diff --git a/packages/codev/src/agent-farm/__tests__/bugfix-584-send-multiline-pacing.test.ts b/packages/codev/src/agent-farm/__tests__/bugfix-584-send-multiline-pacing.test.ts
new file mode 100644
index 00000000..aa37942b
--- /dev/null
+++ b/packages/codev/src/agent-farm/__tests__/bugfix-584-send-multiline-pacing.test.ts
@@ -0,0 +1,189 @@
+/**
+ * Regression test for Bugfix #584: afx send multi-line messages (>3 lines)
+ * treated as paste, final Enter swallowed.
+ *
+ * Verifies that writeMessageToSession paces multi-line output line-by-line
+ * with delays to prevent paste detection, while short messages are still
+ * written in a single call. Also tests delayOffset serialization to prevent
+ * interleaved writes when multiple messages flush to the same session.
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { writeMessageToSession } from '../servers/message-write.js';
+import type { PtySession } from '../../terminal/pty-session.js';
+
+function makeSession(): PtySession & { writeCalls: string[] } {
+  const writeCalls: string[] = [];
+  return {
+    write: vi.fn((data: string) => writeCalls.push(data)),
+    writeCalls,
+  } as unknown as PtySession & { writeCalls: string[] };
+}
+
+describe('writeMessageToSession (Bugfix #584)', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('writes short messages (≤3 lines) in a single call', () => {
+    const session = makeSession();
+    const msg = 'line1\nline2\nline3';
+
+    const endTime = writeMessageToSession(session, msg, false);
+
+    // Message written in one shot
+    expect(session.writeCalls).toEqual([msg]);
+
+    // Enter arrives after 50ms
+    vi.advanceTimersByTime(50);
+    expect(session.writeCalls).toEqual([msg, '\r']);
+    expect(endTime).toBe(50);
+  });
+
+  it('paces multi-line messages (>3 lines) line-by-line with delays', () => {
+    const session = makeSession();
+    const msg = 'line1\nline2\nline3\nline4';
+
+    const endTime = writeMessageToSession(session, msg, false);
+
+    // First line written immediately
+    expect(session.writeCalls).toEqual(['line1\n']);
+
+    // Lines 2-4 arrive with 10ms, 20ms, 30ms delays
+    vi.advanceTimersByTime(10);
+    expect(session.writeCalls).toEqual(['line1\n', 'line2\n']);
+
+    vi.advanceTimersByTime(10);
+    expect(session.writeCalls).toEqual(['line1\n', 'line2\n', 'line3\n']);
+
+    vi.advanceTimersByTime(10);
+    expect(session.writeCalls).toEqual(['line1\n', 'line2\n', 'line3\n', 'line4']);
+
+    // Enter arrives after totalPacing (30ms) + 80ms = 110ms from start
+    vi.advanceTimersByTime(80);
+    expect(session.writeCalls).toEqual(['line1\n', 'line2\n', 'line3\n', 'line4', '\r']);
+    expect(endTime).toBe(110);
+  });
+
+  it('respects noEnter=true for short messages', () => {
+    const session = makeSession();
+    const endTime = writeMessageToSession(session, 'short', true);
+
+    vi.advanceTimersByTime(200);
+    expect(session.writeCalls).toEqual(['short']);
+    expect(endTime).toBe(50); // duration still reported
+  });
+
+  it('respects noEnter=true for multi-line messages', () => {
+    const session = makeSession();
+    const msg = 'l1\nl2\nl3\nl4\nl5';
+
+    const endTime = writeMessageToSession(session, msg, true);
+    vi.advanceTimersByTime(500);
+
+    // All lines written, but no \r
+    expect(session.writeCalls).toEqual(['l1\n', 'l2\n', 'l3\n', 'l4\n', 'l5']);
+    expect(endTime).toBe(40); // (5-1) * 10 = 40ms for last line
+  });
+
+  it('handles formatted architect message (realistic multi-line)', () => {
+    const session = makeSession();
+    // Realistic formatted message: header + 2 content lines + footer = 4 lines
+    const msg = '### [ARCHITECT INSTRUCTION | 2026-04-04T00:00:00.000Z] ###\nDo this thing\nAnd that thing\n###############################';
