diff --git a/README.md b/README.md
index 27598cc..df77aff 100644
--- a/README.md
+++ b/README.md
@@ -106,7 +106,7 @@ The .NET and Node builds are wire-compatible (same port, same `X-Mailbox` header
 
 ## Use from Claude Code (plugin)
 
-Easiest path — everything happens inside Claude Code:
+Easiest path — three prompts, all inside Claude Code:
 
 ```
 /plugin marketplace add https://git.kuns.dev/releases/ClaudeMailbox
@@ -114,11 +114,16 @@ Easiest path — everything happens inside Claude Code:
 /claude-mailbox:mailbox-doctor
 ```
 
-The doctor command checks for the `claude-mailbox` binary, installs it if missing (via npm), runs `install-autostart` if the daemon isn't registered, prompts you for a mailbox name and writes it to `.claude/settings.json`, and finishes with a self → self smoke test.
+The doctor command auto-installs the daemon binary via npm (asks first), registers autostart, optionally takes a base prefix (e.g. `backend`), and runs a smoke test. Subsequent slash commands:
 
-After that, unread messages appear in context before every prompt. If the daemon is unreachable later, the hook emits a one-line setup hint instead of staying silent — missing setup is loud, not invisible.
+- `/claude-mailbox:mailbox-status` — read-only health check
+- `/claude-mailbox:mailbox-update` — pull the latest daemon version and restart
 
-See [`plugin/README.md`](./plugin/README.md) for details, including why each Claude session needs its own mailbox name.
+**Each Claude session gets its own mailbox identity** derived from the session's UUID — `claude-a8b3c1d2` by default, or `<base>-a8b3c1d2` if you set a prefix. Parallel sessions in the same project automatically get distinct names. The `SessionStart` hook announces the current session's name in context so Claude knows its own identity. Peers discover each other via `mcp__mailbox__list_mailboxes`.
+
+If the daemon goes down later, the hook emits a one-line setup hint instead of staying silent.
+
+See [`plugin/README.md`](./plugin/README.md) for the full walkthrough.
 
 ## Use from a Claude session
 
diff --git a/node/src/cli.ts b/node/src/cli.ts
index f2135d6..0a6177f 100644
--- a/node/src/cli.ts
+++ b/node/src/cli.ts
@@ -10,8 +10,11 @@ import {
   applyInstall,
   applyUninstall,
   buildHookCommand,
+  deriveSessionName,
   formatMessagesForHook,
+  parseHookStdin,
   readSettings,
+  readStdinIfPiped,
   settingsPathFor,
   writeSettings,
   type HookMessage,
@@ -125,19 +128,36 @@ program
     }
   });
 
+function resolveHookMailboxName(explicit: string | undefined): string | null {
+  if (explicit && explicit.trim()) return explicit.trim();
+  const stdin = parseHookStdin(readStdinIfPiped());
+  const sid = stdin?.session_id?.trim();
+  if (sid) {
+    const base = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
+    return deriveSessionName(sid, base);
+  }
+  const envName = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim();
+  return envName || null;
+}
+
 program
   .command("check")
   .description(
-    "Pull pending messages and mark delivered. In --hook mode the name can come from CLAUDE_MAILBOX_NAME.",
+    "Pull pending messages and mark delivered. In --hook mode the name is auto-derived from the SessionStart/UserPromptSubmit stdin (session_id), optionally flavored by $CLAUDE_MAILBOX_NAME.",
+  )
+  .option(
+    "--name <name>",
+    "Explicit mailbox name. Overrides hook stdin and $CLAUDE_MAILBOX_NAME.",
   )
-  .option("--name <name>", "Mailbox name (also sent as X-Mailbox). Falls back to $CLAUDE_MAILBOX_NAME.")
   .option("--url <url>", "Daemon base URL", DEFAULT_URL)
   .option(
     "--hook",
-    "Hook mode: human-readable output, silent when no mailbox configured or daemon unreachable (exit 0). Emits a one-line setup hint when name is set but daemon is unreachable.",
+    "Hook mode: human-readable output, silent when no mailbox configured or daemon unreachable. Emits a one-line setup hint when name resolves but daemon is unreachable.",
   )
   .action(async (opts: { name?: string; url: string; hook?: boolean }) => {
-    const name = (opts.name ?? process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim();
+    const name = opts.hook
+      ? resolveHookMailboxName(opts.name)
+      : (opts.name ?? process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
     if (!name) {
       if (opts.hook) return;
       console.error("Missing --name (or set CLAUDE_MAILBOX_NAME).");
@@ -170,6 +190,23 @@ program
     }
   });
 
