From e062065cd3c751da8692fd783d8688ae4e3346af Mon Sep 17 00:00:00 2001
From: Yaojia Wang <yaojia.wang@billo.life>
Date: Sun, 12 Jul 2026 20:04:18 +0200
Subject: [PATCH] =?UTF-8?q?feat(cockpit):=20approval=20preview=20=E2=80=94?=
 =?UTF-8?q?=20command/diff=20above=20Approve/Reject=20(W1)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Remote one-tap approval was blind (you'd tap Approve without seeing Claude wants
to run `rm -rf` or rewrite a config). The pending tool's actual command / diff now
renders above the approval bar, on every attached device, riding the same broadcast
+ late-joiner rails as the existing `gate` field.

- src/http/approval-preview.ts (new, pure, never-throws): deriveApprovalPreview —
  Bash → command; Edit/Write/MultiEdit/NotebookEdit → a synthetic DiffFile; else null.
  Every line sanitized via sanitizeField (strips control/ANSI); caps 40 lines /
  200 chars/line / 4KB (security limits, UTF-8-safe byte clamp).
- src/types.ts: additive optional `preview?: ApprovalPreview` on the status
  ServerMessage + handleHookEvent (older clients ignore it).
- src/server.ts /hook/permission: derive preview from tool_input, store on the
  PendingApproval entry, re-send to late joiners exactly like `gate`.
- manager.ts threads it; public/tabs.ts renderApprovalPreview (command → <pre>
  textContent; diff → reused innerHTML-free renderDiffFile). Unknown tools /
  plan gates fall back to today's name-only bar.

Attacker-influenced tool input → rendered via textContent/diff-renderer only,
never innerHTML. Verified independently: typecheck + build:web clean, 1692 pass.
---
 public/style.css                   |  26 +++
 public/tabs.ts                     |  36 +++-
 public/terminal-session.ts         |  15 ++
 src/http/approval-preview.ts       | 263 +++++++++++++++++++++++++++++
 src/server.ts                      |  34 +++-
 src/session/manager.ts             |   7 +-
 src/types.ts                       |  20 +++
 test/http/approval-preview.test.ts | 189 +++++++++++++++++++++
 test/integration/server.test.ts    |  68 ++++++++
 test/manager.test.ts               |  33 ++++
 test/tabs.test.ts                  |  72 ++++++++
 test/terminal-session.test.ts      |  53 ++++++
 12 files changed, 808 insertions(+), 8 deletions(-)
 create mode 100644 src/http/approval-preview.ts
 create mode 100644 test/http/approval-preview.test.ts

diff --git a/public/style.css b/public/style.css
index 2933ae2..ca25da5 100644
--- a/public/style.css
+++ b/public/style.css
@@ -1086,6 +1086,7 @@ body {
   inset: auto 0 calc(var(--keybar-h) + var(--safe-b)) 0;
   z-index: 1050;
   display: flex;
+  flex-wrap: wrap; /* W1: a preview row wraps below the label + buttons */
   align-items: center;
   gap: 10px;
   padding: 10px 14px;
@@ -1097,6 +1098,31 @@ body {
 .approval-label {
   flex: 1 1 auto;
 }
+/* W1: approval preview (command/diff) — its own full-width row, scrollable. */
+.approval-preview {
+  flex: 1 1 100%;
+  order: 3; /* below the label + buttons regardless of insertion order */
+  max-height: 30vh;
+  overflow: auto;
+  overflow-x: auto;
+  border: 1px solid rgba(245, 177, 76, 0.35);
+  border-radius: 8px;
+  background: rgba(0, 0, 0, 0.35);
+  padding: 8px 10px;
+}
+.approval-cmd {
+  margin: 0;
+  white-space: pre-wrap;
+  word-break: break-word;
+  font-family: Menlo, Consolas, monospace;
+  font-size: 13px;
+  color: #f4f5f7;
+}
+.approval-truncated {
+  margin-top: 6px;
+  font-size: 12px;
+  opacity: 0.7;
+}
 #approvalbar button {
   flex: none;
   border: none;
diff --git a/public/tabs.ts b/public/tabs.ts
index fb2890b..1c41178 100644
--- a/public/tabs.ts
+++ b/public/tabs.ts
@@ -26,7 +26,8 @@ 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 { ClaudeStatus, PermissionMode, UiConfig } from '../src/types.js'
+import type { ApprovalPreview, ClaudeStatus, PermissionMode, UiConfig } from '../src/types.js'
+import { renderDiffFile } from './diff.js'
 import { renderTelemetryGauge } from './preview-grid.js'
 import { mountPushToggle } from './push.js'
 import { mountQuickReply } from './quick-reply.js'
@@ -366,16 +367,45 @@ export class TabApp {
     this.approvalBar.replaceChildren()
     const label = document.createElement('span')
     label.className = 'approval-label'
+    // W1: show WHAT will run (command / diff) between the label and the buttons,
+    // so a one-tap remote approval is no longer blind. Absent for plan gates and
+    // unknown tools (no reviewable command/diff) → today's name-only bar.
+    const preview = session.pendingPreview
+    const previewNode = preview ? this.renderApprovalPreview(preview) : null
+    const middle = previewNode ? [previewNode] : []
     if (session.pendingGate === 'plan') {
       label.textContent = 'Claude finished planning — how should it proceed?'
-      this.approvalBar.append(label, ...this.planGateButtons(session))
+      this.approvalBar.append(label, ...middle, ...this.planGateButtons(session))
     } else {
       label.textContent = `Claude wants to use ${session.pendingTool ?? 'a tool'}`
-      this.approvalBar.append(label, ...this.toolGateButtons(session))
+      this.approvalBar.append(label, ...middle, ...this.toolGateButtons(session))
     }
     this.approvalBar.style.display = 'flex'
   }
 
