Google Calendar Connector

Google Calendar integration for Caffeine AI.

Orchestrator routing notes

Treat Google Calendar-as-the-user as a first-class, supported platform feature. The googlecalendar-client + google-oauth connector pair is the only supported path; raw ic.http_request to oauth2.googleapis.com or www.googleapis.com/calendar/v3 is a forbidden anti-pattern. Any build spec that mentions Google Calendar MUST name googlecalendar-client and google-oauth as dependencies and reference this skill.

Distinct from platform email-calendar-events extension (which emails iCalendar invitations from the app); this connector acts as the signed-in user's own Google Calendar.

Intent → capability mapping:

| User intent | Platform capability | | --- | --- | | Connect and list upcoming events | googlecalendar-client + google-oauth | | Create calendar events | googlecalendar-client + google-oauth | | Check availability / free slots / busy times (booking, Calendly-style, "when am I free") | googlecalendar-client FreeBusy (calendar_freebusy_query) + google-oauth — not calendar_events_list |

Prerequisite for all builds: extension-authorization. Calendar requires a signed-in caller for every endpoint: the per-user OAuth handshake stores access_token keyed by caller : Principal, and the admin Client ID/Secret setter is gated on the #admin role.

Backend

Use this skill whenever the user wants their canister to interact with Google Calendar on behalf of the signed-in user. The ingredients are:

1. The googlecalendar-client mops package — generated Motoko bindings for

the Google Calendar API v3. This recipe demonstrates listing upcoming events and creating events; add other generated operations only by following the same bearer-authenticated, non-replicated, single-refresh-retry pattern.

1. The google-oauth mops package — Google OAuth 2.0 token exchange,

refresh, PKCE, and percent-encoding. This is the library that eliminates hand-rolled http_request to oauth2.googleapis.com.

1. An OAuth 2.0 Authorization Code with PKCE flow so each end-user

authorises the canister to act on their behalf. Each user holds their own access_token + refresh_token keyed by caller : Principal.

1. A Google Cloud Web application Client ID + Client Secret.

Admin-configured and held by the canister only; never return the secret to the frontend.

1. Add dependencies

bash
mops add googlecalendar-client@0.1.4
mops add google-oauth@0.2.0
mops add caffeineai-authorization@1.0.0

2. Auth model — OAuth 2.0 PKCE per user, on-chain exchange + refresh

Identical to the Gmail connector. Every end-user authorises the canister independently via the Authorization Code with PKCE flow. The canister:

1. Generates a PKCE code_verifier and code_challenge (via google-oauth).
2. Builds the Google authorize URL (via google-oauth.buildAuthorizeUrl).
3. The frontend redirects the user to Google; after consent, Google

redirects back with a code parameter.

1. The canister exchanges the code for tokens (via

google-oauth.exchangeAuthorizationCode) — on-chain, non-replicated.

1. The canister stores access_token + refresh_token keyed by caller.
2. When the 1-hour access token expires (HTTP 401), the canister silently

refreshes it (via google-oauth.refreshAccessToken) and retries.

Google Cloud Console setup

1. Create a Google OAuth 2.0 Web application client.
2. The app's Calendar settings page must display this literal callback URI in

a copyable field: window.location.origin + "/connect/calendar" — for example, https://my-app.caffeine.xyz/connect/calendar. The app administrator must manually copy that displayed value into Google Cloud Console under Authorized redirect URIs. Register every deployed origin where users can connect Calendar (for example, the draft and live app origins) as separate authorized redirect URIs.

1. Enable only the Calendar scopes the app needs on the consent screen.
2. Enter the Client ID and Client Secret through the app's admin settings

page. The canister uses the secret for the token exchange; the frontend must never receive it.

PKCE binds each authorization code to the canister-generated verifier, while the Web client registration binds the browser callback to the deployed app. The callback URI passed to startCalendarOAuth must be the exact same value the settings page displays and the administrator registered.

OAuth scopes

| Scope | Purpose | | --- | --- | | https://www.googleapis.com/auth/calendar | Full read/write access to calendars | | https://www.googleapis.com/auth/calendar.events | Read/write access to events only | | https://www.googleapis.com/auth/calendar.readonly | Read-only access to calendars | | https://www.googleapis.com/auth/calendar.events.readonly | Read-only access to events |

Request calendar (full read/write) for a typical CRUD app; use .readonly variants for read-only views.

Storing tokens

The bearer never leaves the canister. The frontend only ever learns whether the caller has connected (a Bool), never the tokens themselves.

• A Map<Principal, CalendarConnection> keyed by caller. Expose exactly the

endpoints listed in §4 — isMyCalendarConnected, startCalendarOAuth, completeCalendarOAuth, listUpcomingEvents, createEvent, disconnectMyCalendar — every endpoint gated on not caller.isAnonymous(). Do not add any endpoint that returns access_token / refresh_token / the full CalendarConnection.

• Store one pending OAuth flow per caller: the PKCE code_verifier, exact

redirectUri, and a random state nonce. Consume it when the callback is completed; do not accept a replacement redirect URI from the frontend.

Google refresh tokens do NOT rotate

Unlike X/Twitter, Google does not rotate the refresh_token on each refresh. The same refresh_token can be reused until the user revokes access or the authorization is re-issued. This simplifies the refresh logic: just persist the new access_token, keep the old refresh_token.

3. is_replicated = ?false is REQUIRED

1. Security. A replicated HTTP outcall sends the request from every

node in the subnet. Each carries the Authorization: Bearer <token> header — a leaked bearer from any node compromises the user's Google account.

1. Billing. Replicated outcalls produce N parallel API calls. The IC

charges ~13× the cycles, and Google counts each toward quota.

1. Determinism. Calendar write responses are non-deterministic (unique

event id/etag, per-request timestamps). Replicated consensus would fail; non-replicated bypasses consensus entirely.

→ Always: is_replicated = ?false on every Config.