+program
+  .command("session-announce")
+  .description(
+    "SessionStart-hook helper: reads the hook stdin JSON, derives the session's mailbox name from session_id, and prints a one-line announcement to stdout so Claude sees its mailbox identity.",
+  )
+  .action(() => {
+    const stdin = parseHookStdin(readStdinIfPiped());
+    const sid = stdin?.session_id?.trim();
+    if (!sid) return;
+    const base = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
+    const name = deriveSessionName(sid, base);
+    process.stdout.write(
+      `Claude-Mailbox: this session is mailbox \`${name}\`. ` +
+        `Peers can send to it with: claude-mailbox send --from <peer> --to ${name} --body "..."\n`,
+    );
+  });
+
 program
   .command("list")
   .description("List known mailboxes.")
diff --git a/node/src/hook.ts b/node/src/hook.ts
index 7d3cc91..520cb1d 100644
--- a/node/src/hook.ts
+++ b/node/src/hook.ts
@@ -2,6 +2,45 @@ import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { homedir } from "node:os";
 import { dirname, join } from "node:path";
 
+export interface HookStdinPayload {
+  session_id?: string;
+  hook_event_name?: string;
+  [key: string]: unknown;
+}
+
+export function parseHookStdin(raw: string | null | undefined): HookStdinPayload | null {
+  if (!raw || !raw.trim()) return null;
+  try {
+    const parsed = JSON.parse(raw) as unknown;
+    if (parsed && typeof parsed === "object") return parsed as HookStdinPayload;
+    return null;
+  } catch {
+    return null;
+  }
+}
+
+export function readStdinIfPiped(): string | null {
+  if (process.stdin.isTTY) return null;
+  try {
+    return readFileSync(0, "utf8");
+  } catch {
+    return null;
+  }
+}
+
+export function shortSessionId(sessionId: string): string {
+  const hex = sessionId.replace(/[^0-9a-fA-F]/g, "").toLowerCase();
+  if (hex.length >= 8) return hex.slice(0, 8);
+  return sessionId.toLowerCase().replace(/[^a-z0-9]/g, "").slice(0, 8) || "00000000";
+}
+
+export function deriveSessionName(sessionId: string, base?: string | null): string {
+  const short = shortSessionId(sessionId);
+  const trimmed = (base ?? "").trim();
+  if (trimmed) return `${trimmed}-${short}`;
+  return `claude-${short}`;
+}
+
 export interface HookMessage {
   id: number;
   from: string;
diff --git a/node/tests/cli-hook.test.ts b/node/tests/cli-hook.test.ts
index 23ce365..e0f6f2e 100644
--- a/node/tests/cli-hook.test.ts
+++ b/node/tests/cli-hook.test.ts
@@ -5,14 +5,14 @@ import { resolve } from "node:path";
 
 const cliPath = resolve(__dirname, "..", "dist", "cli.js");
 
-function runCli(args: string[], env: Record<string, string | undefined> = {}): {
-  status: number;
-  stdout: string;
-  stderr: string;
-} {
+function runCli(
+  args: string[],
+  opts: { env?: Record<string, string | undefined>; stdin?: string } = {},
+): { status: number; stdout: string; stderr: string } {
   const r = spawnSync(process.execPath, [cliPath, ...args], {
     encoding: "utf8",
-    env: { ...process.env, ...env },
+    env: { ...process.env, ...(opts.env ?? {}) },
+    input: opts.stdin,
   });
   return {
     status: r.status ?? -1,
@@ -21,6 +21,13 @@ function runCli(args: string[], env: Record<string, string | undefined> = {}): {
   };
 }
 
