diff --git a/src/frontend/apps/calendars/src/features/calendar/services/dav/caldav-helpers.ts b/src/frontend/apps/calendars/src/features/calendar/services/dav/caldav-helpers.ts
new file mode 100644
index 0000000..f6215a3
--- /dev/null
+++ b/src/frontend/apps/calendars/src/features/calendar/services/dav/caldav-helpers.ts
@@ -0,0 +1,418 @@
+/**
+ * CalDAV Helper Functions
+ *
+ * Factorized utilities for XML building, DAV requests, and error handling.
+ */
+
+import { davRequest, propfind, DAVNamespaceShort } from 'tsdav'
+import type { DAVMethods } from 'tsdav'
+
+type HTTPMethods = 'GET' | 'HEAD' | 'POST' | 'PUT' | 'DELETE' | 'CONNECT' | 'OPTIONS' | 'TRACE' | 'PATCH'
+type AllowedMethods = DAVMethods | HTTPMethods
+import type { SharePrivilege, CalDavResponse } from './types/caldav-service'
+
+// ============================================================================
+// XML Helpers
+// ============================================================================
+
+/** Escape special XML characters */
+export function escapeXml(str: string | undefined | null): string {
+  if (str === undefined || str === null) {
+    return '';
+  }
+  if (typeof str !== 'string') {
+    str = String(str);
+  }
+  return str
+    .replace(/&/g, '&')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&apos;')
+}
+
+/** XML namespaces used in CalDAV */
+export const XML_NS = {
+  DAV: 'xmlns:D="DAV:"',
+  CALDAV: 'xmlns:C="urn:ietf:params:xml:ns:caldav"',
+  APPLE: 'xmlns:A="http://apple.com/ns/ical/"',
+  CS: 'xmlns:CS="http://calendarserver.org/ns/"',
+} as const
+
+/** Build XML prop element */
+export function xmlProp(namespace: string, name: string, value: string): string {
+  return `<${namespace}:${name}>${escapeXml(value)}</${namespace}:${name}>`
+}
+
+/** Build XML with optional value (returns empty string if value is undefined) */
+export function xmlPropOptional(namespace: string, name: string, value: string | undefined): string {
+  return value !== undefined ? xmlProp(namespace, name, value) : ''
+}
+
+// ============================================================================
+// Calendar Property Builders
+// ============================================================================
+
+export type CalendarProps = {
+  displayName?: string
+  description?: string
+  color?: string
+  components?: string[]
+}
+
+/** Build calendar property XML elements */
+export function buildCalendarPropsXml(props: CalendarProps): string[] {
+  const elements: string[] = []
+
+  if (props.displayName !== undefined && props.displayName !== null && typeof props.displayName === 'string') {
+    elements.push(xmlProp('D', 'displayname', props.displayName))
+  }
+  if (props.description !== undefined && props.description !== null && typeof props.description === 'string') {
+    elements.push(xmlProp('C', 'calendar-description', props.description))
+  }
+  if (props.color !== undefined && props.color !== null && typeof props.color === 'string') {
+    elements.push(xmlProp('A', 'calendar-color', props.color))
+  }
+  if (props.components && props.components.length > 0) {
+    const comps = props.components.map((c) => `<C:comp name="${escapeXml(c)}"/>`).join('')
+    elements.push(`<C:supported-calendar-component-set>${comps}</C:supported-calendar-component-set>`)
+  }
+
+  return elements
+}
+
+/** Build MKCALENDAR request body */
+export function buildMkCalendarXml(props: CalendarProps): string {
+  const propsXml = buildCalendarPropsXml(props)
+  return `<?xml version="1.0" encoding="utf-8"?>
+<C:mkcalendar ${XML_NS.DAV} ${XML_NS.CALDAV} ${XML_NS.APPLE}>
+  <D:set>
+    <D:prop>
+      ${propsXml.join('\n      ')}
+    </D:prop>
+  </D:set>
+</C:mkcalendar>`
+}
+
