diff --git a/public/fanout.ts b/public/fanout.ts
new file mode 100644
index 0000000..0a1aa6f
--- /dev/null
+++ b/public/fanout.ts
@@ -0,0 +1,74 @@
+/**
+ * public/fanout.ts (W5 fan-out board) — PURE launch-command + branch-slug helpers.
+ *
+ * No DOM, no fetch, no side effects — every export is unit-testable in isolation.
+ * The load-bearing invariant is the prompt shell-quoting (shellSingleQuote): the
+ * prompt is typed as raw bytes into a lane's shell BEFORE `claude` parses it, so
+ * it must be wrapped as a single literal argv element — no `$(...)`, backtick,
+ * `;`, `&&`, or redirection may execute. NEVER String-concat the prompt into the
+ * command unquoted (see the plan's Security section).
+ */
+
+import type { PermissionMode } from '../src/types.js'
+
+/** Max prompt length typed into a lane (bounded DoS + keeps the command sane). */
+export const FANOUT_PROMPT_MAX = 4000
+
+/** Default/max fan-out lanes (mirrors the server MAX_FANOUT_LANES default and the
+ *  grid-6 board capacity). The FE stepper max and the launch clamp both use it. */
+export const FANOUT_MAX_LANES = 6
+
+/**
+ * POSIX single-quote a string so the shell passes it verbatim as ONE argv element.
+ * Wrap in single quotes and replace every embedded `'` with the classic
+ * `'\''` sequence (close-quote, escaped-quote, re-open-quote). Pure.
+ */
+export function shellSingleQuote(s: string): string {
+  return `'${s.replace(/'/g, `'\\''`)}'`
+}
+
+/**
+ * Normalize a prompt for typing into a PTY: collapse every line break (`\r`, `\n`,
+ * `\r\n`) to a single space — a bare newline keystroke would submit a partial line
+ * before `claude` starts — then trim and cap at FANOUT_PROMPT_MAX. Pure.
+ */
+export function sanitizePrompt(s: string): string {
+  return s.replace(/[\r\n]+/g, ' ').trim().slice(0, FANOUT_PROMPT_MAX)
+}
+
+/**
+ * Build the `claude` launch command for one lane. Single-quotes the sanitized
+ * prompt (injection-safe), prefixes `--permission-mode <m>` for non-default modes,
+ * and ends with `\r` so the shell auto-executes it. 'auto' is downgraded to
+ * 'default' when the server forbids it (SEC-M5 parity with resolveMode). Pure.
+ */
+export function buildFanoutCmd(
+  prompt: string,
+  mode: PermissionMode,
+  allowAutoMode: boolean,
+): string {
+  const effective: PermissionMode = mode === 'auto' && !allowAutoMode ? 'default' : mode
+  const flag = effective === 'default' ? '' : `--permission-mode ${effective} `
+  return `claude ${flag}${shellSingleQuote(sanitizePrompt(prompt))}\r`
+}
+
+/** Lane branch name: `${base}-lane-${i}` (i is 1-based). Pure. */
+export function laneBranch(base: string, i: number): string {
+  return `${base}-lane-${i}`
+}
+
+/**
+ * Derive a safe git-branch-base slug from a free-text prompt: lowercase, non-
+ * alphanumeric runs → single `-`, strip leading/trailing dashes, cap at 40 chars.
+ * Falls back to 'fanout' when nothing survives. The result is a valid branch base
+ * (passes validateBranchNameClient). Pure.
+ */
+export function slugify(prompt: string): string {
+  const slug = prompt
+    .toLowerCase()
+    .replace(/[^a-z0-9]+/g, '-')
+    .replace(/^-+|-+$/g, '')
+    .slice(0, 40)
+    .replace(/-+$/g, '')
+  return slug || 'fanout'
+}
diff --git a/public/projects.ts b/public/projects.ts
index 6e17f6d..5cc21d8 100644
Binary files a/public/projects.ts and b/public/projects.ts differ
diff --git a/public/style.css b/public/style.css
index 695cc6d..8c2fa04 100644
--- a/public/style.css
+++ b/public/style.css
@@ -456,6 +456,20 @@ body {
   color: var(--text);
   background: var(--surface-3);
 }
+.cell-keep {
+  border: 0;
+  background: transparent;
+  color: var(--text-faint);
+  cursor: pointer;
+  font-size: 13px;
+  line-height: 1;
+  padding: 2px 4px;
+  border-radius: 5px;
+}
+.cell-keep:hover {
+  color: var(--text);
+  background: var(--surface-3);
+}
 .cell-monitor-btn {
   border: 0;
   background: transparent;
@@ -1944,6 +1958,60 @@ body {
   text-transform: uppercase;
   color: var(--text-faint);
 }
+/* W5 fan-out form — sibling of the New Worktree form. */
+.proj-fanout-form {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+.proj-fanout-prompt {
+  width: 100%;
+  box-sizing: border-box;
+  resize: vertical;
+  min-height: 44px;
+  font: inherit;
+}
+.proj-fanout-row {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+  flex-wrap: wrap;
+}
+.proj-fanout-lanes {
+  width: 72px;
+}
+.proj-fanout-branch {
+  flex: 1 1 160px;
+  min-width: 120px;
+}
+.proj-fanout-error {
+  color: var(--danger, #e5534b);
+  font-size: 12px;
+}
+.proj-fanout-submit {
+  align-self: flex-start;
+}
+.proj-fanout-submit:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+/* W5 fan-out status/error banner — fixed, dismissible, textContent only. */
+.fanout-banner {
+  position: fixed;
+  left: 50%;
+  bottom: 16px;
+  transform: translateX(-50%);
+  max-width: min(680px, 92vw);
+  z-index: 60;
+  padding: 10px 14px;
+  border-radius: 8px;
+  background: var(--surface-3, #2a2c33);
+  color: var(--text, #e7e8ec);
+  border: 1px solid var(--border, #3a3d46);
+  box-shadow: 0 6px 24px rgba(0, 0, 0, 0.35);
+  font-size: 13px;
+  cursor: pointer;
+}
 /* Section header with an inline action (CLAUDE.md generate/update). */
 .proj-section-row {
   display: flex;
diff --git a/public/tabs.ts b/public/tabs.ts
index 2c0031e..25a3180 100644
--- a/public/tabs.ts
+++ b/public/tabs.ts
@@ -25,8 +25,21 @@ import { TerminalSession } from './terminal-session.js'
 import { ICON_TIMELINE } from './icons.js'
 import { THEMES, DEFAULT_SETTINGS, type Settings } from './settings.js'
 import { mountLauncher, type Launcher } from './launcher.js'
-import { mountProjects, type ProjectsPanel } from './projects.js'
-import type { ApprovalPreview, ClaudeStatus, PermissionMode, UiConfig } from '../src/types.js'
+import {
+  mountProjects,
+  createWorktreeReq,
+  removeWorktreeReq,
+  validateBranchNameClient,
+  type ProjectsPanel,
+} from './projects.js'
+import { buildFanoutCmd, laneBranch, FANOUT_MAX_LANES } from './fanout.js'
+import type {
+  ApprovalPreview,
+  ClaudeStatus,
+  FanoutLaunchOpts,
+  PermissionMode,
+  UiConfig,
+} from '../src/types.js'
 import { renderDiffFile } from './diff.js'
 import { renderTelemetryGauge } from './preview-grid.js'
 import { mountPushToggle } from './push.js'
@@ -72,12 +85,35 @@ const ALL_PERMISSION_MODES: readonly PermissionMode[] = ['default', 'acceptEdits
 
 type HomeView = 'sessions' | 'projects'
 
+/** W5: one lane of a fan-out group — its worktree + branch (the session that runs
+ *  in it is tracked via the tagged TabEntry, matched by TabEntry.fanoutGroupId). */
+interface FanoutLane {
+  branch: string
+  worktreePath: string
+}
+
+/** W5: an in-memory fan-out group — one prompt fanned across N lanes of one repo.
+ *  Lives only for the session (v1): reload does not reconstruct the board (matches
+ *  v0.5 "no auto-restore"); keepFanoutWinner operates on the currently-open lanes. */
+interface FanoutGroup {
+  id: string
+  repoPath: string
+  repoName: string
+  prompt: string
+  lanes: FanoutLane[]
+}
+
 interface TabEntry {
   session: TerminalSession
   customTitle: string | null // user-set; null = use auto/fallback (persisted)
   autoTitle: string | null // current folder from the terminal title
   hasActivity: boolean // inactive tab got output since last viewed
   el: HTMLDivElement | null // the .tab element (updated in place)
+  // W5: when set, this tab is a lane of a fan-out group (id) with its own worktree
+  // + branch; the cell gets a 🏆 Keep button that resolves the race for this lane.
+  fanoutGroupId?: string
+  worktreePath?: string
+  branch?: string
   // Split-grid: the .term-cell wrapper around session.el (header + terminal +
   // optional inline-approve footer). One per tab; the grid lays these out.
   cell: HTMLDivElement | null
@@ -144,6 +180,12 @@ export class TabApp {
   // W3(b): the server COST_BUDGET_USD (from /config/ui); 0 = disabled. The per-tab
   // gauge warn-styles the cost chip once costUsd >= this budget.
   private costBudgetUsd = 0
+  // W5 fan-out board: the server MAX_FANOUT_LANES (from /config/ui) — the hard cap
+  // launchFanout clamps to (alongside grid-6 capacity + the server DoS cap).
+  private maxFanoutLanes = FANOUT_MAX_LANES
+  // W5: in-memory fan-out group model (session-scoped; not persisted in v1).
+  private readonly fanoutGroups = new Map<string, FanoutGroup>()
+  private fanoutBanner: HTMLElement | null = null
   private pushHost!: HTMLElement // A1: 🔔 host, mounted once, re-parented per rebuild
   private timelinePanel!: HTMLElement // A4: shared timeline panel (one mounted at a time)
   private timelineOpen = false
@@ -180,6 +222,7 @@ export class TabApp {
     this.projects = mountProjects(this.paneHost, {
       onOpenProject: (repoPath, repoName, cmd) => this.openProject(repoPath, repoName, cmd),
       onEnterSession: (id) => this.openSession(id),
+      onFanout: (repoPath, repoName, opts) => void this.launchFanout(repoPath, repoName, opts),
     })
 