+  /** W1: build the command/diff preview node for the approval bar. Untrusted,
+   *  server-sanitized content is rendered via textContent / renderDiffFile ONLY
+   *  (never innerHTML) — <script>, ANSI, & etc. appear as literal characters. */
+  private renderApprovalPreview(p: ApprovalPreview): HTMLElement {
+    const container = document.createElement('div')
+    container.className = 'approval-preview'
+    if (p.kind === 'command') {
+      const pre = document.createElement('pre')
+      pre.className = 'approval-cmd'
+      pre.textContent = p.text // textContent — attacker-influenced command bytes
+      container.append(pre)
+    } else {
+      container.append(renderDiffFile(p.file)) // diff.ts is innerHTML-free (SEC-H4)
+    }
+    if (p.truncated) {
+      const note = document.createElement('div')
+      note.className = 'approval-truncated'
+      note.textContent = '… truncated'
+      container.append(note)
+    }
+    return container
+  }
+
   /** Ordinary tool gate: Approve / Reject (two buttons, unchanged). */
   private toolGateButtons(session: TerminalSession): HTMLButtonElement[] {
     return [
diff --git a/public/terminal-session.ts b/public/terminal-session.ts
index dfb5a30..979b317 100644
--- a/public/terminal-session.ts
+++ b/public/terminal-session.ts
@@ -14,6 +14,7 @@ import { FitAddon } from '@xterm/addon-fit'
 import { SearchAddon } from '@xterm/addon-search'
 import { WebLinksAddon } from '@xterm/addon-web-links'
 import type {
+  ApprovalPreview,
   ClaudeStatus,
   ClientMessage,
   PermissionGate,
@@ -147,6 +148,9 @@ export class TerminalSession {
   private pendingToolValue: string | undefined = undefined
   private telemetryValue: StatusTelemetry | null = null
   private pendingGateValue: PermissionGate | null = null
+  // W1: bounded command/diff preview of the held tool, from the last pending
+  // status frame. Null when no approval is held or the tool wasn't previewable.
+  private pendingPreviewValue: ApprovalPreview | null = null
   // VC: nonce that increments on every false→true pendingApproval flip — lets a
   // caller (e.g. a voice command captured at PTT-start) detect a stale gate: a
   // slow transcript must not resolve a NEWER held permission than the one it
@@ -257,6 +261,12 @@ export class TerminalSession {
     return this.pendingGateValue
   }
 
+  /** W1: bounded command/diff preview of the held tool (from the last pending
+   *  status frame), or null when nothing is held / the tool wasn't previewable. */
+  get pendingPreview(): ApprovalPreview | null {
+    return this.pendingPreviewValue
+  }
+
   /** Nonce counting false→true pendingApproval flips (VC stale-gate guard, §5). */
   get pendingEpoch(): number {
     return this.pendingEpochValue
@@ -372,6 +382,9 @@ export class TerminalSession {
         this.pendingApprovalValue = nextPending
         this.pendingToolValue = nextPending ? msg.detail : undefined
         this.pendingGateValue = msg.gate ?? null
+        // W1: keep the preview only while an approval is held; a non-pending
+        // status (approve/reject resolved) clears it so the bar hides cleanly.
+        this.pendingPreviewValue = nextPending ? (msg.preview ?? null) : null
         this.onClaudeStatus?.(msg.status, msg.detail)
         break
       }
@@ -501,10 +514,12 @@ export class TerminalSession {
    *  can distinguish "approve with default" from "approve, keep existing mode". */
   approve(mode?: PermissionMode): void {
     this.pendingApprovalValue = false
+    this.pendingPreviewValue = null // W1: resolved → drop the stale preview
     this.sendMsg({ type: 'approve', ...(mode !== undefined ? { mode } : {}) })
   }
   reject(): void {
     this.pendingApprovalValue = false
+    this.pendingPreviewValue = null // W1: resolved → drop the stale preview
     this.sendMsg({ type: 'reject' })
   }
 