+const HOOK_STDIN = JSON.stringify({
+  session_id: "abc12345-de67-89f0-1234-567890abcdef",
+  hook_event_name: "UserPromptSubmit",
+  cwd: "/tmp",
+  prompt: "test",
+});
+
 describe("`check --hook` CLI behavior", () => {
   beforeAll(() => {
     if (!existsSync(cliPath)) {
@@ -28,36 +35,87 @@ describe("`check --hook` CLI behavior", () => {
     }
   });
 
-  it("exits 0 silently when no name resolved (no --name, no env)", () => {
-    const r = runCli(["check", "--hook"], { CLAUDE_MAILBOX_NAME: undefined });
+  it("exits 0 silently when no stdin, no --name, no env", () => {
+    const r = runCli(["check", "--hook"], { env: { CLAUDE_MAILBOX_NAME: undefined } });
     expect(r.status).toBe(0);
     expect(r.stdout).toBe("");
     expect(r.stderr).toBe("");
   });
 
-  it("emits daemon-not-reachable hint when name is set but daemon is down", () => {
-    const r = runCli(
-      ["check", "--hook", "--url", "http://127.0.0.1:1"],
-      { CLAUDE_MAILBOX_NAME: "alice" },
-    );
+  it("derives session-id-based name from stdin and emits daemon hint when down", () => {
+    const r = runCli(["check", "--hook", "--url", "http://127.0.0.1:1"], {
+      env: { CLAUDE_MAILBOX_NAME: undefined },
+      stdin: HOOK_STDIN,
+    });
     expect(r.status).toBe(0);
     expect(r.stdout).toContain("[Claude-Mailbox] Daemon not reachable");
-    expect(r.stdout).toContain("http://127.0.0.1:1");
-    expect(r.stdout).toContain("claude-mailbox install-autostart");
   });
 
-  it("`--name` arg wins over CLAUDE_MAILBOX_NAME env (visible via hint URL/contents)", () => {
+  it("uses base prefix from CLAUDE_MAILBOX_NAME when both env and stdin present", () => {
+    // We can't directly assert the name from --hook output (it's only in the unreachable hint URL).
+    // The hint always contains the URL we passed, so this just confirms the path runs without error.
+    const r = runCli(["check", "--hook", "--url", "http://127.0.0.1:1"], {
+      env: { CLAUDE_MAILBOX_NAME: "backend" },
+      stdin: HOOK_STDIN,
+    });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("[Claude-Mailbox] Daemon not reachable");
+  });
+
+  it("explicit --name overrides session-id derivation", () => {
     const r = runCli(
-      ["check", "--hook", "--name", "bob", "--url", "http://127.0.0.1:1"],
-      { CLAUDE_MAILBOX_NAME: "alice" },
+      ["check", "--hook", "--name", "explicit", "--url", "http://127.0.0.1:1"],
+      { env: { CLAUDE_MAILBOX_NAME: "ignored" }, stdin: HOOK_STDIN },
     );
     expect(r.status).toBe(0);
     expect(r.stdout).toContain("[Claude-Mailbox] Daemon not reachable");
   });
 
   it("non-hook mode errors out when no name resolved", () => {
-    const r = runCli(["check"], { CLAUDE_MAILBOX_NAME: undefined });
+    const r = runCli(["check"], { env: { CLAUDE_MAILBOX_NAME: undefined } });
     expect(r.status).not.toBe(0);
     expect(r.stderr).toContain("CLAUDE_MAILBOX_NAME");
   });
 });
+
+describe("`session-announce` CLI behavior", () => {
+  beforeAll(() => {
+    if (!existsSync(cliPath)) {
+      throw new Error(`CLI not built. Run \`npm run build\` first. Missing: ${cliPath}`);
+    }
+  });
+
+  it("prints the derived mailbox name from a SessionStart payload", () => {
+    const r = runCli(["session-announce"], {
+      env: { CLAUDE_MAILBOX_NAME: undefined },
+      stdin: HOOK_STDIN,
+    });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("`claude-abc12345`");
+    expect(r.stdout).toContain("claude-mailbox send");
+  });
+
+  it("uses base prefix when set", () => {
+    const r = runCli(["session-announce"], {
+      env: { CLAUDE_MAILBOX_NAME: "backend" },
+      stdin: HOOK_STDIN,
+    });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("`backend-abc12345`");
+  });
+
+  it("stays silent when no session_id in stdin", () => {
+    const r = runCli(["session-announce"], {
+      env: { CLAUDE_MAILBOX_NAME: undefined },
+      stdin: JSON.stringify({ hook_event_name: "SessionStart" }),
+    });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toBe("");
+  });
+
+  it("stays silent when no stdin at all", () => {
+    const r = runCli(["session-announce"], { env: { CLAUDE_MAILBOX_NAME: undefined } });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toBe("");
+  });
+});
diff --git a/node/tests/hook.test.ts b/node/tests/hook.test.ts
index de46166..1ed6b97 100644
--- a/node/tests/hook.test.ts
+++ b/node/tests/hook.test.ts
@@ -6,8 +6,11 @@ import {
   applyInstall,
   applyUninstall,
   buildHookCommand,
+  deriveSessionName,
   formatMessagesForHook,
+  parseHookStdin,
   readSettings,
+  shortSessionId,
   writeSettings,
 } from "../src/hook.js";
 
