diff --git a/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.jsx b/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.jsx
index d7a1cdb526..44ccfa8ed5 100644
--- a/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.jsx
+++ b/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.jsx
@@ -1,11 +1,12 @@
 import { useState } from 'react';
 import { Loader2, X, BookText } from 'lucide-react';
 import { WORLD_PREMISE_MAX } from '../../../services/apiUniverseBuilder.js';
+import { ARC_LIMITS } from '../../../../../server/lib/storyArcLimits.js';
 
-// Field-count guard for derived synopsis textareas — mirrors the server caps so
-// the user isn't surprised by a 400 on commit.
+// UI ceiling for derived issue synopsis textareas; the server accepts a larger
+// stage input, but this review panel keeps pasted suggestions bounded.
 const DERIVE_SYNOPSIS_MAX = 8000;
-const DERIVE_TITLE_MAX = 300;
+const ISSUE_TITLE_MAX = 300;
 
 // Review/edit panel for the derive-from-manuscript proposal. The arc + bible
 // fields and the single-volume title/synopsis are editable; each existing issue
@@ -48,10 +49,10 @@ export default function DeriveFromManuscriptPreview({ preview, committing, onCan
     onConfirm({
       arc,
       bible,
-      volume,
+      volume: { ...volume, title: volume.title.slice(0, ARC_LIMITS.SEASON_TITLE_MAX) },
       issues: issues.map((it) => ({
         id: it.id,
-        title: it.title.slice(0, DERIVE_TITLE_MAX),
+        title: it.title.slice(0, ISSUE_TITLE_MAX),
         synopsis: it.ideaLocked ? '' : it.synopsis.slice(0, DERIVE_SYNOPSIS_MAX),
       })),
     });
@@ -78,7 +79,7 @@ export default function DeriveFromManuscriptPreview({ preview, committing, onCan
       <div className="grid gap-3 @md:grid-cols-2">
         <label className="block space-y-1">
           <span className="text-[11px] uppercase tracking-wider text-gray-500">Series logline</span>
-          <input className={inputCls} value={bible.logline} maxLength={500}
+          <input className={inputCls} value={bible.logline} maxLength={ARC_LIMITS.LOGLINE_MAX}
             onChange={(e) => { setBible((b) => ({ ...b, logline: e.target.value })); setArc((a) => ({ ...a, logline: e.target.value })); }} />
         </label>
         <label className="block space-y-1">
@@ -94,13 +95,13 @@ export default function DeriveFromManuscriptPreview({ preview, committing, onCan
       </label>
       <label className="block space-y-1">
         <span className="text-[11px] uppercase tracking-wider text-gray-500">Protagonist arc</span>
-        <textarea className={`${inputCls} resize-y`} rows={2} value={arc.protagonistArc} maxLength={8000}
+        <textarea className={`${inputCls} resize-y`} rows={2} value={arc.protagonistArc} maxLength={ARC_LIMITS.PROTAGONIST_ARC_MAX}
           onChange={(e) => setArc((a) => ({ ...a, protagonistArc: e.target.value }))} />
       </label>
 
       <label className="block border-t border-port-border pt-2 space-y-2">
         <span className="text-[11px] uppercase tracking-wider text-gray-500">Volume</span>
-        <input className={inputCls} value={volume.title} maxLength={DERIVE_TITLE_MAX} placeholder="Volume title"
+        <input className={inputCls} value={volume.title} maxLength={ARC_LIMITS.SEASON_TITLE_MAX} placeholder="Volume title"
           onChange={(e) => setVolume((v) => ({ ...v, title: e.target.value }))} />
       </label>
 