+/** Build PROPPATCH request body */
+export function buildProppatchXml(props: CalendarProps): string {
+  const propsXml = buildCalendarPropsXml(props)
+  return `<?xml version="1.0" encoding="utf-8"?>
+<D:propertyupdate ${XML_NS.DAV} ${XML_NS.CALDAV} ${XML_NS.APPLE}>
+  <D:set>
+    <D:prop>
+      ${propsXml.join('\n      ')}
+    </D:prop>
+  </D:set>
+</D:propertyupdate>`
+}
+
+// ============================================================================
+// Sharing XML Builders
+// ============================================================================
+
+/** Convert SharePrivilege to XML element */
+export function sharePrivilegeToXml(privilege: SharePrivilege): string {
+  const map: Record<SharePrivilege, string> = {
+    read: '<CS:read/>',
+    'read-write': '<CS:read-write/>',
+    'read-write-noacl': '<CS:read-write-noacl/>',
+    admin: '<CS:admin/>',
+  }
+  return map[privilege] ?? '<CS:read/>'
+}
+
+/** Parse access object to SharePrivilege */
+export function parseSharePrivilege(access: unknown): SharePrivilege {
+  if (!access) return 'read'
+  const accessObj = access as Record<string, unknown>
+  if (accessObj['read-write']) return 'read-write'
+  if (accessObj['admin']) return 'admin'
+  if (accessObj['read-write-noacl']) return 'read-write-noacl'
+  return 'read'
+}
+
+export type ShareeXmlParams = {
+  href: string
+  displayName?: string
+  privilege: SharePrivilege
+  summary?: string
+}
+
+/** Build share set XML for a single sharee */
+export function buildShareeSetXml(params: ShareeXmlParams): string {
+  const privilege = sharePrivilegeToXml(params.privilege)
+  const commonName = params.displayName
+    ? `<CS:common-name>${escapeXml(params.displayName)}</CS:common-name>`
+    : ''
+  const summary = params.summary ? `<CS:summary>${escapeXml(params.summary)}</CS:summary>` : ''
+
+  return `
+    <CS:set>
+      <D:href>${escapeXml(params.href)}</D:href>
+      ${commonName}
+      ${summary}
+      ${privilege}
+    </CS:set>`
+}
+
+/** Build CS:share request body */
+export function buildShareRequestXml(sharees: ShareeXmlParams[]): string {
+  const shareesXml = sharees.map(buildShareeSetXml).join('')
+  return `<?xml version="1.0" encoding="utf-8"?>
+<CS:share ${XML_NS.DAV} ${XML_NS.CS}>
+  ${shareesXml}
+</CS:share>`
+}
+
+/** Build CS:share remove request body */
+export function buildUnshareRequestXml(shareeHref: string): string {
+  return `<?xml version="1.0" encoding="utf-8"?>
+<CS:share ${XML_NS.DAV} ${XML_NS.CS}>
+  <CS:remove>
+    <D:href>${escapeXml(shareeHref)}</D:href>
+  </CS:remove>
+</CS:share>`
+}
+
+/** Build invite-reply request body */
+export function buildInviteReplyXml(inReplyTo: string, accept: boolean): string {
+  return `<?xml version="1.0" encoding="utf-8"?>
+<CS:invite-reply ${XML_NS.DAV} ${XML_NS.CS}>
+  <CS:in-reply-to>${escapeXml(inReplyTo)}</CS:in-reply-to>
+  <CS:invite-${accept ? 'accepted' : 'declined'}/>
+</CS:invite-reply>`
+}
+
+// ============================================================================
+// Sync XML Builders
+// ============================================================================
+
+export type SyncCollectionParams = {
+  syncToken: string
+  syncLevel?: number | 'infinite'
+}
+
+/** Build sync-collection REPORT body */
+export function buildSyncCollectionXml(params: SyncCollectionParams): string {
+  return `<?xml version="1.0" encoding="utf-8"?>
+<D:sync-collection ${XML_NS.DAV} ${XML_NS.CALDAV}>
+  <D:sync-token>${escapeXml(params.syncToken)}</D:sync-token>
+  <D:sync-level>${params.syncLevel ?? 1}</D:sync-level>
+  <D:prop>
+    <D:getetag/>
+    <C:calendar-data/>
+  </D:prop>
+</D:sync-collection>`
+}
+
+// ============================================================================
