diff --git a/.filesize-baseline.json b/.filesize-baseline.json
index 9ecaf998..81732df0 100644
--- a/.filesize-baseline.json
+++ b/.filesize-baseline.json
@@ -1,165 +1,46 @@
 {
   "limit": 350,
   "entries": {
-    "scripts/audit-sdk-dependency.ts": {
-      "loc": 450,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/background-registry.ts": {
-      "loc": 374,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/daemon/scheduler.ts": {
-      "loc": 543,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/memory/memory-store.ts": {
-      "loc": 687,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/memory/memory-tools.ts": {
-      "loc": 437,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/providers/openai-compatible/index.ts": {
-      "loc": 396,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/providers/openai-compatible/query.ts": {
-      "loc": 803,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/session/agent-session.ts": {
-      "loc": 837,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/session/stream-consumer.ts": {
-      "loc": 359,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/tools/compose-executor.ts": {
-      "loc": 559,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/tools/dispatcher.ts": {
-      "loc": 611,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/tools/readonly-bash.ts": {
-      "loc": 415,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/tools/schemas.ts": {
-      "loc": 1277,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/tools/subagent-executor.ts": {
-      "loc": 481,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/trace/events.ts": {
-      "loc": 424,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/trace/receipt.ts": {
-      "loc": 412,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/agent/worktree-sweep.ts": {
-      "loc": 512,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/_lib/testing/virtual-screen.ts": {
-      "loc": 422,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/chat.ts": {
-      "loc": 570,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/daemon.ts": {
-      "loc": 392,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/farm.ts": {
-      "loc": 436,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/interactive.ts": {
-      "loc": 599,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/interactive/loop-iteration.ts": {
-      "loc": 464,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/interactive/tool-lane.ts": {
-      "loc": 448,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/interactive/turn-handler.ts": {
-      "loc": 365,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/interactive/worktree.ts": {
-      "loc": 478,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/commands/trace.ts": {
-      "loc": 573,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/elicitation/agent-question.ts": {
-      "loc": 399,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/slash/commands/info.ts": {
-      "loc": 443,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/cli/terminal-compositor.input-dispatch.ts": {
-      "loc": 512,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/config/env.ts": {
-      "loc": 1846,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/config/import-sources.ts": {
-      "loc": 373,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/improve/eval-run/contracts.ts": {
-      "loc": 491,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/improve/eval-run/runner.ts": {
-      "loc": 372,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/improve/propose/template-engine.ts": {
-      "loc": 459,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/skills/audit-fit/index.ts": {
-      "loc": 421,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/telegram/bot.ts": {
-      "loc": 369,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/telegram/handlers/farm-callbacks.ts": {
-      "loc": 392,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/telegram/handlers/message.ts": {
-      "loc": 581,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    },
-    "src/telegram/session-manager.ts": {
-      "loc": 468,
-      "reason": "legacy: predates the ceiling gate; pending concern extraction"
-    }
+    "scripts/audit-sdk-dependency.ts": { "loc": 450, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/background-registry.ts": { "loc": 374, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/daemon/scheduler.ts": { "loc": 543, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/memory/memory-store.ts": { "loc": 687, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/memory/memory-tools.ts": { "loc": 437, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/providers/openai-compatible/index.ts": { "loc": 396, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/providers/openai-compatible/query.ts": { "loc": 803, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/session/agent-session.ts": { "loc": 837, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/session/stream-consumer.ts": { "loc": 359, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/subagent.ts": { "loc": 351, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/tools/compose-executor.ts": { "loc": 559, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/tools/dispatcher.ts": { "loc": 612, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/tools/readonly-bash.ts": { "loc": 415, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/tools/schemas.ts": { "loc": 1277, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/tools/subagent-executor.ts": { "loc": 481, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/trace/events.ts": { "loc": 424, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/trace/receipt.ts": { "loc": 412, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/agent/worktree-sweep.ts": { "loc": 512, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/_lib/testing/virtual-screen.ts": { "loc": 422, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/chat.ts": { "loc": 570, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/daemon.ts": { "loc": 392, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/farm.ts": { "loc": 436, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/interactive.ts": { "loc": 599, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/interactive/loop-iteration.ts": { "loc": 465, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/interactive/tool-lane.ts": { "loc": 457, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/interactive/turn-handler.ts": { "loc": 366, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/interactive/worktree.ts": { "loc": 478, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/commands/trace.ts": { "loc": 573, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/elicitation/agent-question.ts": { "loc": 399, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/slash/commands/info.ts": { "loc": 443, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/cli/terminal-compositor.input-dispatch.ts": { "loc": 512, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/config/env.ts": { "loc": 1848, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/config/import-sources.ts": { "loc": 373, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/improve/eval-run/contracts.ts": { "loc": 491, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/improve/eval-run/runner.ts": { "loc": 372, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/improve/propose/template-engine.ts": { "loc": 459, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/skills/audit-fit/index.ts": { "loc": 421, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/telegram/bot.ts": { "loc": 369, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/telegram/handlers/farm-callbacks.ts": { "loc": 392, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/telegram/handlers/message.ts": { "loc": 581, "reason": "legacy: predates the ceiling gate; pending concern extraction" },
+    "src/telegram/session-manager.ts": { "loc": 443, "reason": "legacy: predates the ceiling gate; pending concern extraction" }
   }
 }
diff --git a/src/agent/default-hook-registry.ts b/src/agent/default-hook-registry.ts
index ab29df97..2504420b 100644
--- a/src/agent/default-hook-registry.ts
+++ b/src/agent/default-hook-registry.ts
@@ -24,6 +24,7 @@ import {
   type PathApprovalSurface,
 } from './tools/hooks/path-approval-hook.js';
 import { createBashRestrictionHook } from './tools/hooks/bash-restriction-hook.js';
+import { createEditPreviewHook, type PreviewDiffRef } from './tools/hooks/edit-preview-hook.js';
 import type { PermissionMode } from './types/sdk-types.js';
 import type { TraceSink } from './trace/index.js';
 import type { LoadedHooksConfig } from './hooks/config-loader.js';
@@ -39,6 +40,13 @@ export interface SubagentCompleteInfo {
 export interface DefaultHookRegistryResult {
   registry: HookRegistry;
   memoryStore: MemoryStore;
+  /**
+   * Mutable ref the StreamRenderer arms at turn start. The edit-preview hook
+   * calls `addPreviewDiffRef.current(toolUseId, diff)` to deliver previews
+   * to the tool lane. Remains a no-op `() => {}` on non-interactive surfaces
+   * (the ref is never populated outside the REPL bootstrap).
+   */
+  addPreviewDiffRef: PreviewDiffRef;
 }
 
