diff --git a/BUILD-LOG.md b/BUILD-LOG.md
index 92e5deb1..cafcc8c0 100644
--- a/BUILD-LOG.md
+++ b/BUILD-LOG.md
@@ -2133,3 +2133,22 @@ provider in the hover, and a Model detail row. Blank when the log names
 none. Live check against the real data root on a scratch board: all 42
 rows carried a model. Board suite 91/91 (was 90). Sonnet review recorded
 in the plan page entry.
+
+## 2026-09-12 — Control board piece 2: reply-from-board (#1505)
+
+Before: the board could show a seat waiting on Jason but he had to find
+its tmux pane to answer. After: the detail of a row with a live
+registration has a one-line reply box; `POST /api/reply` runs
+`tools/tmux/agent-send.sh -s <session> -S <host>:control-board [-L
+<socket>] -m <text>` once, for one seat, and returns the tool's exit code,
+stdout and stderr; the page shows `delivered <time>` or `failed (exit N):
+<stderr>`. Rows without a live registration say "reply needs a registered
+seat". No queue, retries, history, broadcast or raw send-keys; no change
+to `packages/seat` or `agent-send.sh`. Live check through the real tool on
+a scratch tmux session: delivered, and a refused target reported verbatim.
+Board suite 98/98 (was 91). Gate C's first exchange on 2026-09-12:
+Jason's "pizza?" reached darkwing and the row showed the answer within
+one scan; the seat's attempt to agent-send back to `control-board` was
+refused, correctly, since the board has no pane. Fix: every board message
+ends with a fixed trailer saying to answer in the seat's own session.
+
diff --git a/docs/SESSIONS.md b/docs/SESSIONS.md
index 34cab0d2..6dca3d7c 100644
--- a/docs/SESSIONS.md
+++ b/docs/SESSIONS.md
@@ -223,3 +223,4 @@ are never rewritten or removed; corrections are new entries.
 - 2026-09-12 — darkwing (Claude Code, coordinator; formerly jarvis) — Seat registration (#1504), Jason's go on the professor session's brief: new `packages/seat` and `scripts/mosaic` (`launch <seat>` registers then execs the launch script unchanged; `seat task` edits the task); board reads `<dataRoot>/seats/<layout>/<seat>/registration.json` and lets registered task/project/workspace override derived values with a source tag; the four repo launch scripts register themselves; fleet one-liner written for orch-01. Tests: seat 15, board 79 → 89, launch scripts 5, registry 69, config 24. Sonnet review found a seat-name collision across layouts; fixed by keying on layout, second pass APPROVED. Live check on a scratch server against a temp data root. Also found Jason's launch refused on the Pi pin (node_modules 0.84.4 vs pin 0.85.1 since 557aba0f); `npm ci` fixed it, the refusal now names both versions; the pin stays unless Jason decides otherwise.
 - 2026-09-12 — darkwing (Claude Code, coordinator) — #1504 follow-up on two defects the professor session reported and this session verified: the darkwing launch test leaked fixture registrations into the real data root (flock spawn without the fixture config; fixed and guarded so a leak fails), and a refused launch left a dead-pid record the board honoured (scanner now probes the pid; stale records are shown but do not override). Fleet launchers marked not planned per Jason. Board 90/90, seat 15/15, launch scripts 5/5. Committed and pushed to refactor.
 - 2026-09-12 — darkwing (Claude Code, coordinator) — Control board: model per row (#1503) on Jason's ask: readSession tracks model/provider from the latest model_change entry or assistant turn; page shows the model under the agent name and a Model detail row. Live check on a scratch board: all 42 real rows named a model. Board 91/91. Committed and pushed to refactor.
+- 2026-09-12 — darkwing (Claude Code, coordinator) — Control board piece 2, reply-from-board (#1505, brief by Jason): `POST /api/reply` runs `tools/tmux/agent-send.sh` for one registered, live seat and returns the exit code and streams; the page has a reply box in registered live rows' detail with delivered/failed receipts. Live check on a scratch tmux session through the real agent-send.sh: delivered, then a failure receipt with the tool's stderr. Board 98/98. Gate C first exchange ("pizza?") delivered and read back within one scan; the seat's send-back to control-board was refused as expected, so every board message now ends with a fixed trailer (Jason's ruling via the professor session). Committed and pushed to refactor.
diff --git a/docs/plans/2026-09-12_control-board-mvp.md b/docs/plans/2026-09-12_control-board-mvp.md
index 06ea8e38..6acc39ca 100644
--- a/docs/plans/2026-09-12_control-board-mvp.md
+++ b/docs/plans/2026-09-12_control-board-mvp.md
@@ -407,3 +407,67 @@ of `gpt-5.6-*` and `glm-5.3`). Board suite 91/91. Jason's board on 7331
 needs a restart to pick this up. Sonnet review found the model hover title
 escaped an already escaped string (double encoding for names with `&` or
 `<`); fixed before commit, then APPROVED on that basis.