@@ -110,7 +111,7 @@ export default function DeriveFromManuscriptPreview({ preview, committing, onCan
           <div key={it.id} className="bg-port-card border border-port-border rounded p-2 space-y-1">
             <div className="flex items-center gap-2">
               <span className="text-[11px] text-gray-500 shrink-0">#{it.number}</span>
-              <input className={inputCls} value={it.title} maxLength={DERIVE_TITLE_MAX} placeholder="Issue title"
+              <input className={inputCls} value={it.title} maxLength={ISSUE_TITLE_MAX} placeholder="Issue title"
                 aria-label={`Title for issue ${it.number}`}
                 onChange={(e) => setIssueField(it.id, 'title', e.target.value)} />
             </div>
diff --git a/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.test.jsx b/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.test.jsx
new file mode 100644
index 0000000000..1764991ebf
--- /dev/null
+++ b/client/src/components/pipeline/arcCanvas/DeriveFromManuscriptPreview.test.jsx
@@ -0,0 +1,32 @@
+import { describe, expect, it, vi } from 'vitest';
+import { fireEvent, render, screen } from '@testing-library/react';
+import DeriveFromManuscriptPreview from './DeriveFromManuscriptPreview.jsx';
+
+const preview = {
+  arc: { protagonistArc: '' },
+  bible: {},
+  volume: {},
+  issues: [{ id: 'issue-1', number: 1, title: '', currentSynopsis: '' }],
+};
+
+describe('DeriveFromManuscriptPreview field caps', () => {
+  it('uses server caps and trims the volume title before confirming', () => {
+    const onConfirm = vi.fn();
+    render(<DeriveFromManuscriptPreview preview={preview} committing={false} onCancel={vi.fn()} onConfirm={onConfirm} />);
+
+    const volumeTitle = screen.getByPlaceholderText('Volume title');
+    const protagonistArc = screen.getByText('Protagonist arc').parentElement.querySelector('textarea');
+    const issueTitle = screen.getByPlaceholderText('Issue title');
+    expect(volumeTitle.maxLength).toBe(200);
+    expect(protagonistArc.maxLength).toBe(4000);
+    expect(issueTitle.maxLength).toBe(300);
+
+    const longTitle = 'x'.repeat(250);
+    fireEvent.change(volumeTitle, { target: { value: longTitle } });
+    fireEvent.click(screen.getByRole('button', { name: /apply/i }));
+
+    expect(onConfirm).toHaveBeenCalledWith(expect.objectContaining({
+      volume: { title: longTitle.slice(0, 200), logline: '', synopsis: '' },
+    }));
+  });
+});
diff --git a/server/lib/README.md b/server/lib/README.md
index 63c0c0852e..945bfd0ba5 100644
--- a/server/lib/README.md
+++ b/server/lib/README.md
@@ -128,6 +128,7 @@ The barrel `server/lib/index.js` is a machine-checkable enumeration of every pub
 | `castIntegrityPrompt.js` | The prompt-facing render of a `characterIntegrity.js` report (#6415): `renderCastIntegrity` (gap rows first, header always kept, so a budget-truncated block can never read as a clean cast) and `INTEGRITY_DEPTH_NOTES` (the `explained` / `light` / `full` rulings in the reviewer's own words). Shared by the Series foundation judge and the FableLoom whole-series editor so the two cannot drift on what a `light` character owes. |
 | `bibleLimits.js` | `BIBLE_LIMITS` — the canon field length/count caps every story-bible sanitizer, Zod schema, and catalog payload upgrade measures against. A pure leaf split out of `storyBible.js` (which pulls `crypto` + `fileUtils`) so `catalogTypes.js` and the browser bundle can read the numbers alone. |
 | `storyArc.js` | Canonical Arc + Season + Reader-Map shapes for pipeline arc planning. |
+| `storyArcLimits.js` | Pure canonical length and count caps shared by story arc sanitizers and browser previews. |
 | `styleGuide.js` | Per-series house style (tense/POV/audience/rating/reading-level/tone/conventions): `sanitizeStyleGuide` + `renderStyleGuide` generation block + enums. |
 | `storyBuilderSteps.js` | Unified Story Builder ordered step definitions + helpers (`STEPS`, `STEP_IDS`, `STEP_STATUSES`, `isValidStepId`, `stepIndex`). |
 | `streamLines.js` | `createLineReader(onLine, {splitRe?, maxCarry?})` + `createOutputTail({budget?})` — buffered chunk→line splitter for child-process stdout/stderr. Carries the partial trailing line across `data` chunks and `flush()`es the final unterminated line on `close`; carry clamp guards a newline-less runaway stream. One reader per stream (a shared buffer corrupts marker lines). `splitRe: /[\r\n]+/` handles torch/tqdm bare-`\r` progress redraws. `createOutputTail` is the sibling rolling buffer of recent lines (char-budgeted, decoration-only lines skipped so an ASCII-art banner cannot push the real error out) that turns "exited with code 1" into what the tool actually printed — used by `streamingSpawn.js`. |
diff --git a/server/lib/index.js b/server/lib/index.js
index 42ff6e4977..1e061ae3ea 100644
--- a/server/lib/index.js
+++ b/server/lib/index.js
@@ -113,6 +113,7 @@ export * from './seriesCharacterArc.js';
 export * from './llmRoutePin.js';
 export * from './seriesLlmOverride.js';
 export * from './storyArc.js';
+export * from './storyArcLimits.js';
 export * from './styleGuide.js';
 export * from './storyBuilderIntegrity.js';
 export * from './storyBuilderSteps.js';
diff --git a/server/lib/storyArc.js b/server/lib/storyArc.js
index 990b7c4ed0..a801a497f6 100644
--- a/server/lib/storyArc.js
+++ b/server/lib/storyArc.js
@@ -19,36 +19,8 @@
 import { randomUUID } from 'crypto';
 import { isStr, trimTo, trimToClause } from './storyBible.js';
 import { sanitizeCoverLike } from './renderSlot.js';
-
-export const ARC_LIMITS = Object.freeze({
-  LOGLINE_MAX: 500,
-  SUMMARY_MAX: 8000,
-  PROTAGONIST_ARC_MAX: 4000,
-  THEME_MAX: 100,
-  THEMES_PER_ARC_MAX: 20,
-  // Season
-  SEASON_TITLE_MAX: 200,
-  SEASON_LOGLINE_MAX: 500,
-  // A season synopsis covers a whole season's worth of episodes (8+ issues on a
-  // multi-season series), so it needs the same room as the arc-level SUMMARY_MAX
-  // (8000). The old 4000 cap clipped a full synopsis mid-sentence — and because
-  // the arc-verify→resolve loop re-flags a mid-sentence truncation and the
-  // resolver regenerates a >4000 synopsis that gets re-clipped, the loop could
-  // never converge (it burned all its rounds and paused). See arc-verify
-  // "truncated mid-sentence" finding, 2026-06-21.
-  SEASON_SYNOPSIS_MAX: 8000,
-  SEASON_ENDING_HOOK_MAX: 1000,
-  SEASON_NUMBER_MAX: 99,
-  SEASON_EPISODE_COUNT_MAX: 999,
-  SEASONS_PER_SERIES_MAX: 50,
-  // One issue/episode planning synopsis. This is deliberately smaller than a
-  // whole-volume synopsis: it is a drafting seed, not a place to accumulate
-  // every continuity exception the verifier has ever raised. Keep the value in
-  // the shared arc limits so initial episode generation and later arc repairs
-  // cannot silently disagree about how much text one episode may own.
-  EPISODE_LOGLINE_MAX: 500,
-  EPISODE_SYNOPSIS_MAX: 4000,
-});
+import { ARC_LIMITS } from './storyArcLimits.js';
+export { ARC_LIMITS } from './storyArcLimits.js';
 