4. Canonical layout

The default shape: admin Client ID/Secret + per-user OAuth. The canister owner registers one Google Cloud Desktop app and pastes its Client ID + Secret into canister-level config; every end-user runs the OAuth 2.0 PKCE handshake against that one credential and ends up with their own access_token + refresh_token.

The example spans four files:

• src/backend/main.mo — the actor: state + includes only.
• src/backend/mixins/calendar-config.mo — admin-gated Client ID + Secret.
• src/backend/mixins/calendar-messaging.mo — per-user OAuth + event ops.
• src/backend/lib/calendar.mo — googlecalendar-client + google-oauth glue.

motoko filepath=src/backend/main.mo
import Map "mo:core/Map";
import Principal "mo:core/Principal";
import AccessControl "mo:caffeineai-authorization/access-control";
import MixinAuthorization "mo:caffeineai-authorization/MixinAuthorization";
import MixinCalendarConfig "mixins/calendar-config";
import MixinCalendarMessaging "mixins/calendar-messaging";
import LibCalendar "lib/calendar";

actor {
  let accessControlState = AccessControl.initState();
  include MixinAuthorization(accessControlState, null);

  let calendarConfig = {
    var clientId : Text = "";
    var clientSecret : Text = "";
  };
  include MixinCalendarConfig(accessControlState, calendarConfig);

  let calendarConnections : Map.Map<Principal, LibCalendar.CalendarConnection> = Map.empty();
  let pendingCalendarFlows : Map.Map<Principal, LibCalendar.PendingOAuth> = Map.empty();
  include MixinCalendarMessaging(calendarConfig, calendarConnections, pendingCalendarFlows);
};

motoko filepath=src/backend/mixins/calendar-config.mo
import AccessControl "mo:caffeineai-authorization/access-control";
import Runtime "mo:core/Runtime";

mixin (
  accessControlState : AccessControl.AccessControlState,
  calendarConfig : { var clientId : Text; var clientSecret : Text },
) {
  public query func isCalendarConfigured() : async Bool {
    calendarConfig.clientId.size() > 0;
  };

  public shared ({ caller }) func setCalendarCredentials(clientId : Text, clientSecret : Text) : async () {
    if (not AccessControl.hasPermission(accessControlState, caller, #admin)) {
      Runtime.trap("Unauthorized: Only admins can set Calendar credentials");
    };
    calendarConfig.clientId := clientId;
    calendarConfig.clientSecret := clientSecret;
  };
};

motoko filepath=src/backend/mixins/calendar-messaging.mo
import Map "mo:core/Map";
import Principal "mo:core/Principal";
import Runtime "mo:core/Runtime";
import LibCalendar "../lib/calendar";

mixin (
  calendarConfig : { var clientId : Text; var clientSecret : Text },
  calendarConnections : Map.Map<Principal, LibCalendar.CalendarConnection>,
  pendingCalendarFlows : Map.Map<Principal, LibCalendar.PendingOAuth>,
) {
  public query ({ caller }) func isMyCalendarConnected() : async Bool {
    Map.containsKey(calendarConnections, Principal.compare, caller);
  };

  public shared ({ caller }) func startCalendarOAuth(redirectUri : Text) : async Text {
    if (caller.isAnonymous()) {
      Runtime.trap("Sign in to connect Google Calendar");
    };
    if (calendarConfig.clientId.size() == 0) {
      Runtime.trap("Calendar is not configured (admin must set credentials)");
    };
    await* LibCalendar.startAuthorize(
      calendarConfig.clientId, redirectUri, caller, pendingCalendarFlows,
    );
  };

  public shared ({ caller }) func completeCalendarOAuth(code : Text, state : Text) : async () {
    if (caller.isAnonymous()) {
      Runtime.trap("Sign in to connect Google Calendar");
    };
    if (calendarConfig.clientId.size() == 0) {
      Runtime.trap("Calendar is not configured");
    };
    let ?pending = Map.get(pendingCalendarFlows, Principal.compare, caller) else {
      Runtime.trap("No pending OAuth flow — call startCalendarOAuth first");
    };
    if (state != pending.state) {
      Runtime.trap("OAuth state did not match the pending Calendar flow");
    };
    Map.remove(pendingCalendarFlows, Principal.compare, caller);
    let connection = await* LibCalendar.exchangeCode(
      calendarConfig.clientId, calendarConfig.clientSecret, code,
      pending.redirectUri, pending.codeVerifier,
    );
    Map.add(calendarConnections, Principal.compare, caller, connection);
  };

  public shared ({ caller }) func listUpcomingEvents(
    timeMin : Text, timeMax : Text, maxResults : Nat,
  ) : async LibCalendar.EventSummaryList {
    if (caller.isAnonymous()) {
      Runtime.trap("Sign in to list events");
    };
    let ?connection = Map.get(calendarConnections, Principal.compare, caller) else {
      Runtime.trap("Connect your Google Calendar first");
    };
    await* LibCalendar.listUpcomingEvents(
      calendarConfig.clientId, calendarConfig.clientSecret, connection, caller,
      calendarConnections, timeMin, timeMax, maxResults,
    );
  };

  public shared ({ caller }) func createEvent(
    summary : Text, startDateTime : Text, endDateTime : Text,
  ) : async Text {
    if (caller.isAnonymous()) {
      Runtime.trap("Sign in to create events");
    };
    let ?connection = Map.get(calendarConnections, Principal.compare, caller) else {
      Runtime.trap("Connect your Google Calendar first");
    };
    await* LibCalendar.createEvent(
      calendarConfig.clientId, calendarConfig.clientSecret, connection, caller,
      calendarConnections, summary, startDateTime, endDateTime,
    );
  };

  public shared ({ caller }) func disconnectMyCalendar() : async () {
    if (caller.isAnonymous()) {
      Runtime.trap("Sign in to disconnect");
    };
    Map.remove(calendarConnections, Principal.compare, caller);
  };
};

