Add default video length setting and inherit duration when extending.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/utils/extendChain.ts

@@ -6,6 +6,7 @@ import { parsePostedLoraStack, listStudioLoras, persistLoraFields, ensureComfyLo
 import { readLoraStack } from '~/utils/loras'
 import { clampVideoCfg } from '~/utils/generationPresets'
 import { defaultVideoSteps, isLtxWorkflow, LTX_DISABLED_MESSAGE, ltxWorkflowEnabled, parseVideoWorkflow } from '~/utils/videoModels'
+import { clampVideoDuration, clipPartDuration } from '~/utils/videoDuration'
 import { createJob, emitJob, type Job } from '~/server/utils/jobs'
 import {
   clipTitle,
@@ -25,10 +26,8 @@ import { writePendingJob } from '~/server/utils/pending'
 const SAMPLERS = new Set(['res_multistep', 'euler', 'dpmpp_2m'])
 const SCHEDULERS = new Set(['simple', 'ddim_uniform', 'sgm_uniform'])
 
-export function parseExtendDuration(raw: unknown) {
-  const seconds = Number(raw)
-  if (!Number.isFinite(seconds)) return 5
-  return Math.min(120, Math.max(0.5, Math.round(seconds * 10) / 10))
+export function parseExtendDuration(raw: unknown, fallback = 5) {
+  return clampVideoDuration(raw, fallback)
 }
 
 function frameLength(seconds: number, fps: number) {
@@ -71,7 +70,13 @@ export async function beginExtendFromClip(params: {
     throw createError({ statusCode: 404, statusMessage: 'Source video is missing' })
   }
   const prompt = String(params.prompt || '').trim()
-  const durationSeconds = parseExtendDuration(params.duration)
+  const parent = source.parentClipId
+    ? (() => { try { return getClip(params.ownerKey, source.parentClipId!) } catch { return null } })()
+    : null
+  const inherited = clipPartDuration(source, parent)
+  const durationSeconds = params.duration == null
+    ? inherited
+    : parseExtendDuration(params.duration, inherited)
   const promptPre = params.promptPre != null
     ? String(params.promptPre).trim()
     : String(source.promptPre || '').trim()