 export const ARC_STATUSES = Object.freeze(['draft', 'verified']);
 export const SEASON_STATUSES = Object.freeze(['draft', 'verified', 'in-production', 'complete']);
diff --git a/server/lib/storyArcLimits.js b/server/lib/storyArcLimits.js
new file mode 100644
index 0000000000..6cb142dba9
--- /dev/null
+++ b/server/lib/storyArcLimits.js
@@ -0,0 +1,29 @@
+/** Canonical length and count caps for story arcs, seasons, and episodes. */
+export const ARC_LIMITS = Object.freeze({
+  LOGLINE_MAX: 500,
+  SUMMARY_MAX: 8000,
+  PROTAGONIST_ARC_MAX: 4000,
+  THEME_MAX: 100,
+  THEMES_PER_ARC_MAX: 20,
+  SEASON_TITLE_MAX: 200,
+  SEASON_LOGLINE_MAX: 500,
+  // A season synopsis covers a whole season's worth of episodes (8+ issues on a
+  // multi-season series), so it needs the same room as the arc-level SUMMARY_MAX
+  // (8000). The old 4000 cap clipped a full synopsis mid-sentence — and because
+  // the arc-verify→resolve loop re-flags a mid-sentence truncation and the
+  // resolver regenerates a >4000 synopsis that gets re-clipped, the loop could
+  // never converge (it burned all its rounds and paused). See arc-verify
+  // "truncated mid-sentence" finding, 2026-06-21.
+  SEASON_SYNOPSIS_MAX: 8000,
+  SEASON_ENDING_HOOK_MAX: 1000,
+  SEASON_NUMBER_MAX: 99,
+  SEASON_EPISODE_COUNT_MAX: 999,
+  SEASONS_PER_SERIES_MAX: 50,
+  // One issue/episode planning synopsis. This is deliberately smaller than a
+  // whole-volume synopsis: it is a drafting seed, not a place to accumulate
+  // every continuity exception the verifier has ever raised. Keep the value in
+  // the shared arc limits so initial episode generation and later arc repairs
+  // cannot silently disagree about how much text one episode may own.
+  EPISODE_LOGLINE_MAX: 500,
+  EPISODE_SYNOPSIS_MAX: 4000,
+});