 /**
@@ -151,6 +159,15 @@ export function createDefaultHookRegistry(
     );
   }
 
+  // Edit-preview hook: computes a diff preview at PreToolUse time for
+  // edit_file calls and delivers it to the tool lane via a mutable callback
+  // ref. Non-blocking, display-only. The ref starts as a no-op; the
+  // StreamRenderer arms it each turn on TTY surfaces.
+  const addPreviewDiffRef: PreviewDiffRef = {
+    current: () => {}, // no-op until StreamRenderer arms it each turn
+  };
+  registry.register('PreToolUse', createEditPreviewHook({ addPreviewDiffRef }));
+
   // Path-approval + bash-restriction hooks. Both share a mutable grant-manager
   // ref that the surface bootstrap populates after the provider exists.
   // `AFK_DISABLE_PATH_APPROVAL=1` skips registration entirely — escape hatch
@@ -295,7 +312,7 @@ export function createDefaultHookRegistry(
     });
   }
 
-  return { registry, memoryStore: store };
+  return { registry, memoryStore: store, addPreviewDiffRef };
 }
 
 /**
diff --git a/src/agent/hooks.ts b/src/agent/hooks.ts
index 456ff2c8..947641fc 100644
--- a/src/agent/hooks.ts
+++ b/src/agent/hooks.ts
@@ -219,6 +219,12 @@ export interface PreToolUseContext {
    * their per-session closure state, preserving prior behavior.
    */
   grantManager?: GrantManager;
+  /**
+   * The tool-call id from the model's request. Injected by
+   * {@link SessionToolDispatcher.runPreDispatchGates} so hooks can correlate
+   * the PreToolUse event with an in-flight tool-lane entry.
+   */
+  toolUseId?: string;
 }
 
 export interface PostToolUseContext {
diff --git a/src/agent/hooks/config-bridge.test.ts b/src/agent/hooks/config-bridge.test.ts
index 3dcc8ca2..112e2b6e 100644
--- a/src/agent/hooks/config-bridge.test.ts
+++ b/src/agent/hooks/config-bridge.test.ts
@@ -370,13 +370,18 @@ describe('createDefaultHookRegistry integration', () => {
   });
 
   it('createDefaultHookRegistry without hookConfig → 0 config hooks registered', () => {
-    const { registry } = createDefaultHookRegistry();
-    // Built-in handlers exist for SubagentStop and SessionEnd, plus the THREE
+    const bundle = createDefaultHookRegistry();
+    const { registry } = bundle;
+    // Built-in handlers exist for SubagentStop and SessionEnd, plus the FOUR
     // always-on built-in PreToolUse handlers (the ask-question gate, the
-    // observe-only safe-destruct detector, and the observe-only release-boundary
-    // detector), all registered unconditionally. No further PreToolUse hooks
-    // since we passed no hookConfig (path-approval disabled above).
-    expect(registry.count('PreToolUse')).toBe(3);
+    // observe-only safe-destruct detector, the observe-only release-boundary
+    // detector, and the edit-preview hook), all registered unconditionally. No
+    // further PreToolUse hooks since we passed no hookConfig (path-approval
+    // disabled above).
+    expect(registry.count('PreToolUse')).toBe(4);
+    // addPreviewDiffRef must be present on the bundle so the StreamRenderer can
+    // arm it each turn.
+    expect(bundle).toHaveProperty('addPreviewDiffRef');
   });
 
   it('createDefaultHookRegistry with hookConfig → config hooks ARE registered', () => {
@@ -401,9 +406,9 @@ describe('createDefaultHookRegistry integration', () => {
       hookConfig,
       { cwd: projectCwd },
     );
-    // 3 built-ins (ask-question gate + safe-destruct detector + release-boundary
-    // detector) + 1 config hook
-    expect(registry.count('PreToolUse')).toBe(4);
+    // 4 built-ins (ask-question gate + safe-destruct detector + release-boundary
+    // detector + edit-preview hook) + 1 config hook
+    expect(registry.count('PreToolUse')).toBe(5);
   });
 