@@ -169,6 +172,73 @@ describe("applyUninstall", () => {
   });
 });
 
+describe("parseHookStdin", () => {
+  it("returns null for empty or whitespace input", () => {
+    expect(parseHookStdin(null)).toBeNull();
+    expect(parseHookStdin("")).toBeNull();
+    expect(parseHookStdin("   \n  ")).toBeNull();
+  });
+
+  it("returns null for non-JSON input", () => {
+    expect(parseHookStdin("not json")).toBeNull();
+    expect(parseHookStdin("{")).toBeNull();
+  });
+
+  it("returns null for JSON primitives (only objects allowed)", () => {
+    expect(parseHookStdin("42")).toBeNull();
+    expect(parseHookStdin("\"foo\"")).toBeNull();
+    expect(parseHookStdin("null")).toBeNull();
+  });
+
+  it("parses a hook payload", () => {
+    const out = parseHookStdin(
+      JSON.stringify({
+        session_id: "abc12345-de67-89f0-1234-567890abcdef",
+        hook_event_name: "UserPromptSubmit",
+        prompt: "hi",
+      }),
+    );
+    expect(out?.session_id).toBe("abc12345-de67-89f0-1234-567890abcdef");
+    expect(out?.hook_event_name).toBe("UserPromptSubmit");
+  });
+});
+
+describe("shortSessionId / deriveSessionName", () => {
+  it("takes first 8 hex chars from a UUID", () => {
+    expect(shortSessionId("abc12345-de67-89f0-1234-567890abcdef")).toBe("abc12345");
+  });
+
+  it("normalizes case and ignores hyphens", () => {
+    expect(shortSessionId("ABC12345-DE67-89F0-1234-567890ABCDEF")).toBe("abc12345");
+  });
+
+  it("falls back to a sanitized prefix for non-hex ids", () => {
+    expect(shortSessionId("session-Test123")).toBe("sessiont");
+  });
+
+  it("derives anonymous name when no base", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef")).toBe("claude-abc12345");
+  });
+
+  it("prepends base prefix when given", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "backend")).toBe(
+      "backend-abc12345",
+    );
+  });
+
+  it("treats whitespace-only base as no base", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "  ")).toBe(
+      "claude-abc12345",
+    );
+  });
+
+  it("derives different names for different sessions with the same base", () => {
+    const a = deriveSessionName("aaaa1111-de67-89f0-1234-567890abcdef", "shared");
+    const b = deriveSessionName("bbbb2222-de67-89f0-1234-567890abcdef", "shared");
+    expect(a).not.toBe(b);
+  });
+});
+
 describe("readSettings / writeSettings roundtrip", () => {
   it("survives an install → write → read cycle", () => {
     const dir = mkdtempSync(join(tmpdir(), "claude-mailbox-hook-"));
diff --git a/plugin/README.md b/plugin/README.md
index 8f2e6a1..901c57d 100644
--- a/plugin/README.md
+++ b/plugin/README.md
@@ -1,8 +1,8 @@
 # claude-mailbox plugin
 
-Lets Claude Code pull unread messages from a local `claude-mailbox` daemon before every prompt and inject them into the conversation context.
+Lets Claude Code pull unread messages from a local `claude-mailbox` daemon before every prompt and inject them into the conversation context. Each Claude session gets a **unique mailbox identity** auto-derived from its session id, so two sessions in the same project never collide.
 