     this.segControl = this.buildSegControl()
@@ -195,6 +238,7 @@ export class TabApp {
     this.setupQuickReply() // A3 chips above the key bar
     this.setupTimelinePanel() // A4 activity timeline
     this.setupVoiceOverlay() // A2 interim-transcript overlay
+    this.setupFanoutBanner() // W5 fan-out status/error banner
     void this.loadUiConfig() // B4 allowAutoMode gate (best-effort)
 
     // v0.5: do NOT auto-create or auto-restore tabs. Land on the home screen;
@@ -264,6 +308,35 @@ export class TabApp {
     this.voiceConfirmOverlay = confirmOverlay
   }
 
+  /** W5: a dismissible banner for fan-out status/errors (partial-launch, keep-
+   *  winner failures). textContent only (SEC-L3/H6); tap or auto-hide to clear. */
+  private setupFanoutBanner(): void {
+    const banner = document.createElement('div')
+    banner.id = 'fanout-banner'
+    banner.className = 'fanout-banner'
+    banner.style.display = 'none'
+    banner.addEventListener('click', () => {
+      banner.style.display = 'none'
+    })
+    document.body.appendChild(banner)
+    this.fanoutBanner = banner
+  }
+
+  private fanoutBannerTimer: ReturnType<typeof setTimeout> | null = null
+
+  /** Show a fan-out banner message (textContent only). Auto-hides after 8s. */
+  private showFanoutBanner(msg: string): void {
+    const banner = this.fanoutBanner
+    if (!banner) return
+    banner.textContent = msg // SEC-L3/H6: textContent, never innerHTML
+    banner.style.display = 'block'
+    if (this.fanoutBannerTimer !== null) clearTimeout(this.fanoutBannerTimer)
+    this.fanoutBannerTimer = setTimeout(() => {
+      banner.style.display = 'none'
+      this.fanoutBannerTimer = null
+    }, 8000)
+  }
+
   /** B4: fetch the server UI config (allowAutoMode). Best-effort, never throws. */
   private async loadUiConfig(): Promise<void> {
     try {
@@ -283,6 +356,11 @@ export class TabApp {
       if (typeof budget === 'number' && Number.isFinite(budget) && budget > 0) {
         this.costBudgetUsd = budget
       }
+      // W5: read the fan-out lane cap (optional; older servers omit → keep default).
+      const lanes = (data as Record<string, unknown>)?.['maxFanoutLanes']
+      if (typeof lanes === 'number' && Number.isFinite(lanes) && lanes >= 2) {
+        this.maxFanoutLanes = Math.floor(lanes)
+      }
     } catch {
       // best-effort — leave allowAutoMode false (auto hidden) on any failure
     }
@@ -798,7 +876,23 @@ export class TabApp {
       e.stopPropagation()
       this.toggleMonitor(this.tabs.indexOf(entry))
     })
-    head.append(monBtn, maxBtn)
+    // W5: 🏆 keep-winner — only shown (via renderCell) when this tab is a fan-out
+    // lane. Resolves the race: keep this lane, discard the others' worktrees.
+    const keepBtn = document.createElement('button')
+    keepBtn.type = 'button'
+    keepBtn.className = 'cell-keep'
+    keepBtn.textContent = '🏆'
+    keepBtn.title = 'Keep this lane, discard the others'
+    keepBtn.setAttribute('aria-label', 'Keep this fan-out lane as the winner')
+    keepBtn.style.display = 'none'
+    keepBtn.addEventListener('pointerdown', (e) => e.stopPropagation())
+    keepBtn.addEventListener('click', (e) => {
+      e.stopPropagation()
+      if (entry.fanoutGroupId !== undefined) {
+        void this.keepFanoutWinner(entry.fanoutGroupId, entry.session.id ?? '')
+      }
+    })
+    head.append(monBtn, keepBtn, maxBtn)
     // v2: drag a tab from the tab bar onto this quadrant to assign it here.
     this.wireCellDropTarget(cell, () => this.tabs.indexOf(entry))
     cell.append(head, session.el)
@@ -852,6 +946,134 @@ export class TabApp {
     ).length
   }
 
+  /* ── W5 fan-out board ────────────────────────────────────────────────── */
+
+  /**
+   * Fan ONE prompt across N lanes of `repoPath`: create N worktrees SEQUENTIALLY
+   * (git's `worktree add` takes a repo lock — parallel adds race), then open one
+   * Claude session per created lane pre-injected with the same shell-quoted prompt,
+   * and lay them out on the split-grid board. Composition of shipped parts —
+   * createWorktreeReq + addEntry (openProject shape) + setGridLayout. Partial
+   * success is surfaced in the banner (no auto-rollback in v1); never throws.
+   */
+  async launchFanout(repoPath: string, repoName: string, opts: FanoutLaunchOpts): Promise<void> {
+    const base = opts.branchBase.trim()
+    const branchErr = validateBranchNameClient(base)
+    if (branchErr !== null) {
+      this.showFanoutBanner(`Cannot fan out — ${branchErr}.`)
+      return
+    }
+    // Effective N: min(requested, server cap, grid-6 capacity). The remaining
+    // maxSessions − liveCount bound is enforced server-side (assertUnderSessionCap
+    // throws → that lane shows exit(-1); reported as "started K of N" below).
+    const n = Math.min(
+      Math.max(2, Math.floor(opts.lanes)),
+      this.maxFanoutLanes,
+      FANOUT_MAX_LANES,
+    )
+    // Build the launch command ONCE — every lane runs the identical prompt.
+    const cmd = buildFanoutCmd(opts.prompt, this.resolveMode(opts.mode ?? 'default'), this.allowAutoMode)
+
+    const groupId = `fanout-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`
+    const lanes: FanoutLane[] = []
+    const failures: string[] = []
+    for (let i = 1; i <= n; i++) {
+      const branch = laneBranch(base, i)
+      const r = await createWorktreeReq(repoPath, branch) // sequential (repo lock)
+      if (r.ok && typeof r.path === 'string') {
+        lanes.push({ branch, worktreePath: r.path })
+      } else {
+        failures.push(`${branch}: ${r.error ?? 'failed'}`)
+      }
+    }
+
+    if (lanes.length === 0) {
+      this.showFanoutBanner(`Fan-out failed — no lanes created. ${failures.join('; ')}`)
+      return
+    }
+
+    const group: FanoutGroup = { id: groupId, repoPath, repoName, prompt: opts.prompt, lanes }
+    this.fanoutGroups.set(groupId, group)
+
+    const created: TabEntry[] = []
+    for (const lane of lanes) {
+      const entry = this.addEntry(null, `${repoName}·${lane.branch}`, lane.worktreePath || undefined, cmd)
+      entry.fanoutGroupId = groupId
+      entry.worktreePath = lane.worktreePath
+      entry.branch = lane.branch
+      created.push(entry)
+    }
+    this.persist()
+    this.rebuild()
+    // Watch board: 2×2 up to 4 lanes, else 2×3 (grid-6 holds up to 6).
+    this.setGridLayout(lanes.length <= 4 ? 'grid-4' : 'grid-6')
+    const first = created[0]
+    if (first) this.activate(this.tabs.indexOf(first))
+
+    if (failures.length > 0) {
+      this.showFanoutBanner(`Started ${lanes.length} of ${n} lanes. Failed: ${failures.join('; ')}`)
+    }
+  }
+
+  /**
+   * Keep the winning lane and discard the others: a single batch confirm, then
+   * each losing lane's tab is closed (detach — its PTY is reclaimed by IDLE_TTL)
+   * and its worktree removed (auto-forcing on a 409 dirty tree, inside the one
+   * confirm — no per-lane prompt). The winner's tab + worktree stay so the user
+   * runs the merge inside it (one-click merge is out of v1 scope). Never throws.
+   */
+  async keepFanoutWinner(groupId: string, winnerSessionId: string): Promise<void> {
+    const group = this.fanoutGroups.get(groupId)
+    if (!group) return
+    const laneTabs = this.tabs.filter((t) => t.fanoutGroupId === groupId)
+    const winner = laneTabs.find((t) => t.session.id === winnerSessionId)
+    if (!winner) {
+      // No lane matches — refuse rather than treat every lane (incl. the intended
+      // winner) as a loser and delete them all. Happens if the winner hasn't
+      // attached yet (session.id still null).
+      this.showFanoutBanner('Cannot keep this lane yet — it has not connected.')
+      return
+    }
+    const losers = laneTabs.filter((t) => t !== winner)
+    const winnerBranch = winner.branch ?? winnerSessionId
+    if (
+      !window.confirm(
+        `Keep ${winnerBranch} and discard the other ${losers.length} lane(s) (delete their worktrees)?`,
+      )
+    ) {
+      return
+    }
+
+    const failures: string[] = []
+    for (const loser of losers) {
+      const idx = this.tabs.indexOf(loser)
+      if (idx >= 0) this.closeTab(idx) // detach; PTY reaped by IDLE_TTL
+      const wt = loser.worktreePath
+      if (wt !== undefined && wt !== '') {
+        let r = await removeWorktreeReq(group.repoPath, wt, false)
+        if (!r.ok && r.status === 409) {
+          r = await removeWorktreeReq(group.repoPath, wt, true) // dirty → force (batch-confirmed)
+        }
+        if (!r.ok) failures.push(`${loser.branch ?? wt}: ${r.error ?? 'failed'}`)
+      }
+    }
+
+    this.fanoutGroups.delete(groupId)
+    winner.fanoutGroupId = undefined // drop the 🏆 button — the race is resolved
+
+    if (losers.length > 0) {
+      this.setGridLayout('single')
+      const wi = this.tabs.indexOf(winner)
+      if (wi >= 0) this.activate(wi)
+    }
+    this.persist()
+    this.rebuild()
+
+    if (failures.length > 0) {
+      this.showFanoutBanner(`Kept ${winnerBranch}. Some worktrees could not be removed: ${failures.join('; ')}`)
+    }
+  }
+
   activate(i: number): void {
     if (i < 0 || i >= this.tabs.length) return
     // Board-aware focus: in a split grid the focused pane must be ON the board,
@@ -1344,6 +1566,9 @@ export class TabApp {
     if (maxBtn) maxBtn.textContent = maximized ? '⤡' : '⛶'
     const monBtn = cell.querySelector<HTMLElement>('.cell-monitor-btn')
     if (monBtn) monBtn.classList.toggle('active', grid && entry.monitor)
+    // W5: the 🏆 keep button appears only on a fan-out lane's cell (any layout).
+    const keepBtn = cell.querySelector<HTMLElement>('.cell-keep')
+    if (keepBtn) keepBtn.style.display = entry.fanoutGroupId !== undefined ? '' : 'none'
 