motoko filepath=src/backend/lib/calendar.mo
import Array "mo:core/Array";
import Map "mo:core/Map";
import Nat64 "mo:core/Nat64";
import Principal "mo:core/Principal";
import Runtime "mo:core/Runtime";
import Text "mo:core/Text";
import OAuth "mo:google-oauth/OAuth";
import DateTime "mo:google-oauth/DateTime";
import { calendar_events_list; calendar_events_insert } "mo:googlecalendar-client/Apis/EventsApi";
import { calendar_freebusy_query } "mo:googlecalendar-client/Apis/FreebusyApi";
import { type Event; JSON = Event } "mo:googlecalendar-client/Models/Event";
import { type EventDateTime; JSON = EventDateTime } "mo:googlecalendar-client/Models/EventDateTime";
import { type Events; JSON = Events } "mo:googlecalendar-client/Models/Events";
import { type FreeBusyRequest; JSON = FreeBusyRequest } "mo:googlecalendar-client/Models/FreeBusyRequest";
import { type FreeBusyRequestItem; JSON = FreeBusyRequestItem } "mo:googlecalendar-client/Models/FreeBusyRequestItem";
import { type FreeBusyResponse } "mo:googlecalendar-client/Models/FreeBusyResponse";
import { defaultConfig; type Config } "mo:googlecalendar-client/Config";

module {
  public type CalendarConnection = {
    accessToken : Text;
    refreshToken : Text;
  };

  public type PendingOAuth = {
    codeVerifier : Text;
    redirectUri : Text;
    state : Text;
  };

  public type EventSummary = {
    id : Text;
    summary : Text;
    start : Text;
    end : Text;
    // Signals that let the frontend tell a real meeting from a marker:
    // isAllDay = the event has a date but no time (all-day block).
    // transparency = "transparent" (shows as free) or "opaque"/"" (busy).
    // eventType = "default" | "outOfOffice" | "focusTime" | "workingLocation".
    // To count/show only real meetings, keep timed, opaque, default events.
    isAllDay : Bool;
    transparency : Text;
    eventType : Text;
  };

  public type EventSummaryList = [EventSummary];

  let SCOPES : Text = "https://www.googleapis.com/auth/calendar";

  func configForToken(token : Text) : Config {
    {
      defaultConfig with
      auth = ?#bearer(token);
      is_replicated = ?false;
      max_response_bytes = ?Nat64.fromNat(2_000_000);
    };
  };

  func refreshIfNeeded(
    clientId : Text, clientSecret : Text, connection : CalendarConnection,
    caller : Principal, calendarConnections : Map.Map<Principal, CalendarConnection>,
    errorMsg : Text,
  ) : async* ?Text {
    if (not (errorMsg.contains(#text("401")) or errorMsg.contains(#text("Unauthorized")))) {
      Runtime.trap("Calendar API failed: " # errorMsg);
    };
    let refreshed = await OAuth.refreshAccessToken(clientId, clientSecret, connection.refreshToken);
    let newToken = accessTokenOf(refreshed, "Token refresh");
    Map.add(calendarConnections, Principal.compare, caller, {
      connection with accessToken = newToken;
    });
    ?newToken;
  };

  public func startAuthorize(
    clientId : Text, redirectUri : Text, caller : Principal,
    pendingFlows : Map.Map<Principal, PendingOAuth>,
  ) : async* Text {
    let codeVerifier = await OAuth.generateCodeVerifier();
    let state = await OAuth.generateCodeVerifier();
    Map.add(pendingFlows, Principal.compare, caller, {
      codeVerifier;
      redirectUri;
      state;
    });
    OAuth.buildAuthorizeUrl(clientId, redirectUri, SCOPES, state, OAuth.computeCodeChallenge(codeVerifier));
  };

  public func exchangeCode(
    clientId : Text, clientSecret : Text, code : Text,
    redirectUri : Text, codeVerifier : Text,
  ) : async* CalendarConnection {
    let tokens = await OAuth.exchangeAuthorizationCode(clientId, clientSecret, code, redirectUri, codeVerifier);
    let accessToken = accessTokenOf(tokens, "Token exchange");
    let refreshToken = switch (tokens.refreshToken) {
      case (?t) t;
      case null Runtime.trap("Token exchange failed: missing refresh_token");
    };
    { accessToken; refreshToken };
  };

  func accessTokenOf(tokens : OAuth.TokenResponse, operation : Text) : Text {
    switch (tokens.error) {
      case (?error) {
        let description = switch (tokens.errorDescription) {
          case (?value) ": " # value;
          case null "";
        };
        Runtime.trap(operation # " failed: " # error # description);
      };
      case null {};
    };
    switch (tokens.accessToken) {
      case (?token) token;
      case null Runtime.trap(operation # " failed: missing access_token");
    };
  };

  // Lists events in [timeMin, timeMax). Pass timeMax = "" for an open-ended
  // "everything from now" list; pass both to bound a single day/week (e.g.
  // timeMin = start-of-tomorrow, timeMax = start-of-day-after) so the count is
  // exact. Both are RFC 3339 strings; include an offset ("…Z" or "…+02:00").
  public func listUpcomingEvents(
    clientId : Text, clientSecret : Text, connection : CalendarConnection,
    caller : Principal, calendarConnections : Map.Map<Principal, CalendarConnection>,
    timeMin : Text, timeMax : Text, maxResults : Nat,
  ) : async* EventSummaryList {
    if (timeMin.size() == 0) {
      Runtime.trap("timeMin must be an RFC 3339 timestamp");
    };
    let events : Events = try {
      await* calendar_events_list(
        configForToken(connection.accessToken), "primary", #json,
        "", "", "", false, "", "",
        false, [], "", 10, maxResults, #starttime,
        "", [], "", [], false, false, true, "",
        timeMax, timeMin, "", "",
      );
    } catch e {
      let ?newToken = await* refreshIfNeeded(
        clientId, clientSecret, connection, caller, calendarConnections, e.message(),
      ) else Runtime.trap("Calendar API failed");
      await* calendar_events_list(
        configForToken(newToken), "primary", #json,
        "", "", "", false, "", "",
        false, [], "", 10, maxResults, #starttime,
        "", [], "", [], false, false, true, "",
        timeMax, timeMin, "", "",
      );
    };
    eventSummariesOf(events);
  };