+
+**2026-09-12 — Piece 2: reply-from-board (#1505).** Built to the brief
+above. `POST /api/reply {agent, text}` rescans, finds the row, and runs
+`tools/tmux/agent-send.sh -s <session> -S <host>:control-board -m <text>`,
+plus `-L <socket>` when the registration names one, with
+`MOSAIC_TMUX_SOCKET` stripped from the tool's environment so the
+registration is the only source of the socket. The answer carries
+`delivered`, the exit code, stdout and stderr; a non-zero exit is
+reported as is. Refusals before the tool runs: bad or empty or long text
+400, unknown row 404, no registration or stale registration or no tmux
+session 409, tool not startable 500. The page puts a one-line box and
+Send in the detail of rows whose registration is live and has a tmux
+session; other rows say "reply needs a registered seat". The receipt
+reads `delivered <time> to tmux <session>` or `failed (exit N): <stderr>`;
+drafts and receipts survive the periodic refresh and the caret is given
+back to the box; a delivered reply clears the box and rescans at once.
+Not built, per the brief: raw send-keys, queue, retries, history,
+broadcast, more than one seat per request; no change to `packages/seat`,
+the registration record or `agent-send.sh`.
+
+Live check on a scratch server (port 7332) with a scratch tmux session
+`cb-reply-test` running `cat` and a temp data root: `agent-send.sh`
+accepted `dragon-lin:control-board` as sender, the text arrived in the
+pane as `[dragon-lin:control-board -> dragon-lin:cb-reply-test] ...`, and
+the page showed `delivered`. Killing the session also killed the
+registered pid, so the row went stale and the box gave way to "reply
+needs a registered seat", as intended. With a live pid pointed at a
+missing session the receipt read `failed (exit 1): ERROR: invalid
+resolved pane identity` and the draft stayed in the box. Tests: a fake
+`agent-send.sh` on the server's configured path captures argv and the
+environment; cases for `-s`/`-S`/`-m` without `-L`, with `-L`, exit 4 with
+stderr, each refusal with the tool never called, a missing tool, and
+static checks on the page (box only when `alive !== false` and a tmux
+session, JSON POST, escaped receipts). Board suite 98/98 (was 91). Gate C
+is Jason's to run against a real waiting seat; his board on 7331 needs a
+restart first.
+
+Sonnet review of the piece: one finding, the route comment at the top of
+`serve.mjs` had the `/api/reply` line spliced into the middle of the
+`/api/seen` description, so it read as if a reply rescanned and returned
+the index; the comment was split back into two entries before commit.
+Noted but not changed: the body limit is bytes and the text limit is
+characters, so a non-ASCII text near 2000 characters is refused as an
+oversized body rather than as long text; and a row open in both "Waiting
+on you" and its project group has two forms with the same id, so the
+refocus after a refresh may land on the other copy.
+
+**2026-09-12 — Gate C, first exchange, and the trailer.** Jason answered
+darkwing from his restarted board with "pizza?". The delivery worked
+(`[dragon-lin:control-board -> dragon-lin:darkwing] pizza?` in darkwing's
+session log) and the row showed the answer as its last message within one
+scan, so the return path exists: the board reads the transcript. What
+failed was the seat's side: the communications skill told the seat a reply
+only exists once `agent-send.sh` runs, the seat aimed at `-s
+control-board`, and the tool refused because `control-board` is not a
+tmux session ("invalid resolved pane identity"). That refusal is correct;
+the board is not made a pane. Fix in the board, relayed as Jason's ruling
+by the professor session: every message the board sends ends with a fixed
+line, `(control-board: answer in your own session as usual; the board
+reads your transcript. Do not agent-send to control-board.)`, exported as
+`REPLY_TRAILER`; `-S` stays `<host>:control-board` so the preamble stays
+honest about the origin. The fake agent-send test now checks the message
+text ends with the trailer; the receipt is unchanged. Committed with the
+piece.
diff --git a/docs/plans/CURRENT.md b/docs/plans/CURRENT.md
index 7365cc8a..d9ac9963 100644
--- a/docs/plans/CURRENT.md
+++ b/docs/plans/CURRENT.md
@@ -32,13 +32,27 @@ Package: `packages/seat`. Fleet seats stay on their own launchers (Jason's
 ruling, 2026-09-12); only `agents/` seats register. A record whose pid is
 gone is stale and does not override the derived values.
 
-Next action: step 3 of #1503 continues — daily use with #1504 landed. Jason
-launches his seats through the new path (or the launch scripts, which now
-register), sets tasks with `seat task`, and reports what is wrong, missing
-or confusing; each report becomes a small fix with a test, committed and
-pushed to `refactor`. The sequence Jason set stays: seat registration (done),
-then reply-from-board, then the WebUI on Dewey's Console design absorbs the
-board as its first screen. Close #1503 when Jason says the page answers "who
+Gate B passed 2026-09-12 (seat registration shown and used on the board).
+
+Piece 2, reply-from-board (#1505; brief in the plan page, section "Piece
+2: reply-from-board", approved by Jason 2026-09-12), is built and pushed
+2026-09-12: a text box and Send in the detail of registered, live rows;
+`POST /api/reply` runs `tools/tmux/agent-send.sh -s <session> -S
+"<host>:control-board" [-L <socket>] -m <text>` and returns the exit code,
+stdout and stderr; the page shows `delivered` or `failed` with the stderr.
+No send-keys, queue, history or broadcast; no change to `packages/seat` or
+`agent-send.sh`. Board suite 98/98.
+
+Gate C's first exchange happened 2026-09-12 ("pizza?" to darkwing:
+delivered, read back within one scan; the seat's send-back to
+`control-board` was refused as expected, so every board message now ends
+with a fixed trailer telling the seat to answer in its own session).
+
+Next action: Jason confirms Gate C on the trailer build (restart the board
+on 7331, answer a `waiting` seat, the row turns `working` with the reply
+as its newest user message and the seat answers in its own session).