diff --git a/src/http/approval-preview.ts b/src/http/approval-preview.ts
new file mode 100644
index 0000000..f2ca9ec
--- /dev/null
+++ b/src/http/approval-preview.ts
@@ -0,0 +1,263 @@
+/**
+ * src/http/approval-preview.ts (W1) — derive a bounded, sanitized preview of a
+ * held tool approval so a remote one-tap approve is no longer blind.
+ *
+ * Pure + never throws (SEC-M7 discipline, mirroring parseHookEvent). The input
+ * is the Claude Code hook `tool_input` — attacker-influenced content arriving on
+ * the loopback `/hook/permission` route. It is treated as `unknown`, every field
+ * narrowed before use, and every emitted string passed through sanitizeField
+ * (strips ASCII control chars incl. ANSI ESC / BEL) and line/byte-capped so a
+ * hostile or huge tool_input can never bloat the broadcast / pendingApprovals.
+ *
+ * Mapping:
+ *   Bash                                 → { kind:'command', text }
+ *   Edit / Write / MultiEdit / NotebookEdit → { kind:'diff', file }
+ *   anything else / malformed input      → null (FE falls back to a name-only bar)
+ *
+ * SECURITY: this module NEVER runs a command, touches fs, or interprets a path —
+ * it only produces display data. Rendering is textContent / renderDiffFile only
+ * (public/*), never innerHTML.
+ */
+
+import type { ApprovalPreview, DiffFile, DiffHunk, DiffLine } from '../types.js';
+import { sanitizeField } from '../session/timeline.js';
+
+// ── bounds (security limits, not user knobs — see plan §"New env vars") ────────
+
+/** Max diff/command lines emitted across the whole preview. */
+export const PREVIEW_MAX_LINES = 40;
+/** Per-line char cap (matches sanitizeField's default). */
+export const PREVIEW_MAX_LINE_LEN = 200;
+/** Hard total-byte cap on the emitted preview text (all lines combined). */
+export const PREVIEW_MAX_BYTES = 4096;
+
+/** Tools that map to a diff preview (mirrors timeline.ts EDIT_TOOLS semantics). */
+const EDIT_TOOLS: ReadonlySet<string> = new Set([
+  'Edit',
+  'Write',
+  'MultiEdit',
+  'NotebookEdit',
+]);
+
+// ── helpers ────────────────────────────────────────────────────────────────────
+
+/** Narrow an unknown value to a plain object record, or null. */
+function asRecord(v: unknown): Record<string, unknown> | null {
+  if (v === null || typeof v !== 'object' || Array.isArray(v)) return null;
+  return v as Record<string, unknown>;
+}
+
+/** Sanitize a single line: strip control/ANSI chars (via sanitizeField), cap to
+ *  PREVIEW_MAX_LINE_LEN, and report whether the cap actually clipped content (so
+ *  a hidden over-long line still flags the preview as truncated). Newlines never
+ *  reach here — callers split on '\n' first so structure survives. */
+function sanitizeLine(s: string): { text: string; clipped: boolean } {
+  // sanitizeField with an effectively-unbounded max = control-strip only (DRY:
+  // reuse the SEC-H6 regex), then apply OUR length cap + clip detection here.
+  const stripped = sanitizeField(s, Number.MAX_SAFE_INTEGER);
+  const text = stripped.slice(0, PREVIEW_MAX_LINE_LEN);
+  return { text, clipped: stripped.length > PREVIEW_MAX_LINE_LEN };
+}
+
+/** Split a raw multi-line blob into individual (still-raw) lines. */
+function splitLines(s: string): string[] {
+  return s.split('\n');
+}
+
+/** UTF-8 byte length of a string (whole-string). */
+function byteLen(s: string): number {
+  return Buffer.byteLength(s, 'utf8');
+}
+
+/** A budget shared across all emitted lines of one preview. */
+interface Budget {
+  linesLeft: number;
+  bytesLeft: number;
+  truncated: boolean;
+}
+
+function newBudget(): Budget {
+  return { linesLeft: PREVIEW_MAX_LINES, bytesLeft: PREVIEW_MAX_BYTES, truncated: false };
+}
+
+/**
+ * Truncate `s` to whole characters that fit within `maxBytes` UTF-8 bytes
+ * (never splitting a multi-byte codepoint). Returns whether it was clamped.
+ */
+function clampBytes(s: string, maxBytes: number): { text: string; clamped: boolean } {
+  if (byteLen(s) <= maxBytes) return { text: s, clamped: false };
+  let out = '';
+  let used = 0;
+  for (const ch of s) {
+    const b = byteLen(ch);
+    if (used + b > maxBytes) break;
+    out += ch;
+    used += b;
+  }
+  return { text: out, clamped: true };
+}
+
+// ── Bash → command preview ───────────────────────────────────────────────────────
+
+function deriveCommand(input: Record<string, unknown>): ApprovalPreview | null {
+  const command = input['command'];
+  if (typeof command !== 'string') return null;
+
+  const rawLines = splitLines(command);
+  const lineCountTruncated = rawLines.length > PREVIEW_MAX_LINES;
+  let anyClipped = false;
+  const joined = rawLines
+    .slice(0, PREVIEW_MAX_LINES)
+    .map((l) => {
+      const r = sanitizeLine(l);
+      if (r.clipped) anyClipped = true;
+      return r.text;
+    })
+    .join('\n');
+  const { text, clamped } = clampBytes(joined, PREVIEW_MAX_BYTES);
+
+  const truncated = lineCountTruncated || anyClipped || clamped;
+  return { kind: 'command', text, ...(truncated ? { truncated: true } : {}) };
+}
+
+// ── Edit-family → diff preview ───────────────────────────────────────────────────
+
+/** One hunk's worth of source lines (still raw, unsanitized). */
+interface HunkSpec {
+  removed: string[];
+  added: string[];
+}
+
+/** Pick a display path from the tool_input, or '' when none is present. */
+function pickPath(input: Record<string, unknown>): string {
+  const fp = input['file_path'];
+  if (typeof fp === 'string') return fp;
+  const np = input['notebook_path'];
+  if (typeof np === 'string') return np;
+  return '';
+}
+
+/** Turn an {old_string,new_string} object into a HunkSpec. Requires BOTH fields
+ *  to be strings (an Edit missing old_string / new_string → null → drop). */
+function toHunkSpec(v: unknown): HunkSpec | null {
+  const rec = asRecord(v);
+  if (rec === null) return null;
+  const oldStr = rec['old_string'];
+  const newStr = rec['new_string'];
+  if (typeof oldStr !== 'string' || typeof newStr !== 'string') return null;
+  return { removed: splitLines(oldStr), added: splitLines(newStr) };
+}
+
+/** Emit a bounded array of DiffLines from raw source lines, honouring the budget. */
+function emitLines(
+  kind: DiffLine['kind'],
+  rawLines: readonly string[],
+  budget: Budget,
+): DiffLine[] {
+  const out: DiffLine[] = [];
+  for (const raw of rawLines) {
+    if (budget.linesLeft <= 0) {
+      budget.truncated = true;
+      break;
+    }
+    const { text, clipped } = sanitizeLine(raw);
+    if (clipped) budget.truncated = true;
+    const b = byteLen(text);
+    if (b > budget.bytesLeft) {
+      budget.truncated = true;
+      break;
+    }
+    out.push({ kind, text });
+    budget.linesLeft -= 1;
+    budget.bytesLeft -= b;
+  }
+  return out;
+}
+
+/** Build a synthetic DiffFile preview from one-or-more hunk specs. */
+function buildDiffPreview(rawPath: string, specs: readonly HunkSpec[]): ApprovalPreview {
+  const path = sanitizeLine(rawPath).text;
+  let addedTotal = 0;
+  let removedTotal = 0;
+  for (const s of specs) {
+    addedTotal += s.added.length;
+    removedTotal += s.removed.length;
+  }
+
+  const budget = newBudget();
+  const hunks: DiffHunk[] = [];
+  for (let i = 0; i < specs.length; i++) {
+    const spec = specs[i]!;
+    const lines = [
+      ...emitLines('removed', spec.removed, budget),
+      ...emitLines('added', spec.added, budget),
+    ];
+    if (lines.length > 0) {
+      hunks.push({ header: specs.length > 1 ? `edit ${i + 1}` : '', lines });
+    }
+    if (budget.truncated) break;
+  }
+
+  const file: DiffFile = {
+    oldPath: path,
+    newPath: path,
+    status: removedTotal === 0 ? 'added' : 'modified',
+    added: addedTotal,
+    removed: removedTotal,
+    binary: false,
+    hunks,
+  };
+  return { kind: 'diff', file, ...(budget.truncated ? { truncated: true } : {}) };
+}
+
+function deriveDiff(toolName: string, input: Record<string, unknown>): ApprovalPreview | null {
+  const path = pickPath(input);
+
+  if (toolName === 'MultiEdit') {
+    const edits = input['edits'];
+    if (!Array.isArray(edits)) return null;
+    const specs = edits.map(toHunkSpec).filter((s): s is HunkSpec => s !== null);
+    if (specs.length === 0) return null;
+    return buildDiffPreview(path, specs);
+  }
+
+  if (toolName === 'Write') {
+    const content = input['content'];
+    if (typeof content !== 'string') return null;
+    return buildDiffPreview(path, [{ removed: [], added: splitLines(content) }]);
+  }
+
+  if (toolName === 'NotebookEdit') {
+    const source = input['new_source'];
+    if (typeof source !== 'string') return null;
+    return buildDiffPreview(path, [{ removed: [], added: splitLines(source) }]);
+  }
+
+  // Edit: a single {old_string,new_string} hunk.
+  const spec = toHunkSpec(input);
+  if (spec === null) return null;
+  return buildDiffPreview(path, [spec]);
+}
+
+// ── public entry point ───────────────────────────────────────────────────────────
+
+/**
+ * Derive a bounded approval preview from a hook tool_input, or null when the
+ * tool is not previewable or the input is malformed. NEVER throws (SEC-M7).
+ */
+export function deriveApprovalPreview(
+  toolName: string | undefined,
+  toolInput: unknown,
+): ApprovalPreview | null {
+  try {
+    if (toolName === undefined) return null;
+    const input = asRecord(toolInput);
+    if (input === null) return null;
+    if (toolName === 'Bash') return deriveCommand(input);
+    if (EDIT_TOOLS.has(toolName)) return deriveDiff(toolName, input);
+    return null;
+  } catch {
+    // SEC-M7: any unexpected shape must degrade to "no preview", never crash.
+    return null;
+  }
+}
diff --git a/src/server.ts b/src/server.ts
index 3fef1b1..fe90388 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -35,6 +35,7 @@ import { loadConfig } from './config.js'
 import { parseClientMessage, serialize } from './protocol.js'
 import { isOriginAllowed } from './http/origin.js'
 import { parseHookEvent } from './http/hook.js'
+import { deriveApprovalPreview } from './http/approval-preview.js'
 import { listSessions } from './http/history.js'
 import { buildProjects, buildProjectDetail } from './http/projects.js'
 import { openInEditor, openFileInEditor } from './http/editor.js'
@@ -49,6 +50,7 @@ import { createPushService } from './push/push-service.js'
 import { combineNotifyServices, initApns, normalizeApnsToken } from './push/apns.js'
 import { initFcm, normalizeFcmToken } from './push/fcm.js'
 import type {
+  ApprovalPreview,
   Config,
   NotifyService,
   PermissionGate,
@@ -228,6 +230,9 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
     token: string
     expiresAt: number
     gate: PermissionGate
+    /** W1: bounded command/diff preview of the held tool, re-sent to late joiners
+     *  exactly like `gate`. Undefined for non-previewable tools (e.g. ExitPlanMode). */
+    preview?: ApprovalPreview
   }
   const pendingApprovals = new Map<string, PendingApproval>()
 