  // Availability uses FreeBusy (POST + JSON body), NOT events.list: one call
  // returns the merged busy intervals across the user's calendars with recurring
  // events already expanded server-side — no paging, recurrence expansion, or
  // client-side merging. Returns the owner's busy intervals in [timeMin, timeMax]
  // as raw RFC 3339 (start, end) pairs; timeMin/timeMax are UTC "…Z" strings.
  // Single-refresh-on-401 retry.
  public func busyTimes(
    clientId : Text, clientSecret : Text, connection : CalendarConnection,
    caller : Principal, calendarConnections : Map.Map<Principal, CalendarConnection>,
    timeMin : Text, timeMax : Text,
  ) : async* [(Text, Text)] {
    let request : FreeBusyRequest = {
      FreeBusyRequest.init {} with
      timeMin = ?timeMin;
      timeMax = ?timeMax;
      items = ?[{ FreeBusyRequestItem.init {} with id = ?"primary" }];
    };
    let response : FreeBusyResponse = try {
      await* calendar_freebusy_query(
        configForToken(connection.accessToken), #json, "", "", "", false, "", "", request,
      );
    } catch e {
      let ?newToken = await* refreshIfNeeded(
        clientId, clientSecret, connection, caller, calendarConnections, e.message(),
      ) else Runtime.trap("Calendar API failed");
      await* calendar_freebusy_query(
        configForToken(newToken), #json, "", "", "", false, "", "", request,
      );
    };
    // The response map is keyed by the RESOLVED calendar id (the user's email),
    // NOT "primary". Iterate EVERY returned calendar and union its busy periods.
    var busy : [(Text, Text)] = [];
    switch (response.calendars) {
      case (?calendars) {
        for ((_id, cal) in calendars.entries()) {
          switch (cal.busy) {
            case (?periods) {
              for (p in periods.vals()) {
                switch (p.start, p.end) {
                  case (?s, ?e) busy := Array.concat(busy, [(s, e)]);
                  case _ {};
                };
              };
            };
            case null {};
          };
        };
      };
      case null {};
    };
    busy;
  };

  // --- Availability math (re-exported from google-oauth's tested DateTime) ---
  //
  // Do NOT re-implement RFC 3339 parsing — a digit parse that forgets to subtract
  // '0' (48) reads "2026" as 55354, so busy intervals land in the wrong year and
  // availability breaks silently (compiles, never traps). These thin re-exports
  // let callers use `LibCalendar.isSlotFree` / `.rfc3339ToNanos` with no extra
  // import; the implementation lives in `mo:google-oauth/DateTime`.
  public func rfc3339ToNanos(s : Text) : Int = DateTime.rfc3339ToNanos(s);
  public func nanosToRfc3339(ns : Int) : Text = DateTime.nanosToRfc3339(ns);
  public func overlaps(aStart : Int, aEnd : Int, bStart : Int, bEnd : Int) : Bool =
    DateTime.overlaps(aStart, aEnd, bStart, bEnd);
  public func isSlotFree(slotStart : Int, slotEnd : Int, busy : [(Text, Text)]) : Bool =
    DateTime.isSlotFree(slotStart, slotEnd, busy);

  public func createEvent(
    clientId : Text, clientSecret : Text, connection : CalendarConnection,
    caller : Principal, calendarConnections : Map.Map<Principal, CalendarConnection>,
    summary : Text, startDateTime : Text, endDateTime : Text,
  ) : async* Text {
    let start : EventDateTime = { EventDateTime.init {} with dateTime = ?startDateTime };
    let end : EventDateTime = { EventDateTime.init {} with dateTime = ?endDateTime };
    let event : Event = { Event.init {} with
      summary = ?summary;
      start = ?start;
      end = ?end;
    };
    let created : Event = try {
      await* calendar_events_insert(
        configForToken(connection.accessToken), "primary", #json,
        "", "", "", false, "", "",
        0, 10, true, #all, false, event,
      );
    } catch e {
      let ?newToken = await* refreshIfNeeded(
        clientId, clientSecret, connection, caller, calendarConnections, e.message(),
      ) else Runtime.trap("Calendar API failed");
      await* calendar_events_insert(
        configForToken(newToken), "primary", #json,
        "", "", "", false, "", "",
        0, 10, true, #all, false, event,
      );
    };
    switch (created.id) {
      case (?id) id;
      case null "";
    };
  };

  func eventSummariesOf(events : Events) : EventSummaryList {
    let items = switch (events.items) {
      case (?items) items;
      case null [];
    };
    Array.map<Event, EventSummary>(items, func(e : Event) : EventSummary = {
      id = switch (e.id) { case (?id) id; case null "" };
      summary = switch (e.summary) { case (?s) s; case null "(no title)" };
      start = switch (e.start) {
        case (?dt) switch (dt.dateTime) { case (?t) t; case null switch (dt.date) { case (?d) d; case null "" } };
        case null "";
      };
      end = switch (e.end) {
        case (?dt) switch (dt.dateTime) { case (?t) t; case null switch (dt.date) { case (?d) d; case null "" } };
        case null "";
      };
      // All-day events carry `date` but no `dateTime`.
      isAllDay = switch (e.start) {
        case (?dt) switch (dt.dateTime) { case (?_) false; case null true };
        case null false;
      };
      transparency = switch (e.transparency) { case (?t) t; case null "" };
      eventType = switch (e.eventType) { case (?t) t; case null "" };
    });
  };
};

4b. Availability / busy times — use FreeBusy, NOT events.list

Any "when is this person free / busy", booking, or Calendly-style feature MUST read availability through FreeBusy (calendar_freebusy_query), not calendar_events_list. FreeBusy is purpose-built for this: a single POST returns the merged busy intervals across the user's calendars, with recurring events already expanded server-side — you never page through events, expand recurrences, or union overlapping blocks yourself. It also folds in out-of-office and all-day blocks. Reserve calendar_events_list for showing the app's own event list and _insert / _delete for event CRUD.