+Record the pass or the failure in the plan page and here. Then the WebUI on Dewey's Console design absorbs
+the board as its first screen. Close #1503 when Jason says the page answers "who
 is waiting on me" without him opening a terminal. Out of scope until he
 asks: auth or provider registry, roster schema changes, hooks/plugins, comms,
 memory, multiple sessions per seat, stopping seats, new root files. The
diff --git a/packages/control-board/README.md b/packages/control-board/README.md
index d5465bd3..bc757ff6 100644
--- a/packages/control-board/README.md
+++ b/packages/control-board/README.md
@@ -64,8 +64,37 @@ Routes served:
   more than 4096 bytes): `{"project", "agent", "lastActivity", "seen"}`.
   `seen` defaults to `true`; pass `false` to unsee. Bad input, a missing
   header, or an oversized body gets a `400`.
+- `POST /api/reply` — send one line of text to one registered seat through
+  `tools/tmux/agent-send.sh` (#1505). Body: `{"agent": "<project>/<agent>",
+  "text"}`, JSON, at most 4096 bytes, text at most 2000 characters. The
+  server rescans, finds the row, and runs
+  `agent-send.sh -s <session> -S <host>:control-board [-L <socket>] -m <text>`
+  with the session and socket from the row's registration; `MOSAIC_TMUX_SOCKET`
+  is stripped from the tool's environment so the registration is the only
+  source. The text is sent as typed, followed on its own line by a fixed
+  trailer: `(control-board: answer in your own session as usual; the board
+  reads your transcript. Do not agent-send to control-board.)`. The board is
+  a sender without a pane: replies to it are read from the seat's
+  transcript, never sent, and `agent-send.sh -s control-board` refuses
+  because no such session exists (Jason's refinement after the first real
+  exchange, 2026-09-12). It answers `200` with `{delivered, exitCode, signal, stdout,
+  stderr, agent, session, socket, sentAt}` whenever the tool ran, delivered
+  or not; a non-zero exit is reported with the tool's stderr, never retried
+  and never routed around. Refusals before the tool runs: `400` bad body,
+  empty or blank text, text too long; `404` unknown row; `409` the row has
+  no registration, the registration is stale (pid gone), or it has no tmux
+  session; `500` the tool could not be started. One seat per request; no
+  queue, no history, no broadcast, no raw `send-keys`.
 - `GET /healthz`.
 
+On the page, the detail of a row with a live registration and a tmux
+session has a one-line reply box and a Send button; every other row's
+detail says "reply needs a registered seat". After Send the row shows
+`delivered <time> to tmux <session>` or `failed (exit N): <stderr>`. A
+draft that has not been sent, and the last receipt, survive the page's
+periodic refresh; a delivered reply clears the box and triggers a rescan
+so the seat's reaction shows without waiting for the next tick.
+
 ## Exit codes
 
 - `0` — scan completed and status files were written, or the server stopped cleanly.
diff --git a/packages/control-board/src/page.html b/packages/control-board/src/page.html
index 6b866a57..94db3436 100644
--- a/packages/control-board/src/page.html
+++ b/packages/control-board/src/page.html
@@ -66,6 +66,10 @@
   .model{display:block;color:var(--muted);font-size:.75em;white-space:nowrap}
   .task-text{display:block;max-width:28ch;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}
   .unknown{color:var(--muted);font-style:italic}
+  .reply-form{display:flex;gap:.4em;align-items:center;max-width:60ch}
+  .reply-form input{flex:1;min-width:12ch;font:inherit;padding:.2em .4em}
+  .reply-receipt{display:block;margin-top:.3em;font-size:.85em}
+  .reply-ok{color:var(--muted)}
   .source{color:var(--muted);font-size:.75em;margin-left:.35em;white-space:nowrap}
   .detail-row td{background:var(--raised)}
   .detail-list{display:grid;grid-template-columns:auto 1fr;gap:4px 14px;margin:0;font-size:.85rem;font-family:var(--mono)}
@@ -195,6 +199,28 @@
   // Empty when the log has not named one yet.
   function modelTag(rec) { return rec.model ? '<span class="model" title="' + modelText(rec) + '">' + esc(rec.model) + "</span>" : ""; }
   function modelText(rec) { return rec.model ? (rec.provider ? esc(rec.provider) + "/" : "") + esc(rec.model) : "unknown"; }
+  // Reply-from-board (#1505): a box and Send only where the board has an
+  // address, i.e. a live registration naming a tmux session. Receipts and
+  // unsent drafts are kept per row id so a refresh does not lose them.