   it('built-in SubagentStop handler still present when hookConfig is provided', () => {
diff --git a/src/agent/tools/dispatcher.ts b/src/agent/tools/dispatcher.ts
index 9992fb86..bfa1b3bc 100644
--- a/src/agent/tools/dispatcher.ts
+++ b/src/agent/tools/dispatcher.ts
@@ -927,6 +927,9 @@ export class SessionToolDispatcher implements ToolDispatcher {
         ...(this.sessionGrantManager !== undefined
           ? { grantManager: this.sessionGrantManager }
           : {}),
+        // Inject the tool-call id so hooks (e.g. edit-preview-hook) can
+        // correlate this PreToolUse event with an in-flight tool-lane entry.
+        ...(call.id !== undefined ? { toolUseId: call.id } : {}),
       };
       try {
         await dispatchPreToolUse(this.hookRegistry, preCtx, {
diff --git a/src/agent/tools/hooks/edit-preview-hook.test.ts b/src/agent/tools/hooks/edit-preview-hook.test.ts
new file mode 100644
index 00000000..0390e59f
--- /dev/null
+++ b/src/agent/tools/hooks/edit-preview-hook.test.ts
@@ -0,0 +1,74 @@
+/**
+ * Tests for createEditPreviewHook — pre-execution diff preview hook.
+ */
+import { describe, it, expect, vi } from 'vitest';
+import { createEditPreviewHook } from './edit-preview-hook.js';
+import type { DiffPayload } from '../../../utils/diff.js';
+import type { PreToolUseContext } from '../../hooks.js';
+
+function makeCtx(overrides: Partial<PreToolUseContext> = {}): PreToolUseContext {
+  return {
+    event: 'PreToolUse',
+    toolName: 'edit_file',
+    input: { old_string: 'a', new_string: 'b', file_path: 'foo.ts' },
+    toolUseId: 'tu_001',
+    ...overrides,
+  };
+}
+
+describe('createEditPreviewHook', () => {
+  it('(a) fires callback with DiffPayload for a normal edit', () => {
+    const cb = vi.fn();
+    const ref = { current: cb };
+    const hook = createEditPreviewHook({ addPreviewDiffRef: ref });
+    const decision = hook(makeCtx());
+    expect(cb).toHaveBeenCalledOnce();
+    const [id, diff] = cb.mock.calls[0] as [string, DiffPayload];
+    expect(id).toBe('tu_001');
+    expect(diff.hunks.length).toBeGreaterThan(0);
+    expect(decision).toEqual({});
+  });
+
+  it('(b) no-op when old_string === new_string', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    hook(makeCtx({ input: { old_string: 'same', new_string: 'same', file_path: 'x.ts' } }));
+    expect(cb).not.toHaveBeenCalled();
+  });
+
+  it('(c) skips subagents (parentSessionId set)', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    hook(makeCtx({ parentSessionId: 'parent-sess' }));
+    expect(cb).not.toHaveBeenCalled();
+  });
+
+  it('(d) skips non-edit_file tools', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    hook(makeCtx({ toolName: 'bash' }));
+    expect(cb).not.toHaveBeenCalled();
+  });
+
+  it('(e) skips non-PreToolUse events', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    // Cast to satisfy type — hook must guard event at runtime
+    hook({ event: 'PostToolUse', toolName: 'edit_file' } as any);
+    expect(cb).not.toHaveBeenCalled();
+  });
+
+  it('(f) returned decision has no injectContext', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    const decision = hook(makeCtx());
+    expect(decision).not.toHaveProperty('injectContext');
+  });
+
+  it('(g) no-op when toolUseId absent', () => {
+    const cb = vi.fn();
+    const hook = createEditPreviewHook({ addPreviewDiffRef: { current: cb } });
+    hook(makeCtx({ toolUseId: undefined }));
+    expect(cb).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/agent/tools/hooks/edit-preview-hook.ts b/src/agent/tools/hooks/edit-preview-hook.ts
new file mode 100644
index 00000000..231abe3c
--- /dev/null
+++ b/src/agent/tools/hooks/edit-preview-hook.ts
@@ -0,0 +1,80 @@
+/**
+ * Edit-preview hook: computes a diff preview at PreToolUse time for
+ * edit_file calls and communicates it to the tool lane via a callback ref.
+ *
+ * Non-blocking. Returns {} (passthrough) always — this is display-only.
+ * No I/O. Skips subagents (parentSessionId set) — they have no overlay.
+ *
+ * @module agent/tools/hooks/edit-preview-hook
+ */
+
+import { computeLineDiff } from '../../../utils/diff.js';
+import type { HookContext, HookDecision, HookHandler } from '../../hooks.js';
+import type { DiffPayload } from '../../../utils/diff.js';
+
+// ---------------------------------------------------------------------------
+// Public types
+// ---------------------------------------------------------------------------
+
+/** Mutable ref type for delivering preview diffs from the hook to the tool lane. */
+export type PreviewDiffRef = { current: (toolUseId: string, diff: DiffPayload) => void };
+
+export interface EditPreviewHookOptions {
+  /**
+   * Mutable ref populated by the StreamRenderer when it arms. The hook
+   * calls `ref.current(toolUseId, diff)` to deliver the preview to the
+   * tool lane without importing any CLI-layer module.
+   *
+   * Remains a no-op `() => {}` until the StreamRenderer arms it each turn.
+   * On non-interactive surfaces (daemon, Telegram) it is never armed.
+   */
+  addPreviewDiffRef: PreviewDiffRef;
+}
+
+// ---------------------------------------------------------------------------
+// Factory
+// ---------------------------------------------------------------------------
+
+/**
+ * Create a `PreToolUse` hook that computes a pre-execution diff for
+ * `edit_file` calls and delivers it to the tool lane via the supplied ref.
+ *
+ * @param opts - {@link EditPreviewHookOptions}
+ * @returns A {@link HookHandler} to register on the `PreToolUse` event.
+ */
+export function createEditPreviewHook(opts: EditPreviewHookOptions): HookHandler {
+  return (context: HookContext): HookDecision => {
+    // Guard: only PreToolUse
+    if (context.event !== 'PreToolUse') return {};
+    // Guard: only edit_file
+    if (context.toolName !== 'edit_file') return {};
+    // Guard: subagents have no overlay — skip silently
+    if (context.parentSessionId !== undefined) return {};
+    // Guard: need a toolUseId to key the lane entry
+    if (!context.toolUseId) return {};
+
+    const input = context.input as Record<string, unknown> | undefined;
+    if (!input) return {};
+    const oldStr = typeof input['old_string'] === 'string' ? input['old_string'] : undefined;
+    const newStr = typeof input['new_string'] === 'string' ? input['new_string'] : undefined;
+    if (oldStr === undefined || newStr === undefined) return {};
+    // No-op edit: mirrors computeLineDiff null-on-identical behavior
+    if (oldStr === newStr) return {};
+
+    // replace_all preview would require reading the file to know how many
+    // occurrences exist and where they are — not available to this hook (no I/O
+    // by design). Skip the preview entirely to avoid a misleading single-hunk
+    // diff that doesn't reflect the full multi-occurrence replacement.
+    if (input['replace_all']) return {};
+
+    // TODO(#1366): preview may diverge from final input when canUseTool returns
+    // updatedInput — requires hook re-fire or post-rewrite preview refresh.
+
+    // computeLineDiff returns non-null for distinct strings; the null case
+    // (identical input) is handled by the oldStr === newStr guard above.
+    const diff = computeLineDiff(oldStr, newStr)!;
+
+    opts.addPreviewDiffRef.current(context.toolUseId, diff);
+    return {};
+  };
+}
diff --git a/src/cli/_lib/stream-renderer-options.ts b/src/cli/_lib/stream-renderer-options.ts
index d6f3a37f..b33762f1 100644
--- a/src/cli/_lib/stream-renderer-options.ts
+++ b/src/cli/_lib/stream-renderer-options.ts
@@ -10,6 +10,7 @@ import type { Writer } from '../slash/types.js';
 import type { IHistoryRing } from '../input/types.js';
 import type { AutocompleteState } from '../input/autocomplete-state.js';
 import type { TerminalCompositor } from '../terminal-compositor.js';
+import type { PreviewDiffRef } from '../../agent/tools/hooks/edit-preview-hook.js';
 