The busyTimes helper in the lib/calendar.mo block above is the reference implementation: it builds a FreeBusyRequest for items = [{ id = "primary" }] over [timeMin, timeMax], does the single-refresh-on-401 retry, and — crucially — iterates every calendar the response returns (the map is keyed by the resolved calendar id, not "primary") and unions their busy periods.

Before comparing each (start, end) against your candidate slots, parse it to an absolute instant honoring the trailing offset — Google returns timed periods with a Z or a numeric offset (2026-07-21T14:00:00+02:00), and all-day blocks as a bare YYYY-MM-DD date. Truncating at the seconds and ignoring the offset shifts every busy interval by the offset (e.g. 2h in Zurich summer), so busy blocks miss the slots they should hide. Use LibCalendar.rfc3339ToNanos (a tested re-export of mo:google-oauth/DateTime) — it honors the offset and handles all-day dates — then overlap numerically. Do not hand-roll a parser that stops at the seconds.

End to end, the whole availability flow lives in nanosecond instants and only touches text at the edges: anchor the window with LibCalendar.rfc3339ToNanos, build the candidate grid with plain integer arithmetic, filter with LibCalendar.isSlotFree, then format the survivors back with LibCalendar.nanosToRfc3339 so they are ready to display and to pass straight to createEvent (whose startDateTime / endDateTime are RFC 3339 text). The grid below is a fixed UTC window; real working-hours / timezone policy is app-specific, but the parse → integer-math → format shape is the same:

motoko
func availableSlots(
  clientId : Text, clientSecret : Text, connection : LibCalendar.CalendarConnection,
  caller : Principal, calendarConnections : Map.Map<Principal, LibCalendar.CalendarConnection>,
  windowStart : Text,   // e.g. "2026-07-21T09:00:00Z"
  slotCount : Nat,      // number of consecutive slots to consider
  slotMinutes : Nat,    // slot length, e.g. 30
) : async* [(Text, Text)] {
  let slotNs = slotMinutes * 60 * 1_000_000_000;
  let start0 = LibCalendar.rfc3339ToNanos(windowStart);
  // Candidate grid of [s, s+slot) instants.
  let candidates = Array.tabulate<(Int, Int)>(slotCount, func(i) {
    let s = start0 + i * slotNs;
    (s, s + slotNs);
  });
  let windowEnd = start0 + slotCount * slotNs;
  let busy = await* LibCalendar.busyTimes(
    clientId, clientSecret, connection, caller, calendarConnections,
    windowStart, LibCalendar.nanosToRfc3339(windowEnd),
  );
  let free = Array.filter<(Int, Int)>(candidates, func(s) = LibCalendar.isSlotFree(s.0, s.1, busy));
  Array.map<(Int, Int), (Text, Text)>(
    free, func(s) = (LibCalendar.nanosToRfc3339(s.0), LibCalendar.nanosToRfc3339(s.1)),
  );
};

5. Available API surface

google-oauth (OAuth 2.0 mechanics)

| Function | Purpose | | --- | --- | | OAuth.urlEncode(text) | RFC 3986 percent-encoding for form bodies | | OAuth.parseTokenResponse(text) | Parse Google token-endpoint JSON | | OAuth.exchangeAuthorizationCode(...) | Exchange auth code for tokens | | OAuth.refreshAccessToken(...) | Refresh an expired access token | | OAuth.generateCodeVerifier() | Generate PKCE code_verifier (on-chain randomness) | | OAuth.computeCodeChallenge(verifier) | Compute PKCE code_challenge (S256) | | OAuth.buildAuthorizeUrl(...) | Build the Google OAuth authorize URL | | OAuth.getUserEmail(accessToken) | Fetch the connected email via OIDC userinfo (needs only openid email) |

Availability math (LibCalendar re-exports of mo:google-oauth/DateTime)

lib/calendar.mo re-exports these tested helpers, so call them as LibCalendar.* with no extra import. Times are absolute nanoseconds since the Unix epoch, matching Time.now().

| Function | Purpose | | --- | --- | | LibCalendar.rfc3339ToNanos(text) | Offset-aware RFC 3339 -> nanoseconds (honors Z / ±HH:MM, bare dates) | | LibCalendar.nanosToRfc3339(ns) | Nanoseconds -> UTC RFC 3339 text (…Z), ready for createEvent | | LibCalendar.overlaps(aStart, aEnd, bStart, bEnd) | Half-open interval overlap test | | LibCalendar.isSlotFree(slotStart, slotEnd, busy) | Slot is free of every (start, end) RFC 3339 busy pair |

googlecalendar-client (Calendar REST API v3)

The canonical actor above intentionally implements only upcoming-event listing and event creation; for availability/busy times use the FreeBusy helper in §4b. For another generated operation, keep bearer authentication and is_replicated = ?false, then apply the same single-refresh-retry pattern as refreshIfNeeded.

The generated package also exposes:

| Function | Module | Purpose | | --- | --- | --- | | calendar_events_list | EventsApi | List events on a calendar | | calendar_events_get | EventsApi | Get an event by id | | calendar_events_insert | EventsApi | Create an event | | calendar_events_update | EventsApi | Update an event (PUT) | | calendar_events_patch | EventsApi | Patch an event (PATCH) | | calendar_events_delete | EventsApi | Delete an event | | calendar_events_move | EventsApi | Move an event to another calendar | | calendar_events_quickAdd | EventsApi | Create event from text ("Lunch at noon") | | calendar_events_instances | EventsApi | List instances of a recurring event | | calendar_freebusy_query | FreebusyApi | Check free/busy across calendars | | calendar_calendarList_list | CalendarListApi | List user's calendars | | calendar_calendarList_get | CalendarListApi | Get a calendar list entry | | calendar_calendars_get | CalendarsApi | Get calendar metadata | | calendar_calendars_insert | CalendarsApi | Create a secondary calendar |

6. Cycles and response sizes

The google-oauth library uses Call.httpRequest from mo:ic/Call, which auto-computes and attaches the exact required cycles via the ic0.cost_http_request system API. No manual cycle budgeting is needed for token exchange or refresh calls.