+  var replyReceipts = {};
+  var replyDrafts = {};
+  function canReply(rec) {
+    var reg = rec.registered;
+    return !!(reg && reg.alive !== false && reg.tmux && reg.tmux.session);
+  }
+  function replyReceipt(id) {
+    var r = replyReceipts[id];
+    if (!r) return "";
+    if (r.delivered) return '<span class="reply-receipt reply-ok">delivered ' + esc(r.sentAt) + " to tmux " + esc(r.session) + "</span>";
+    return '<span class="reply-receipt msg-error" style="white-space:normal;max-width:none">failed' + (r.exitCode === null || r.exitCode === undefined ? "" : " (exit " + esc(r.exitCode) + ")") + ": " + esc(r.stderr || r.error || "no output") + "</span>";
+  }
+  function replyControl(rec) {
+    var id = [rec.project, rec.agent].join("/");
+    if (!canReply(rec)) return '<span class="unknown">reply needs a registered seat</span>' + replyReceipt(id);
+    return '<form class="reply-form" data-id="' + esc(id) + '">' +
+      '<input type="text" class="reply-text" maxlength="2000" autocomplete="off" aria-label="Reply to ' + esc(rec.agent) + '" placeholder="Reply to ' + esc(rec.agent) + '" value="' + esc(replyDrafts[id] || "") + '">' +
+      '<button type="submit">Send</button></form>' + replyReceipt(id);
+  }
   function registeredText(reg) {
     if (!reg) return "no (not started through mosaic launch)";
     var parts = ["started " + esc(reg.startedAt || "—")];
@@ -249,6 +275,7 @@
       "<dt>Workspace</dt><dd>" + (rec.workspace ? esc(rec.workspace) : "unknown") + fromSource(rec.workspaceSource) + "</dd>" +
       "<dt>Model</dt><dd>" + modelText(rec) + "</dd>" +
       "<dt>Registered</dt><dd>" + registeredText(rec.registered) + "</dd>" +
+      "<dt>Reply</dt><dd>" + replyControl(rec) + "</dd>" +
       "<dt>Session cwd</dt><dd>" + esc(rec.cwd || "—") + "</dd>" +
       "<dt>Tmux session</dt><dd>" + tmux + "</dd>" +
       "<dt>Last activity</dt><dd>" + esc(rec.lastActivity || "—") + "</dd>" +
@@ -360,12 +387,30 @@
     footer.innerHTML = "<p>" + esc(parts) + " — generated " + esc(timeAgo(data.generatedAt)) + " (" + esc(data.generatedAt || "") + ")</p>";
   }
 
+  // A refresh rebuilds the tables. If Jason is typing a reply, put the
+  // caret back where it was so the periodic refresh does not steal it.
+  function withReplyFocus(render) {
+    var active = document.activeElement;
+    var focus = active && active.classList && active.classList.contains("reply-text")
+      ? { id: active.form.dataset.id, pos: active.selectionStart }
+      : null;
+    render();
+    if (!focus) return;
+    var forms = main.querySelectorAll('.reply-form[data-id="' + focus.id.replace(/"/g, '\\"') + '"]');
+    var input = forms.length ? forms[0].querySelector(".reply-text") : null;
+    if (!input) return;
+    input.focus();
+    try { input.setSelectionRange(focus.pos, focus.pos); } catch (e) {}
+  }
+
   function renderAll() {
-    rowIdx = 0;
-    renderWaiting(lastData);
-    renderSeen(lastData);
-    renderProjects(lastData);
-    renderFooter(lastData);
+    withReplyFocus(function () {
+      rowIdx = 0;
+      renderWaiting(lastData);
+      renderSeen(lastData);
+      renderProjects(lastData);
+      renderFooter(lastData);
+    });
   }
 
   function renderHeaderStatus() {
@@ -462,6 +507,53 @@
       });
   }
 
+  function postReply(form) {
+    var id = form.dataset.id;
+    var input = form.querySelector(".reply-text");
+    var text = input.value;
+    if (!text.trim()) return;
+    var sendBtn = form.querySelector("button");
+    sendBtn.disabled = true;
+    fetch("/api/reply", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ agent: id, text: text })
+    })
+      .then(function (res) {
+        return res.json().catch(function () {
+          throw new Error("the server sent a response that was not valid JSON");
+        }).then(function (body) {
+          if (!res.ok) throw new Error((body && body.error) || "HTTP " + res.status);
+          return body;
+        });
+      })
+      .then(function (body) {
+        replyReceipts[id] = body;
+        if (body.delivered) delete replyDrafts[id];
+        renderAll();
+        // The seat's log shows the effect; fetch now instead of waiting a tick.
+        if (body.delivered && !paused) runFetch();
+      })
+      .catch(function (err) {
+        console.error("reply failed", err);
+        replyReceipts[id] = { delivered: false, exitCode: null, error: err && err.message ? err.message : "unknown error" };
+        renderAll();
+      });
+  }
+
+  main.addEventListener("submit", function (e) {
+    var form = e.target.closest(".reply-form");
+    if (!form) return;
+    e.preventDefault();
+    postReply(form);
+  });
+
+  main.addEventListener("input", function (e) {
+    var input = e.target.closest(".reply-text");
+    if (!input) return;
+    replyDrafts[input.form.dataset.id] = input.value;
+  });
+
   main.addEventListener("click", function (e) {
     var seenBtn = e.target.closest(".seen-toggle");
     if (seenBtn) return postSeen(seenBtn);
diff --git a/packages/control-board/src/serve.mjs b/packages/control-board/src/serve.mjs
index e23b031f..ab485263 100644
--- a/packages/control-board/src/serve.mjs
+++ b/packages/control-board/src/serve.mjs
@@ -4,7 +4,10 @@
 //   GET /            the page (src/page.html)
 //   GET /api/board   re-runs the scanner and returns index.json as JSON
 //   POST /api/seen   {project, agent, lastActivity, seen?} marks a row as seen
-//                    (or clears the mark with seen:false), rescans, returns index
+//                    (or clears the mark with seen:false); rescans, returns index
+//   POST /api/reply  {agent: "<project>/<agent>", text} delivers text to the
+//                    seat's tmux pane through tools/tmux/agent-send.sh (#1505);
+//                    answers {delivered, exitCode, stdout, stderr, ...}
 //   GET /healthz     {"ok":true}
 //
 // POST requires Content-Type: application/json. A plain form post from another
@@ -16,12 +19,72 @@
 
 import { createServer as createHttpServer } from "node:http";
 import { readFileSync } from "node:fs";
-import { join } from "node:path";
+import { join, resolve } from "node:path";
 import { isIP } from "node:net";
-import { scan, markSeen, ConfigError } from "./scan.mjs";
+import { hostname } from "node:os";
+import { spawnSync } from "node:child_process";
+import { scan, markSeen, seenKey, ConfigError } from "./scan.mjs";
 
 const MAX_BODY = 4096;
 
+// Reply-from-board (#1505). The board owns no transport: a reply is handed to
+// the repository's own inter-agent channel, tools/tmux/agent-send.sh, which
+// prepends the "[<sender> -> <host>:<session>]" preamble and pastes into the
+// seat's pane. The tool's exit code is the receipt; the board never retries,
+// queues or broadcasts, and never calls tmux send-keys itself.
