fix(tools/tmux): unique per-invocation paste buffer; track auto-submit-drafts.sh

Upstream the 2026-07-09 live hot-fix from web1: send-message.sh used a fixed
tmux buffer name (__mosaic_send) shared by all concurrent senders on one
server — load-buffer overwrote load-buffer and paste-buffer -d deleted
underneath peers, cross-delivering or dropping messages. Bit the fleet during
the simultaneous restart (standing briefs swapped between sessions). Buffer
name is now unique per invocation (PID + nanoseconds).

Also brings auto-submit-drafts.sh (coordinator draft-flush watchdog, until now
only a loose file in the deployed ~/.config/mosaic/tools/tmux/) under source
control, and extends test-send-message-socket.sh with a concurrency lock:
5 parallel sends to distinct panes, asserting no drop and no cross-delivery.
The lock fails 3/3 runs against the pre-fix script and passes post-fix.

Without this, the next framework redeploy would regress the live fix.

docs/scratchpads/561-python-is-python3.md

@@ -0,0 +1,33 @@
+# Issue #561 — Bare python on agent hosts
+
+## Objective
+
+Make the durable bootstrap/provisioning guidance ensure agent hosts provide a bare `python` command that resolves to Python 3.
+
+## Scope
+
+- Add Debian/Ubuntu `python-is-python3` to agent-host prerequisites in bootstrap docs.
+- Check for actual OS package provisioning scripts and update only if an existing agent-host package install path exists.
+- Do not touch live host state.
+- Do not update `docs/TASKS.md`; repo guidance says workers read it but never modify it.
+
+## Recon
+
+- Issue #561 confirms repeated `python: command not found` failures from fleet agents that emit `python foo.py`.
+- `guides/BOOTSTRAP.md` and `packages/mosaic/framework/guides/BOOTSTRAP.md` are the source and packaged framework copies of the bootstrap guide.
+- Targeted repo sweep found no agent-host Debian package provisioning script. Existing `apt-get install` hits are CI/test helper paths or unrelated deployment docs.
+
+## Plan
+
+1. Add a host prerequisite section to both bootstrap guide copies.
+2. Include `python-is-python3` in the Debian/Ubuntu package list with an issue comment.
+3. Note the non-Debian equivalent as a `/usr/bin/python -> python3` symlink.
+4. Validate markdown/diff, run shell syntax checks where applicable, run required review, commit, queue guard, and push.
+
+## Validation Log
+
+- `rg` recon: no existing agent-host Debian package provisioning script; only CI/test helper `apt-get install` paths and unrelated deployment docs.
+- `git diff --check`: passed.
+- `bash -n packages/mosaic/framework/install.sh tools/install.sh packages/mosaic/framework/tools/bootstrap/init-project.sh packages/mosaic/framework/tools/_scripts/mosaic-bootstrap-repo`: passed. No touched shell scripts.
+- `~/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted`: approved, 0 findings.
+- `pnpm format:check`: initially blocked because `node_modules` was absent and `prettier` was unavailable; `pnpm install --frozen-lockfile` initially hit an invalid `/root` pnpm store path. Reran install with `--store-dir /home/hermes/agent-work/.pnpm-store`, then `pnpm format:check` passed.

guides/BOOTSTRAP.md

@@ -15,6 +15,22 @@ This guide covers how to bootstrap a project so AI agents (Claude, Codex, etc.)
 7. Branching/merging is consistent: `branch -> main` via PR with squash-only merges
 8. Steered-autonomy execution is enabled so agents can run end-to-end with escalation-only human intervention
 