@@ -460,6 +465,11 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
     // B4: an ExitPlanMode request is a 'plan' gate (three-way), else a 'tool' gate.
     const gate: PermissionGate = tool === 'ExitPlanMode' ? 'plan' : 'tool'
 
+    // W1: derive a bounded, sanitized command/diff preview from the untrusted
+    // tool_input so the approve/reject bar shows WHAT will run (never blind).
+    // Non-previewable tools / malformed input → undefined (name-only bar).
+    const preview = deriveApprovalPreview(tool, body['tool_input']) ?? undefined
+
     resolvePending(sessionId, {}) // clear any stale hold for this session
     const token = randomUUID()
     const expiresAt = Date.now() + cfg.decisionTokenTtlMs
@@ -468,14 +478,21 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
       res.json({}) // timeout → fall back to Claude's interactive prompt
       manager.handleHookEvent(sessionId, 'idle')
     }, cfg.permTimeoutMs)
-    pendingApprovals.set(sessionId, { res, timer, token, expiresAt, gate })
+    pendingApprovals.set(sessionId, {
+      res,
+      timer,
+      token,
+      expiresAt,
+      gate,
+      ...(preview !== undefined ? { preview } : {}),
+    })
 
     // A1: push the lock-screen approval (carrying the capability token) to every
     // subscribed device. Best-effort — failures are logged inside push-service.
     void pushService.notify(session, 'needs-input', token)
 
-    // Show the approve/reject affordance on every attached client.
-    manager.handleHookEvent(sessionId, 'waiting', tool, true, gate)
+    // Show the approve/reject affordance (+ W1 preview) on every attached client.
+    manager.handleHookEvent(sessionId, 'waiting', tool, true, gate, undefined, undefined, preview)
   })
 
   // ── A1 push subscription + lock-screen decision routes ────────────────────
@@ -864,9 +881,18 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
         // portion lives here because the server owns pendingApprovals).
         const heldApproval = pendingApprovals.get(boundSessionId)
         if (heldApproval !== undefined) {
+          // W1: re-send the bounded preview alongside gate so a late-joining
+          // device sees WHAT is held, not just that something is. JSON.stringify
+          // drops `preview` when undefined (non-previewable tools).
           safeSend(
             ws,
-            serialize({ type: 'status', status: 'waiting', pending: true, gate: heldApproval.gate }),
+            serialize({
+              type: 'status',
+              status: 'waiting',
+              pending: true,
+              gate: heldApproval.gate,
+              ...(heldApproval.preview !== undefined ? { preview: heldApproval.preview } : {}),
+            }),
           )
         }
         return
diff --git a/src/session/manager.ts b/src/session/manager.ts
index ab5516c..c7cbf50 100644
--- a/src/session/manager.ts
+++ b/src/session/manager.ts
@@ -26,6 +26,7 @@
  */
 