+export const DEFAULT_AGENT_SEND = resolve(import.meta.dirname, "..", "..", "..", "tools", "tmux", "agent-send.sh");
+export const REPLY_LIMIT = 2000;
+export const REPLY_SENDER = "control-board";
+// Appended to every message on its own line. The board is a sender without
+// a pane: it reads the seat's transcript, so a seat must answer in its own
+// session as usual and never agent-send back to "control-board" (that
+// target does not exist and the tool refuses it). Jason's refinement after
+// the first real exchange, 2026-09-12.
+export const REPLY_TRAILER = "(control-board: answer in your own session as usual; the board reads your transcript. Do not agent-send to control-board.)";
+const REPLY_TIMEOUT_MS = 15000;
+
+// Decide and, when allowed, send. Returns { status, body } for the HTTP layer.
+// Refusals (4xx) happen before the tool runs and carry { error }. Once the
+// tool has run the answer is 200 with delivered true/false, the exit code and
+// both output streams verbatim, whatever the code was.
+export function replyToRow({ index, key, text, agentSend = DEFAULT_AGENT_SEND, exec = spawnSync, now = () => new Date(), host = hostname().split(".")[0] }) {
+  if (typeof key !== "string" || !key) return { status: 400, body: { error: "agent must be the row id <project>/<agent>" } };
+  if (typeof text !== "string" || !text.trim()) return { status: 400, body: { error: "text must be a non-empty string" } };
+  if (text.length > REPLY_LIMIT) return { status: 400, body: { error: `text is longer than ${REPLY_LIMIT} characters` } };
+  const rec = index.sessions.find((r) => seenKey(r) === key);
+  if (!rec) return { status: 404, body: { error: `unknown row: ${key}` } };
+  const reg = rec.registered;
+  if (!reg) return { status: 409, body: { error: "reply needs a registered seat (start it through scripts/mosaic launch)" } };
+  if (reg.alive === false) return { status: 409, body: { error: `registration is stale: pid ${reg.pid} is gone` } };
+  if (!reg.tmux || !reg.tmux.session) return { status: 409, body: { error: "registration has no tmux session to address" } };
+  const session = reg.tmux.session;
+  const socket = reg.tmux.socket || null;
+  const args = ["-s", session, "-S", `${host}:${REPLY_SENDER}`];
+  if (socket) args.push("-L", socket);
+  args.push("-m", `${text}\n${REPLY_TRAILER}`);
+  // The registration says which socket the seat is on. A launcher-exported
+  // MOSAIC_TMUX_SOCKET in this server's own environment must not override it.
+  const env = { ...process.env };
+  delete env.MOSAIC_TMUX_SOCKET;
+  const r = exec(agentSend, args, { encoding: "utf8", timeout: REPLY_TIMEOUT_MS, env });
+  if (r.error) return { status: 500, body: { error: `could not run ${agentSend}: ${r.error.message}` } };
+  const exitCode = r.status;
+  return {
+    status: 200,
+    body: {
+      delivered: exitCode === 0,
+      exitCode,
+      signal: r.signal ?? null,
+      stdout: String(r.stdout ?? ""),
+      stderr: String(r.stderr ?? ""),
+      agent: key,
+      session,
+      socket,
+      sentAt: now().toISOString(),
+    },
+  };
+}
+
 function sendJson(res, status, body) {
   res.writeHead(status, { "content-type": "application/json", "cache-control": "no-store" });
   res.end(JSON.stringify(body) + "\n");
@@ -67,9 +130,24 @@ export function loadPage(path = join(import.meta.dirname, "page.html")) {
 }
 
 // specs: agent specs to scan on each request. boardDir: where scan writes.