 export interface StreamRendererOptions {
   /** Where line-based output goes (non-TTY fallback + always-emitted compact lines). */
@@ -157,4 +158,15 @@ export interface StreamRendererOptions {
    * streaming content chunk arrives (see `notifyFirstContent`).
    */
   turnStartedAt?: number;
+  /**
+   * Mutable ref the StreamRenderer arms each turn in {@link StreamRenderer.arm}.
+   * The edit-preview hook calls `addPreviewDiffRef.current(toolUseId, diff)` to
+   * deliver a pre-execution diff preview to the tool lane without importing any
+   * CLI-layer module.
+   *
+   * On non-interactive surfaces (daemon, Telegram, tests) this ref is never
+   * armed and remains a no-op. The REPL bootstrap threads it from
+   * `DefaultHookRegistryResult.addPreviewDiffRef`.
+   */
+  addPreviewDiffRef?: PreviewDiffRef;
 }
diff --git a/src/cli/_lib/stream-renderer.ts b/src/cli/_lib/stream-renderer.ts
index 07eb9565..3beef9c5 100644
--- a/src/cli/_lib/stream-renderer.ts
+++ b/src/cli/_lib/stream-renderer.ts
@@ -27,6 +27,7 @@
  */
 
 import type { OutputEvent, SubagentProgressMeta, ProgressEvent } from '../../agent/types.js';
+import type { PreviewDiffRef } from '../../agent/tools/hooks/edit-preview-hook.js';
 import type { Message } from '../../agent/types/message-types.js';
 import type { Writer } from '../slash/types.js';
 import { TerminalCompositor } from '../terminal-compositor.js';
@@ -177,6 +178,9 @@ export class StreamRenderer {
   /** Last annotation string rendered for the TTFB line — drives 1 Hz change detection. */
   private lastTtfbAnnotation = '';
 
+  /** Ref wired in arm() so the edit-preview hook can push diffs to the tool lane. */
+  private readonly addPreviewDiffRef: PreviewDiffRef | undefined;
+
   /**
    * Pre-bound sink — pass directly to `runWithSink(...)` from callers.
    * Equivalent to `(event, meta) => this.process(event, meta)`.
@@ -243,6 +247,7 @@ export class StreamRenderer {
     }
     this.ttfbStartedAt = opts.turnStartedAt;
     this.ttfbDone = opts.turnStartedAt === undefined;
+    this.addPreviewDiffRef = opts.addPreviewDiffRef;
 
     this.sink = (event, meta) => this.process(event, meta);
   }
@@ -317,6 +322,16 @@ export class StreamRenderer {
     }
     this.compositor = compositor;
 
+    // Wire the edit-preview ref so the hook can push diff previews into the
+    // tool lane during this turn. No-op when ref is absent (non-REPL surfaces).
+    if (this.addPreviewDiffRef) {
+      this.addPreviewDiffRef.current = (toolUseId, diff) => {
+        this.toolLane.addPreviewDiff(toolUseId, diff);
+        this.overlayComposer?.markDirty('tool-lane');
+        this.overlayComposer?.flush();
+      };
+    }
+
     // Construct the OverlayComposer with the five overlay slot types in z-order.
     // The slots read live state at flush time, so there's no initialization
     // needed beyond construction and registration. 'interrupt' is bottom-most
@@ -529,6 +544,9 @@ export class StreamRenderer {
   async dispose(): Promise<void> {
     if (this.disposed) return;
     this.disposed = true;
+    // Reset the preview-diff ref to a no-op so the disposed turn's toolLane
+    // reference is released and the hook cannot write into a stale lane.
+    if (this.addPreviewDiffRef) this.addPreviewDiffRef.current = () => {};
     // Clear the subagent elapsed-time ticker immediately — it guards against
     // `this.disposed` but clearing here is cleaner and avoids one extra tick.
     if (this.subagentTickInterval !== null) {
diff --git a/src/cli/commands/interactive/bootstrap-hooks.ts b/src/cli/commands/interactive/bootstrap-hooks.ts
index bb099b89..a1110606 100644
--- a/src/cli/commands/interactive/bootstrap-hooks.ts
+++ b/src/cli/commands/interactive/bootstrap-hooks.ts
@@ -8,6 +8,7 @@ import type { CompletionWriter } from './shared.js';
 import { emitSubagentCompletion } from './progress-banner.js';
 import { createTerminalStateGate } from './terminal-state-gate.js';
 import { loadConfig } from '../../config.js';
+import type { PreviewDiffRef } from '../../../agent/tools/hooks/edit-preview-hook.js';
 