+## Agent Host Prerequisites
+
+Agent hosts must provide the Python runtime shape that runtime agents and
+Mosaic automation assume is present.
+
+For Debian/Ubuntu hosts:
+
+```bash
+sudo apt-get update
+# #561: bare python invocations from agents must resolve.
+sudo apt-get install -y python3 python-is-python3
+```
+
+For non-Debian hosts, install the equivalent Python 3 runtime and ensure
+`/usr/bin/python` resolves to `python3` (for example, via a managed symlink).
+
 ## Quick Start
 
 ```bash

packages/mosaic/framework/guides/BOOTSTRAP.md

@@ -15,6 +15,22 @@ This guide covers how to bootstrap a project so AI agents (Claude, Codex, etc.)
 7. Branching/merging is consistent: `branch -> main` via PR with squash-only merges
 8. Steered-autonomy execution is enabled so agents can run end-to-end with escalation-only human intervention
 
+## Agent Host Prerequisites
+
+Agent hosts must provide the Python runtime shape that runtime agents and
+Mosaic automation assume is present.
+
+For Debian/Ubuntu hosts:
+
+```bash
+sudo apt-get update
+# #561: bare python invocations from agents must resolve.
+sudo apt-get install -y python3 python-is-python3
+```
+
+For non-Debian hosts, install the equivalent Python 3 runtime and ensure
+`/usr/bin/python` resolves to `python3` (for example, via a managed symlink).
+
 ## Quick Start
 
 ```bash

packages/mosaic/framework/tools/tmux/README.md

@@ -87,6 +87,10 @@ message crosses the wire as base64 (`-b`) to avoid all shell-quoting hazards.
 
 - `agent-send.sh` — inter-agent wrapper (preamble + local/remote dispatch).
 - `send-message.sh` — low-level reliable single-pane submitter (`-b` base64 input).
+- `auto-submit-drafts.sh` — watchdog that flushes stable unsubmitted prompt
+  drafts on a coordinator pane (default target `mos-claude`); run it as a
+  long-lived process alongside the coordinator session.
+- `agent-send.test.sh` — regression + grammar lock for `agent-send.sh`.
 - `test-send-message-socket.sh` — smoke test for named-socket isolation.
 
 ## Distribution

packages/mosaic/framework/tools/tmux/auto-submit-drafts.sh

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+# auto-submit-drafts.sh — watchdog for Claude Code panes that receive channel
+# messages but leave them as unsubmitted prompt drafts. Intended for Mos only.
+set -uo pipefail
+
+TARGET="${1:-mos-claude}"
+INTERVAL="${INTERVAL:-2}"
+STABLE_SECONDS="${STABLE_SECONDS:-4}"
+LOG_PREFIX="[auto-submit-drafts:$TARGET]"
+
+last_prompt=""
+first_seen=0
+
+prompt_text() {
+  tmux capture-pane -t "$TARGET" -p 2>/dev/null | python3 -c '
+import sys, re
+lines = sys.stdin.read().splitlines()
+idx = None
+for i in range(len(lines)-1, -1, -1):
+    if "❯" in lines[i]:
+        idx = i
+        break
+if idx is None:
+    raise SystemExit
+parts = []
+after = lines[idx].split("❯", 1)[1]
+parts.append(after)
+for line in lines[idx+1:]:
+    # Stop at Claude Code separator/border lines.
+    if "─" in line or "╰" in line or "╭" in line:
+        break
+    s = line.replace("\u00a0", " ")
+    s = re.sub(r"[\x00-\x1f\x7f]", "", s).strip()
+    if s:
+        parts.append(s)
+text = " ".join(parts).replace("\u00a0", " ")
+text = re.sub(r"[\x00-\x1f\x7f]", "", text).strip()
+print(text)
+'
+}
+
+while true; do
+  if ! tmux has-session -t "$TARGET" 2>/dev/null; then
+    echo "$LOG_PREFIX target missing; waiting" >&2
+    sleep "$INTERVAL"
+    last_prompt=""
+    first_seen=0
+    continue
+  fi
+
+  current="$(prompt_text || true)"
+  now="$(date +%s)"
+
+  if [[ -z "$current" ]]; then
+    last_prompt=""
+    first_seen=0
+    sleep "$INTERVAL"
+    continue
+  fi
+
+  if [[ "$current" != "$last_prompt" ]]; then
+    last_prompt="$current"
+    first_seen="$now"
+    sleep "$INTERVAL"
+    continue
+  fi
+
+  age=$(( now - first_seen ))
+  if (( age >= STABLE_SECONDS )); then
+    echo "$LOG_PREFIX submitting stable draft after ${age}s: ${current:0:120}" >&2
+    tmux send-keys -t "$TARGET" C-j
+    sleep 0.8
+    tmux send-keys -t "$TARGET" C-m
+    sleep 2
+    last_prompt=""
+    first_seen=0
+  else
+    sleep "$INTERVAL"
+  fi
+done

packages/mosaic/framework/tools/tmux/send-message.sh

@@ -77,10 +77,20 @@ snippet=$(printf '%s' "$MSG" | tr '\n' ' ' | tr -s ' ' | sed 's/[^[:print:]]//g'
 