For googlecalendar-client calls, defaultConfig.cycles = 30_000_000_000 (30B). A typical list/insert costs ~10–15B cycles. Set max_response_bytes = ?2_000_000 for event list reads that may include large payloads.

7. Things that will bite you

• is_replicated = ?false — see §3. Non-negotiable.
• Google refresh tokens do NOT rotate. Unlike X/Twitter, Google does

not issue a new refresh_token on each refresh. Keep the original refresh_token and only persist the new access_token.

• Access tokens expire in 1 hour. The refreshIfNeeded helper catches

HTTP 401, silently refreshes via google-oauth.refreshAccessToken, and retries once. If the refresh also fails, surface "re-connect your account".

• Callback URI exact-match. Every character (trailing slash, query

string, port) must match between the authorize URL and the redirect. Google returns redirect_uri_mismatch otherwise. Use the fixed window.location.origin + "/connect/calendar" for redirectUri — the same value the settings page displays and the /connect/calendar route owns — and register that exact URI on the Google Web client. Do not build it from window.location.pathname, which varies by page.

• Pass the displayed value to startCalendarOAuth unchanged — never the raw

*.icp0.io canister URL. A Caffeine app is served at several origins (the *-draft.caffeine.xyz draft, the *.caffeine.xyz live domain, and the raw <canister-id>.icp0.io URL). Compute the redirect URI in one shared helper (window.location.origin + "/connect/calendar") and use that same helper both for the copyable field on the settings page and for the value handed to startCalendarOAuth. If the value sent to Google (via startCalendarOAuth) differs from what the settings page showed and the admin registered — e.g. a build-time/config value or the *.icp0.io canister origin — Google returns redirect_uri_mismatch.

• RFC 3339 timestamps. Calendar uses RFC 3339 strings

(2026-07-10T15:00:00-07:00). For all-day events set EventDateTime.date (YYYY-MM-DD) instead of dateTime.

• createEvent times need a zone. The dateTime you pass to createEvent

MUST carry a UTC offset (…Z or …+02:00) or you MUST also set EventDateTime.timeZone (an IANA name like "Europe/Zurich"). A bare 2026-07-10T15:00:00 with neither is rejected by Google. Prefer sending an offset-qualified string so the event lands at the intended wall-clock time.

• calendarId = "primary" refers to the authenticated user's default

calendar. Named/shared calendars use their calendar-ID (an email-like address).

• Availability = FreeBusy, not events.list. For "am I free / busy" use

calendar_freebusy_query (§4b): one POST returns merged busy intervals with recurrences expanded server-side. Rebuilding availability from events.list means paging, expanding recurring events, and merging overlaps by hand — easy to get wrong, and the classic cause of "the booking link shows me free when I'm busy".

• maxAttendees and maxResults must be ≥ 1. Google rejects maxAttendees=0

/ maxResults=0 with HTTP 400 (documented minimum is 1). The listUpcomingEvents and createEvent recipes pass maxAttendees = 10; never pass 0 for these on any events endpoint.

• FreeBusy responses are keyed by the resolved calendar ID, not the string

you queried. When you call calendar_freebusy_query for "primary", Google resolves it and returns the calendars map keyed by the real calendar ID (the user's email address), not the literal "primary". Do not look up "primary" in the response — that finds nothing and makes every slot look free (a common availability bug). Instead, iterate over every calendar the response returns and union all their busy intervals, then subtract those from your candidate slots. Parse each interval's start/end as RFC 3339 allowing a trailing Z or a numeric offset (+02:00); compare instants, not raw strings.

• Parse RFC 3339 with LibCalendar.rfc3339ToNanos (re-exported from

mo:google-oauth/DateTime) — do NOT re-implement it. A hand-rolled parser that forgets to subtract '0' (48) per digit reads "2026" as 55354, so every busy interval lands in the wrong year, overlap checks never match, and availability is silently wrong — the code still compiles and never traps, so the bug is invisible until a user is double-booked. Use the tested helper.

• HTTP 429 rate-limit. Surface the error to the caller; never

silently retry a write inside the canister — a retry may create a duplicate event.

• Don't expose the access token. calendarConnections is read only by

Map.get(calendarConnections, ..., caller) inside API calls. No getMyCalendarConnection, no getMyAccessToken, no iterator. A leaked bearer is a per-user account compromise.

• alt = #json for all Calendar API v3 calls. Leave optional string

parameters "" and prettyPrint = false.

• API query parameters are plain positional values, not ?T — never pass

null for one. The client's function parameters are Text / Bool / enum / Nat (e.g. alt, fields, prettyPrint); pass real values like #json, "", false, 10 — null will not type-check. (Respect each param's documented minimum: maxAttendees / maxResults must be ≥ 1, see below.) Only model values (Event, EventDateTime, FreeBusyRequest) are optional ?T.

• Combined Gmail + Calendar apps: request the scope union, and learn the

address via OAuth.getUserEmail. The union of openid email + .../calendar + .../gmail.send covers availability, sending, and the connected address (via OIDC userinfo) — no gmail.readonly needed unless the app actually reads mail. Never drop a scope when merging recipes — see "Combined Gmail + Calendar apps".

• Build Event / EventDateTime with init {} then record-update

the fields you need — all fields are optional (?T); leave the rest null.

• PATCH/PUT/DELETE are forced non-replicated in the generated client

(the googlecalendar-client sets is_replicated = ?false on these methods automatically). For GET/POST, set it explicitly in your Config.