+// Principal Search XML Builder
+// ============================================================================
+
+/** Build principal-property-search REPORT body */
+export function buildPrincipalSearchXml(query: string): string {
+  return `<?xml version="1.0" encoding="utf-8"?>
+<D:principal-property-search ${XML_NS.DAV}>
+  <D:property-search>
+    <D:prop>
+      <D:displayname/>
+    </D:prop>
+    <D:match>${escapeXml(query)}</D:match>
+  </D:property-search>
+  <D:prop>
+    <D:displayname/>
+    <C:calendar-home-set ${XML_NS.CALDAV}/>
+  </D:prop>
+</D:principal-property-search>`
+}
+
+// ============================================================================
+// DAV Request Helpers
+// ============================================================================
+
+export type DavRequestOptions = {
+  url: string
+  method: AllowedMethods
+  body: string
+  headers?: Record<string, string>
+  fetchOptions?: RequestInit
+  contentType?: string
+}
+
+/** Execute a DAV request with standard error handling */
+export async function executeDavRequest(options: DavRequestOptions): Promise<CalDavResponse> {
+  try {
+    // Use fetch directly for methods that davRequest doesn't handle well
+    // POST is included because CalDAV sharing requires specific Content-Type handling
+    const useDirectFetch = ['PROPPATCH', 'DELETE', 'POST'].includes(options.method);
+
+    if (useDirectFetch) {
+      const response = await fetch(options.url, {
+        method: options.method,
+        headers: {
+          'Content-Type': options.contentType ?? 'application/xml; charset=utf-8',
+          ...options.headers,
+        },
+        body: options.body || undefined,
+        ...options.fetchOptions,
+      });
+
+      if (!response.ok && response.status !== 204 && response.status !== 207) {
+        const errorText = await response.text().catch(() => '');
+        console.error(`[CalDAV] ${options.method} request failed:`, {
+          url: options.url,
+          status: response.status,
+          error: errorText,
+        });
+        return {
+          success: false,
+          error: `Request failed: ${response.status} ${errorText}`,
+          status: response.status,
+        };
+      }
+
+      return { success: true };
+    }
+
+    // Use davRequest for standard WebDAV methods
+    const responses = await davRequest({
+      url: options.url,
+      init: {
+        method: options.method as DAVMethods,
+        headers: {
+          'Content-Type': options.contentType ?? 'application/xml; charset=utf-8',
+          ...options.headers,
+        },
+        body: options.body,
+      },
+      fetchOptions: options.fetchOptions,
+    })
+
+    const response = responses[0]
+    if (!response?.ok && response?.status !== 204) {
+      return {
+        success: false,
+        error: `Request failed: ${response?.status}`,
+        status: response?.status,
+      }
+    }
+
+    return { success: true }
+  } catch (error) {
+    return {
+      success: false,
+      error: `Request failed: ${error instanceof Error ? error.message : String(error)}`,
+    }
+  }
+}
+
+/** Standard PROPFIND props for calendar fetching */
+export const CALENDAR_PROPS = {
+  [`${DAVNamespaceShort.CALDAV}:calendar-description`]: {},
+  [`${DAVNamespaceShort.CALDAV}:calendar-timezone`]: {},
+  [`${DAVNamespaceShort.DAV}:displayname`]: {},
+  [`${DAVNamespaceShort.CALDAV_APPLE}:calendar-color`]: {},
+  [`${DAVNamespaceShort.CALENDAR_SERVER}:getctag`]: {},
+  [`${DAVNamespaceShort.DAV}:resourcetype`]: {},
+  [`${DAVNamespaceShort.CALDAV}:supported-calendar-component-set`]: {},
+  [`${DAVNamespaceShort.DAV}:sync-token`]: {},
+} as const