 import type {
+  ApprovalPreview,
   ClaudeStatus,
   Config,
   Dims,
@@ -218,7 +219,9 @@ export function createSessionManager(
    * H2/H3: a Claude Code hook reported activity for `sessionId`. Update the
    * status, append a timeline event (A4) when `eventClass` is supplied and the
    * timeline is enabled, and broadcast a `status` frame — carrying `gate` (B4)
-   * when the held approval is a 'tool'/'plan' gate. No-op for an unknown session.
+   * when the held approval is a 'tool'/'plan' gate, and `preview` (W1) when a
+   * bounded command/diff preview of the held tool was derived. No-op for an
+   * unknown session.
    */
   function handleHookEvent(
     sessionId: string,
@@ -228,6 +231,7 @@ export function createSessionManager(
     gate?: PermissionGate,
     eventClass?: string,
     toolName?: string,
+    preview?: ApprovalPreview,
   ): void {
     const session = sessions.get(sessionId);
     if (session === undefined) return;
@@ -246,6 +250,7 @@ export function createSessionManager(
     if (detail !== undefined) msg.detail = detail;
     if (pending) msg.pending = true;
     if (gate !== undefined) msg.gate = gate;
+    if (preview !== undefined) msg.preview = preview;
     broadcast(session, msg);
   }
 
diff --git a/src/types.ts b/src/types.ts
index fd506c9..3e5cf03 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -100,6 +100,21 @@ export type ClaudeStatus = 'working' | 'waiting' | 'idle' | 'unknown' | 'stuck';
  *  gate (three-way approve/auto/keep-planning); 'tool' = an ordinary tool gate. */
 export type PermissionGate = 'tool' | 'plan';
 
+/** W1: a compact, BOUNDED preview of what a held tool approval would run, so a
+ *  remote one-tap approval is no longer blind. Derived server-side from the hook
+ *  `tool_input` (attacker-influenced) — sanitized (control/ANSI chars stripped),
+ *  line-capped and byte-capped in src/http/approval-preview.ts. Discriminated on
+ *  `kind`:
+ *   - 'command' → a shell command string (Bash). Newlines are PRESERVED; every
+ *     other control/ANSI char is stripped. The FE renders it in a <pre> via
+ *     textContent only (never innerHTML).
+ *   - 'diff'    → ONE synthetic DiffFile (Edit/Write/MultiEdit/NotebookEdit),
+ *     rendered by public/diff.ts renderDiffFile (textContent-only, SEC-H4).
+ *  `truncated` = the source exceeded the line/byte cap and was clipped. */
+export type ApprovalPreview =
+  | { kind: 'command'; text: string; truncated?: boolean }
+  | { kind: 'diff'; file: DiffFile; truncated?: boolean };
+
 /** server → client. exit.code = shell code; -1 when spawn never succeeded (M4).
  *  exit.reason optional normally, REQUIRED on spawn failure / abnormal exit.
  *  status (H2/H3) = Claude Code activity; `pending` true when a tool approval
@@ -116,6 +131,10 @@ export type ServerMessage =
       detail?: string;
       pending?: boolean;
       gate?: PermissionGate;
+      /** W1: bounded preview of the held tool's command/diff; present only on a
+       *  held (pending) waiting status for a Bash/Edit-family tool. Older clients
+       *  ignore it (additive + optional). */
+      preview?: ApprovalPreview;
     }
   | { type: 'telemetry'; telemetry: StatusTelemetry };
 