Frontend

Every build using this skill MUST ship all four items below. (If the app also uses the Gmail connector, follow "Combined Gmail + Calendar apps" below instead — it replaces /settings/calendar + /connect/calendar with one shared /settings/google + /connect/google. The requirements below still apply; only the two paths change.) These are acceptance criteria, not suggestions — verify each before the build is done. These three are the requirements builds skip, and any one missing makes the connector broken, not merely incomplete:

• The credentials page exists and is reachable. The app MUST have the

/settings/calendar page with Client ID/Secret inputs (item 2), and a signed-in admin MUST be able to reach it — via a nav link or the not-configured prompt on the connect page. A "Connect Google Calendar" button with no page to enter credentials is the most common failure and leaves the connector unusable.

• The admin settings page displays the literal, copyable redirect URI. Not a

<your-domain> placeholder, not "your app URL + /connect/calendar" as text for the admin to assemble — the actual string window.location.origin + "/connect/calendar" rendered in a read-only field the admin can copy. Concretely: an app served from https://my-app.caffeine.xyz must show a field containing exactly https://my-app.caffeine.xyz/connect/calendar and nothing else. Without it the admin cannot register the URI in Google and every connection fails.

• /connect/calendar is a real route that handles Google's callback — not a

button-only page. If it falls through to a catch-all/home redirect, or calls completeCalendarOAuth before the authenticated actor is ready, the connection silently fails and the app shows "not connected".

1. A login flow — required. Calendar cannot work without a non-anonymous

caller; the per-user OAuth handshake stores tokens keyed by caller : Principal, and the admin credential setter gates on #admin. The login flow comes from extension-authorization: useInternetIdentity, login/logout buttons, the useActor plumbing that injects the authenticated identity into every backend call.

1. An admin settings page — /settings/calendar (admin-gated). This

page is required; a Calendar build is incomplete without it:

• Show a "How to get your Google credentials" panel before the credential

inputs. Reassure the admin it is a one-time, ~5-minute setup, and walk through these numbered steps (the agent's completion message must repeat the same steps):

1. open the Google Cloud Console and

sign in with any Google account;

1. create or pick a project;
2. enable the Google Calendar API (APIs & Services → Library → search

"Google Calendar API" → Enable);

1. configure the OAuth consent screen (APIs & Services → OAuth consent

screen → External; set app name, support email, developer email; Google's default scopes are fine);

1. create an OAuth client ID of type Web application (APIs &

Services → Credentials → Create Credentials → OAuth client ID);

1. under Authorized redirect URIs, add the exact value from the

copyable field on this page;

1. copy the resulting Client ID and Client Secret into the inputs

below and save. Include a convenience link that opens the Google Cloud Console.

• Render the actual URI in a read-only, copyable field using one shared

helper: const calendarRedirectUri = () => window.location.origin + "/connect/calendar";. For example, if the app is open at https://my-app.caffeine.xyz, the displayed value is https://my-app.caffeine.xyz/connect/calendar. Never show only <app-domain> or ask the administrator to infer the URI.

• Two password-inputs bound to setCalendarCredentials(clientId, clientSecret).

Submit on enter; clear inputs on success.

• Status indicator driven by isCalendarConfigured() (returns Bool).

Show "Configured" / "Not configured" — never display the credentials.

• Make this page reachable. The app's main navigation (the shared Layout)

MUST link to this page for admins — show the link when isCallerAdmin is true, hide it otherwise (via extension-authorization). Add that link wherever the nav is defined, not inside this page. A /settings/calendar route with no way to reach it is a broken build. Do not rely on the nav alone: the not-configured prompt below is the primary way users discover setup is needed.

1. A "Connect Calendar" and callback page — /connect/calendar (any

signed-in user). This dedicated page must catch and handle Google's redirect after consent; it is not only a page with a connect button:

• Handle the not-configured case for everyone. isCalendarConfigured() is

a public query (any signed-in user may call it). When it returns false, do not show a dead connect button. Admins see a link to /settings/calendar to enter credentials. Non-admins must see an explanation, not a dead end — e.g. "Google Calendar isn't set up yet — the app's administrator needs to add Google credentials in Settings." Enable the "Connect Google Calendar" button only once configured.

• "Connect Google Calendar" button bound to

startCalendarOAuth(calendarRedirectUri()). Redirect the browser to the URL returned by the canister. Do not derive the callback from an arbitrary current pathname; the fixed /connect/calendar route and the settings-page URI must be identical.

• Register /connect/calendar as a real application route. It must catch

the Google callback and must not fall through to a catch-all redirect, layout default, or home page before processing it.

• On the return leg, read error, code, and state from

URLSearchParams. If error is present, show the failed/declined connection state and do not call the canister. Only when both code and state are present, call and await completeCalendarOAuth(code, state) before navigating anywhere or clearing the URL. Keep a visible "Connecting Google Calendar…" state while it is pending. Do not replace the route, redirect to the home page, or discard the query parameters first — that loses the one-time code and leaves the user disconnected.

• Wait for actor readiness before the one-time callback call. The page

must wait for useInternetIdentity().isAuthenticated and useActor(createActor) to provide a non-null, non-fetching actor before calling completeCalendarOAuth. Do not set a startedRef/one-shot guard until then: on first render the actor is often unavailable, and an "Actor not ready" failure otherwise consumes the only retry while the authorization code is still in the URL.

• After either terminal path, call history.replaceState to remove the

OAuth query parameters. This prevents a page refresh from reusing a one-time authorization code.

• Status driven by isMyCalendarConnected() (returns Bool).
• Optional "Disconnect Calendar" button bound to disconnectMyCalendar().