 # 1) Paste the body as a bracketed paste so multi-line content does not submit
 #    line-by-line. load-buffer/paste-buffer is far safer than `send-keys -l`.
-printf '%s' "$MSG" | "${tmux_cmd[@]}" load-buffer -b __mosaic_send -
+# Buffer name MUST be unique per invocation: concurrent senders on the shared
+# tmux server race a fixed name (load overwrites load, -d deletes underneath),
+# cross-delivering or dropping messages — bit the fleet on the 2026-07-09
+# simultaneous restart (briefs swapped between sessions).
+BUF="__mosaic_send_$$_$(date +%s%N)"
+printf '%s' "$MSG" | "${tmux_cmd[@]}" load-buffer -b "$BUF" -
 # -p = bracketed paste when the client supports it; fall back if not.
-"${tmux_cmd[@]}" paste-buffer -d -p -b __mosaic_send -t "$EFFECTIVE_TARGET" 2>/dev/null \
+"${tmux_cmd[@]}" paste-buffer -d -p -b "$BUF" -t "$EFFECTIVE_TARGET" 2>/dev/null \
-  || "${tmux_cmd[@]}" paste-buffer -d -b __mosaic_send -t "$EFFECTIVE_TARGET"
+  || "${tmux_cmd[@]}" paste-buffer -d -b "$BUF" -t "$EFFECTIVE_TARGET" \
+  || "${tmux_cmd[@]}" delete-buffer -b "$BUF" 2>/dev/null
+# ^ -d deletes the buffer only on a SUCCESSFUL paste; if both attempts fail
+#   (e.g. the target vanished since the liveness check), delete explicitly —
+#   named buffers are exempt from tmux's buffer-limit eviction, so orphans
+#   would otherwise accumulate forever.
 sleep 0.5
 
 # 2) Submit, then verify; flush with another Enter if it is still a draft.

packages/mosaic/framework/tools/tmux/test-send-message-socket.sh

@@ -47,4 +47,32 @@ if capture_default | grep -qF "agent socket hello"; then
   fail "agent-send.sh leaked named-socket message to default tmux server"
 fi
 
+# Concurrency: parallel senders on one server must not cross-deliver or drop.
+# Locks the unique-per-invocation paste buffer (a fixed buffer name raced:
+# load overwrote load, -d deleted underneath — messages swapped between panes).
+CONC_N=5
+for i in $(seq 1 "$CONC_N"); do
+  tmux -L "$SOCKET" new-session -d -s "conc-$i" -c "$TMPDIR" 'bash --noprofile --norc -i'
+done
+pids=()
+for i in $(seq 1 "$CONC_N"); do
+  "$SEND_MESSAGE" -L "$SOCKET" -t "=conc-$i" -m "CONCPAYLOAD-${i}-END" >/dev/null &
+  pids+=($!)
+done
+for pid in "${pids[@]}"; do
+  wait "$pid" || fail "concurrent send-message.sh invocation exited non-zero"
+done
+sleep 0.2
+for i in $(seq 1 "$CONC_N"); do
+  pane=$(tmux -L "$SOCKET" capture-pane -t "=conc-$i:0.0" -p)
+  printf '%s' "$pane" | grep -qF "CONCPAYLOAD-${i}-END" \
+    || fail "concurrent send dropped payload for pane conc-$i"
+  for j in $(seq 1 "$CONC_N"); do
+    [ "$j" = "$i" ] && continue
+    if printf '%s' "$pane" | grep -qF "CONCPAYLOAD-${j}-END"; then
+      fail "concurrent send cross-delivered payload $j to pane conc-$i"
+    fi
+  done
+done
+
 echo "ok - named tmux socket send tools"