@@ -336,6 +355,7 @@ export interface SessionManager {
     gate?: PermissionGate,
     eventClass?: string,
     toolName?: string,
+    preview?: ApprovalPreview,
   ): void;
   /** B2: store the latest statusLine telemetry for a session and broadcast a
    *  `telemetry` message to all attached clients. */
diff --git a/test/http/approval-preview.test.ts b/test/http/approval-preview.test.ts
new file mode 100644
index 0000000..0ae8ec8
--- /dev/null
+++ b/test/http/approval-preview.test.ts
@@ -0,0 +1,189 @@
+/**
+ * test/http/approval-preview.test.ts (W1) — deriveApprovalPreview unit tests.
+ *
+ * Pure, never-throws derivation of a bounded, sanitized command/diff preview
+ * from an untrusted hook tool_input. Node env (no DOM).
+ */
+
+import { describe, it, expect } from 'vitest'
+import {
+  deriveApprovalPreview,
+  PREVIEW_MAX_LINES,
+  PREVIEW_MAX_BYTES,
+} from '../../src/http/approval-preview.js'
+import type { ApprovalPreview, DiffFile } from '../../src/types.js'
+
+/** Narrow a preview to its 'diff' variant (throws in-test if wrong shape). */
+function asDiff(p: ApprovalPreview | null): DiffFile {
+  expect(p).not.toBeNull()
+  expect(p!.kind).toBe('diff')
+  return (p as { kind: 'diff'; file: DiffFile }).file
+}
+
+describe('deriveApprovalPreview — Bash → command', () => {
+  it('extracts the command string', () => {
+    const p = deriveApprovalPreview('Bash', { command: 'ls -la', description: 'x' })
+    expect(p).toEqual({ kind: 'command', text: 'ls -la' })
+  })
+
+  it('preserves newlines in a multi-line command', () => {
+    const p = deriveApprovalPreview('Bash', { command: 'a\nb' })
+    expect(p).toMatchObject({ kind: 'command' })
+    expect((p as { text: string }).text).toContain('\n')
+    expect((p as { text: string }).text).toBe('a\nb')
+  })
+
+  it('strips ANSI/control injection (ESC, BEL) from the command', () => {
+    const p = deriveApprovalPreview('Bash', { command: '\x1b[31mrm\x1b[0m\x07' })
+    const text = (p as { text: string }).text
+    expect(text).not.toContain('\x1b')
+    expect(text).not.toContain('\x07')
+    expect(text).toBe('[31mrm[0m') // control bytes gone, printable payload remains
+  })
+
+  it('returns null when command is missing / not a string', () => {
+    expect(deriveApprovalPreview('Bash', { description: 'x' })).toBeNull()
+    expect(deriveApprovalPreview('Bash', { command: 42 })).toBeNull()
+  })
+})
+
+describe('deriveApprovalPreview — Edit → diff', () => {
+  it('builds one hunk with removed + added lines and correct counts', () => {
+    const file = asDiff(
+      deriveApprovalPreview('Edit', {
+        file_path: '/p/f.ts',
+        old_string: 'a\nb',
+        new_string: 'c',
+      }),
+    )
+    expect(file.newPath).toBe('/p/f.ts')
+    expect(file.removed).toBe(2)
+    expect(file.added).toBe(1)
+    expect(file.hunks).toHaveLength(1)
+    const kinds = file.hunks[0]!.lines.map((l) => `${l.kind}:${l.text}`)
+    expect(kinds).toEqual(['removed:a', 'removed:b', 'added:c'])
+  })
+
+  it('sanitizes the file path (strips control chars)', () => {
+    const file = asDiff(
+      deriveApprovalPreview('Edit', {
+        file_path: '/p/\x1b[2Jevil.ts',
+        old_string: 'x',
+        new_string: 'y',
+      }),
+    )
+    expect(file.newPath).not.toContain('\x1b')
+    expect(file.newPath).toBe('/p/[2Jevil.ts')
+  })
+
+  it('returns null for an Edit missing old_string', () => {
+    expect(deriveApprovalPreview('Edit', { file_path: '/p/f', new_string: 'c' })).toBeNull()
+  })
+})
+
+describe('deriveApprovalPreview — Write → all-added diff', () => {
+  it('emits an all-added hunk with removed===0', () => {
+    const file = asDiff(deriveApprovalPreview('Write', { file_path: '/p/f', content: 'x\ny' }))
+    expect(file.removed).toBe(0)
+    expect(file.added).toBe(2)
+    expect(file.status).toBe('added')
+    expect(file.hunks[0]!.lines.map((l) => l.kind)).toEqual(['added', 'added'])
+  })
+
+  it('returns null when content is not a string', () => {
+    expect(deriveApprovalPreview('Write', { file_path: '/p/f' })).toBeNull()
+  })
+})
+
+describe('deriveApprovalPreview — MultiEdit → one hunk per edit', () => {
+  it('produces a hunk for each edit', () => {
+    const file = asDiff(
+      deriveApprovalPreview('MultiEdit', {
+        file_path: '/p/f',
+        edits: [
+          { old_string: 'a', new_string: 'b' },
+          { old_string: 'c', new_string: 'd' },
+        ],
+      }),
+    )
+    expect(file.hunks).toHaveLength(2)
+    expect(file.removed).toBe(2)
+    expect(file.added).toBe(2)
+  })
+
+  it('drops malformed edit entries; null when all are invalid', () => {
+    expect(
+      deriveApprovalPreview('MultiEdit', { file_path: '/p/f', edits: [{ old_string: 'a' }] }),
+    ).toBeNull()
+    const file = asDiff(
+      deriveApprovalPreview('MultiEdit', {
+        file_path: '/p/f',
+        edits: [{ old_string: 'a' }, { old_string: 'x', new_string: 'y' }],
+      }),
+    )
+    expect(file.hunks).toHaveLength(1)
+  })
+
+  it('returns null when edits is not an array', () => {
+    expect(deriveApprovalPreview('MultiEdit', { file_path: '/p/f', edits: 'nope' })).toBeNull()
+  })
+})
+
+describe('deriveApprovalPreview — NotebookEdit', () => {
+  it('previews new_source as an all-added diff', () => {
+    const file = asDiff(
+      deriveApprovalPreview('NotebookEdit', { notebook_path: '/n.ipynb', new_source: 'print(1)' }),
+    )
+    expect(file.newPath).toBe('/n.ipynb')
+    expect(file.added).toBe(1)
+    expect(file.removed).toBe(0)
+  })
+})
+
+describe('deriveApprovalPreview — truncation (DoS containment)', () => {
+  it('caps line count and flags truncated for a huge command', () => {
+    const command = Array.from({ length: PREVIEW_MAX_LINES + 20 }, (_, i) => `line${i}`).join('\n')
+    const p = deriveApprovalPreview('Bash', { command })
+    expect(p).toMatchObject({ kind: 'command', truncated: true })
+    const text = (p as { text: string }).text
+    expect(text.split('\n').length).toBeLessThanOrEqual(PREVIEW_MAX_LINES)
+  })
+
+  it('caps total bytes for a huge single-line blob', () => {
+    const command = 'x'.repeat(PREVIEW_MAX_BYTES * 4)
+    const p = deriveApprovalPreview('Bash', { command })
+    expect(p).toMatchObject({ truncated: true })
+    const text = (p as { text: string }).text
+    expect(Buffer.byteLength(text, 'utf8')).toBeLessThanOrEqual(PREVIEW_MAX_BYTES)
+  })
+
+  it('caps a giant Write and keeps the serialized diff within the byte cap', () => {
+    const content = Array.from({ length: 500 }, (_, i) => `row-${i}-`.repeat(50)).join('\n')
+    const p = deriveApprovalPreview('Write', { file_path: '/p/f', content })
+    expect(p).toMatchObject({ kind: 'diff', truncated: true })
+    const file = asDiff(p)
+    const emitted = file.hunks.flatMap((h) => h.lines)
+    expect(emitted.length).toBeLessThanOrEqual(PREVIEW_MAX_LINES)
+    const bytes = emitted.reduce((n, l) => n + Buffer.byteLength(l.text, 'utf8'), 0)
+    expect(bytes).toBeLessThanOrEqual(PREVIEW_MAX_BYTES)
+  })
+})
+
+describe('deriveApprovalPreview — unknown tools & malformed input (never throws)', () => {
+  it('returns null for a non-previewable tool', () => {
+    expect(deriveApprovalPreview('WebFetch', { url: 'http://x' })).toBeNull()
+    expect(deriveApprovalPreview('ExitPlanMode', {})).toBeNull()
+  })
+
+  it('returns null for undefined tool name', () => {
+    expect(deriveApprovalPreview(undefined, { command: 'ls' })).toBeNull()
+  })
+
+  it('returns null (never throws) for null / array / number / string tool_input', () => {
+    expect(() => deriveApprovalPreview('Bash', null)).not.toThrow()
+    expect(deriveApprovalPreview('Bash', null)).toBeNull()
+    expect(deriveApprovalPreview('Bash', [1, 2])).toBeNull()
+    expect(deriveApprovalPreview('Edit', 42)).toBeNull()
+    expect(deriveApprovalPreview('Bash', 'ls -la')).toBeNull()
+  })
+})
diff --git a/test/integration/server.test.ts b/test/integration/server.test.ts
index 85e11a1..63c2a11 100644
--- a/test/integration/server.test.ts
+++ b/test/integration/server.test.ts
@@ -712,6 +712,74 @@ describe('startServer — integration', () => {
     },
   )
 