-export function createServer({ specs, boardDir, isAlive, now, seatsDir = null, page = loadPage() }) {
+export function createServer({ specs, boardDir, isAlive, now, seatsDir = null, page = loadPage(), isPidAlive, agentSend = DEFAULT_AGENT_SEND, exec = spawnSync }) {
+  const rescan = () => scan(specs, { boardDir, isAlive, now, seatsDir, isPidAlive });
   return createHttpServer((req, res) => {
     const url = new URL(req.url, "http://localhost");
+    if (req.method === "POST" && url.pathname === "/api/reply") {
+      return readJsonBody(req)
+        .then((body) => {
+          let index;
+          try {
+            index = rescan();
+          } catch (err) {
+            return sendJson(res, 500, { error: err.message });
+          }
+          const out = replyToRow({ index, key: body.agent, text: body.text, agentSend, exec, now });
+          sendJson(res, out.status, out.body);
+        })
+        .catch((err) => sendJson(res, 400, { error: err.message }));
+    }
     if (req.method === "POST" && url.pathname === "/api/seen") {
       return readJsonBody(req)
         .then((body) => {
@@ -79,7 +157,7 @@ export function createServer({ specs, boardDir, isAlive, now, seatsDir = null, p
             return sendJson(res, 400, { error: err.message });
           }
           try {
-            sendJson(res, 200, scan(specs, { boardDir, isAlive, now, seatsDir }));
+            sendJson(res, 200, rescan());
           } catch (err) {
             sendJson(res, 500, { error: err.message });
           }
@@ -97,7 +175,7 @@ export function createServer({ specs, boardDir, isAlive, now, seatsDir = null, p
     if (url.pathname === "/api/board") {
       let index;
       try {
-        index = scan(specs, { boardDir, isAlive, now, seatsDir });
+        index = rescan();
       } catch (err) {
         res.writeHead(500, { "content-type": "application/json", "cache-control": "no-store" });
         return res.end(JSON.stringify({ error: err.message }) + "\n");
diff --git a/packages/control-board/tests/serve.test.mjs b/packages/control-board/tests/serve.test.mjs
index 54c8c280..0d7418a6 100644
--- a/packages/control-board/tests/serve.test.mjs
+++ b/packages/control-board/tests/serve.test.mjs
@@ -7,13 +7,15 @@ import {
   rmSync,
   readFileSync,
   existsSync,
+  chmodSync,
+  statSync,
 } from "node:fs";
 import { tmpdir } from "node:os";
 import { join, resolve, basename } from "node:path";
 import { spawnSync, spawn } from "node:child_process";
 import { createServer as createNetServer } from "node:net";
 import { ConfigError, markSeen } from "../src/scan.mjs";
-import { isLoopbackHost, startServer } from "../src/serve.mjs";
+import { isLoopbackHost, startServer, DEFAULT_AGENT_SEND, REPLY_LIMIT, REPLY_TRAILER } from "../src/serve.mjs";
 import { writeRegistration, makeRegistration } from "../../seat/src/seat.mjs";
 
 const pkgRoot = resolve(import.meta.dirname, "..");
@@ -689,3 +691,193 @@ test("page.html: task and active project cells show their source via sourceTag()
     assert.match(line, /esc\(/, `every dynamic value read off reg must be escaped: ${line.trim()}`);
   }
 });
+
+// ---------------------------------------------------------------------------
+// 10. POST /api/reply: reply-from-board through a fake agent-send.sh (#1505)
+// ---------------------------------------------------------------------------
+
+// A fake tools/tmux/agent-send.sh: records argv and the env it saw, exits
+// with FAKE_SEND_EXIT (default 0) and writes FAKE_SEND_STDERR to stderr.
+function writeFakeAgentSend(root) {
+  const path = join(root, "agent-send.sh");
+  writeFile(path, `#!/usr/bin/env node
+const fs = require("node:fs");
+fs.writeFileSync(process.env.FAKE_SEND_CAPTURE, JSON.stringify({ argv: process.argv.slice(2), tmuxSocketEnv: process.env.MOSAIC_TMUX_SOCKET ?? null }));
+process.stdout.write("fake sent\\n");
+if (process.env.FAKE_SEND_STDERR) process.stderr.write(process.env.FAKE_SEND_STDERR);
+process.exit(Number(process.env.FAKE_SEND_EXIT || 0));
+`);
+  chmodSync(path, 0o755);
+  return path;
+}
+
+function replyFixture({ tmux = { socket: null, session: "agent1" }, pid = process.pid, registered = true } = {}) {
+  const root = makeRoot();
+  const sessionsDir = join(root, "sessions");
+  writeSessionFile(sessionsDir, "s.jsonl", [
+    sessionLine({ id: "s1", timestamp: "2026-09-01T00:00:00Z", cwd: "/w" }),
+    messageLine({ timestamp: "2026-09-01T00:00:01Z", role: "assistant", stopReason: "stop", texts: ["done?"] }),
+  ]);
+  const seatsDir = join(root, "seats");
+  if (registered) {
+    writeRegistration(seatsDir, makeRegistration({
+      resolved: { seat: "agent1", project: "proj", sessionsDir, seatDir: join(root, "agent1"), launchScript: join(root, "agent1", "launch.sh"), layout: "repo", defaultWorkspace: null },
+      task: "t", tmux, pid,
+    }));
+  }
+  const capture = join(root, "capture.json");
+  process.env.FAKE_SEND_CAPTURE = capture;