     const nameEl = cell.querySelector('.cell-name')
     if (nameEl) nameEl.textContent = this.displayTitle(entry, idx)
diff --git a/src/config.ts b/src/config.ts
index 6d2b4a6..156b73c 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -65,6 +65,8 @@ const DEFAULT_GH_TIMEOUT_MS = 8_000
 const DEFAULT_STATUSLINE_TTL_MS = 30_000
 // B3 worktrees
 const DEFAULT_WORKTREE_TIMEOUT_MS = 10_000
+// W5 fan-out board — max lanes one task may be fanned across (matches grid-6 capacity)
+const DEFAULT_MAX_FANOUT_LANES = 6
 // W4 git write (stage / commit / push)
 const DEFAULT_GIT_OPS_TIMEOUT_MS = 10_000 // stage/commit exec bound (local, like worktree)
 const DEFAULT_GIT_PUSH_TIMEOUT_MS = 120_000 // push is network-bound → longer bound
@@ -410,6 +412,14 @@ export function loadConfig(env: EnvLike): Config {
     DEFAULT_WORKTREE_TIMEOUT_MS,
   )
 
+  // W5 fan-out board — cap on lanes one task may be fanned across (DoS guard,
+  // like MAX_SESSIONS). 0 is allowed (disables fan-out) via parseNonNegativeInt.
+  const maxFanoutLanes = parseNonNegativeInt(
+    env['MAX_FANOUT_LANES'],
+    'MAX_FANOUT_LANES',
+    DEFAULT_MAX_FANOUT_LANES,
+  )
+
   // W4 git write (stage / commit / push) — the highest-risk write channel
   const gitOpsEnabled = parseBool(env['GIT_OPS_ENABLED'], true) // master kill-switch
   const gitOpsTimeoutMs = parseNonNegativeInt(
@@ -497,6 +507,8 @@ export function loadConfig(env: EnvLike): Config {
     worktreeEnabled,
     worktreeRoot,
     worktreeTimeoutMs,
+    // W5 fan-out board
+    maxFanoutLanes,
     // W4 git write (stage / commit / push)
     gitOpsEnabled,
     gitOpsTimeoutMs,
diff --git a/src/http/session-groups.ts b/src/http/session-groups.ts
new file mode 100644
index 0000000..751999c
--- /dev/null
+++ b/src/http/session-groups.ts
@@ -0,0 +1,57 @@
+/**
+ * src/http/session-groups.ts (W5 fan-out board) — cluster running sessions by
+ * the repo they belong to, for the fan-out discovery endpoint.
+ *
+ * PURE + STRING-ONLY (no `git` exec, no filesystem I/O) — fully node-unit-testable.
+ * Fan-out worktrees always live under `<repo>-worktrees/` (createWorktree's base,
+ * worktrees.ts computeWorktreeDir), so a session whose cwd is
+ * `<repo>-worktrees/<lane>` shares a group with the repo and its sibling lanes.
+ * A session that is NOT under a `*-worktrees` parent groups under its own cwd.
+ */
+
+import path from 'node:path'
+import type { LiveSessionInfo, SessionGroup } from '../types.js'
+
+/** The suffix createWorktree appends to derive the worktree base dir name. */
+const WORKTREES_SUFFIX = '-worktrees'
+
+/**
+ * Derive the repo root for a session cwd. If the cwd's PARENT directory is a
+ * `<name>-worktrees` folder, the repo root is `<dirname-of-parent>/<name>` (strip
+ * the `-worktrees` suffix). Otherwise the cwd is its own repo root. `null` in →
+ * `null` out (a session with no known cwd has no repo to group under). Pure.
+ */
+export function deriveRepoRoot(cwd: string | null): string | null {
+  if (cwd === null || cwd === '') return null
+  const parent = path.dirname(cwd)
+  const parentName = path.basename(parent)
+  if (parentName.length > WORKTREES_SUFFIX.length && parentName.endsWith(WORKTREES_SUFFIX)) {
+    const repoName = parentName.slice(0, -WORKTREES_SUFFIX.length)
+    return path.join(path.dirname(parent), repoName)
+  }
+  return cwd
+}
+
+/**
+ * Group live sessions by derived repo root. Sessions whose cwd resolves to the
+ * same repo (a repo + its `*-worktrees/*` lanes) land in one SessionGroup.
+ * Sessions with `cwd === null` are SKIPPED (no repo to attribute them to).
+ * Group order and per-group member order both follow the input order (which
+ * manager.list() already yields newest-first). Never mutates the input. Pure.
+ */
+export function groupSessionsByRepo(sessions: readonly LiveSessionInfo[]): SessionGroup[] {
+  const byRoot = new Map<string, SessionGroup>()
+  const order: string[] = []
+  for (const session of sessions) {
+    const repoRoot = deriveRepoRoot(session.cwd)
+    if (repoRoot === null) continue // no cwd → not attributable to a repo
+    let group = byRoot.get(repoRoot)
+    if (group === undefined) {
+      group = { repoRoot, label: path.basename(repoRoot), sessions: [] }
+      byRoot.set(repoRoot, group)
+      order.push(repoRoot)
+    }
+    group.sessions.push(session)
+  }
+  return order.map((root) => byRoot.get(root)!)
+}
diff --git a/src/server.ts b/src/server.ts
index ae8ef5e..138a315 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -45,6 +45,7 @@ import { buildDigest, clampSince } from './http/digest.js'
 import { getPrStatus } from './http/gh.js'
 import { parseStatusLine } from './http/statusline.js'
 import { createWorktree, removeWorktree, pruneWorktrees } from './http/worktrees.js'
+import { groupSessionsByRepo } from './http/session-groups.js'
 import { stageFiles, commit as gitCommit, push as gitPush } from './http/git-ops.js'
 import { createSessionManager } from './session/manager.js'
 import { detachWs, writeInput, setClientDims } from './session/session.js'
@@ -333,6 +334,14 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
     res.json(manager.list())
   })
 