-## Setup (two steps, all inside Claude Code)
+## Setup (three prompts, all inside Claude Code)
 
 ```
 /plugin marketplace add https://git.kuns.dev/releases/ClaudeMailbox
@@ -10,50 +10,59 @@ Lets Claude Code pull unread messages from a local `claude-mailbox` daemon befor
 /claude-mailbox:mailbox-doctor
 ```
 
-The doctor command walks the rest:
+The doctor walks the rest:
 
-1. checks whether the `claude-mailbox` binary is on `PATH` — installs it (`npm install -g @kuns/claude-mailbox`) if missing, asks before doing anything that might need elevation
-2. checks the daemon status — runs `install-autostart` and/or `start` until it reports `Running`
-3. ensures `CLAUDE_MAILBOX_NAME` is set in `.claude/settings.json` env — prompts for a name if not, writes it idempotently
-4. runs a self → self smoke test to verify the round-trip works
+1. installs the `claude-mailbox` binary via `npm install -g @kuns/claude-mailbox` if missing (asks first)
+2. registers the daemon for autostart and starts it if needed
+3. health-probes `http://127.0.0.1:47822/health`
+4. optionally lets you set a **base prefix** (e.g., `backend`) — without one, mailbox names are anonymous (`claude-XXXXXXXX`)
+5. runs a self → self smoke test
 
-Restart Claude Code after the doctor finishes (only needed if the mailbox name was newly written). Unread messages will then appear in context before every prompt.
+Restart Claude Code only if step 4 wrote a new prefix. After that, every prompt auto-pulls unread messages.
 
-## Why a mailbox name?
+## Mailbox identity (the important bit)
 
-Each Claude session has an identity used to address peer sessions — like an email address. If you run a `backend` session and a `frontend` session in parallel, they need different names so they can send messages to each other.
+Each Claude Code session gets its own mailbox name, derived from the session's UUID:
 
-For a single Claude Code instance just wanting notifications, any stable kebab-case name works. The name lives in **per-project** `.claude/settings.json` env, so different worktrees / projects automatically get different mailboxes.
+| Configuration | Resulting mailbox name |
+|---|---|
+| No `CLAUDE_MAILBOX_NAME` set | `claude-a8b3c1d2` (first 8 hex chars of session_id) |
+| `CLAUDE_MAILBOX_NAME=backend` in `.claude/settings.json` env | `backend-a8b3c1d2` |
 
-## What the hook actually does
+So if you open two Claude Code sessions in the same project, they'll be e.g. `backend-a8b3c1d2` and `backend-d4e5f6a7` — distinct, addressable, no manual setup.
 
-Before every prompt the plugin runs `claude-mailbox check --hook`, which:
+The `SessionStart` hook announces the current session's mailbox name in the conversation context on startup. Peers discover each other via `claude-mailbox list` or the `mcp__mailbox__list_mailboxes` MCP tool.
 
-- prints unread mailbox messages in a Claude-friendly format and marks them delivered,
-- stays **silent** when the inbox is empty or `CLAUDE_MAILBOX_NAME` is not set,
-- emits a one-line setup hint when the daemon is unreachable, so a missing daemon is loud, not invisible.
+## What the hooks do
 
-Cost: one local HTTP round-trip plus Node coldstart per prompt (~100ms on Windows).
+| Hook | Command | Effect |
+|---|---|---|
+| `SessionStart` | `claude-mailbox session-announce` | Prints `"Claude-Mailbox: this session is mailbox \`X\`"` so Claude knows its own identity. |
+| `UserPromptSubmit` | `claude-mailbox check --hook` | Pulls unread messages for the session's mailbox and injects them as context. Silent on empty inbox; emits a one-line setup hint when the daemon is unreachable. |
 
-## Commands
+Cost: one local HTTP round-trip per prompt + Node coldstart (~100ms on Windows).
+
+## Slash commands
 
 | Command | What it does |
 |---|---|
 | `/claude-mailbox:mailbox-doctor` | Diagnose + auto-fix the full setup. |
 | `/claude-mailbox:mailbox-status` | Read-only health check. No changes. |
+| `/claude-mailbox:mailbox-update` | Update the daemon to the latest npm version and restart it. |
 