 /**
  * Build the stable hook registry shared across sessions (including
@@ -16,9 +17,6 @@ import { loadConfig } from '../../config.js';
  *
  * The hook callbacks close over `stats` and `completionWriter`, which are
  * also stable, so a rebuilt session gets the same routing without re-wiring.
- * `pathApprovalGrantRef` is populated by the caller once the provider
- * exists — the path-approval hook fails open until then (mirroring
- * `setAllowDirDispatcher` wiring order).
  *
  * The terminal-state gate (issue #237) is a post-turn `Stop` hook that
  * bounces a self-certified `Done` with no corroborating evidence back into
@@ -36,7 +34,10 @@ export function createReplHookRegistry(a: {
   stats: SessionStats;
   effectiveCwd: string | undefined;
   traceWriter: TraceSink | undefined;
-}): { hookRegistry: HookRegistry } {
+}): {
+  hookRegistry: HookRegistry;
+  addPreviewDiffRef: PreviewDiffRef;
+} {
   const hookRegistryBundle = createDefaultHookRegistry(
     (info) => { emitSubagentCompletion(a.completionWriter, info); },
     'cli',
@@ -47,6 +48,7 @@ export function createReplHookRegistry(a: {
     () => a.effectiveCwd ?? process.cwd(),
   );
   const hookRegistry = hookRegistryBundle.registry;
+  const addPreviewDiffRef = hookRegistryBundle.addPreviewDiffRef;
 
   hookRegistry.register(
     'Stop',
@@ -56,5 +58,5 @@ export function createReplHookRegistry(a: {
     }),
   );
 
-  return { hookRegistry };
+  return { hookRegistry, addPreviewDiffRef };
 }
diff --git a/src/cli/commands/interactive/bootstrap.ts b/src/cli/commands/interactive/bootstrap.ts
index 2e31c27a..e9c337b8 100644
--- a/src/cli/commands/interactive/bootstrap.ts
+++ b/src/cli/commands/interactive/bootstrap.ts
@@ -123,7 +123,7 @@ export async function bootstrapSession(
 
   // Stable hookRegistry shared across sessions (including swaps), plus the
   // terminal-state Stop gate registered on top of it.
-  const { hookRegistry } = createReplHookRegistry({
+  const { hookRegistry, addPreviewDiffRef } = createReplHookRegistry({
     completionWriter, memoryStore: sharedMemoryStore, stats, effectiveCwd, traceWriter: trace?.writer,
   });
 
@@ -342,6 +342,7 @@ export async function bootstrapSession(
       ? { suggestGhostConfig: cliConfig.interactive.suggestGhost }
       : {}),
     hookRegistry,
+    addPreviewDiffRef,
   };
 
   // Trusted-skill event subscriptions — emit in-flight + completion badges
diff --git a/src/cli/commands/interactive/loop-iteration.ts b/src/cli/commands/interactive/loop-iteration.ts
index a88aa216..35af0e88 100644
--- a/src/cli/commands/interactive/loop-iteration.ts
+++ b/src/cli/commands/interactive/loop-iteration.ts
@@ -809,6 +809,9 @@ export async function runInputLoop(
               },
             }
           : {}),
+        // Thread the edit-preview ref so the StreamRenderer can wire the hook
+        // callback into the tool lane during arm(). Absent on non-REPL callers.
+        ...(ctx.addPreviewDiffRef ? { addPreviewDiffRef: ctx.addPreviewDiffRef } : {}),
       }, ctx.stats.thinkingUi ?? ctx.options.thinkingUi, ctx.completionWriter,
         // Surface refs threaded into the per-turn StreamRenderer for the
         // legacy non-borrow path (non-TTY, when surface.getCompositor()
diff --git a/src/cli/commands/interactive/shared.ts b/src/cli/commands/interactive/shared.ts
index 2a0e49c4..2dc83e23 100644
--- a/src/cli/commands/interactive/shared.ts
+++ b/src/cli/commands/interactive/shared.ts
@@ -2,6 +2,7 @@ import * as readline from 'node:readline';
 import { statSync } from 'node:fs';
 import { getQuotaSnapshot } from '../../../agent/quota-cache.js';
 import type { HookRegistry } from '../../../agent/hooks.js';
+import type { PreviewDiffRef } from '../../../agent/tools/hooks/edit-preview-hook.js';
 import type { SessionRef } from '../../../agent/session-ref.js';
 import type { MemoryStore } from '../../../agent/memory/index.js';
 import type { AgentModelInput } from '../../../agent/types.js';
@@ -526,6 +527,12 @@ export interface InteractiveCtx {
    * call (enabling per-prompt policy hooks) and Stop after each completed turn.
    */
   hookRegistry?: HookRegistry;
+  /**
+   * Mutable ref threaded from `createReplHookRegistry` into the per-turn
+   * StreamRenderer so the edit-preview hook can deliver diff previews to the
+   * tool lane without importing CLI-layer modules. Absent on non-REPL callers.
+   */
+  addPreviewDiffRef?: PreviewDiffRef;
 }
 
 /**
@@ -760,6 +767,12 @@ export interface TurnHandles {
     stage: import('./loop-stage.js').LoopStage,
     signals?: import('./loop-stage.js').StageSignals,
   ): void;
+  /**
+   * Mutable ref threaded from `createReplHookRegistry` into the per-turn
+   * StreamRenderer so the edit-preview hook can deliver diff previews to the
+   * tool lane without importing CLI-layer modules. Absent on non-REPL callers.
+   */
+  addPreviewDiffRef?: PreviewDiffRef;
 }
 
 // `discardStdin: false` is load-bearing — ora's default wraps process.stdin
diff --git a/src/cli/commands/interactive/tool-lane-format-diff.ts b/src/cli/commands/interactive/tool-lane-format-diff.ts
index 77dc3106..4bc7d273 100644
--- a/src/cli/commands/interactive/tool-lane-format-diff.ts
+++ b/src/cli/commands/interactive/tool-lane-format-diff.ts
@@ -2,6 +2,7 @@ import { env } from '../../../config/env.js';
 import type { DiffPayload, DiffLine } from '../../../utils/diff.js';
 import { palette } from '../../palette.js';
 import { stripAnsi } from '../../display.js';
+import { previewDiff } from '../../render/preview-diff.js';
 