+  // ── W5 fan-out board: running sessions clustered by repo (read-only) ──────
+  // Pure string grouping over manager.list() (no git/FS work); same threat model
+  // as /live-sessions and /digest → no Origin guard. Registered BEFORE the
+  // /live-sessions/:id routes so "grouped" is never captured as an :id.
+  app.get('/live-sessions/grouped', (_req, res) => {
+    res.json(groupSessionsByRepo(manager.list()))
+  })
+
   // ── W3 quick-wins (c): "while you were away" reconnect digest (read-only) ──
   // A pure read-side aggregate over manager.list() + in-memory telemetry/status;
   // no Origin guard (same threat model as /live-sessions). `?since=<epochMs>` is
@@ -1102,6 +1111,8 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
       // W3(b): expose the cost budget only when set (>0) so the FE can derive
       // cost-overage warn styling; a non-secret number, safe over /config/ui.
       ...(cfg.costBudgetUsd > 0 ? { costBudgetUsd: cfg.costBudgetUsd } : {}),
+      // W5 fan-out board: let the FE stepper max mirror MAX_FANOUT_LANES.
+      maxFanoutLanes: cfg.maxFanoutLanes,
     }
     res.json(uiConfig)
   })
diff --git a/src/types.ts b/src/types.ts
index f0dd6a0..c806b7d 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -72,6 +72,8 @@ export interface Config {
   readonly worktreeEnabled: boolean; // WORKTREE_ENABLED, default true
   readonly worktreeRoot: string | undefined; // WORKTREE_ROOT (undefined → computed)
   readonly worktreeTimeoutMs: number; // WORKTREE_TIMEOUT_MS, default 10000
+  // W5 fan-out board — cap on lanes one task may be fanned across (grid-6 capacity)
+  readonly maxFanoutLanes: number; // MAX_FANOUT_LANES, default 6
   // W4 git write (stage / commit / push) — the highest-risk write channel
   readonly gitOpsEnabled: boolean; // GIT_OPS_ENABLED, default true (kill-switch → all 3 routes 403)
   readonly gitOpsTimeoutMs: number; // GIT_OPS_TIMEOUT_MS, default 10000 (stage/commit exec bound)
@@ -311,6 +313,27 @@ export interface LiveSessionInfo {
   readonly queueLength?: number;
 }
 
+/* ───────────────── W5 fan-out board (parallel agent lanes) ───────────────── */
+
+/** A group of running sessions sharing a repo/worktree-root (fan-out discovery).
+ *  Derived STRING-ONLY from each session's cwd (no git exec): fan-out worktrees
+ *  live under `<repo>-worktrees/`, so sessions whose cwd shares that parent
+ *  cluster into one group. impl: src/http/session-groups.ts groupSessionsByRepo. */
+export interface SessionGroup {
+  repoRoot: string; // derived repo dir (parent of the *-worktrees folder, or the cwd)
+  label: string; // basename(repoRoot)
+  sessions: LiveSessionInfo[]; // members, newest-first (already the manager.list order)
+}
+
+/** Launch options the FE passes to TabApp.launchFanout (FE-internal, but typed
+ *  shared so the form and the launcher agree on ONE shape). */
+export interface FanoutLaunchOpts {
+  prompt: string;
+  lanes: number; // 2..maxFanoutLanes (clamped to grid-6 capacity + the DoS cap)
+  branchBase: string; // slug; lanes get `${branchBase}-lane-${i}`
+  mode?: PermissionMode; // reuse PermissionMode
+}
+
 /* ───────────────── project manager (v0.6, §4.3 FEATURE doc) ──────────────── */
 
 /** One running session belonging to a project (live-sessions归并 by cwd).
@@ -658,6 +681,9 @@ export interface UiConfig {
   /** W3 quick-wins (b): the cost-budget threshold (USD). Present when > 0 so the
    *  FE can derive cost-overage warn styling client-side; omitted when disabled. */
   costBudgetUsd?: number;
+  /** W5 fan-out board: server-controlled cap on fan-out lanes so the FE stepper
+   *  max mirrors MAX_FANOUT_LANES. Additive OPTIONAL (older clients default to 6). */
+  maxFanoutLanes?: number;
 }
 
 /* ── W3 quick-wins (c) reconnect digest (GET /digest) ── */
diff --git a/test/config.test.ts b/test/config.test.ts
index 7951433..4b9de49 100644
--- a/test/config.test.ts
+++ b/test/config.test.ts
@@ -299,6 +299,18 @@ describe('loadConfig — session/rate/timer constants', () => {
     expect(() => loadConfig({ MAX_SESSIONS: 'lots' })).toThrow()
   })
 