-## Smoke test (manually, after doctor finishes)
+## Sending a message to a peer session
+
+From inside Claude Code, use the MCP tool (the daemon already exposes `mcp__mailbox__*`). From any shell:
 
 ```sh
-claude-mailbox send --from probe --to <your-mailbox-name> --body "hello"
+claude-mailbox list                                  # find the recipient's mailbox name
+claude-mailbox send --from probe --to backend-a8b3c1d2 --body "hi"
 ```
 
-Then start a new Claude Code prompt — the message should appear in context before Claude's first reply.
-
 ## Uninstall
 
 ```
 /plugin uninstall claude-mailbox@claude-mailbox
 npm uninstall -g @kuns/claude-mailbox
-claude-mailbox uninstall-autostart    # if you used it
+claude-mailbox uninstall-autostart    # if you registered it
 ```
diff --git a/plugin/commands/mailbox-doctor.md b/plugin/commands/mailbox-doctor.md
index 78c9edc..c84fdbc 100644
--- a/plugin/commands/mailbox-doctor.md
+++ b/plugin/commands/mailbox-doctor.md
@@ -1,85 +1,92 @@
 ---
-description: Diagnose and auto-fix the Claude-Mailbox setup (binary install, daemon autostart, mailbox name, smoke test).
+description: Diagnose and auto-fix the Claude-Mailbox setup (binary install, daemon autostart, smoke test, optional base-prefix).
 allowed-tools: Bash, Read, Edit, Write
 ---
 
-You are running the **Claude-Mailbox doctor**. Walk through these checks in order. After each check, print a one-line status (✓ / ✗) and the action you took. At the very end, print a final summary block.
+You are running the **Claude-Mailbox doctor**. Walk through these checks in order. After each step, print a one-line `✓` / `✗` with the action you took. End with a summary block.
 
-Throughout, prefer the dedicated tools (`Read`, `Edit`, `Write`) for files. Use `Bash` only for `claude-mailbox` subcommands, `npm`, `where`/`which`, and `cat /etc/os-release` style lookups. Never run `sudo` automatically — if elevation is needed, stop and ask the user how to proceed.
-
----
+Use `Bash` only for `claude-mailbox` subcommands, `npm`, `where`/`which`, and HTTP probes. Use `Read`/`Edit`/`Write` for `.claude/settings.json`. Never run `sudo` automatically — if elevation is needed, stop and ask.
 
 ## Step 1 — daemon binary on PATH
 
 Run: `claude-mailbox --version`
 
-- **Exit 0** → binary present. Record the version string. ✓ continue.
-- **Command not found / non-zero exit** → binary missing. Tell the user the install command for their platform and ask before running it:
+- **Exit 0** → ✓ record the version. Continue.
+- **Command not found** → binary missing. Install path:
 
-  | Platform | Install command |
+  | Platform | Command |
   |---|---|
   | Windows | `npm install -g @kuns/claude-mailbox` (no admin) |
-  | macOS / Linux | `npm install -g @kuns/claude-mailbox` (may need `sudo` depending on Node setup; ask the user if `npm install` fails with EACCES) |
+  | macOS / Linux | `npm install -g @kuns/claude-mailbox` (may fail with EACCES — never run sudo automatically; ask the user) |
 
-  Prerequisite: `npm config get @kuns:registry` should return `https://git.kuns.dev/api/packages/releases/npm/`. If it doesn't, set it first:
+  Prerequisite: `npm config get @kuns:registry` must point at `https://git.kuns.dev/api/packages/releases/npm/`. If not:
 
   ```
   npm config set @kuns:registry=https://git.kuns.dev/api/packages/releases/npm/
   ```
 
-  After install, re-run `claude-mailbox --version`. If it still fails, stop and report the error.
+  After install, re-run `claude-mailbox --version`. If it still fails, stop and report.
 
 ## Step 2 — daemon autostart and running state
 
 Run: `claude-mailbox status`
 
-- **`Running`** → ✓ continue.
-- **`Stopped`** → run `claude-mailbox start`. Re-check status.
-- **`NotInstalled`** → run `claude-mailbox install-autostart`, then `claude-mailbox start`. Re-check status.
+- `Running` → ✓ continue.
+- `Stopped` → `claude-mailbox start`, re-check.
+- `NotInstalled` → `claude-mailbox install-autostart`, then `claude-mailbox start`, re-check.
 