 /**
  * Maximum number of diff body lines to render in the live overlay before
@@ -62,7 +63,7 @@ function diffFlushMaxLines(): number {
  * — this only suppresses the render. That keeps the JSON-output / Telegram
  * surfaces unaffected if they choose to surface diffs independently.
  */
-function diffsDisabled(): boolean {
+export function diffsDisabled(): boolean {
   const raw = env.AFK_SHOW_DIFFS;
   if (raw === undefined) return false;
   const v = raw.trim().toLowerCase();
@@ -277,3 +278,13 @@ function _diffBlockCacheStore(diff: DiffPayload, key: string, result: string[]):
   inner.set(key, result);
 }
 
+
+/**
+ * Render a pre-execution diff preview into an array of indented lines for the
+ * live overlay. Returns `[]` when `AFK_SHOW_DIFFS=0` (same gate as
+ * {@link formatDiffBlock}). Each returned line already includes `indent`.
+ */
+export function formatPreviewDiffBlock(diff: DiffPayload, indent: string): string[] {
+  if (diffsDisabled()) return [];
+  return previewDiff(diff).split('\n').map((l) => indent + l);
+}
diff --git a/src/cli/commands/interactive/tool-lane-format.ts b/src/cli/commands/interactive/tool-lane-format.ts
index 138f3361..ac8d5f09 100644
--- a/src/cli/commands/interactive/tool-lane-format.ts
+++ b/src/cli/commands/interactive/tool-lane-format.ts
@@ -23,6 +23,8 @@ export {
   MAX_OVERLAY_DIFF_LINES,
   FLUSH_DIFF_LINES_DEFAULT,
   formatDiffBlock,
+  diffsDisabled,
+  formatPreviewDiffBlock,
 } from './tool-lane-format-diff.js';
 
 /**
diff --git a/src/cli/commands/interactive/tool-lane-render-children.ts b/src/cli/commands/interactive/tool-lane-render-children.ts
index 71b71677..e8d19ed0 100644
--- a/src/cli/commands/interactive/tool-lane-render-children.ts
+++ b/src/cli/commands/interactive/tool-lane-render-children.ts
@@ -6,6 +6,7 @@ import {
   inProgressVerb,
   formatOutcome,
   formatDiffBlock,
+  formatPreviewDiffBlock,
   doneGlyph,
   sanitizeLabel,
 } from './tool-lane-format.js';
@@ -286,6 +287,14 @@ function renderOverlayChildren(
         // connector + tool name). Computed at repaint time from child.startedAt;
         // suppressed under ELAPSED_GRACE_MS (2s) to avoid flicker on fast tools.
         lines.push(clampLineToTerminal(indentColored + connector + child.prefix + palette.dim(' …') + formatElapsed(child.startedAt), cols));
+        if (child.previewDiff) {
+          // Pre-execution diff preview for nested edit_file. formatPreviewDiffBlock
+          // renders ⟳ Proposed and applies the AFK_SHOW_DIFFS=0 opt-out.
+          const previewIndent = indentColored + (isLast ? g.spineClosed : palette.dim(g.spine)) + '  ';
+          for (const line of formatPreviewDiffBlock(child.previewDiff, previewIndent)) {
+            lines.push(clampLineToTerminal(line, cols));
+          }
+        }
         // In-progress / thinking continuation hangs under the prefix at the
         // same "past-connector" column the diff path uses.
         const continuationIndent = indentColored + (isLast ? g.spineClosed : palette.dim(g.spine)) + '  ';
diff --git a/src/cli/commands/interactive/tool-lane-render.ts b/src/cli/commands/interactive/tool-lane-render.ts
index 253382e0..4165fe31 100644
--- a/src/cli/commands/interactive/tool-lane-render.ts
+++ b/src/cli/commands/interactive/tool-lane-render.ts
@@ -90,6 +90,14 @@ interface ToolEntryFields {
    * (truncated to 8 lines) and flush (full diff) paths.
    */
   diff?: DiffPayload;
+  /**
+   * Optional pre-execution diff preview attached by the edit-preview hook
+   * at PreToolUse time. Rendered in the live overlay while the call is
+   * in-flight (guarded by `!entry.result`). Never appears in scrollback —
+   * once the tool_result arrives, the post-execution diff (via the existing
+   * `tool_diff` sidecar path) takes over in the flush path.
+   */
+  previewDiff?: DiffPayload;
   /**
    * Set to `true` by {@link ToolLane.flushSource} the first time it emits
    * this entry's header line eagerly (before the entry's own done-event).
diff --git a/src/cli/commands/interactive/tool-lane.test.ts b/src/cli/commands/interactive/tool-lane.test.ts
index b9e38df5..58f394cc 100644
--- a/src/cli/commands/interactive/tool-lane.test.ts
+++ b/src/cli/commands/interactive/tool-lane.test.ts
@@ -16,6 +16,7 @@ import { displayWidth, stripAnsi } from '../../display.js';
 import { palette } from '../../palette.js';
 import type { ToolResultChunk } from '../../../agent/types/message-types.js';
 import type { OutputEvent, SubagentProgressMeta } from '../../../agent/types.js';
+import type { DiffPayload } from '../../../utils/diff.js';
 