+  return { root, sessionsDir, seatsDir, boardDir: join(root, "board"), agentSend: writeFakeAgentSend(root), capture,
+    specs: [{ agent: "agent1", project: "proj", sessionsDir, tmux: {} }] };
+}
+
+async function postReply(base, body, headers = { "content-type": "application/json" }) {
+  const res = await fetch(`${base}/api/reply`, { method: "POST", headers, body: typeof body === "string" ? body : JSON.stringify(body) });
+  return { status: res.status, body: await res.json() };
+}
+
+test("POST /api/reply: runs agent-send.sh with -s from the registration, -S <host>:control-board, -m text plus the fixed trailer, no -L on the default socket, MOSAIC_TMUX_SOCKET stripped; answers delivered with the exit code and both streams", async () => {
+  const f = replyFixture();
+  const server = await startServer({ host: "127.0.0.1", port: 0, specs: f.specs, boardDir: f.boardDir, seatsDir: f.seatsDir, isAlive: () => true, page: "<html></html>", agentSend: f.agentSend, now: () => new Date("2026-09-12T18:00:00Z") });
+  const base = `http://127.0.0.1:${server.address().port}`;
+  process.env.MOSAIC_TMUX_SOCKET = "leaked-from-launcher";
+  delete process.env.FAKE_SEND_EXIT;
+  delete process.env.FAKE_SEND_STDERR;
+  try {
+    const { status, body } = await postReply(base, { agent: "proj/agent1", text: "yes, go ahead" });
+    assert.equal(status, 200);
+    assert.equal(body.delivered, true);
+    assert.equal(body.exitCode, 0);
+    assert.equal(body.stdout, "fake sent\n");
+    assert.equal(body.stderr, "");
+    assert.equal(body.session, "agent1");
+    assert.equal(body.socket, null);
+    assert.equal(body.sentAt, "2026-09-12T18:00:00.000Z");
+    const seen = JSON.parse(readFileSync(f.capture, "utf8"));
+    assert.equal(seen.tmuxSocketEnv, null, "the launcher's socket variable must not reach the tool");
+    const argv = seen.argv;
+    assert.equal(argv[argv.indexOf("-s") + 1], "agent1");
+    assert.match(argv[argv.indexOf("-S") + 1], /^[^:\s]+:control-board$/);
+    assert.equal(argv[argv.indexOf("-m") + 1], "yes, go ahead\n" + REPLY_TRAILER, "the text, then the fixed trailer on its own line");
+    assert.match(REPLY_TRAILER, /^\(control-board: .*Do not agent-send to control-board\.\)$/);
+    assert.equal(argv.includes("-L"), false);
+    assert.equal(argv.length, 6, "exactly -s, -S and -m: no send-keys, class or retries");
+  } finally {
+    delete process.env.MOSAIC_TMUX_SOCKET;
+    await closeServer(server);
+  }
+});
+
+test("POST /api/reply: a registration with a tmux socket adds -L <socket>", async () => {
+  const f = replyFixture({ tmux: { socket: "mosaic-fleet", session: "agent1" } });
+  const server = await startServer({ host: "127.0.0.1", port: 0, specs: f.specs, boardDir: f.boardDir, seatsDir: f.seatsDir, isAlive: () => true, page: "<html></html>", agentSend: f.agentSend });
+  const base = `http://127.0.0.1:${server.address().port}`;
+  try {
+    const { status, body } = await postReply(base, { agent: "proj/agent1", text: "hi" });
+    assert.equal(status, 200);
+    assert.equal(body.socket, "mosaic-fleet");
+    const argv = JSON.parse(readFileSync(f.capture, "utf8")).argv;
+    assert.equal(argv[argv.indexOf("-L") + 1], "mosaic-fleet");
+  } finally {
+    await closeServer(server);
+  }
+});
+
+test("POST /api/reply: a non-zero tool exit is a 200 with delivered false, the exit code and the stderr verbatim", async () => {
+  const f = replyFixture();
+  const server = await startServer({ host: "127.0.0.1", port: 0, specs: f.specs, boardDir: f.boardDir, seatsDir: f.seatsDir, isAlive: () => true, page: "<html></html>", agentSend: f.agentSend });
+  const base = `http://127.0.0.1:${server.address().port}`;
+  process.env.FAKE_SEND_EXIT = "4";
+  process.env.FAKE_SEND_STDERR = "agent-send.sh: REFUSING - session 'agent1' exists on multiple sockets\n";
+  try {
+    const { status, body } = await postReply(base, { agent: "proj/agent1", text: "hi" });
+    assert.equal(status, 200);
+    assert.equal(body.delivered, false);
+    assert.equal(body.exitCode, 4);
+    assert.match(body.stderr, /REFUSING/);
+  } finally {
+    delete process.env.FAKE_SEND_EXIT;
+    delete process.env.FAKE_SEND_STDERR;
+    await closeServer(server);
+  }
+});
+
+test("POST /api/reply: refusals before the tool runs: empty or blank or long text 400, unknown row 404, no registration 409, stale registration 409, no tmux session 409, bad JSON 400; the tool is never called", async () => {
+  const cases = [
+    { fixture: replyFixture(), body: { agent: "proj/agent1", text: "" }, status: 400, error: /non-empty/ },
+    { fixture: replyFixture(), body: { agent: "proj/agent1", text: "   \n" }, status: 400, error: /non-empty/ },
+    { fixture: replyFixture(), body: { agent: "proj/agent1", text: "x".repeat(REPLY_LIMIT + 1) }, status: 400, error: /longer than/ },
+    { fixture: replyFixture(), body: { agent: "proj/nobody", text: "hi" }, status: 404, error: /unknown row/ },
+    { fixture: replyFixture(), body: { text: "hi" }, status: 400, error: /row id/ },