-If status doesn't become `Running` after the fix, stop and report what `status` and `start` printed.
+If status doesn't reach `Running`, stop and report.
 
-Sanity check: `curl -sf http://127.0.0.1:47822/health` (or use Bash to fetch it). Expect a JSON body with `"status":"ok"`.
+## Step 3 — health probe
 
-## Step 3 — mailbox name in project settings
+Hit `http://127.0.0.1:47822/health`. Expect a JSON body with `"status":"ok"`. If unreachable, stop and report — the daemon claims it's running but isn't accepting connections.
 
-The hook reads the mailbox name from `$CLAUDE_MAILBOX_NAME`. Claude Code injects env vars from `.claude/settings.json` into hook commands, so the cleanest place to set it is per-project.
+## Step 4 — mailbox identity
 
-1. Read `.claude/settings.json` in the current working directory (it may not exist yet — that's fine).
-2. Check if `env.CLAUDE_MAILBOX_NAME` is set.
-3. If **set**: ✓ continue, record the name.
-4. If **not set**:
-   - Ask the user for a mailbox name. Suggest a default based on the cwd basename (e.g., for `C:\Private\Claude-Mailbox` suggest `claude-mailbox`). Names should be short, kebab-case-ish, unique among parallel Claude sessions.
-   - Read existing `.claude/settings.json` if present, otherwise start with `{}`.
-   - Set/merge `env.CLAUDE_MAILBOX_NAME` to the chosen name. Preserve any other existing settings.
-   - Write back with 2-space indentation.
-   - Tell the user they need to **restart this Claude Code session** for the env to take effect in the hook — but the smoke test below can still run because we'll pass `--name` explicitly.
+**No prompt by default.** Each Claude Code session now gets a unique mailbox name auto-derived from its `session_id` (e.g., `claude-a8b3c1d2`), so two parallel sessions can never collide.
 
-## Step 4 — smoke test
+Read `.claude/settings.json` in the current working directory and look for `env.CLAUDE_MAILBOX_NAME`.
 
-Use the resolved name from step 3 (either pre-existing or just chosen). Run:
+- If set → ✓ this is a **base prefix**. The real name will be `<base>-<short_session_id>`. Tell the user "Mailbox prefix is set to `X`."
+- If unset → ✓ tell the user "Mailbox name will be auto-derived (`claude-<short_session_id>`)."
+
+Then **ask** the user (one question, not a deep prompt):
+
+> "Want to flavor your mailbox names with a memorable prefix like `backend` or `frontend`? (yes / no / change to `<x>`)"
+
+If they say yes or give a value:
+1. Read `.claude/settings.json` (empty `{}` if missing).
+2. Merge `env.CLAUDE_MAILBOX_NAME` = chosen value, preserving anything else.
+3. Write back with 2-space indentation.
+4. Mark this as `restart_needed = true`.
+
+If they say no or skip → leave as-is.
+
+## Step 5 — smoke test
+
+Use two ephemeral names (`doctor-probe-a` / `doctor-probe-b`) — we don't need the real session name here, we just need to prove the daemon round-trips:
 
 ```
-claude-mailbox send  --from doctor-probe --to <name> --body "ping from /claude-mailbox:mailbox-doctor"
-claude-mailbox check --name <name>
+claude-mailbox send  --from doctor-probe-a --to doctor-probe-b --body "ping from doctor"
+claude-mailbox check --name doctor-probe-b
 ```
 
-- The `check` output should be a JSON array containing exactly one message with `"from": "doctor-probe"` and that body.
-- If yes: ✓ smoke test passed.
-- If no (empty array, error, or wrong message): ✗ report what was returned.
+The `check` output must be a JSON array with one message: `from: doctor-probe-a`, body matches. If yes ✓. If no ✗ and report what came back.
 