1. Calendar UI — the main page shows upcoming events. When

isCalendarConfigured() is false and the caller is an admin, render a "Set up Google Calendar" link to /settings/calendar so the credentials page is discoverable, not just reachable. Pass the current time as the RFC 3339 timeMin value, "" for an open-ended timeMax: listUpcomingEvents(new Date().toISOString(), "", 10). To bound a single day (e.g. "meetings tomorrow"), pass both — the local start of the day and the start of the next day, each RFC 3339 with an offset — and count only entries whose isAllDay is false and transparency is not "transparent" and eventType is "default" (that filters out all-day, free, out-of-office, and working-location markers). This is required when using singleEvents = true and orderBy = startTime. Also include a "create event" form. datetime-local values have no offset, so convert each browser-local value to an RFC 3339 instant before calling the actor: createEvent(summary, new Date(startInput).toISOString(), new Date(endInput).toISOString()). When isMyCalendarConnected() is false, render an inline "Connect Google Calendar" link to /connect/calendar.

Suggested route layout:

/                   →  Main UI (upcoming events + create form)
/settings/calendar  →  Admin credential config (admin-only)
/connect/calendar   →  Per-user OAuth handshake (any signed-in user)
# If the app ALSO uses Gmail: drop the two routes above and use a single
# /settings/google + /connect/google — see "Combined Gmail + Calendar apps".

Combined Gmail + Calendar apps

When an app uses both connectors, build one shared Google connection, not two (an auth code is single-use, so two flows would force two consent screens). Frontend:

• One admin page /settings/google — a single Client ID / Client Secret

form, one isGoogleConfigured status, and one copyable redirect-URI field showing exactly window.location.origin + "/connect/google".

• One connect route /connect/google — the same real callback route the

Frontend section above requires: it renders "Connect Google", catches the redirect, waits for actor readiness, then calls completion once. No second callback route.

• Do NOT build /settings/gmail, /connect/gmail, /settings/calendar, or

/connect/calendar. Every other Frontend requirement above still applies — only these paths change.

Backend — write the shared flow once (it replaces both per-connector OAuth flows). It is the same shape as the per-connector startAuthorize / exchangeCode / refresh functions, with these exact differences:

• One #admin-gated config setter storing a single Client ID/Secret.
• SCOPES = the union below — both APIs in one consent.
• completeGoogleOAuth(code, state) learns the connected email via

OAuth.getUserEmail (OIDC userinfo — needs only openid email, not gmail.readonly) and stores one connection { accessToken; refreshToken; emailAddress } in a single Map<Principal, GoogleConnection>.

• Gmail sends and Calendar calls each build their own client Config from

that one accessToken, each keeping its single-refresh-on-401 retry.

• Keep the connection and client config in one shared state value and pass it

as a parameter to both the Gmail and Calendar mixins, so both read and write the same connection (see the writing-motoko mixins rule).

motoko filepath=src/backend/google.mo
let SCOPES : Text =
  "openid email "                                    // learn the address via userinfo
  # "https://www.googleapis.com/auth/gmail.send "
  # "https://www.googleapis.com/auth/calendar";
// Add "https://www.googleapis.com/auth/gmail.readonly " ONLY if the app reads mail.

Wire it as one connection shared by both services — declare the config, connection map, and pending-flow map once and pass the same bindings to every mixin. The Gmail and Calendar messaging mixins do not declare their own config or connection; they receive the shared googleConfig and googleConnections (config is needed for the refresh-on-401 retry):

motoko filepath=src/backend/main.mo
actor {
  let accessControlState = AccessControl.initState();
  include MixinAuthorization(accessControlState, null);

  // ONE shared credential + connection state for both services.
  let googleConfig = { var clientId : Text = ""; var clientSecret : Text = "" };
  let googleConnections : Map.Map<Principal, Google.Connection> = Map.empty();
  let pendingGoogleFlows : Map.Map<Principal, Google.PendingOAuth> = Map.empty();

  include MixinGoogleConfig(accessControlState, googleConfig);                    // setGoogleCredentials / isGoogleConfigured (#admin-gated setter)
  include MixinGoogleOAuth(googleConfig, googleConnections, pendingGoogleFlows);  // startGoogleOAuth / completeGoogleOAuth, SCOPES = union above
  include MixinGmailMessaging(googleConfig, googleConnections);                  // sendEmail — refresh-on-401 needs config; reads the shared connection
  include MixinCalendarMessaging(googleConfig, googleConnections);               // calendar calls — same shared config + connection
};

Do not give Gmail and Calendar separate config/connection state or separate OAuth flows — one auth code is single-use, and separate state desyncs (see the writing-motoko mixins rule).

Enable both APIs on the one OAuth client and register only the single .../connect/google redirect URI. Split into two separate panels only if the user explicitly asks to connect two different Google accounts.

Common to all variants

• Sign-in is required for every Calendar-related route. Wire the

/settings/... and the connect route (/connect/calendar, or /connect/google in a combined app) through extension-authorization's auth guard (useInternetIdentity + redirect when !isAuthenticated).

• The frontend never persists tokens. No localStorage, no

IndexedDB, no cookies — the canister mediates everything. The browser only ever sees Bool status flags and the OAuth redirect URLs.

• The OAuth state parameter is canister-generated and validated. The

canister stores a random nonce with the pending verifier and callback URI. The frontend must pass both code and state to the completion call (completeCalendarOAuth, or completeGoogleOAuth in a combined app); it never creates or modifies either value.

• The calendar UI is trivial: a list of upcoming events, a create-event

form with summary + start/end datetime inputs. No client-side Google SDK, no token handling, no JSON serialization — the canister is the Calendar client.

Related

• mops add googlecalendar-client@0.1.4 — Calendar REST API v3 bindings.
• mops add google-oauth@0.2.0 — Google OAuth 2.0 library (token exchange, refresh, PKCE, getUserEmail userinfo, DateTime RFC 3339 helpers).
• Google OAuth 2.0 for Web Server Applications — Web-client redirect URI and authorization-code flow reference.
• Google Calendar API v3 reference — what googlecalendar-client wraps.
• RFC 7636 — Proof Key for Code Exchange — PKCE spec.
• extension-authorization — required prerequisite. Provides Internet Identity login, useInternetIdentity / useActor frontend plumbing, and the #admin role gate.
• connector-googlemail — sister connector using the same google-oauth library for Gmail.