7. Security design

The bugfix spec's boundary is self-reply ingestion; this design enforces it at the producer rather than adding a new consumer-side heuristic:

• The check and the stamp are one unit. is_self_authored (consumer) and mark_self_authored (producer) share a module and a constant, so a future edit cannot mark bodies in a form the trigger paths no longer recognise.
• Least authority for the helper. Pure, str -> str, no config, no I/O, no subprocess. It cannot widen routing, cannot fail open, and cannot be reached by untrusted input on any current path.
• No laundering (AC4). Documented and enforced by call-site discipline: the sole caller composes its body from registry fields already validated (_NAME_RE on owner/repo, the regex-sanitised loop-<slug> tmux target). Marking is deliberately not applied in _argv or in a generic posting wrapper that could one day be handed payload text.
• The marker remains non-authorizing. Anyone may type it; the only effect is that the-loop ignores that comment. routing.authorizedUsers is untouched and still runs, in the same order, on both paths.
• Fail-closed direction. The change strictly reduces what re-enters a session. Its worst-case failure — the marker not being appended — is exactly today's behaviour, not a new exposure.
• Abuse case as a test. The poller regression (AC5) is the negative test proving the boundary: the-loop's own announcement fed back through the ingress must be resolved, never delivered.

8. Minimalism

One function (six lines) and one constant, reusing the module, the marker and the predicate that already exist; the alternative — a posting wrapper class or a per-call-site literal — either adds an abstraction with one implementation or re-creates the very drift this bug is. No new dependency, no config surface.

9. UI/UX artifacts

None — CLI/daemon change with no user-facing surface (design.uiArtifacts applies to user-facing work items only). The one visible effect is the attribution line at the foot of the announcement comment, shown verbatim above.