-## Step 5 — final summary
-
-Print a compact block with these fields, one per line:
+## Step 6 — summary
 
 ```
 Claude-Mailbox doctor
-  binary:       <version>
-  daemon:       Running | Stopped | NotInstalled  (and what you did)
-  health:       ok | unreachable
-  mailbox name: <name>  (source: existing | newly written to .claude/settings.json)
-  smoke test:   passed | failed
-  restart hint: <yes if name was newly written, otherwise no>
+  binary:        <version>
+  daemon:        Running    (and what you did, if anything)
+  health:        ok
+  base prefix:   <name from settings, or "auto-derived (anonymous)">
+  smoke test:    passed | failed
+  restart hint:  yes if restart_needed, otherwise no
 ```
 
-If everything is ✓ and `restart hint: yes`, end with: "Restart Claude Code (or open a new session) so the UserPromptSubmit hook picks up `CLAUDE_MAILBOX_NAME`." If `restart hint: no`, end with: "You're good to go — unread messages will appear before your next prompt."
+End with one of:
+
+- All ✓ and no restart needed → "Setup is healthy. Your mailbox name this session will be revealed by the SessionStart hook on next session start (or run `claude-mailbox list` to see active mailboxes)."
+- All ✓ and restart needed → "Restart Claude Code (or open a new session) so the SessionStart hook picks up the new base prefix."
+- Anything ✗ → "Setup incomplete: <first failure>."
diff --git a/plugin/commands/mailbox-update.md b/plugin/commands/mailbox-update.md
new file mode 100644
index 0000000..f4f22cc
--- /dev/null
+++ b/plugin/commands/mailbox-update.md
@@ -0,0 +1,56 @@
+---
+description: Update the Claude-Mailbox daemon to the latest published npm version and restart it.
+allowed-tools: Bash
+---
+
+You are running the **Claude-Mailbox update** command. Update the `@kuns/claude-mailbox` npm package and restart the daemon. Never run `sudo` automatically — if elevation is needed, stop and ask.
+
+## Step 1 — current version
+
+Run: `claude-mailbox --version`
+
+- Exit 0 → record the version string as `CURRENT`.
+- Non-zero → tell the user the daemon binary is not installed yet. Suggest `/claude-mailbox:mailbox-doctor` to do a full setup and stop.
+
+## Step 2 — latest published version
+
+Run: `npm view @kuns/claude-mailbox version`
+
+If the npm registry config is missing, the call may fail with a 404. Fall back to:
+
+```
+npm view --registry=https://git.kuns.dev/api/packages/releases/npm/ @kuns/claude-mailbox version
+```
+
+Record the result as `LATEST`.
+
+## Step 3 — compare
+
+- If `CURRENT === LATEST`: print "Already up to date (vX.Y.Z)." and stop. Do not run any further steps.
+- Otherwise: tell the user `CURRENT` → `LATEST` and ask for confirmation before proceeding.
+
+## Step 4 — perform the update
+
+On user confirmation, run these in order. Stop on the first failure and report it:
+
+1. `claude-mailbox stop`
+2. `npm install -g @kuns/claude-mailbox@latest`
+   - On Linux/macOS this may fail with EACCES. **Do not run sudo automatically.** Ask the user how they want to proceed (e.g., `sudo npm install -g …`, or switch to a user-scoped Node setup with nvm/fnm).
+3. `claude-mailbox start`
+4. `claude-mailbox --version` to verify the upgrade landed.
+5. `claude-mailbox status` to verify the daemon is `Running`.
+
+## Step 5 — summary
+
+Print exactly this block:
+
+```
+Claude-Mailbox update
+  previous version: <CURRENT>
+  new version:      <whatever --version now reports>
+  daemon:           Running | Stopped | NotInstalled
+  pending messages survived: <count of messages still in inbox via `claude-mailbox list`, if applicable>
+```
+
+If the new version matches `LATEST` and daemon is `Running`, end with: "Update complete."
+Otherwise, end with the first thing that went wrong.
diff --git a/plugin/hooks/hooks.json b/plugin/hooks/hooks.json
index 68c2d1d..45fbcdb 100644
--- a/plugin/hooks/hooks.json
+++ b/plugin/hooks/hooks.json
@@ -1,5 +1,15 @@
 {
   "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "claude-mailbox session-announce"
+          }
+        ]
+      }
+    ],
     "UserPromptSubmit": [
       {
         "hooks": [