+
+/** Execute PROPFIND with error handling */
+export async function executePropfind<T>(
+  url: string,
+  props: Record<string, unknown>,
+  options?: {
+    headers?: Record<string, string>
+    fetchOptions?: RequestInit
+    depth?: '0' | '1' | 'infinity'
+  }
+): Promise<CalDavResponse<T>> {
+  try {
+    const response = await propfind({
+      url,
+      props,
+      headers: options?.headers,
+      fetchOptions: options?.fetchOptions,
+      depth: options?.depth ?? '0',
+    })
+
+    const rs = response[0]
+    if (!rs.ok) {
+      return {
+        success: false,
+        error: `PROPFIND failed: ${rs.status}`,
+        status: rs.status,
+      }
+    }
+
+    return { success: true, data: rs.props as T }
+  } catch (error) {
+    return {
+      success: false,
+      error: `PROPFIND failed: ${error instanceof Error ? error.message : String(error)}`,
+    }
+  }
+}
+
+// ============================================================================
+// Response Parsing Helpers
+// ============================================================================
+
+/** Parse supported-calendar-component-set from PROPFIND response */
+export function parseCalendarComponents(supportedCalendarComponentSet: unknown): string[] | undefined {
+  if (!supportedCalendarComponentSet) return undefined
+
+  const comp = (supportedCalendarComponentSet as Record<string, unknown>).comp
+  if (Array.isArray(comp)) {
+    return comp
+      .map((sc: Record<string, unknown>) => (sc._attributes as Record<string, string>)?.name)
+      .filter(Boolean)
+  }
+  const name = (comp as Record<string, unknown>)?._attributes as Record<string, string> | undefined
+  return name?.name ? [name.name] : undefined
+}
+
+/** Parse share status from invite response */
+export function parseShareStatus(
+  accepted: unknown,
+  noResponse: unknown
+): 'pending' | 'accepted' | 'declined' {
+  if (accepted) return 'accepted'
+  if (noResponse) return 'pending'
+  return 'declined'
+}
+
+/** Extract calendar URL from event URL */
+export function getCalendarUrlFromEventUrl(eventUrl: string): string {
+  const parts = eventUrl.split('/')
+  parts.pop() // Remove filename
+  return parts.join('/') + '/'
+}
+
+/** Extract calendar ID from calendar URL (e.g., /calendars/user/calendar-id/ -> calendar-id) */
+export function getCalendarIdFromUrl(calendarUrl: string): string {
+  const parts = calendarUrl.replace(/\/$/, '').split('/')
+  return parts[parts.length - 1]
+}
+
+// ============================================================================
+// Error Handling
+// ============================================================================
+
+/** Wrap async operation with standard error handling */
+export async function withErrorHandling<T>(
+  operation: () => Promise<T>,
+  errorPrefix: string
+): Promise<CalDavResponse<T>> {
+  try {
+    const data = await operation()
+    return { success: true, data }
+  } catch (error) {
+    return {
+      success: false,
+      error: `${errorPrefix}: ${error instanceof Error ? error.message : String(error)}`,
+    }
+  }
+}
diff --git a/src/frontend/apps/calendars/src/features/calendar/services/dav/constants.ts b/src/frontend/apps/calendars/src/features/calendar/services/dav/constants.ts
new file mode 100644
index 0000000..8e5db47
--- /dev/null
+++ b/src/frontend/apps/calendars/src/features/calendar/services/dav/constants.ts
@@ -0,0 +1,16 @@
+export const attendeeRoleTypes = [
+  'CHAIR',
+  'REQ-PARTICIPANT',
+  'OPT-PARTICIPANT',
+  'NON-PARTICIPANT',
+] as const
+
+export const availableViews = [
+  'timeGridDay',
+  'timeGridWeek',
+  'dayGridMonth',
+  'listDay',
+  'listWeek',
+  'listMonth',
+  'listYear',
+] as const