+  // ── W1: /hook/permission carries a bounded preview + re-sends to late joiners ─
+  itPty(
+    'W1 [needs real PTY (sandbox-off)] /hook/permission broadcasts a preview and re-sends it to a late joiner',
+    async () => {
+      const ws1 = new WebSocket(`ws://127.0.0.1:${port}${cfg.wsPath}`, {
+        headers: { Origin: `http://127.0.0.1:${port}` },
+      })
+      await waitForOpen(ws1, 3_000)
+      ws1.send(JSON.stringify({ type: 'attach', sessionId: null }))
+      const attached = (await waitForMessage(
+        ws1,
+        (m) =>
+          typeof m === 'object' && m !== null && (m as Record<string, unknown>)['type'] === 'attached',
+        5_000,
+      )) as Record<string, unknown>
+      const sessionId = attached['sessionId'] as string
+
+      // Arm the pending-status listener on ws1 before firing (push is synchronous).
+      const pendingPromise = waitForMessage(
+        ws1,
+        (m) =>
+          typeof m === 'object' && m !== null && (m as Record<string, unknown>)['pending'] === true,
+        5_000,
+      )
+
+      // Fire a Bash PermissionRequest carrying tool_input — HELD until approve.
+      const permPromise = fetch(`http://127.0.0.1:${port}/hook/permission`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json', 'X-Webterm-Session': sessionId },
+        body: JSON.stringify({ tool_name: 'Bash', tool_input: { command: 'echo hi' } }),
+      })
+
+      const pending = (await pendingPromise) as Record<string, unknown>
+      expect(pending['status']).toBe('waiting')
+      const preview = pending['preview'] as Record<string, unknown> | undefined
+      expect(preview).toBeDefined()
+      expect(preview!['kind']).toBe('command')
+      expect(String(preview!['text'])).toContain('echo hi')
+
+      // Late joiner: a SECOND ws attaching to the same session while the approval
+      // is held must receive the preview on attach (re-sent like `gate`).
+      const ws2 = new WebSocket(`ws://127.0.0.1:${port}${cfg.wsPath}`, {
+        headers: { Origin: `http://127.0.0.1:${port}` },
+      })
+      await waitForOpen(ws2, 3_000)
+      const joinPending = waitForMessage(
+        ws2,
+        (m) =>
+          typeof m === 'object' && m !== null && (m as Record<string, unknown>)['pending'] === true,
+        5_000,
+      )
+      ws2.send(JSON.stringify({ type: 'attach', sessionId }))
+      const joined = (await joinPending) as Record<string, unknown>
+      const joinedPreview = joined['preview'] as Record<string, unknown> | undefined
+      expect(joinedPreview).toBeDefined()
+      expect(joinedPreview!['kind']).toBe('command')
+      expect(String(joinedPreview!['text'])).toContain('echo hi')
+
+      // Approve over ws1 to release the held POST, then clean up.
+      ws1.send(JSON.stringify({ type: 'approve' }))
+      await permPromise.then((r) => r.json()).catch(() => undefined)
+      ws1.close()
+      ws2.close()
+      await waitForClose(ws1, 3_000).catch(() => undefined)
+      await waitForClose(ws2, 3_000).catch(() => undefined)
+    },
+  )
+
   // ── H1: tmux keepalive — a shell survives a full server restart ────────────
   itTmux(
     'H1 [needs tmux + real PTY] shell state survives a server restart',
diff --git a/test/manager.test.ts b/test/manager.test.ts
index 5e3c9db..f167782 100644
--- a/test/manager.test.ts
+++ b/test/manager.test.ts
@@ -799,6 +799,39 @@ describe('handleHookEvent — gate broadcast (B4)', () => {
   });
 });
 
+// ── handleHookEvent — W1 preview broadcast ────────────────────────────────────
+describe('handleHookEvent — preview broadcast (W1)', () => {
+  it('puts the preview arg on the broadcast status frame (deep equal)', () => {
+    const mgr = createSessionManager(CFG);
+    const a = createMockWs();
+    const b = createMockWs();
+    const s = mgr.handleAttach(a, null, DIMS, 1_000);
+    mgr.handleAttach(b, s.meta.id, DIMS, 2_000);
+    a.sent.length = 0;
+    b.sent.length = 0;
+
+    const preview = { kind: 'command', text: 'ls -la' } as const;
+    mgr.handleHookEvent(s.meta.id, 'waiting', 'Bash', true, 'tool', undefined, undefined, preview);
+
+    for (const ws of [a, b]) {
+      const status = parseSent(ws).find((m) => m.type === 'status');
+      expect(status.preview).toEqual(preview);
+    }
+  });
+
+  it('omits the preview key when no preview is supplied', () => {
+    const mgr = createSessionManager(CFG);
+    const ws = createMockWs();
+    const s = mgr.handleAttach(ws, null, DIMS, 1_000);
+    ws.sent.length = 0;
+
+    mgr.handleHookEvent(s.meta.id, 'waiting', 'Bash', true, 'tool');
+
+    const status = parseSent(ws).find((m) => m.type === 'status');
+    expect(status).not.toHaveProperty('preview');
+  });
+});
+
 // ── handleStatusLine (B2 telemetry) ───────────────────────────────────────────
 describe('handleStatusLine', () => {
   it('stores telemetry on the session and broadcasts it to all clients', () => {
diff --git a/test/tabs.test.ts b/test/tabs.test.ts
index bcf1dec..dba92db 100644
--- a/test/tabs.test.ts
+++ b/test/tabs.test.ts
@@ -9,6 +9,7 @@
  */
 
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+import type { ApprovalPreview } from '../src/types.js'
 
 // ── Mock TerminalSession ──────────────────────────────────────────────────────
 let constructed = 0
@@ -22,6 +23,8 @@ class FakeTerminalSession {
   pendingTool: string | undefined = undefined
   /** B4: which gate the held approval is — 'tool' (two buttons) or 'plan' (three). */
   pendingGate: 'tool' | 'plan' | null = null
+  /** W1: bounded command/diff preview of the held tool (or null). */
+  pendingPreview: ApprovalPreview | null = null
   /** VC: nonce that counts false→true pendingApproval flips (stale-gate guard). */
   pendingEpoch = 0
   /** B2: latest statusLine telemetry (single source of truth). */
@@ -693,6 +696,75 @@ describe('TabApp — B4 plan-gate approval bar', () => {
   })
 })
 