 function makeResult(content: string, isError = false): ToolResultChunk {
   return {
@@ -3034,3 +3035,61 @@ describe('ToolLane.peekTrailingCompletedRootToolName — cross-flush run gate',
     expect(lane.peekTrailingCompletedRootToolName()).toBe('Read');
   });
 });
+
+describe('ToolLane.addPreviewDiff — pre-execution diff preview', () => {
+  const sampleDiff: DiffPayload = {
+    addedLines: 1, removedLines: 1,
+    hunks: [{
+      oldStart: 1, oldLines: 1, newStart: 1, newLines: 1,
+      lines: [{ kind: '-', text: 'before' }, { kind: '+', text: 'after' }],
+    }],
+  };
+
+  it('(a) preview renders in overlay while in-flight (no result yet)', () => {
+    const lane = new ToolLane();
+    lane.addStart('tu_pre', 'edit_file', '(foo.ts)');
+    lane.addPreviewDiff('tu_pre', sampleDiff);
+    const overlay = stripAnsi(lane.getOverlay());
+    expect(overlay).toContain('before');
+    expect(overlay).toContain('after');
+  });
+
+  it('(b) preview absent from overlay after result arrives', () => {
+    const lane = new ToolLane();
+    lane.addStart('tu_pre', 'edit_file', '(foo.ts)');
+    lane.addPreviewDiff('tu_pre', sampleDiff);
+    lane.addResult('tu_pre', makeResult('Replaced 1 occurrence'));
+    const overlay = stripAnsi(lane.getOverlay());
+    // result branch: shows outcome line, not preview diff
+    expect(overlay).not.toContain('@@ -1,1 +1,1 @@');
+    // positive: outcome line is present
+    expect(overlay).toContain('Replaced 1 occurrence');
+  });
+
+  it('(c) no-op for unknown toolUseId', () => {
+    const lane = new ToolLane();
+    expect(() => lane.addPreviewDiff('ghost_id', sampleDiff)).not.toThrow();
+  });
+
+  it('(d) AFK_SHOW_DIFFS=0 suppresses preview in overlay', () => {
+    process.env['AFK_SHOW_DIFFS'] = '0';
+    try {
+      const lane = new ToolLane();
+      lane.addStart('tu_env', 'edit_file', '(x.ts)');
+      lane.addPreviewDiff('tu_env', sampleDiff);
+      const overlay = stripAnsi(lane.getOverlay());
+      expect(overlay).not.toContain('@@ -1,1 +1,1 @@');
+    } finally {
+      delete process.env['AFK_SHOW_DIFFS'];
+    }
+  });
+
+  it('(e) preview does not appear in flush/scrollback (overlay-only)', () => {
+    const lane = new ToolLane();
+    lane.addStart('tu_fl', 'edit_file', '(bar.ts)');
+    lane.addPreviewDiff('tu_fl', sampleDiff);
+    // No addResult — entry stays in-flight; flush should not emit preview
+    const flushed = stripAnsi(lane.flush().join('\n'));
+    expect(flushed).not.toContain('@@ -1,1 +1,1 @@');
+  });
+});
diff --git a/src/cli/commands/interactive/tool-lane.ts b/src/cli/commands/interactive/tool-lane.ts
index 9422062c..b171ba62 100644
--- a/src/cli/commands/interactive/tool-lane.ts
+++ b/src/cli/commands/interactive/tool-lane.ts
@@ -1,7 +1,7 @@
 import type { ToolResultChunk } from '../../../agent/types/message-types.js';
 import { palette } from '../../palette.js';
 import { SUBAGENT_TOOLS, NESTING_TOOLS, SKILL_TOOLS } from '../../tool-category.js';
-import { formatToolLine, formatToolResultLine, formatOutcome, formatDiffBlock, doneGlyph, sanitizeLabel, batchBadge } from './tool-lane-format.js';
+import { formatToolLine, formatToolResultLine, formatOutcome, formatDiffBlock, formatPreviewDiffBlock, doneGlyph, sanitizeLabel, batchBadge } from './tool-lane-format.js';
 import type { DiffPayload } from '../../../utils/diff.js';
 import { truncateDisplayWidth, stripAnsi } from '../../display.js';
 import { formatElapsed, ELAPSED_GRACE_MS } from '../../terminal-compositor.scrollback.js';
@@ -178,7 +178,8 @@ export class ToolLane {
 
   addResult(toolUseId: string, chunk: ToolResultChunk): void {
     const entry = this.entries.get(toolUseId);
-    if (entry?.kind === 'tool') entry.result = chunk;
+    // Clear previewDiff on the same tick as result — preview is now obsolete.
+    if (entry?.kind === 'tool') { entry.result = chunk; entry.previewDiff = undefined; }
     if (this.agentIdStack.at(-1) === toolUseId) {
       this.agentIdStack.pop();
     }
@@ -196,6 +197,21 @@ export class ToolLane {
     if (entry?.kind === 'tool') entry.diff = diff;
   }
 
+  /**
+   * Attach a pre-execution diff preview to an in-flight tool entry. Called by
+   * the edit-preview hook at PreToolUse time, before the edit executes.
+   * No-op if the entry doesn't exist or is not a tool entry.
+   *
+   * The preview is rendered in the live overlay only (while `!entry.result`).
+   * It does NOT appear in scrollback (flush). Once the tool_result arrives
+   * and the post-execution diff sidechannel takes over, the preview is no
+   * longer shown.
+   */
+  addPreviewDiff(toolUseId: string, diff: DiffPayload): void {
+    const entry = this.entries.get(toolUseId);
+    if (entry?.kind === 'tool') entry.previewDiff = diff;
+  }
+
   /**
    * Create or replace the `text` of a {@link TextEntry} child. Used by the
    * streaming renderer to drive a subagent's live narration under its
@@ -559,6 +575,13 @@ export class ToolLane {
           // Live elapsed counter: same pattern as the NESTING branch above —
           // computed at repaint time, suppressed under ELAPSED_GRACE_MS (2s).
           lines.push(clamp(flatRootLead + entry.prefix + palette.dim(' …') + formatElapsed(entry.startedAt)));
+          if (entry.previewDiff) {
+            // Pre-execution diff preview: formatPreviewDiffBlock renders ⟳ Proposed
+            // and applies the AFK_SHOW_DIFFS=0 opt-out (returns [] when disabled).
+            for (const line of formatPreviewDiffBlock(entry.previewDiff, '    ')) {
+              lines.push(clamp(line));
+            }
+          }
           if (entry.thinkingTail) {
             // Childless Agent entries (a child just opened its thinking block
             // and hasn't yet emitted content or a tool_use) get the tail right
diff --git a/src/cli/commands/interactive/turn-handler.ts b/src/cli/commands/interactive/turn-handler.ts
index 7dd24d9e..177513fc 100644
--- a/src/cli/commands/interactive/turn-handler.ts
+++ b/src/cli/commands/interactive/turn-handler.ts
@@ -209,6 +209,7 @@ export async function runTurn(
     // before the first streaming content token arrives. The renderer clears
     // the line automatically when notifyFirstContent() is called below.
     ...ttfbRendererOptions(turnTtfb),
+    ...(h.addPreviewDiffRef ? { addPreviewDiffRef: h.addPreviewDiffRef } : {}),
   });
 
