Google Calendar

Google Calendar is Google's online calendar service.

Authentication types

Google Calendar supports two authentication methods:

• OAuth: Create your own Google Cloud OAuth app. You control the app, scopes, and credentials. Each user connects with their Google account. Use this method for production. Google Cloud setup is required.

• Instant OAuth: Use Willow's preconfigured Google app. Setup is faster, but fewer scopes are available. Use this method for testing, not production. No Google Cloud setup is required.

Before you start

You will need:

• A Google account that can create projects in the Google Cloud Console. A personal Gmail account is enough for testing.
• Your Willow Redirect URL (see Create the OAuth client).

If your company uses Google Workspace, your admin may block third-party apps by default. In that case a Workspace super admin has to allow your app before anyone can connect. See Approving the app for your organization.

Order matters

Enable the Google Calendar API before you pick scopes. The scope picker only lists scopes for APIs that are already enabled in the project, so if you skip ahead you will not be able to find the Calendar scopes.

Setting up OAuth

1. Create or select a project

Go to https://console.cloud.google.com/ and either create a new project or select an existing one using the project picker in the top bar.

Every step below applies to the selected project, so make sure the right project name is showing before you continue.

2. Enable the Google Calendar API

Go to APIs & Services → Library, search for "Google Calendar API", open it, and click Enable.

If the API is already turned on, this page shows API Enabled and a Manage button instead of Enable.

Direct link: console.cloud.google.com/apis/library/calendar-json.googleapis.com

3. Configure the consent screen (Branding)

Google now places OAuth settings under Google Auth Platform instead of the old "OAuth consent screen" wizard. Open console.cloud.google.com/auth/branding. You can also go to APIs & Services → OAuth consent screen, which redirects to the same page.

Fill in the required fields:

• App name: What users see on the Google consent screen
• User support email
• Developer contact email

Click Save. Until these fields are filled in, the Audience page will warn that your OAuth configuration is incomplete.

4. Choose your audience

Open Audience (console.cloud.google.com/auth/audience) and set the User type.

Use Internal if you can. Click Make internal. An Internal app can be used only by members of your organization and does not need Google's External app verification.

Google offers Internal only when the Google Cloud project belongs to a Google Cloud Organization. Google Workspace or Cloud Identity provides an organization, but a personal Gmail account does not. If your project has no organization, Make internal is greyed out and External is your only option.

Internal apps may still need admin approval

An Internal app may still need admin approval for restricted Google Workspace services, including Gmail and Drive. See Approving the app for your organization.

If you use External, the app starts in Testing. Only accounts listed under Test users can connect. Add each user under Test users → Add users. Anyone who is not on the list gets an error when they try to connect.

Two things to know about Testing before you rely on it:

• The cap is 100 test users over the entire lifetime of the project. It cannot be reset or changed. Each person counts toward the limit as soon as you add them, even if they never connect.
• The user's authorization and refresh token expire after seven days. Users will have to reconnect every week.

Adding a test user takes effect immediately. Until the account is on the list, Google returns Error 403: access_denied with the message "Access blocked: … has not completed the Google verification process". Although the message mentions verification, it usually means the account is missing from Test users.

Complete Branding before publishing. If the Branding page is incomplete, the Audience page shows "Your app's OAuth configuration is incomplete" and Publish app stays greyed out.

5. Add scopes

Open Data Access (console.cloud.google.com/auth/scopes) and click Add or remove scopes.

Add the Calendar scopes you need:

• https://www.googleapis.com/auth/calendar: full access
• https://www.googleapis.com/auth/calendar.readonly: read-only

If you do not see them in the list, the Calendar API is not enabled yet. Go back to step 2. You can also paste a scope directly into the Manually add scopes box.

Click Update, then Save.

Check how Google classifies your scopes

The Data Access page sorts your scopes into three groups. The group determines what Google requires before you can publish an External app:

Group	What it means for you
Non-sensitive	No additional review.
Sensitive	An External app must pass Google's OAuth verification before you can publish it. Internal apps and apps that remain in Testing do not need this verification.
Restricted	Verification, plus an annual third-party security assessment (CASA), for External apps that are published.

.../auth/calendar is a sensitive scope, not a restricted one. An Internal app does not need Google's External app verification.

6. Create the OAuth client

Open Clients (console.cloud.google.com/auth/clients) and click Create client.

1. Set Application type to Web application
2. Give the client a name
3. Under Authorized redirect URIs, click Add URI and add your Willow redirect URL:
   • For SaaS deployments: https://{org}.mcp-s.com/{org}/api/auth/callback
   • For On-Premise deployments: {connectUrl}/{org}/api/auth/callback
4. Click Create
5. Copy the Client ID and Client Secret

The redirect URI must match exactly, including https://, and must not have a trailing slash. A mismatch produces Error 400: redirect_uri_mismatch when you connect.

7. Finish in Willow

Open your Google connector in Willow and go to the Setup tab.