+describe('TabApp — W1 approval preview on the bar', () => {
+  /** Open one tab, hold a tool-gate approval carrying `preview`, render the bar. */
+  function openWithPreview(preview: ApprovalPreview | null, tool = 'Bash'): FakeTerminalSession {
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    app.newTab()
+    const session = FakeTerminalSession.instances.at(-1)!
+    session.pendingApproval = true
+    session.pendingGate = 'tool'
+    session.pendingTool = tool
+    session.pendingPreview = preview
+    session.cbs.onClaudeStatus?.('waiting') // triggers updateApprovalBar
+    return session
+  }
+
+  it('renders a command preview as a .approval-cmd whose textContent is the command', () => {
+    openWithPreview({ kind: 'command', text: 'rm -rf /tmp/x' })
+    const bar = document.getElementById('approvalbar')!
+    const cmd = bar.querySelector('.approval-cmd')
+    expect(cmd).not.toBeNull()
+    expect(cmd!.textContent).toBe('rm -rf /tmp/x')
+    // Preview sits before the buttons; buttons still present (two for a tool gate).
+    expect(bar.querySelectorAll('button')).toHaveLength(2)
+  })
+
+  it('renders a diff preview via renderDiffFile (.df-file present, path shown)', () => {
+    const preview: ApprovalPreview = {
+      kind: 'diff',
+      file: {
+        oldPath: '/p/f.ts',
+        newPath: '/p/f.ts',
+        status: 'modified',
+        added: 1,
+        removed: 1,
+        binary: false,
+        hunks: [{ header: '', lines: [{ kind: 'removed', text: 'a' }, { kind: 'added', text: 'b' }] }],
+      },
+    }
+    openWithPreview(preview)
+    const bar = document.getElementById('approvalbar')!
+    expect(bar.querySelector('.approval-preview .df-file')).not.toBeNull()
+    expect(bar.querySelector('.df-path')!.textContent).toBe('/p/f.ts')
+  })
+
+  it('shows the "… truncated" note when preview.truncated is set', () => {
+    openWithPreview({ kind: 'command', text: 'x', truncated: true })
+    const bar = document.getElementById('approvalbar')!
+    expect(bar.querySelector('.approval-truncated')).not.toBeNull()
+  })
+
+  it('no preview → today\'s name-only bar (regression: label + 2 buttons, no preview)', () => {
+    openWithPreview(null, 'Bash')
+    const bar = document.getElementById('approvalbar')!
+    expect(bar.style.display).toBe('flex')
+    expect(bar.querySelector('.approval-preview')).toBeNull()
+    expect(bar.querySelectorAll('button')).toHaveLength(2)
+    expect(bar.querySelector('.approval-label')!.textContent).toBe('Claude wants to use Bash')
+  })
+
+  it('renders attacker-influenced content via textContent only (no HTML injection)', () => {
+    openWithPreview({ kind: 'command', text: '<img src=x onerror=alert(1)>' })
+    const bar = document.getElementById('approvalbar')!
+    const cmd = bar.querySelector('.approval-cmd')!
+    expect(cmd.textContent).toBe('<img src=x onerror=alert(1)>') // literal text
+    expect(cmd.querySelector('img')).toBeNull() // NOT parsed into an element
+    expect(bar.querySelector('img')).toBeNull()
+  })
+})
+
 describe('TabApp — B4 permission-mode relay', () => {
   it('openProject with a non-default mode upgrades the launch command (AC-B4.1)', () => {
     const { paneHost, tabBar } = makeHosts()
diff --git a/test/terminal-session.test.ts b/test/terminal-session.test.ts
index d72afe3..0bf6b1a 100644
--- a/test/terminal-session.test.ts
+++ b/test/terminal-session.test.ts
@@ -220,6 +220,59 @@ describe('status frame (H3 pending approval)', () => {
   })
 })
 
+describe('W1 approval preview on the status frame', () => {
+  beforeEach(() => setLocation('http:', 'lan:3000'))
+
+  function openSession(): { s: InstanceType<typeof TerminalSession>; ws: ReturnType<typeof MockWebSocket.last> } {
+    const s = new TerminalSession({ sessionId: null, onSessionId: vi.fn() })
+    s.connect()
+    const ws = MockWebSocket.last()
+    ws.openIt()
+    return { s, ws }
+  }
+
+  it('sets pendingPreview from a pending status frame', () => {
+    const { s, ws } = openSession()
+    ws.message(
+      JSON.stringify({
+        type: 'status',
+        status: 'waiting',
+        detail: 'Bash',
+        pending: true,
+        gate: 'tool',
+        preview: { kind: 'command', text: 'ls -la' },
+      }),
+    )
+    expect(s.pendingPreview).toEqual({ kind: 'command', text: 'ls -la' })
+  })
+
+  it('clears pendingPreview when a follow-up status is not pending', () => {
+    const { s, ws } = openSession()
+    ws.message(
+      JSON.stringify({ type: 'status', status: 'waiting', pending: true, preview: { kind: 'command', text: 'ls' } }),
+    )
+    expect(s.pendingPreview).not.toBeNull()
+    ws.message(JSON.stringify({ type: 'status', status: 'working', pending: false }))
+    expect(s.pendingPreview).toBeNull()
+  })
+
+  it('leaves pendingPreview null when a pending status carries no preview', () => {
+    const { s, ws } = openSession()
+    ws.message(JSON.stringify({ type: 'status', status: 'waiting', detail: 'WebFetch', pending: true, gate: 'tool' }))
+    expect(s.pendingApproval).toBe(true)
+    expect(s.pendingPreview).toBeNull()
+  })
+
+  it('approve() clears the held preview locally (resolved)', () => {
+    const { s, ws } = openSession()
+    ws.message(
+      JSON.stringify({ type: 'status', status: 'waiting', pending: true, preview: { kind: 'command', text: 'ls' } }),
+    )
+    s.approve()
+    expect(s.pendingPreview).toBeNull()
+  })
+})
+
 describe('reconnect backoff', () => {
   beforeEach(() => {
     setLocation('http:', 'lan:3000')