diff --git a/packages/cli/bin/tps.ts b/packages/cli/bin/tps.ts
index 0ea2f96..c2e33bf 100755
--- a/packages/cli/bin/tps.ts
+++ b/packages/cli/bin/tps.ts
@@ -843,7 +843,7 @@ async function main() {
       const action = rest[0] as "init" | "start" | "stop" | "status" | "log" | undefined;
       const valid = ["init", "start", "stop", "status", "log"];
       if (!action || !valid.includes(action)) {
-        console.error("Usage:\n  tps branch init [--listen <port>] [--host <hostname>] [--transport ws|tcp]\n  tps branch start\n  tps branch stop\n  tps branch status\n  tps branch log [--lines N] [--follow]");
+        console.error("Usage:\n  tps branch init [--listen <port>] [--host <hostname>] [--transport ws|tcp] [--agent <id>]\n  tps branch start\n  tps branch stop\n  tps branch status\n  tps branch log [--lines N] [--follow]");
         process.exit(1);
       }
       const { runBranch } = await import("../src/commands/branch.js");
@@ -852,6 +852,7 @@ async function main() {
         port: typeof cli.flags.listen === "number" ? Number(cli.flags.listen) : undefined,
         host: cli.flags.host,
         transport: cli.flags.transport === "tcp" ? "tcp" : cli.flags.transport === "ws" ? "ws" : undefined,
+        agent: cli.flags.agent,
         force: cli.flags.force,
         lines: typeof cli.flags.lines === "number" ? Number(cli.flags.lines) : undefined,
         follow: !!cli.flags.follow,
diff --git a/packages/cli/src/commands/branch.ts b/packages/cli/src/commands/branch.ts
index d06178f..1786392 100644
--- a/packages/cli/src/commands/branch.ts
+++ b/packages/cli/src/commands/branch.ts
@@ -21,6 +21,7 @@ export interface BranchArgs {
   port?: number;
   host?: string;
   transport?: "ws" | "tcp";
+  agent?: string;
   force?: boolean;
   lines?: number;
   follow?: boolean;
@@ -63,7 +64,7 @@ function processAlive(pid: number): boolean {
   }
 }
 
-function writeBranchConf(port: number, host: string, transport: "ws" | "tcp", agentsDir?: string): void {
+export function writeBranchConf(port: number, host: string, transport: "ws" | "tcp", agentsDir?: string, agentId?: string): void {
   writeFileSync(
     confPath(),
     JSON.stringify(
@@ -72,6 +73,7 @@ function writeBranchConf(port: number, host: string, transport: "ws" | "tcp", ag
         host,
         transport,
         agentsDir,
+        agentId,
         createdAt: new Date().toISOString(),
       },
       null,
@@ -81,7 +83,7 @@ function writeBranchConf(port: number, host: string, transport: "ws" | "tcp", ag
   );
 }
 
-function readBranchConf(): { port: number; host: string; transport: "ws" | "tcp"; agentsDir?: string } {
+function readBranchConf(): { port: number; host: string; transport: "ws" | "tcp"; agentsDir?: string; agentId?: string } {
   const p = confPath();
   if (!existsSync(p)) throw new Error("branch.conf.json not found. Run `tps branch init` first.");
   const raw = JSON.parse(readFileSync(p, "utf-8"));
@@ -90,7 +92,8 @@ function readBranchConf(): { port: number; host: string; transport: "ws" | "tcp"
     port: Number(raw.port),
     host: String(raw.host || ""),
     transport: t,
-    agentsDir: raw.agentsDir ? String(raw.agentsDir) : undefined
+    agentsDir: raw.agentsDir ? String(raw.agentsDir) : undefined,
+    agentId: raw.agentId ? String(raw.agentId) : undefined
   };
 }
 
@@ -125,7 +128,11 @@ async function runInit(args: BranchArgs): Promise<void> {
   const port = args.port ?? 6458;
   const advertiseHost = args.host ?? hostname();
   const transport = args.transport ?? "ws";
-  writeBranchConf(port, advertiseHost, transport, undefined);
+  // agentId resolves the maildir for branch-delivered messages. Default to
+  // the value at runBranch start time (TPS_AGENT_ID env → conf.agentId →
+  // hostname). Persisting at init time lets `tps branch start` pick it up
+  // without re-setting TPS_AGENT_ID in the environment.
+  writeBranchConf(port, advertiseHost, transport, undefined, args.agent);
 
   const pubkeyB64 = Buffer.from(kp.encryption.publicKey).toString("base64url");
   const sigPubkeyB64 = Buffer.from(kp.signing.publicKey).toString("base64url");
diff --git a/packages/cli/test/branch.test.ts b/packages/cli/test/branch.test.ts
index 34bc71c..b832eb3 100644
--- a/packages/cli/test/branch.test.ts
+++ b/packages/cli/test/branch.test.ts
@@ -2,7 +2,8 @@ import { describe, test, expect, beforeEach, afterEach } from "bun:test";
 import { mkdtempSync, rmSync, existsSync, writeFileSync, mkdirSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
-import { buildJoinToken, isAlreadyJoined } from "../src/commands/branch.js";
+import { readFileSync } from "node:fs";
+import { buildJoinToken, isAlreadyJoined, writeBranchConf } from "../src/commands/branch.js";
 
 describe("tps branch init", () => {
   let tmpDir: string;
@@ -57,6 +58,32 @@ describe("tps branch init", () => {
     writeFileSync(join(dir, "host.json"), JSON.stringify({ hostId: "existing" }));
     expect(isAlreadyJoined(dir)).toBe(true);
   });
+
+  test("writeBranchConf persists agentId when --agent supplied (ops-hs19)", () => {
+    process.env.TPS_ROOT = tmpDir;
+    try {
+      writeBranchConf(33744, "localhost", "ws", undefined, "reed");
+      const conf = JSON.parse(readFileSync(join(tmpDir, "branch.conf.json"), "utf-8"));
+      expect(conf.agentId).toBe("reed");
+      expect(conf.port).toBe(33744);
+      expect(conf.host).toBe("localhost");
+      expect(conf.transport).toBe("ws");
+    } finally {
+      delete process.env.TPS_ROOT;
+    }
+  });
+
+  test("writeBranchConf omits agentId when not supplied (back-compat)", () => {
+    process.env.TPS_ROOT = tmpDir;
+    try {
+      writeBranchConf(6458, "tps-host", "ws");
+      const conf = JSON.parse(readFileSync(join(tmpDir, "branch.conf.json"), "utf-8"));
+      expect(conf.agentId).toBeUndefined();
+      expect(conf.port).toBe(6458);
+    } finally {
+      delete process.env.TPS_ROOT;
+    }
+  });
 });
 
 describe("MSG_JOIN_COMPLETE", () => {