1. Under Authentication, select OAuth to use your own Client ID and Client Secret.
2. Paste the Client ID and Client Secret
3. Confirm the Redirect URL shown here exactly matches the URI you registered in step 6
4. Under Scopes, click Add and select the same scopes you configured in step 5
5. Click Save Changes

Selecting a scope in Willow that you did not add in Google Cloud will fail at connect time, so keep the two lists identical.

One OAuth client can serve several connectors

Every connector in a Willow organization shares the same Redirect URL, so one Google OAuth client works for all of them. You do not need one client per connector. Paste the same Client ID and Secret into each connector, then select only the scopes it needs.

Select the narrowest scopes offered by the connector that meet your needs.

8. Authorize the connection

Click Check connection. It stays disabled until the credentials are saved.

Willow opens Google's consent window. It also shows an Authenticate your MCP dialog with the authorization URL and an Authenticate button in case your browser blocks the window. Complete the Google consent steps, then click I've authenticated.

"Google hasn't verified this app"

An External app that requests sensitive or restricted scopes may show an unverified-app warning. If you are testing your own app, follow the available prompts to continue to consent. Publishing the app does not by itself complete verification.

If the consent window does not appear, use the Authenticate button in the dialog.

Test a read-only tool

To test the connection, go to the Tools tab, open the row menu for a read-only tool, and choose Test Tool → Run test. A successful run returns data from Google as JSON.

Use an existing resource that the authenticated account can access. A 401 response indicates an authentication problem; a 403 or 404 can also indicate missing permissions or an inaccessible resource, so an error alone does not confirm a working connection.

Check Guards if a response is blocked

If a tool returns Tool blocked by organization's guardrails, check the matching Willow Guard. The response was blocked by a Guard; changing OAuth scopes will not fix that block.

Pick a tool that returns no personal data

list-colors is a good quick connection test. list-calendars returns calendar IDs, which are email addresses. A Willow Guard such as Email Address Detection may block the response with Tool blocked by organization's guardrails. If this happens, OAuth is working, but the Guard blocked the result.

Approving the app for your organization

Google Workspace admins can restrict which third-party apps may access organization data. When this restriction is on, users may see an access-blocked message naming their Workspace admin or an admin-policy error, even when the OAuth setup is correct.

A Workspace super admin fixes this in the Admin console. From the Admin console, go to Menu → Security → Access and data control → API controls, then click Manage App Access.

To add an app that is not listed yet:

1. Click Configure new app
2. Enter the app name or the Client ID from step 6, then click Search
3. Select the app and click Continue
4. Under Access to Google data, choose Specific Google data and allow the scopes your connector needs, including any required Google Sign-in scopes. Choose Trusted only if your organization intends to allow the app to request all Google services, including restricted services.
5. Click Continue, then Finish

To change an app that is already listed, point to it and click Change access. To update several apps, select them and click Change access at the top. Use Select org units → Include organizations to choose which parts of your organization receive the change. Leave the top-level organization selected to apply it to everyone. Then confirm with Change access.

The access levels are Trusted, Limited, Specific Google data, and Blocked. Specific Google data permits the scopes you approve, including scopes for restricted services. Trusted permits access across all services.

Internal apps also need approval if your organization restricts unconfigured third-party apps. See Google's guide, Control which third-party & internal apps access Google Workspace data.

Publishing and verification

If your app is Internal, you do not need External app verification or a test-user list, and authorizations do not expire after seven days.

An External app starts in Testing. Only listed test users can connect, and refresh tokens expire after seven days. To use the app in production, click Publish app on the Audience page. Google must verify an External app that uses sensitive Calendar scopes. Submit it from the Verification Center. The review can take days or weeks.

Troubleshooting

Symptom	Cause
Required scopes missing from the scope picker	A required API is not enabled in the selected project. See step 2.
Error 400: redirect_uri_mismatch	The Redirect URI in the OAuth client does not exactly match Willow's Redirect URL.
org_internal error	The app is Internal, but the user does not belong to the organization that owns the Google Cloud project.
Error 403: access_denied: "Access blocked: <domain> has not completed the Google verification process… can only be accessed by developer-approved testers"	The app is External and in Testing, but the Google account is not in Test users. Add the account on the Audience page. This is a test-user error, not a Workspace admin approval error.
"Access blocked" that names your Workspace admin, or persists for an account already listed under Test users	Workspace admin has not trusted the app. See Approving the app for your organization.
Users have to reconnect every 7 days	App is External and still in Testing. Authorizations expire seven days after consent.
Make internal is greyed out	The project does not belong to a Google Cloud Organization
Publish app is greyed out, with "Your app's OAuth configuration is incomplete"	Branding is unfinished. Fill in the Branding page first.
Tool call returns {"error":{"type":"Tool Error","message":"Tool blocked by organization's guardrails"}}	A Willow Guard blocked the response. Check Guards for the matching rule and result.