+
+    const endTime = writeMessageToSession(session, msg, false);
+
+    // First line immediately
+    expect(session.writeCalls[0]).toBe('### [ARCHITECT INSTRUCTION | 2026-04-04T00:00:00.000Z] ###\n');
+
+    // All lines delivered after enough time
+    vi.advanceTimersByTime(30);
+    expect(session.writeCalls).toHaveLength(4);
+
+    // Enter delivered after pacing + 80ms
+    vi.advanceTimersByTime(80);
+    expect(session.writeCalls[session.writeCalls.length - 1]).toBe('\r');
+    expect(endTime).toBe(110); // 30ms pacing + 80ms enter
+  });
+
+  it('single-line message written in one shot without pacing', () => {
+    const session = makeSession();
+    const endTime = writeMessageToSession(session, 'hello', false);
+
+    expect(session.writeCalls).toEqual(['hello']);
+    vi.advanceTimersByTime(50);
+    expect(session.writeCalls).toEqual(['hello', '\r']);
+    expect(endTime).toBe(50);
+  });
+
+  describe('delayOffset serialization (prevents interleaving)', () => {
+    it('short message with delayOffset defers the initial write', () => {
+      const session = makeSession();
+      const endTime = writeMessageToSession(session, 'hello', false, 100);
+
+      // Nothing written yet
+      expect(session.writeCalls).toEqual([]);
+
+      // Message arrives at offset
+      vi.advanceTimersByTime(100);
+      expect(session.writeCalls).toEqual(['hello']);
+
+      // Enter arrives at offset + 50ms
+      vi.advanceTimersByTime(50);
+      expect(session.writeCalls).toEqual(['hello', '\r']);
+      expect(endTime).toBe(150);
+    });
+
+    it('multi-line message with delayOffset defers all lines', () => {
+      const session = makeSession();
+      const msg = 'a\nb\nc\nd';
+      const endTime = writeMessageToSession(session, msg, false, 200);
+
+      // Nothing written before offset
+      expect(session.writeCalls).toEqual([]);
+
+      // First line at 200ms
+      vi.advanceTimersByTime(200);
+      expect(session.writeCalls).toEqual(['a\n']);
+
+      // Remaining lines at 210, 220, 230ms
+      vi.advanceTimersByTime(30);
+      expect(session.writeCalls).toEqual(['a\n', 'b\n', 'c\n', 'd']);
+
+      // Enter at 230 + 80 = 310ms from start
+      vi.advanceTimersByTime(80);
+      expect(session.writeCalls).toEqual(['a\n', 'b\n', 'c\n', 'd', '\r']);
+      expect(endTime).toBe(310);
+    });
+
+    it('two multi-line messages in sequence do not interleave', () => {
+      const session = makeSession();
+      const msg1 = 'A1\nA2\nA3\nA4';
+      const msg2 = 'B1\nB2\nB3\nB4';
+
+      // Simulate what SendBuffer.flush does: chain offsets
+      const end1 = writeMessageToSession(session, msg1, false, 0);
+      const end2 = writeMessageToSession(session, msg2, false, end1);
+
+      // Advance through all timers
+      vi.advanceTimersByTime(end2 + 100);
+
+      // Verify message 1 lines come before message 2 lines
+      const writes = session.writeCalls;
+      const a4Idx = writes.indexOf('A4');
+      const enterAfterA = writes.indexOf('\r');
+      const b1Idx = writes.indexOf('B1\n');
+
+      expect(a4Idx).toBeLessThan(enterAfterA);
+      expect(enterAfterA).toBeLessThan(b1Idx);
+
+      // Both messages fully delivered with their own Enters
+      const enterCount = writes.filter(w => w === '\r').length;
+      expect(enterCount).toBe(2);
+    });
+  });
+});
diff --git a/packages/codev/src/agent-farm/__tests__/send-buffer.test.ts b/packages/codev/src/agent-farm/__tests__/send-buffer.test.ts
index b74697b4..a313547d 100644
--- a/packages/codev/src/agent-farm/__tests__/send-buffer.test.ts
+++ b/packages/codev/src/agent-farm/__tests__/send-buffer.test.ts
@@ -59,7 +59,7 @@ describe('SendBuffer', () => {
 