+  it('defaults maxFanoutLanes to 6 (W5)', () => {
+    expect(loadConfig({}).maxFanoutLanes).toBe(6)
+  })
+
+  it('reads MAX_FANOUT_LANES from env', () => {
+    expect(loadConfig({ MAX_FANOUT_LANES: '3' }).maxFanoutLanes).toBe(3)
+  })
+
+  it('throws for a negative MAX_FANOUT_LANES (fail-fast, like MAX_SESSIONS)', () => {
+    expect(() => loadConfig({ MAX_FANOUT_LANES: '-1' })).toThrow()
+  })
+
   it('defaults maxMsgsPerSec to 2000 and reads MAX_MSGS_PER_SEC', () => {
     expect(loadConfig({}).maxMsgsPerSec).toBe(2000)
     expect(loadConfig({ MAX_MSGS_PER_SEC: '500' }).maxMsgsPerSec).toBe(500)
diff --git a/test/fanout.test.ts b/test/fanout.test.ts
new file mode 100644
index 0000000..f3eb2d0
--- /dev/null
+++ b/test/fanout.test.ts
@@ -0,0 +1,128 @@
+/**
+ * test/fanout.test.ts (W5) — pure launch-command + branch-slug builders.
+ *
+ * The load-bearing case is shellSingleQuote: the prompt is typed as raw bytes into
+ * a lane's shell before `claude` parses it, so it must become one literal argv
+ * element — no shell metacharacter may execute.
+ */
+
+import { describe, it, expect } from 'vitest'
+
+import {
+  shellSingleQuote,
+  sanitizePrompt,
+  buildFanoutCmd,
+  laneBranch,
+  slugify,
+  FANOUT_PROMPT_MAX,
+} from '../public/fanout.js'
+
+// Local mirror of validateBranchNameClient's rules (projects.ts) — inlined so this
+// stays a pure node test (importing projects.js would drag in @xterm/xterm).
+function isValidBranch(branch: string): boolean {
+  if (!branch || branch.length > 250) return false
+  if (/[\x00-\x20\x7f]/.test(branch)) return false
+  if (branch.startsWith('-') || branch.includes('..') || branch.endsWith('.lock')) return false
+  if (/[~^:?*[\\]/.test(branch)) return false
+  if (branch.includes('@{')) return false
+  if (branch.startsWith('/') || branch.endsWith('/') || branch.includes('//')) return false
+  return true
+}
+
+describe('shellSingleQuote', () => {
+  it('single-quotes a plain string', () => {
+    expect(shellSingleQuote('fix bug')).toBe(`'fix bug'`)
+  })
+
+  it("escapes an embedded single quote with the '\\'' sequence", () => {
+    expect(shellSingleQuote("it's ok")).toBe(`'it'\\''s ok'`)
+  })
+
+  it('neutralizes shell metacharacters as literal text', () => {
+    // $(...), backticks, ;, && all sit inside single quotes → passed verbatim.
+    const dangerous = 'a; rm -rf / && echo $(whoami) `id`'
+    const quoted = shellSingleQuote(dangerous)
+    expect(quoted.startsWith("'")).toBe(true)
+    expect(quoted.endsWith("'")).toBe(true)
+    // No unescaped single quote can break out of the quoting.
+    expect(quoted.slice(1, -1).includes("'")).toBe(false)
+  })
+})
+
+describe('sanitizePrompt', () => {
+  it('collapses newlines (\\n, \\r\\n) to single spaces', () => {
+    expect(sanitizePrompt('a\nb\r\nc')).toBe('a b c')
+  })
+
+  it('collapses a bare carriage return too (would submit a partial line)', () => {
+    expect(sanitizePrompt('a\rb')).toBe('a b')
+  })
+
+  it('trims surrounding whitespace', () => {
+    expect(sanitizePrompt('  hello  ')).toBe('hello')
+  })
+
+  it('caps the length at FANOUT_PROMPT_MAX', () => {
+    const long = 'x'.repeat(FANOUT_PROMPT_MAX + 500)
+    expect(sanitizePrompt(long)).toHaveLength(FANOUT_PROMPT_MAX)
+  })
+})
+
+describe('buildFanoutCmd', () => {
+  it('adds --permission-mode for a non-default mode and ends with \\r', () => {
+    expect(buildFanoutCmd('fix bug', 'plan', true)).toBe(`claude --permission-mode plan 'fix bug'\r`)
+  })
+
+  it('omits the flag for the default mode', () => {
+    expect(buildFanoutCmd('fix bug', 'default', true)).toBe(`claude 'fix bug'\r`)
+  })
+
+  it('downgrades auto to default when the server forbids it (SEC-M5)', () => {
+    expect(buildFanoutCmd('fix bug', 'auto', false)).toBe(`claude 'fix bug'\r`)
+  })
+
+  it('honors auto when the server allows it', () => {
+    expect(buildFanoutCmd('fix bug', 'auto', true)).toBe(`claude --permission-mode auto 'fix bug'\r`)
+  })
+
+  it('shell-quotes an injection-style prompt into one argv element', () => {
+    const cmd = buildFanoutCmd('rm -rf $(pwd); echo hi', 'default', false)
+    expect(cmd).toBe(`claude 'rm -rf $(pwd); echo hi'\r`)
+  })
+
+  it('collapses a multi-line prompt before quoting', () => {
+    expect(buildFanoutCmd('line1\nline2', 'default', false)).toBe(`claude 'line1 line2'\r`)
+  })
+})
+
+describe('laneBranch', () => {
+  it('builds `${base}-lane-${i}`', () => {
+    expect(laneBranch('feat-x', 3)).toBe('feat-x-lane-3')
+  })
+
+  it('produces a valid git branch name', () => {
+    expect(isValidBranch(laneBranch('feat-x', 3))).toBe(true)
+    expect(isValidBranch(laneBranch(slugify('Add dark mode!'), 1))).toBe(true)
+  })
+})
+
+describe('slugify', () => {
+  it('lowercases and dash-collapses non-alphanumeric runs', () => {
+    expect(slugify('Add Dark Mode!')).toBe('add-dark-mode')
+  })
+
+  it('strips leading/trailing dashes', () => {
+    expect(slugify('  ...hello...  ')).toBe('hello')
+  })
+
+  it('falls back to "fanout" for an empty/symbol-only prompt', () => {
+    expect(slugify('!!!')).toBe('fanout')
+    expect(slugify('')).toBe('fanout')
+  })
+
+  it('caps the slug length and stays a valid branch base', () => {
+    const slug = slugify('a'.repeat(100))
+    expect(slug.length).toBeLessThanOrEqual(40)
+    expect(isValidBranch(slug)).toBe(true)
+  })
+})
diff --git a/test/http/session-groups.test.ts b/test/http/session-groups.test.ts
new file mode 100644
index 0000000..cb67f16
--- /dev/null
+++ b/test/http/session-groups.test.ts
@@ -0,0 +1,119 @@
+/**
+ * test/http/session-groups.test.ts (W5) — pure grouping helpers, no I/O.
+ *
+ * deriveRepoRoot strips a `<name>-worktrees/<lane>` parent to `<...>/<name>`;
+ * groupSessionsByRepo clusters a repo + its worktree lanes into one SessionGroup,
+ * skips cwd:null, and preserves manager.list() (newest-first) order.
+ */
+
+import { describe, it, expect } from 'vitest'
+
+import { deriveRepoRoot, groupSessionsByRepo } from '../../src/http/session-groups.js'
+import type { LiveSessionInfo } from '../../src/types.js'
+
+function live(id: string, cwd: string | null): LiveSessionInfo {
+  return {
+    id,
+    createdAt: 0,
+    clientCount: 0,
+    status: 'unknown',
+    exited: false,
+    cwd,
+    cols: 80,
+    rows: 24,
+  }
+}
+
+describe('deriveRepoRoot', () => {
+  it('strips a *-worktrees parent to the repo root', () => {
+    expect(deriveRepoRoot('/a/proj-worktrees/lane-1')).toBe('/a/proj')
+  })
+
+  it('returns the cwd unchanged when the parent is not a *-worktrees dir', () => {
+    expect(deriveRepoRoot('/a/proj')).toBe('/a/proj')
+  })
+
+  it('returns null for a null cwd', () => {
+    expect(deriveRepoRoot(null)).toBe(null)
+  })
+
+  it('returns null for an empty cwd', () => {
+    expect(deriveRepoRoot('')).toBe(null)
+  })
+
+  it('handles a nested repo path with a *-worktrees parent', () => {
+    expect(deriveRepoRoot('/home/me/code/web-terminal-worktrees/feat-x-lane-2')).toBe(
+      '/home/me/code/web-terminal',
+    )
+  })
+
+  it('does not treat a bare "-worktrees" segment (no repo name) specially', () => {
+    // parentName === '-worktrees' has no repo name to keep → falls through to cwd.
+    expect(deriveRepoRoot('/a/-worktrees/x')).toBe('/a/-worktrees/x')
+  })
+})
+
+describe('groupSessionsByRepo', () => {
+  it('returns [] for no sessions', () => {
+    expect(groupSessionsByRepo([])).toEqual([])
+  })
+
+  it('clusters a repo and its worktree lanes into ONE group', () => {
+    const sessions = [
+      live('s1', '/a/proj-worktrees/lane-1'),
+      live('s2', '/a/proj-worktrees/lane-2'),
+    ]
+    const groups = groupSessionsByRepo(sessions)
+    expect(groups).toHaveLength(1)
+    expect(groups[0]!.repoRoot).toBe('/a/proj')
+    expect(groups[0]!.label).toBe('proj')
+    expect(groups[0]!.sessions.map((s) => s.id)).toEqual(['s1', 's2'])
+  })
+
+  it('groups the main repo cwd together with its worktree lanes', () => {
+    const sessions = [
+      live('main', '/a/proj'),
+      live('lane', '/a/proj-worktrees/feat-lane-1'),
+    ]
+    const groups = groupSessionsByRepo(sessions)
+    expect(groups).toHaveLength(1)
+    expect(groups[0]!.repoRoot).toBe('/a/proj')
+    expect(groups[0]!.sessions.map((s) => s.id)).toEqual(['main', 'lane'])
+  })
+
+  it('puts an unrelated cwd in its own group', () => {
+    const groups = groupSessionsByRepo([
+      live('a', '/a/proj-worktrees/lane-1'),
+      live('b', '/b/other'),
+    ])
+    expect(groups).toHaveLength(2)
+    expect(groups[0]!.repoRoot).toBe('/a/proj')
+    expect(groups[1]!.repoRoot).toBe('/b/other')
+    expect(groups[1]!.label).toBe('other')
+  })
+
+  it('skips sessions with a null cwd (not attributable to a repo)', () => {
+    const groups = groupSessionsByRepo([
+      live('nocwd', null),
+      live('real', '/a/proj'),
+    ])
+    expect(groups).toHaveLength(1)
+    expect(groups[0]!.repoRoot).toBe('/a/proj')
+    expect(groups[0]!.sessions.map((s) => s.id)).toEqual(['real'])
+  })
+
+  it('preserves the input (newest-first) order across and within groups', () => {
+    const groups = groupSessionsByRepo([
+      live('newest', '/a/proj-worktrees/lane-2'),
+      live('older', '/a/proj-worktrees/lane-1'),
+    ])
+    expect(groups[0]!.sessions.map((s) => s.id)).toEqual(['newest', 'older'])
+  })
+
+  it('does not mutate the input array', () => {
+    const input = [live('a', '/a/proj')]
+    const copy = [...input]
+    groupSessionsByRepo(input)
+    expect(input).toEqual(copy)
+  })
+})
diff --git a/test/integration/grouped-endpoint.test.ts b/test/integration/grouped-endpoint.test.ts
new file mode 100644
index 0000000..4c4217f
--- /dev/null
+++ b/test/integration/grouped-endpoint.test.ts
@@ -0,0 +1,172 @@
+/**
+ * Integration test for GET /live-sessions/grouped (W5 fan-out board) and the
+ * /config/ui maxFanoutLanes field.
+ *
+ * Starts a real HTTP server. The grouped route is read-only (no Origin guard) and
+ * derives groups purely from cwd — the empty-manager path needs no PTY. The
+ * real-PTY case (two sessions under one *-worktrees dir clustering into one group)
+ * is itPty-gated: posix_spawn is blocked in the sandbox, so it auto-skips there
+ * and runs in CI.
+ */
+
+import net from 'node:net'
+import fs from 'node:fs/promises'
+import os from 'node:os'
+import path from 'node:path'
+
+import { afterAll, afterEach, beforeAll, describe, expect, it } from 'vitest'
+import WebSocket from 'ws'
+import * as nodePty from 'node-pty'
+
+import { loadConfig } from '../../src/config.js'
+import { startServer } from '../../src/server.js'
+import type { SessionGroup, UiConfig } from '../../src/types.js'
+
+const PTY_AVAILABLE = (() => {
+  try {
+    const p = nodePty.spawn(process.env['SHELL'] ?? '/bin/sh', [], { cols: 80, rows: 24 })
+    p.kill()
+    return true
+  } catch {
+    return false
+  }
+})()
+const itPty = PTY_AVAILABLE ? it : it.skip
+
+function getFreePort(): Promise<number> {
+  return new Promise((resolve, reject) => {
+    const srv = net.createServer()
+    srv.listen(0, '127.0.0.1', () => {
+      const addr = srv.address()
+      if (addr === null || typeof addr === 'string') {
+        srv.close()
+        reject(new Error('unexpected address type'))
+        return
+      }
+      const port = addr.port
+      srv.close(() => resolve(port))
+    })
+    srv.on('error', reject)
+  })
+}
+
+function waitForOpen(ws: WebSocket, timeoutMs = 3_000): Promise<void> {
+  return new Promise((resolve, reject) => {
+    const timer = setTimeout(() => reject(new Error('WS open timeout')), timeoutMs)
+    ws.once('open', () => {
+      clearTimeout(timer)
+      resolve()
+    })
+    ws.once('error', (err) => {
+      clearTimeout(timer)
+      reject(err)
+    })
+  })
+}
+
+/** Resolve once an 'attached' frame arrives (output frames may interleave). */
+function waitForAttached(ws: WebSocket, timeoutMs = 4_000): Promise<void> {
+  return new Promise((resolve, reject) => {
+    const timer = setTimeout(() => reject(new Error('attached timeout')), timeoutMs)
+    const onMsg = (raw: WebSocket.RawData): void => {
+      let msg: { type?: string }
+      try {
+        msg = JSON.parse(raw.toString('utf8'))
+      } catch {
+        return
+      }
+      if (msg.type === 'attached') {
+        clearTimeout(timer)
+        ws.off('message', onMsg)
+        resolve()
+      }
+    }
+    ws.on('message', onMsg)
+  })
+}
+
+describe('GET /live-sessions/grouped + /config/ui maxFanoutLanes — integration', () => {
+  let port: number
+  let serverHandle: { close(): Promise<void> }
+  const origin = (): string => `http://127.0.0.1:${port}`
+
+  beforeAll(async () => {
+    port = await getFreePort()
+    const cfg = loadConfig({
+      PORT: String(port),
+      BIND_HOST: '127.0.0.1',
+      SHELL_PATH: process.env['SHELL'] ?? '/bin/zsh',
+      ALLOWED_ORIGINS: `http://127.0.0.1:${port}`,
+      USE_TMUX: '0',
+      IDLE_TTL: '86400',
+      MAX_FANOUT_LANES: '5',
+    })
+    serverHandle = startServer(cfg)
+    await new Promise<void>((r) => setTimeout(r, 100))
+  })
+
+  afterAll(async () => {
+    await serverHandle.close()
+  })
+
+  it('returns 200 [] on an empty manager, WITHOUT requiring an Origin header', async () => {
+    const res = await fetch(`${origin()}/live-sessions/grouped`)
+    expect(res.status).toBe(200) // read-only: not 403 even with no Origin
+    const body = (await res.json()) as SessionGroup[]
+    expect(Array.isArray(body)).toBe(true)
+    expect(body).toHaveLength(0)
+  })
+
+  it('does not shadow "grouped" as a session :id (returns an array, not an error)', async () => {
+    const res = await fetch(`${origin()}/live-sessions/grouped`)
+    const body = await res.json()
+    expect(Array.isArray(body)).toBe(true)
+  })
+
+  it('exposes maxFanoutLanes on /config/ui', async () => {
+    const res = await fetch(`${origin()}/config/ui`)
+    expect(res.status).toBe(200)
+    const body = (await res.json()) as UiConfig
+    expect(body.maxFanoutLanes).toBe(5)
+  })
+
+  // ── real-PTY: two sessions under one *-worktrees dir cluster into one group ──
+  let tmpRepo: string | null = null
+  const sockets: WebSocket[] = []
+
+  afterEach(() => {
+    for (const ws of sockets.splice(0)) {
+      try {
+        ws.close()
+      } catch {
+        // ignore
+      }
+    }
+  })
+
+  itPty('clusters two worktree-lane sessions into a single group', async () => {
+    tmpRepo = await fs.mkdtemp(path.join(os.tmpdir(), 'fanout-'))
+    const laneDir = path.join(tmpRepo, 'proj-worktrees')
+    const lane1 = path.join(laneDir, 'lane-1')
+    const lane2 = path.join(laneDir, 'lane-2')
+    await fs.mkdir(lane1, { recursive: true })
+    await fs.mkdir(lane2, { recursive: true })
+
+    for (const cwd of [lane1, lane2]) {
+      const ws = new WebSocket(`ws://127.0.0.1:${port}/term`, { origin: origin() })
+      sockets.push(ws)
+      await waitForOpen(ws)
+      ws.send(JSON.stringify({ type: 'attach', sessionId: null, cwd }))
+      await waitForAttached(ws)
+    }
+    // Give the sessions a beat to register their cwd.
+    await new Promise<void>((r) => setTimeout(r, 100))
+
+    const res = await fetch(`${origin()}/live-sessions/grouped`)
+    const groups = (await res.json()) as SessionGroup[]
+    const proj = groups.find((g) => g.repoRoot === path.join(tmpRepo!, 'proj'))
+    expect(proj).toBeDefined()
+    expect(proj!.label).toBe('proj')
+    expect(proj!.sessions.length).toBe(2)
+  })
+})
diff --git a/test/integration/timeline-events.test.ts b/test/integration/timeline-events.test.ts
index d78e831..e2c6a04 100644
--- a/test/integration/timeline-events.test.ts
+++ b/test/integration/timeline-events.test.ts
@@ -206,13 +206,13 @@ describe('GET /config/ui', () => {
     const { port } = await spawnServer()
     const res = await fetch(`http://127.0.0.1:${port}/config/ui`)
     expect(res.status).toBe(200)
-    expect(await res.json()).toEqual({ allowAutoMode: false })
+    expect(await res.json()).toEqual({ allowAutoMode: false, maxFanoutLanes: 6 })
   })
 
