Message Guard

Version 1.3.0 (2026-09-01) adds patterns.example.json, the public, validator-backed starting point used by synthesis-onboarding init. The scaffolder preserves the user's local copy and can add interview-supplied wording boundaries without putting personal policy in this repository. A fail-closed message gate now ships the route to a valid config instead of requiring adopters to reconstruct one from prose.

Version 1.2.0 (2026-08-28) — adds checkheaderhygiene: the gate now inspects the RFC threading headers (inreplyto, references) in the tool input, not only the message text, and blocks a send whose Message-ID has been HTML-escaped (<id@host>) or left with an unbalanced angle bracket. Such a header matches no message, so the reply orphans in any strict RFC client — but Gmail's own thread_id threading masks it whenever the caller passes both, which is why the error survived review and recurred (2026-08-20, 2026-08-28). A malformed header is now structurally undeliverable rather than silently wrong.

Version 1.1.0 (2026-07-29)

Prose rules do not survive contact with a model under load. This skill is the enforcement layer for correspondence the way commit hooks are the enforcement layer for repositories: the rules live in code, run outside the model, and fail closed.

Why it exists

Two same-night incidents (2026-07-29), both by an agent that had the relevant rules loaded in context:

1. A reply composed without reading the thread it was replying into. The

thread's own quoted history contained the principal's earlier message taking the opposite strategic position, with a better argument. The reply was sent. Correcting it cost a follow-up email and real trust.

1. **A drafted message containing register patterns the principal's written

voice rules explicitly ban** (self-flagellation about a delayed reply; expressing trust in a colleague via the author's own limitation). The principal had warned about exactly this class before.

Both failures were rule-knowledge failures at compose time, not knowledge gaps. The fix is structural: make the send mechanically impossible until the work is attested and the text passes a deterministic scan.

Architecture

composing agent                      engine (stdlib python, fail closed)
--------------                       ----------------------------------
1. research: full thread read,       PreToolUse hook on every send/draft
   history search, claims→sources    tool call:
2. compose                             a. register scan of outgoing text
3. self-scan:   --scan < draft            (block patterns from config)
4. write ledger (sha256 of exact       b. ledger present, fresh (<45 min),
   final text + attestations)             sha256 matches the exact text,
5. call the send/draft tool               attestations complete
                                       c. pass → log + consume ledger
                                          fail → exit 2, send blocked

• Engine: scripts/message_guard.py. Stdlib only — identical behavior

under any python3 (lesson inherited from a PyYAML-dependent guard that failed open for weeks). Any internal error blocks the send.

• Config: ~/.synthesis/message-guard/patterns.json — private, per-person.

Block/warn regexes, gated tool patterns, exemptions, freshness window. The engine refuses to run without it. Start with patterns.example.json or run synthesis-onboarding init; both paths are validated before wiring the guard.

• Ledger: ~/.synthesis/message-guard/ledger/<message-sha256>.json — one

per message, consumed on use (single-shot; no reuse across messages). File it with --write-ledger, which derives the path from the ledger's own message_sha256: you never type a path, so you cannot misfile one. Keying by sha is what lets several seats compose at once. A single shared slot could not: the second seat's write replaced the first's, and the first seat's send was then refused for a sha mismatch — reported as "you edited the text after grounding," which was false and pointed at the wrong repair. Passed sends are appended to log.jsonl with the full ledger for audit.

• Wiring: equivalent PreToolUse entries in Claude Code's

~/.claude/settings.json and Codex's ~/.codex/hooks.json, each matching the send/draft tool family across all MCP servers by name pattern. The doctor requires every installed client to carry the guard.

Peer-session sends (config-adopted)

Agent-to-agent session messaging has a different failure mode from correspondence: not register drift but misdelivery — a target chat session chosen by guessing a title or display label. Adopting peersendresolution in patterns.json gives those tools their own lane, which runs before the exempt list and replaces the ledger lane for matching calls:

"peer_send_resolution": {
  "tool_pattern": "ccd_session_mgmt__send_message$",
  "target_field": "session_id",
  "board": "~/.synthesis/coordination/active-sessions.md"
}

The target session id must appear as an active client session ref on the coordination board (schema v4, registered at claim time) — otherwise the send blocks with instructions to run coordination.py resolve or use the board message bus. An unreadable board blocks (fail closed). Unadopted instances keep the old posture — inter-session tools stay in exempttoolpatterns — and the doctor says which posture is live. Requires the tool's PreToolUse wiring to route these calls to the guard; the doctor checks that too.