   it('delivers messages when session is idle', () => {
     const session = makeSession(true);
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(() => session, deliver, log);
@@ -76,7 +76,7 @@ describe('SendBuffer', () => {
 
   it('does NOT deliver messages when session is actively typing', () => {
     const session = makeSession(false); // not idle
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(() => session, deliver, log);
@@ -90,7 +90,7 @@ describe('SendBuffer', () => {
 
   it('delivers when max buffer age is exceeded even if user is typing', () => {
     const session = makeSession(false); // not idle
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(() => session, deliver, log);
@@ -109,8 +109,9 @@ describe('SendBuffer', () => {
   it('delivers all messages in order within a session', () => {
     const session = makeSession(true);
     const deliveredMsgs: string[] = [];
-    const deliver = (_s: PtySession, msg: BufferedMessage) => {
+    const deliver = (_s: PtySession, msg: BufferedMessage): number => {
       deliveredMsgs.push(msg.formattedMessage);
+      return 0;
     };
     const log = vi.fn();
 
@@ -126,7 +127,7 @@ describe('SendBuffer', () => {
   });
 
   it('discards messages for dead sessions with warning', () => {
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(() => undefined, deliver, log); // session gone
@@ -141,7 +142,7 @@ describe('SendBuffer', () => {
 
   it('stop() delivers all remaining messages (force flush)', () => {
     const session = makeSession(false); // not idle — normally wouldn't deliver
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(() => session, deliver, log);
@@ -158,7 +159,7 @@ describe('SendBuffer', () => {
   it('handles multiple sessions independently', () => {
     const idleSession = makeSession(true);
     const typingSession = makeSession(false);
-    const deliver = vi.fn();
+    const deliver = vi.fn().mockReturnValue(0);
     const log = vi.fn();
 
     buf.start(
@@ -196,7 +197,7 @@ describe('SendBuffer', () => {
       // Bugfix #492: composing gets stuck true after non-Enter keystrokes (Ctrl+C,
       // arrows, Tab). Idle threshold alone is sufficient for delivery.
       const session = makeSession(true, true); // idle=true, composing=true
-      const deliver = vi.fn();
+      const deliver = vi.fn().mockReturnValue(0);
       const log = vi.fn();
 
       buf.start(() => session, deliver, log);
@@ -210,7 +211,7 @@ describe('SendBuffer', () => {
 
     it('delivers when session is idle and NOT composing', () => {
       const session = makeSession(true, false); // idle=true, composing=false
-      const deliver = vi.fn();
+      const deliver = vi.fn().mockReturnValue(0);
       const log = vi.fn();
 
       buf.start(() => session, deliver, log);
@@ -224,7 +225,7 @@ describe('SendBuffer', () => {
 
     it('delivers when composing but max buffer age exceeded', () => {
       const session = makeSession(false, true); // not idle, composing
-      const deliver = vi.fn();
+      const deliver = vi.fn().mockReturnValue(0);
       const log = vi.fn();
 