+    { fixture: replyFixture({ registered: false }), body: { agent: "proj/agent1", text: "hi" }, status: 409, error: /needs a registered seat/ },
+    { fixture: replyFixture({ pid: 4242 }), isPidAlive: () => false, body: { agent: "proj/agent1", text: "hi" }, status: 409, error: /stale/ },
+    { fixture: replyFixture({ tmux: null }), body: { agent: "proj/agent1", text: "hi" }, status: 409, error: /no tmux session/ },
+    { fixture: replyFixture(), body: "{not json", status: 400, error: /invalid JSON/ },
+  ];
+  for (const c of cases) {
+    const f = c.fixture;
+    const server = await startServer({ host: "127.0.0.1", port: 0, specs: f.specs, boardDir: f.boardDir, seatsDir: f.seatsDir, isAlive: () => true, page: "<html></html>", agentSend: f.agentSend, isPidAlive: c.isPidAlive });
+    const base = `http://127.0.0.1:${server.address().port}`;
+    try {
+      const { status, body } = await postReply(base, c.body);
+      assert.equal(status, c.status, `${JSON.stringify(c.body).slice(0, 60)}: ${JSON.stringify(body)}`);
+      assert.match(body.error, c.error);
+      assert.equal(existsSync(f.capture), false, "the tool must not run on a refusal");
+    } finally {
+      await closeServer(server);
+    }
+  }
+});
+
+test("POST /api/reply: a missing agent-send.sh is a 500 with the path in the error, not a crash", async () => {
+  const f = replyFixture();
+  const server = await startServer({ host: "127.0.0.1", port: 0, specs: f.specs, boardDir: f.boardDir, seatsDir: f.seatsDir, isAlive: () => true, page: "<html></html>", agentSend: join(f.root, "missing-agent-send.sh") });
+  const base = `http://127.0.0.1:${server.address().port}`;
+  try {
+    const { status, body } = await postReply(base, { agent: "proj/agent1", text: "hi" });
+    assert.equal(status, 500);
+    assert.match(body.error, /missing-agent-send\.sh/);
+    const health = await fetch(`${base}/healthz`);
+    assert.equal(health.status, 200);
+  } finally {
+    await closeServer(server);
+  }
+});
+
+test("replyToRow: DEFAULT_AGENT_SEND is the repository's tools/tmux/agent-send.sh and it is executable", () => {
+  assert.equal(DEFAULT_AGENT_SEND, resolve(pkgRoot, "..", "..", "tools", "tmux", "agent-send.sh"));
+  assert.ok(existsSync(DEFAULT_AGENT_SEND));
+  assert.ok(statSync(DEFAULT_AGENT_SEND).mode & 0o111);
+});
+
+test("page.html: the reply box appears only where canReply() holds (live registration with a tmux session), the detail has a Reply row, the submit posts JSON to /api/reply, receipts and drafts survive a refresh, and every receipt value is escaped", () => {
+  const html = readFileSync(join(pkgRoot, "src", "page.html"), "utf8");
+  const lines = html.split("\n");
+  const canReplyFn = html.match(/function canReply\(rec\) \{[\s\S]*?\n  \}/);
+  assert.ok(canReplyFn, "canReply() must exist");
+  assert.match(canReplyFn[0], /reg\.alive !== false/, "a stale registration gets no box");
+  assert.match(canReplyFn[0], /reg\.tmux && reg\.tmux\.session/, "a registration without a tmux session gets no box");
+  const replyControlFn = html.match(/function replyControl\(rec\) \{[\s\S]*?\n  \}/);
+  assert.ok(replyControlFn, "replyControl() must exist");
+  assert.match(replyControlFn[0], /if \(!canReply\(rec\)\) return '<span class="unknown">reply needs a registered seat<\/span>'/);
+  assert.match(replyControlFn[0], /class="reply-form" data-id="' \+ esc\(id\)/);
+  assert.match(replyControlFn[0], /value="' \+ esc\(replyDrafts\[id\] \|\| ""\)/, "an unsent draft is put back after a refresh");
+  const rowPair = html.match(/function buildRowPair\(rec, showProject\) \{[\s\S]*?\n  \}/);
+  assert.match(rowPair[0], /<dt>Reply<\/dt><dd>" \+ replyControl\(rec\) \+ "<\/dd>/);
+  const receiptFn = html.match(/function replyReceipt\(id\) \{[\s\S]*?\n  \}/);
+  assert.ok(receiptFn, "replyReceipt() must exist");
+  for (const line of receiptFn[0].split("\n")) {
+    if (!/\br\.[a-zA-Z]/.test(line)) continue;
+    assert.match(line, /esc\(/, `every receipt value must be escaped: ${line.trim()}`);
+  }
+  assert.match(receiptFn[0], /esc\(r\.stderr \|\| r\.error/, "a failure shows the tool's stderr");
+  const postReplyFn = html.match(/function postReply\(form\) \{[\s\S]*?\n  \}/);
+  assert.ok(postReplyFn, "postReply() must exist");
+  assert.match(postReplyFn[0], /fetch\("\/api\/reply", \{\s*method: "POST",\s*headers: \{ "content-type": "application\/json" \}/);
+  assert.match(postReplyFn[0], /JSON\.stringify\(\{ agent: id, text: text \}\)/);
+  assert.match(postReplyFn[0], /replyReceipts\[id\] = body/);
+  assert.ok(lines.some((l) => l.includes('main.addEventListener("submit"')), "the form submits through one delegated handler");
+  assert.ok(lines.some((l) => l.includes('main.addEventListener("input"')), "drafts are captured as they are typed");
+  assert.match(html, /function withReplyFocus\(render\)/, "a refresh gives the caret back to the reply box");
+  assert.doesNotMatch(html, /send-keys/, "the page never talks tmux");
+});