   it('reports allowAutoMode:true when ALLOW_AUTO_MODE=1', async () => {
     const { port } = await spawnServer({ ALLOW_AUTO_MODE: '1' })
     const res = await fetch(`http://127.0.0.1:${port}/config/ui`)
-    expect(await res.json()).toEqual({ allowAutoMode: true })
+    expect(await res.json()).toEqual({ allowAutoMode: true, maxFanoutLanes: 6 })
   })
 
   it('omits costBudgetUsd when the budget is unset (W3 b)', async () => {
@@ -225,6 +225,6 @@ describe('GET /config/ui', () => {
   it('reports costBudgetUsd when COST_BUDGET_USD is set (W3 b)', async () => {
     const { port } = await spawnServer({ COST_BUDGET_USD: '7.5' })
     const res = await fetch(`http://127.0.0.1:${port}/config/ui`)
-    expect(await res.json()).toEqual({ allowAutoMode: false, costBudgetUsd: 7.5 })
+    expect(await res.json()).toEqual({ allowAutoMode: false, costBudgetUsd: 7.5, maxFanoutLanes: 6 })
   })
 })
diff --git a/test/manager.test.ts b/test/manager.test.ts
index 94534cd..5dbc0ac 100644
--- a/test/manager.test.ts
+++ b/test/manager.test.ts
@@ -86,6 +86,7 @@ const CFG: Config = {
   worktreeEnabled: true,
   worktreeRoot: undefined,
   worktreeTimeoutMs: 10_000,
+  maxFanoutLanes: 6,
   defaultPermissionMode: 'default',
   allowAutoMode: false,
   // W2 inject queue
diff --git a/test/push-apns.test.ts b/test/push-apns.test.ts
index f94bcae..fc47f86 100644
--- a/test/push-apns.test.ts
+++ b/test/push-apns.test.ts
@@ -114,6 +114,7 @@ const BASE_CFG: Config = {
   worktreeEnabled: true,
   worktreeRoot: undefined,
   worktreeTimeoutMs: 10_000,
+  maxFanoutLanes: 6,
   defaultPermissionMode: 'default',
   allowAutoMode: false,
 }
diff --git a/test/push-fcm.test.ts b/test/push-fcm.test.ts
index 50d4142..8c1309d 100644
--- a/test/push-fcm.test.ts
+++ b/test/push-fcm.test.ts
@@ -133,6 +133,7 @@ const BASE_CFG: Config = {
   worktreeEnabled: true,
   worktreeRoot: undefined,
   worktreeTimeoutMs: 10_000,
+  maxFanoutLanes: 6,
   defaultPermissionMode: 'default',
   allowAutoMode: false,
 }
diff --git a/test/push/push-service.test.ts b/test/push/push-service.test.ts
index 21c6964..9d4beba 100644
--- a/test/push/push-service.test.ts
+++ b/test/push/push-service.test.ts
@@ -55,6 +55,7 @@ const BASE_CFG: Config = {
   worktreeEnabled: true,
   worktreeRoot: undefined,
   worktreeTimeoutMs: 10_000,
+  maxFanoutLanes: 6,
   defaultPermissionMode: 'default',
   allowAutoMode: false,
 }
diff --git a/test/tabs.test.ts b/test/tabs.test.ts
index ddd2946..f6158fc 100644
--- a/test/tabs.test.ts
+++ b/test/tabs.test.ts
@@ -156,7 +156,26 @@ const mountProjects = vi.fn(() => ({
   },
   refresh: vi.fn(),
 }))
-vi.mock('../public/projects.js', () => ({ mountProjects }))
+// W5: TabApp imports these worktree helpers from projects.js for launchFanout /
+// keepFanoutWinner. Mocked so we drive them without real fetch and assert routing.
+const createWorktreeReqMock = vi.fn(
+  async (_repoPath: string, branch: string) => ({ ok: true, path: `/wt/${branch}` }) as {
+    ok: boolean
+    path?: string
+    status?: number
+    error?: string
+  },
+)
+const removeWorktreeReqMock = vi.fn(
+  async () => ({ ok: true }) as { ok: boolean; status?: number; error?: string },
+)
+const validateBranchNameClientMock = vi.fn((_b: string): string | null => null)
+vi.mock('../public/projects.js', () => ({
+  mountProjects,
+  createWorktreeReq: (...a: unknown[]) => createWorktreeReqMock(...(a as [string, string])),
+  removeWorktreeReq: (...a: unknown[]) => removeWorktreeReqMock(...(a as [])),
+  validateBranchNameClient: (...a: unknown[]) => validateBranchNameClientMock(...(a as [string])),
+}))
 
 // settings.js is light but imports nothing heavy; let it load for real.
 