       buf.start(() => session, deliver, log);
diff --git a/packages/codev/src/agent-farm/servers/message-write.ts b/packages/codev/src/agent-farm/servers/message-write.ts
new file mode 100644
index 00000000..365e0acc
--- /dev/null
+++ b/packages/codev/src/agent-farm/servers/message-write.ts
@@ -0,0 +1,72 @@
+/**
+ * Paced message writing for PTY sessions (Bugfix #584).
+ *
+ * Extracted to a shared module to avoid circular imports between
+ * tower-routes.ts and tower-cron.ts.
+ */
+
+/** Minimal writable session interface — avoids coupling to PtySession. */
+export interface WritableSession {
+  write(data: string): void;
+}
+
+// Messages longer than this threshold are written line-by-line with delays
+// to prevent the receiving terminal from classifying the input as a paste
+// and swallowing the final Enter.
+const PACED_WRITE_LINE_THRESHOLD = 4;
+const INTER_LINE_DELAY_MS = 10;
+const PACED_ENTER_DELAY_MS = 80;
+const SIMPLE_ENTER_DELAY_MS = 50;
+
+/**
+ * Write a message to a PTY session, pacing multi-line output to prevent
+ * the terminal from treating it as a paste (Bugfix #584).
+ *
+ * Short messages (≤3 lines): single write + delayed Enter.
+ * Long messages (>3 lines): line-by-line writes with 10ms gaps, then Enter
+ * after all lines are delivered.
+ *
+ * @param delayOffset  ms offset for all scheduled writes (used to serialize
+ *                     multiple messages to the same session without interleaving)
+ * @returns            ms timestamp (from call time) when all writes complete
+ */
+export function writeMessageToSession(
+  session: WritableSession, message: string, noEnter: boolean, delayOffset = 0,
+): number {
+  const lines = message.split('\n');
+
+  if (lines.length < PACED_WRITE_LINE_THRESHOLD) {
+    // Short messages: single write (existing behavior, works fine)
+    if (delayOffset === 0) {
+      session.write(message);
+    } else {
+      setTimeout(() => session.write(message), delayOffset);
+    }
+    const enterTime = delayOffset + SIMPLE_ENTER_DELAY_MS;
+    if (!noEnter) {
+      setTimeout(() => session.write('\r'), enterTime);
+    }
+    return enterTime;
+  }
+
+  // Multi-line: pace output line-by-line to avoid paste detection.
+  // Writing all lines in a single write() causes the terminal to treat it
+  // as a paste, swallowing the final Enter.
+  for (let i = 0; i < lines.length; i++) {
+    const text = i < lines.length - 1 ? lines[i] + '\n' : lines[i];
+    const lineDelay = delayOffset + i * INTER_LINE_DELAY_MS;
+    if (lineDelay === 0) {
+      session.write(text);
+    } else {
+      setTimeout(() => session.write(text), lineDelay);
+    }
+  }
+
+  const lastLineTime = delayOffset + (lines.length - 1) * INTER_LINE_DELAY_MS;
+  if (!noEnter) {
+    const enterTime = lastLineTime + PACED_ENTER_DELAY_MS;
+    setTimeout(() => session.write('\r'), enterTime);
+    return enterTime;
+  }
+  return lastLineTime;
+}
diff --git a/packages/codev/src/agent-farm/servers/send-buffer.ts b/packages/codev/src/agent-farm/servers/send-buffer.ts
index b1737e37..90ba2ace 100644
--- a/packages/codev/src/agent-farm/servers/send-buffer.ts
+++ b/packages/codev/src/agent-farm/servers/send-buffer.ts
@@ -25,7 +25,8 @@ export interface BufferedMessage {
 }
 
 export type GetSessionFn = (id: string) => PtySession | undefined;
-export type DeliverFn = (session: PtySession, msg: BufferedMessage) => void;
+/** Deliver function returns ms timestamp when all writes complete (for serialization). */
+export type DeliverFn = (session: PtySession, msg: BufferedMessage, delayOffset?: number) => number;
 export type LogFn = (level: 'INFO' | 'ERROR' | 'WARN', message: string) => void;
 
 const DEFAULT_IDLE_THRESHOLD_MS = 3000;
@@ -99,9 +100,12 @@ export class SendBuffer {
       // Bugfix #492: removed composing check — it gets stuck true after non-Enter
       // keystrokes (Ctrl+C, arrows, Tab), causing messages to wait 60s max age.
       if (forceAll || isIdle || maxAgeExceeded) {
-        // Deliver all messages in order
+        // Deliver all messages in order, serializing paced writes (Bugfix #584).
+        // Each delivery returns the ms when its writes complete; the next message
+        // starts after that to prevent interleaved lines.
+        let offset = 0;
         for (const msg of messages) {
-          this.deliver(session, msg);
+          offset = this.deliver(session, msg, offset);
           if (this.log && msg.logMessage) {
             this.log('INFO', msg.logMessage);
           }
diff --git a/packages/codev/src/agent-farm/servers/tower-cron.ts b/packages/codev/src/agent-farm/servers/tower-cron.ts
index 8aa36473..5ec01626 100644
--- a/packages/codev/src/agent-farm/servers/tower-cron.ts
+++ b/packages/codev/src/agent-farm/servers/tower-cron.ts
@@ -13,6 +13,7 @@ import { parseCronExpression, isDue } from './tower-cron-parser.js';
 import type { CronSchedule } from './tower-cron-parser.js';
 import { formatBuilderMessage } from '../utils/message-format.js';
 import { broadcastMessage } from './tower-messages.js';
+import { writeMessageToSession } from './message-write.js';
 import { getGlobalDb } from '../db/index.js';
 