Currency claims carry read freshness (config-adopted, v1.5.0)

A claim such as "still unanswered", "unsent", or "no reply yet" is a statement about NOW that rests on a read taken at some moment. The ledger recorded WHERE such a claim came from but not WHEN the source was read, so on 2026-09-01 a "still unanswered by the principal" claim resting on a read eight hours old passed as verified while the answer had gone out that morning — a false receipt in the layer built to stop exactly that.

Adopt the lane by adding to the config:

"currency_claim_patterns": ["\\b(unanswered|unsent|not (yet )?(replied|responded|answered|sent)|no (reply|response|answer)( yet)?|still (open|waiting|pending|unanswered)|has(n't| not) (replied|responded|answered|sent))\\b"],
"currency_claim_max_age_minutes": 30

With it adopted, every claims[] entry whose text matches a pattern must carry readat (ISO-8601, the moment the source was read THIS run), and the send is blocked when readat is missing or older than the maximum — the remedy is to re-read the source and refresh it. Stable facts ("PR 96 merged") need no read_at. Without the keys the lane is off and the doctor says so. --ledger-template shows the field.

The ledger contract

--ledger-template prints the skeleton. Fields the engine enforces:

The ledger cannot make a model honest, but it converts skipping the research from an invisible omission into a deliberate written lie — auditable in log.jsonl — and the deterministic scan layer is model-independent entirely.

Modes

message_guard.py --gate            # hook mode (stdin: tool-call JSON)
message_guard.py --scan  < draft   # pre-check wording; exit 2 on block hits
message_guard.py --sha   < draft   # sha256 for the ledger
message_guard.py --ledger-template # skeleton
message_guard.py --write-ledger    # stdin: ledger JSON -> files it at its own sha
message_guard.py --ledger-path < draft  # where this text's ledger belongs
message_guard.py --doctor          # config, controls, all client wiring, state dir
message_guard.py --test            # behavioral suite (31 cases)

Guarantees and their proofs

1. Fail closed. No ledger, stale ledger, sha mismatch, unknown tool shape,

unreadable config, or ANY internal exception → the send is blocked with remediation on stderr. Proven by --test cases including a missing-config invocation.

1. Positive controls. --doctor requires a known-bad text to trip the

scanner and a known-clean text to pass — a scanner that stops matching is detected, not trusted. Since 1.6.0 the known-clean text is a canonical SIGNED agent message (the Ragbot signature line in Slack wire form): on 2026-08-03 a generic clean control passed while a retired-branding pattern compiled under IGNORECASE blocked every real signed send. Add your own canonical messages with doctorcleancontrols so a pattern change that blocks real traffic fails the doctor.

1. Calibration. The pattern set must PASS the principal's real sent

messages and BLOCK the incident drafts. Re-run calibration whenever patterns change; a guard that blocks the principal's own voice is miscalibrated, not strict.

1. Monitored across clients. The doctor runs in the day-start ritual

(synthesis-daily-rituals Step 1) alongside the commit-hook doctor. It checks Claude Code and Codex independently whenever each client is installed; one healthy client cannot hide an unwired peer.

Known limits — stated, not hidden

• Judgment failures pass the scan. A condescending-but-pattern-free

sentence, a strategically wrong recommendation, or a subtly mis-scoped legal claim will not trip a regex. Those are caught by the ledger's forced research step and, for high-stakes messages, by adversarial multi-agent review. The scan removes the enumerable failure modes; the ledger makes the research auditable; neither replaces review.

• Bash is not gated. Sending mail via raw shell would bypass the hook.

Doing so is prohibited by rule; the hook covers every legitimate send path (the MCP tools).

• Hook config loads at session start. A newly wired hook protects new

sessions; the wiring session itself must self-enforce.

Composing under the guard (the honest workflow)

1. Read the FULL thread you are replying into — including quoted history.

The thread's own tail is a primary source; a reply that contradicts it is the canonical incident.

1. Search prior correspondence for the recipient AND topic — every mailbox the

principal uses, plus local transcripts. Record the queries.

1. Compose. Run --scan. Fix hits by rewriting the thought, not by

thesaurus-dodging the regex.

1. Map every factual claim to its source. A claim you cannot source becomes a

question to the recipient or gets cut.

1. File the ledger: --sha for the hash, then pipe the JSON to

--write-ledger. Call the tool. The gate verifies.

Related

• synthesis-git-hooks — the same fail-closed philosophy at the commit

boundary; this skill is its correspondence twin.

• The principal's private writing-voice skill — the source of truth the block

patterns are derived from; patterns.json cites it.

• synthesis-daily-rituals — runs --doctor at day-start.