@@ -200,6 +219,16 @@ beforeEach(() => {
   quickReply.onSend = undefined
   quickReply.onQueue = undefined
   enqueueFollowupMock.mockClear()
+  // W5 fan-out mocks: reset to permissive defaults each test.
+  createWorktreeReqMock.mockReset()
+  createWorktreeReqMock.mockImplementation(async (_repoPath: string, branch: string) => ({
+    ok: true,
+    path: `/wt/${branch}`,
+  }))
+  removeWorktreeReqMock.mockReset()
+  removeWorktreeReqMock.mockResolvedValue({ ok: true })
+  validateBranchNameClientMock.mockReset()
+  validateBranchNameClientMock.mockReturnValue(null)
   voice.onTranscript = undefined
   voice.onInterim = undefined
   // Default: server forbids 'auto' (loadUiConfig resolves to allowAutoMode=false).
@@ -208,6 +237,7 @@ beforeEach(() => {
 
 afterEach(() => {
   vi.unstubAllGlobals()
+  vi.restoreAllMocks() // restore window.confirm etc. spied via vi.spyOn (W5 tests)
 })
 
 describe('TabApp — v0.5 launcher chooser', () => {
@@ -1718,3 +1748,169 @@ describe('TabApp — W2 inject queue', () => {
     expect(enqueueFollowupMock).not.toHaveBeenCalled()
   })
 })
+
+// ── W5: fan-out board (launchFanout + keepFanoutWinner) ──────────────────────
+describe('TabApp — W5 fan-out board', () => {
+  const flush = (): Promise<void> => new Promise((r) => setTimeout(r, 0))
+
+  /** Fan out N lanes and stamp server ids on the resulting sessions. */
+  async function launched(
+    app: InstanceType<typeof TabApp>,
+    lanes: number,
+    branchBase = 'feat',
+  ): Promise<void> {
+    await app.launchFanout('/repo', 'repo', { prompt: 'add feature', lanes, branchBase, mode: 'default' })
+    FakeTerminalSession.instances.forEach((inst, i) => (inst.id = `sess-${i}`))
+  }
+
+  const keepBtnOf = (i: number): HTMLButtonElement =>
+    FakeTerminalSession.instances[i]!.el.closest('.term-cell')!.querySelector('.cell-keep')!
+
+  it('launchFanout creates N worktrees SEQUENTIALLY, one tab per lane, on a grid-4 board', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+
+    await app.launchFanout('/repo', 'repo', {
+      prompt: 'add feature',
+      lanes: 3,
+      branchBase: 'feat',
+      mode: 'default',
+    })
+
+    expect(app.snapshot()).toHaveLength(3)
+    expect(FakeTerminalSession.instances).toHaveLength(3)
+    // Sequential, in lane order.
+    expect(createWorktreeReqMock).toHaveBeenCalledTimes(3)
+    expect(createWorktreeReqMock.mock.calls.map((c) => c[1])).toEqual([
+      'feat-lane-1',
+      'feat-lane-2',
+      'feat-lane-3',
+    ])
+    // Same shell-quoted launch command injected into EVERY lane.
+    for (const inst of FakeTerminalSession.instances) {
+      expect(inst.initialInput).toBe("claude 'add feature'\r")
+    }
+    // Each lane spawns in its own worktree cwd.
+    const cwds = FakeTerminalSession.instances.map(
+      (i) => (i.cbs as Record<string, unknown>)['cwd'],
+    )
+    expect(cwds).toEqual(['/wt/feat-lane-1', '/wt/feat-lane-2', '/wt/feat-lane-3'])
+    expect(app.getGridLayout()).toBe('grid-4')
+  })
+
+  it('launchFanout uses grid-6 for more than four lanes', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await launched(app, 5)
+    expect(app.snapshot()).toHaveLength(5)
+    expect(app.getGridLayout()).toBe('grid-6')
+  })
+
+  it('launchFanout refuses an invalid branch base (no worktrees, banner shown)', async () => {
+    validateBranchNameClientMock.mockReturnValue('bad branch')
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await app.launchFanout('/repo', 'repo', { prompt: 'x', lanes: 3, branchBase: '-bad', mode: 'default' })
+    expect(createWorktreeReqMock).not.toHaveBeenCalled()
+    expect(app.snapshot()).toHaveLength(0)
+    expect(document.getElementById('fanout-banner')!.style.display).toBe('block')
+  })
+
+  it('launchFanout tolerates a partial failure — records it in the banner, never throws', async () => {
+    createWorktreeReqMock.mockImplementation(async (_repo: string, branch: string) =>
+      branch.endsWith('-2')
+        ? { ok: false, status: 500, error: 'boom' }
+        : { ok: true, path: `/wt/${branch}` },
+    )
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await expect(
+      app.launchFanout('/repo', 'repo', { prompt: 'x', lanes: 3, branchBase: 'feat', mode: 'default' }),
+    ).resolves.toBeUndefined()
+    expect(app.snapshot()).toHaveLength(2) // lanes 1 and 3 created
+    const banner = document.getElementById('fanout-banner')!
+    expect(banner.style.display).toBe('block')
+    expect(banner.textContent).toContain('Started 2 of 3')
+  })
+
+  it('every created lane cell shows a 🏆 Keep button (fan-out lanes only)', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await launched(app, 3)
+    for (let i = 0; i < 3; i++) expect(keepBtnOf(i).style.display).not.toBe('none')
+  })
+
+  it('a plain (non-fan-out) tab has no visible Keep button', () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    app.newTab()
+    const cell = FakeTerminalSession.instances[0]!.el.closest('.term-cell')!
+    expect((cell.querySelector('.cell-keep') as HTMLElement).style.display).toBe('none')
+  })
+
+  it('keepFanoutWinner keeps the winner, closes losers, removes their worktrees, layout → single', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await launched(app, 3)
+    vi.spyOn(window, 'confirm').mockReturnValue(true)
+
+    keepBtnOf(0).click() // keep sess-0 (feat-lane-1)
+    await flush()
+
+    expect(app.snapshot()).toHaveLength(1) // only the winner remains
+    expect(app.activeSessionId()).toBe('sess-0')
+    // One DELETE per loser, force=false, with the loser's worktree path.
+    expect(removeWorktreeReqMock).toHaveBeenCalledTimes(2)
+    const paths = removeWorktreeReqMock.mock.calls.map((c) => (c as unknown[])[1]).sort()
+    expect(paths).toEqual(['/wt/feat-lane-2', '/wt/feat-lane-3'])
+    expect(removeWorktreeReqMock.mock.calls.every((c) => (c as unknown[])[2] === false)).toBe(true)
+    expect(app.getGridLayout()).toBe('single')
+  })
+
+  it('keepFanoutWinner force-removes a dirty (409) loser without a second confirm', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await launched(app, 2)
+    removeWorktreeReqMock
+      .mockResolvedValueOnce({ ok: false, status: 409, error: 'dirty' })
+      .mockResolvedValueOnce({ ok: true })
+    const confirmSpy = vi.spyOn(window, 'confirm').mockReturnValue(true)
+
+    keepBtnOf(0).click() // keep sess-0; one loser (sess-1)
+    await flush()
+
+    expect(removeWorktreeReqMock).toHaveBeenCalledTimes(2)
+    expect((removeWorktreeReqMock.mock.calls[0] as unknown[])[2]).toBe(false) // first try
+    expect((removeWorktreeReqMock.mock.calls[1] as unknown[])[2]).toBe(true) // forced retry
+    expect(confirmSpy).toHaveBeenCalledTimes(1) // only the batch confirm
+  })
+
+  it('keepFanoutWinner cancelled → no tabs closed, no worktree removed', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    await launched(app, 3)
+    vi.spyOn(window, 'confirm').mockReturnValue(false)
+
+    keepBtnOf(0).click()
+    await flush()
+
+    expect(app.snapshot()).toHaveLength(3) // nothing closed
+    expect(removeWorktreeReqMock).not.toHaveBeenCalled()
+  })
+
+  it('keepFanoutWinner refuses when the winner has not attached (null id) — deletes nothing', async () => {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    // Fan out but do NOT stamp ids (sessions still null).
+    await app.launchFanout('/repo', 'repo', { prompt: 'x', lanes: 3, branchBase: 'feat', mode: 'default' })
+    const confirmSpy = vi.spyOn(window, 'confirm').mockReturnValue(true)
+
+    keepBtnOf(0).click() // entry.session.id is null → winnerSessionId ''
+    await flush()
+
+    expect(confirmSpy).not.toHaveBeenCalled() // refused before the confirm
+    expect(removeWorktreeReqMock).not.toHaveBeenCalled()
+    expect(app.snapshot()).toHaveLength(3)
+    expect(document.getElementById('fanout-banner')!.style.display).toBe('block')
+  })
+})
diff --git a/test/worktree-form.test.ts b/test/worktree-form.test.ts
index c521793..9c5abcd 100644
--- a/test/worktree-form.test.ts
+++ b/test/worktree-form.test.ts
@@ -44,6 +44,7 @@ vi.mock('../public/gh-chip.js', () => ({ mountPrChip: mockMountPrChip }))
 const {
   validateBranchNameClient,
   renderNewWorktreeForm,
+  renderFanoutForm,
   renderProjectDetail,
   makeWorktreeRow,
   confirmAndRemoveWorktree,
@@ -53,7 +54,7 @@ const {
 /* ── Helpers ───────────────────────────────────────────────────────────────── */
 
