feat(application-starter): inject migration guide URLs into final prompt

When the user brief indicates a Next.js or Remix/React Router migration,
append an instruction telling the downstream agent to fetch the relevant
tanstack.com migration guide and follow it. Keeps the guide out of the
analyze stage and out of the UI; only affects the final generation prompt.

Author: tannerlinsley

src/components/application-builder/shared.ts

@@ -52,8 +52,15 @@ export type StarterPartnerButtonStyle = CSSProperties & {
 
 const nextJsMigrationPattern =
   /migrat(?:e|ing|ion).*(next(?:\.js)?)|(next(?:\.js)?).*(migrat(?:e|ing|ion))/i
+const remixMigrationPattern =
+  /migrat(?:e|ing|ion).*(remix|react[\s-]?router)|(remix|react[\s-]?router).*(migrat(?:e|ing|ion))/i
 const migrationRepositoryUrlPattern = /^(https?:\/\/\S+|git@\S+|ssh:\/\/\S+)$/i
 
+export const STARTER_NEXTJS_MIGRATION_GUIDE_URL =
+  'https://tanstack.com/start/latest/docs/framework/react/migrate-from-next-js'
+export const STARTER_REMIX_MIGRATION_GUIDE_URL =
+  'https://tanstack.com/router/latest/docs/guide/how-to/migrate-from-react-router'
+
 export const starterPinnedLibraryIds = [
   'start',
   'router',
@@ -142,6 +149,22 @@ export function isNextJsMigrationInput(input: string) {
   return nextJsMigrationPattern.test(input)
 }
 
+export function isRemixMigrationInput(input: string) {
+  return remixMigrationPattern.test(input)
+}
+
+export function getStarterMigrationGuideUrl(input: string) {
+  if (isNextJsMigrationInput(input)) {
+    return STARTER_NEXTJS_MIGRATION_GUIDE_URL
+  }
+
+  if (isRemixMigrationInput(input)) {
+    return STARTER_REMIX_MIGRATION_GUIDE_URL
+  }
+
+  return null
+}
+
 export function normalizeMigrationRepositoryUrl(value: string) {
   return value.trim()
 }

src/utils/application-starter.server.ts

@@ -17,7 +17,10 @@ import {
   type ApplicationStarterResult,
 } from '~/utils/application-starter'
 import type { LibraryId } from '~/libraries'
-import { starterAddonLibraryIds } from '~/components/application-builder/shared'
+import {
+  getStarterMigrationGuideUrl,
+  starterAddonLibraryIds,
+} from '~/components/application-builder/shared'
 import {
   getApplicationStarterGuidanceLines,
   getApplicationStarterPartnerSuggestions,
@@ -400,6 +403,10 @@ function buildPromptGenerationRequest({
   ].join('\n')
   const userBrief = getApplicationStarterUserBrief(request.input)
   const starterGuidanceLines = getApplicationStarterGuidanceLines(request.input)
+  const migrationGuideUrl = getStarterMigrationGuideUrl(request.input)
+  const migrationGuideInstruction = migrationGuideUrl
+    ? `The prompt must instruct the agent to fetch ${migrationGuideUrl} and use it as the primary reference for the migration, following its steps in order.`
+    : null
 
   return [
     'Write a short, natural final prompt for a stronger coding agent.',
@@ -417,6 +424,7 @@ function buildPromptGenerationRequest({
     'If the user says things like make it cool, keep it minimal, or do not include something, restate those instructions explicitly in the final prompt instead of compressing them away.',
     'Use the resolved starter plan as fixed input. Do not redesign the stack unless the original brief requires sequencing work after scaffolding.',
     'Keep the prompt concise and plain-English. Avoid internal process language like fixed input, resolved plan, objective, implementation notes, or deliverable.',
+    ...(migrationGuideInstruction ? [migrationGuideInstruction] : []),
     '',
     `Context: ${request.context}`,
     `User request: ${userBrief}`,