 // ============================================================================
@@ -318,10 +319,8 @@ function deliverMessage(task: CronTask, message: string): void {
   }
 
   const formatted = formatBuilderMessage('af-cron', message);
-  // Write message, then Enter separately after delay so PTY processes the
-  // multi-line paste before receiving the submission keystroke (Bugfix #492)
-  session.write(formatted);
-  setTimeout(() => session.write('\r'), 50);
+  // Bugfix #584: pace multi-line output to avoid paste detection.
+  writeMessageToSession(session, formatted, false);
 
   broadcastMessage({
     type: 'message',
diff --git a/packages/codev/src/agent-farm/servers/tower-routes.ts b/packages/codev/src/agent-farm/servers/tower-routes.ts
index 2f0c82fa..ffa556b0 100644
--- a/packages/codev/src/agent-farm/servers/tower-routes.ts
+++ b/packages/codev/src/agent-farm/servers/tower-routes.ts
@@ -45,6 +45,7 @@ import { formatArchitectMessage, formatBuilderMessage } from '../utils/message-f
 import { SendBuffer } from './send-buffer.js';
 import type { BufferedMessage } from './send-buffer.js';
 import type { PtySession } from '../../terminal/pty-session.js';
+import { writeMessageToSession } from './message-write.js';
 import {
   getKnownWorkspacePaths,
   getInstances,
@@ -86,14 +87,12 @@ const overviewCache = new OverviewCache();
 // Singleton send buffer for typing-aware message delivery (Spec 403)
 const sendBuffer = new SendBuffer();
 
-/** Deliver a buffered message to a session (write + broadcast + log). */
-function deliverBufferedMessage(session: PtySession, msg: BufferedMessage): void {
-  // Write message, then Enter after delay — see handleSend for rationale (Bugfix #492)
-  session.write(msg.formattedMessage);
-  if (!msg.noEnter) {
-    setTimeout(() => session.write('\r'), 50);
-  }
+/** Deliver a buffered message to a session (write + broadcast + log).
+ *  Returns the ms timestamp when all writes complete (for serialization). */
+function deliverBufferedMessage(session: PtySession, msg: BufferedMessage, delayOffset = 0): number {
+  const endTime = writeMessageToSession(session, msg.formattedMessage, msg.noEnter, delayOffset);
   broadcastMessage(msg.broadcastPayload as Parameters<typeof broadcastMessage>[0]);
+  return endTime;
 }
 
 /** Start the send buffer flush timer (called from tower-server during init). */
@@ -917,14 +916,8 @@ async function handleSend(
     ctx.log('INFO', `Message deferred (user typing): ${from ?? 'unknown'} → ${result.agent} (terminal ${result.terminalId.slice(0, 8)}...)`);
   } else {
     // User is idle (or interrupt) — deliver immediately.
-    // Write message first, then Enter separately after a short delay.
-    // Multi-line formatted messages contain embedded \n which the PTY processes
-    // as line breaks. A trailing \r in the same write submits an empty line after
-    // the footer, not the message. Delayed \r lets the PTY process the paste first.
-    session.write(formattedMessage);
-    if (!noEnter) {
-      setTimeout(() => session.write('\r'), 50);
-    }
+    // Bugfix #584: paces multi-line output to avoid paste detection.
+    writeMessageToSession(session, formattedMessage, noEnter);
     broadcastMessage(broadcastPayload);
     ctx.log('INFO', logMessage);
   }