   // `let` (not `const`) so the resumed-event handler can swap in a fresh
diff --git a/src/cli/render/index.ts b/src/cli/render/index.ts
index 34475b61..14e20143 100644
--- a/src/cli/render/index.ts
+++ b/src/cli/render/index.ts
@@ -24,3 +24,4 @@ export * from './utils.js';
 export * from './tool-card.js';
 export * from './interrupt-peek.js';
 export * from './compact-diff-view.js';
+export * from './preview-diff.js';
diff --git a/src/cli/render/preview-diff.test.ts b/src/cli/render/preview-diff.test.ts
new file mode 100644
index 00000000..9b90c16c
--- /dev/null
+++ b/src/cli/render/preview-diff.test.ts
@@ -0,0 +1,54 @@
+/**
+ * Tests for previewDiff — pre-execution diff preview render component.
+ */
+import { describe, it, expect } from 'vitest';
+import { stripAnsi } from '../display.js';
+import { previewDiff } from './preview-diff.js';
+import type { DiffPayload } from '../../utils/diff.js';
+
+const samplePayload: DiffPayload = {
+  addedLines: 1,
+  removedLines: 1,
+  hunks: [{
+    oldStart: 1, oldLines: 1, newStart: 1, newLines: 1,
+    lines: [{ kind: '-', text: 'old line' }, { kind: '+', text: 'new line' }],
+  }],
+};
+
+describe('previewDiff', () => {
+  it('(a) contains ⟳ Proposed label', () => {
+    const out = previewDiff(samplePayload);
+    expect(out).toContain('⟳ Proposed');
+  });
+
+  it('(b) contains hunk content (after strip-ansi)', () => {
+    const out = stripAnsi(previewDiff(samplePayload));
+    expect(out).toContain('old line');
+    expect(out).toContain('new line');
+  });
+
+  it('(c) maxLines propagation truncates long diffs', () => {
+    const manyLines: DiffPayload = {
+      addedLines: 5,
+      removedLines: 0,
+      hunks: [{
+        oldStart: 1, oldLines: 0, newStart: 1, newLines: 5,
+        lines: Array.from({ length: 5 }, (_, i) => ({ kind: '+' as const, text: `line ${i}` })),
+      }],
+    };
+    const out = stripAnsi(previewDiff(manyLines, { maxLines: 2 }));
+    expect(out).toContain('⟳ Proposed');
+    // Only 2 body lines rendered; the rest elided
+    const bodyLines = out.split('\n').filter(l => l.includes('+ line'));
+    expect(bodyLines.length).toBeLessThanOrEqual(2);
+  });
+
+  it('(d) accepts optional filePath in stat header', () => {
+    const out = stripAnsi(previewDiff(samplePayload, { filePath: 'src/foo.ts' }));
+    expect(out).toContain('src/foo.ts');
+  });
+
+  it('returns a string (type check)', () => {
+    expect(typeof previewDiff(samplePayload)).toBe('string');
+  });
+});
diff --git a/src/cli/render/preview-diff.ts b/src/cli/render/preview-diff.ts
new file mode 100644
index 00000000..6cdcb48f
--- /dev/null
+++ b/src/cli/render/preview-diff.ts
@@ -0,0 +1,66 @@
+/**
+ * PreviewDiff — pre-execution diff preview render component.
+ *
+ * A pure `(payload, opts?) => string` component that renders a diff preview
+ * for display in the live overlay before an `edit_file` write occurs. The
+ * `⟳ Proposed` label distinguishes it from the post-execution diff that
+ * arrives via the `tool_diff` sidecar channel.
+ *
+ * Delegates all diff layout to {@link compactDiffView} so truncation,
+ * box-framing, and line-colouring are handled identically.
+ *
+ * LOC ceiling: 350 (enforced by audit:filesize:check).
+ */
+
+import { palette } from '../palette.js';
+import { compactDiffView } from './compact-diff-view.js';
+import type { DiffPayload } from '../../utils/diff.js';
+
+// ---------------------------------------------------------------------------
+// Public types
+// ---------------------------------------------------------------------------
+
+export interface PreviewDiffOptions {
+  /**
+   * Optional file path shown in the stat header of the compact diff view.
+   * When omitted a generic `'preview'` placeholder is used — the tool-lane
+   * entry's own prefix line already names the file, so this is supplemental.
+   */
+  filePath?: string;
+  /**
+   * Maximum number of diff body lines to render before truncating.
+   * Passed through to {@link compactDiffView}. Default: 20.
+   */
+  maxLines?: number;
+  /**
+   * Terminal width passed through to {@link compactDiffView} for box sizing.
+   * Default: 80.
+   */
+  width?: number;
+}
+
+// ---------------------------------------------------------------------------
+// Public API
+// ---------------------------------------------------------------------------
+
+/**
+ * Render a pre-execution diff preview as a multi-line string for overlay display.
+ *
+ * @param payload - Structured diff produced by {@link computeLineDiff}.
+ * @param opts    - Optional rendering overrides (filePath, maxLines, width).
+ * @returns A styled, multi-line string beginning with `⟳ Proposed`.
+ *          Ends without a trailing newline.
+ */
+export function previewDiff(payload: DiffPayload, opts: PreviewDiffOptions = {}): string {
+  const spec = {
+    filePath: opts.filePath ?? 'preview',
+    hunks: payload.hunks.map((h) => ({
+      header: `@@ -${h.oldStart},${h.oldLines} +${h.newStart},${h.newLines} @@`,
+      lines: h.lines.map((l) => `${l.kind === ' ' ? ' ' : l.kind} ${l.text}`),
+    })),
+    stats: { added: payload.addedLines, removed: payload.removedLines },
+    ...(opts.maxLines !== undefined ? { maxLines: opts.maxLines } : {}),
+    ...(opts.width !== undefined ? { width: opts.width } : {}),
+  };
+  return palette.dim('⟳ Proposed') + '\n' + compactDiffView(spec);
+}