 function makeHooks() {
-  return { onOpenProject: vi.fn(), onEnterSession: vi.fn() }
+  return { onOpenProject: vi.fn(), onEnterSession: vi.fn(), onFanout: vi.fn() }
 }
 
 function makeCbs() {
@@ -286,6 +287,7 @@ describe('renderNewWorktreeForm', () => {
     // Flush async microtasks
     await Promise.resolve()
     await Promise.resolve()
+    await new Promise((r) => setTimeout(r, 0))
 
     expect(hooks.onOpenProject).toHaveBeenCalledWith(
       '/home/user/my-repo-worktrees/feat',
@@ -309,6 +311,7 @@ describe('renderNewWorktreeForm', () => {
 
     await Promise.resolve()
     await Promise.resolve()
+    await new Promise((r) => setTimeout(r, 0))
 
     expect(hooks.onOpenProject).toHaveBeenCalledWith('/repo', 'feat', 'claude\r')
   })
@@ -327,6 +330,7 @@ describe('renderNewWorktreeForm', () => {
 
     await Promise.resolve()
     await Promise.resolve()
+    await new Promise((r) => setTimeout(r, 0))
 
     const errorEl = form.querySelector('.proj-wt-error') as HTMLElement
     expect(errorEl.style.display).not.toBe('none')
@@ -345,6 +349,7 @@ describe('renderNewWorktreeForm', () => {
 
     await Promise.resolve()
     await Promise.resolve()
+    await new Promise((r) => setTimeout(r, 0))
 
     const errorEl = form.querySelector('.proj-wt-error') as HTMLElement
     expect(errorEl.textContent).toBeTruthy()
@@ -365,6 +370,7 @@ describe('renderNewWorktreeForm', () => {
 
     await Promise.resolve()
     await Promise.resolve()
+    await new Promise((r) => setTimeout(r, 0))
 
     const errorEl = form.querySelector('.proj-wt-error') as HTMLElement
     // No <script> element in DOM — textContent was used
@@ -492,6 +498,112 @@ describe('renderProjectDetail — B3 New Worktree Form', () => {
   })
 })
 
+/* ── renderFanoutForm (W5 fan-out board) ──────────────────────────────────── */
+
+describe('renderFanoutForm', () => {
+  it('renders a prompt textarea, lane stepper, branch input, mode select, submit', () => {
+    const form = renderFanoutForm(makeDetail(), makeHooks())
+    expect(form.querySelector('textarea.proj-fanout-prompt')).not.toBeNull()
+    const lanes = form.querySelector('input.proj-fanout-lanes') as HTMLInputElement
+    expect(lanes).not.toBeNull()
+    expect(lanes.min).toBe('2')
+    expect(lanes.max).toBe('6') // default maxFanoutLanes
+    expect(form.querySelector('input.proj-fanout-branch')).not.toBeNull()
+    expect(form.querySelector('select.proj-fanout-mode')).not.toBeNull()
+    expect(form.querySelector('button.proj-fanout-submit')).not.toBeNull()
+  })
+
+  it('honors a server-controlled max lane count', () => {
+    const form = renderFanoutForm(makeDetail(), makeHooks(), 3)
+    expect((form.querySelector('input.proj-fanout-lanes') as HTMLInputElement).max).toBe('3')
+  })
+
+  it('disables submit until a prompt is entered', () => {
+    const form = renderFanoutForm(makeDetail(), makeHooks())
+    const submit = form.querySelector('button.proj-fanout-submit') as HTMLButtonElement
+    expect(submit.disabled).toBe(true)
+    const prompt = form.querySelector('textarea.proj-fanout-prompt') as HTMLTextAreaElement
+    prompt.value = 'add dark mode'
+    prompt.dispatchEvent(new Event('input', { bubbles: true }))
+    expect(submit.disabled).toBe(false)
+  })
+
+  it('auto-fills the branch base from the prompt slug until the user edits it', () => {
+    const form = renderFanoutForm(makeDetail(), makeHooks())
+    const prompt = form.querySelector('textarea.proj-fanout-prompt') as HTMLTextAreaElement
+    const branch = form.querySelector('input.proj-fanout-branch') as HTMLInputElement
+    prompt.value = 'Add Dark Mode!'
+    prompt.dispatchEvent(new Event('input', { bubbles: true }))
+    expect(branch.value).toBe('add-dark-mode')
+    branch.value = 'custom-base'
+    branch.dispatchEvent(new Event('input', { bubbles: true }))
+    prompt.value = 'Add Dark Mode changed'
+    prompt.dispatchEvent(new Event('input', { bubbles: true }))
+    expect(branch.value).toBe('custom-base') // not clobbered after manual edit
+  })
+
+  it('submitting calls onFanout with the repo path, name, and options', () => {
+    const hooks = makeHooks()
+    const detail = makeDetail({ path: '/home/user/my-repo', name: 'my-repo' })
+    const form = renderFanoutForm(detail, hooks)
+    const prompt = form.querySelector('textarea.proj-fanout-prompt') as HTMLTextAreaElement
+    const lanes = form.querySelector('input.proj-fanout-lanes') as HTMLInputElement
+    const mode = form.querySelector('select.proj-fanout-mode') as HTMLSelectElement
+    prompt.value = 'add dark mode'
+    prompt.dispatchEvent(new Event('input', { bubbles: true }))
+    lanes.value = '4'
+    mode.value = 'plan'
+    ;(form.querySelector('button.proj-fanout-submit') as HTMLButtonElement).click()
+    expect(hooks.onFanout).toHaveBeenCalledWith('/home/user/my-repo', 'my-repo', {
+      prompt: 'add dark mode',
+      lanes: 4,
+      branchBase: 'add-dark-mode',
+      mode: 'plan',
+    })
+  })
+
+  it('shows an inline error (textContent) and does not call onFanout with no prompt', () => {
+    const hooks = makeHooks()
+    const form = renderFanoutForm(makeDetail(), hooks)
+    const submit = form.querySelector('button.proj-fanout-submit') as HTMLButtonElement
+    submit.disabled = false // exercise the submit-handler guard directly
+    submit.click()
+    const err = form.querySelector('.proj-fanout-error') as HTMLElement
+    expect(err.textContent).not.toBe('')
+    expect(err.style.display).not.toBe('none')
+    expect(hooks.onFanout).not.toHaveBeenCalled()
+  })
+
+  it('rejects an invalid branch base via inline error, not onFanout', () => {
+    const hooks = makeHooks()
+    const form = renderFanoutForm(makeDetail(), hooks)
+    const prompt = form.querySelector('textarea.proj-fanout-prompt') as HTMLTextAreaElement
+    const branch = form.querySelector('input.proj-fanout-branch') as HTMLInputElement
+    prompt.value = 'ok prompt'
+    prompt.dispatchEvent(new Event('input', { bubbles: true }))
+    branch.value = '-bad' // leading dash → invalid git branch
+    branch.dispatchEvent(new Event('input', { bubbles: true }))
+    ;(form.querySelector('button.proj-fanout-submit') as HTMLButtonElement).click()
+    expect(hooks.onFanout).not.toHaveBeenCalled()
+    expect((form.querySelector('.proj-fanout-error') as HTMLElement).textContent).not.toBe('')
+  })
+
+  it('renders the Fan out section inside a git project detail', () => {
+    const root = renderProjectDetail(makeDetail({ isGit: true }), makeHooks(), makeCbs())
+    expect(root.querySelector('.proj-fanout-form')).not.toBeNull()
+    expect(root.textContent).toContain('Fan out')
+  })
+
+  it('omits the Fan out form for a non-git directory', () => {
+    const root = renderProjectDetail(
+      makeDetail({ isGit: false, branch: undefined }),
+      makeHooks(),
+      makeCbs(),
+    )
+    expect(root.querySelector('.proj-fanout-form')).toBeNull()
+  })
+})
+
 /* ── renderProjectDetail — A4: Activity section ───────────────────────────── */
 
 describe('renderProjectDetail — A4 Activity section', () => {
diff --git a/vitest.config.ts b/vitest.config.ts
index 51d1b83..6b7ce5f 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -26,6 +26,7 @@ export default defineConfig({
         'public/link-paths.ts',
         'public/queue.ts',
         'public/tabs.ts',
+        'public/fanout.ts',
         'public/grid-layout.ts',
         'public/grid-presets.ts',
         'public/cell-monitor.ts',