[BUG] Gmail connector: all tool calls fail with "Precondition check failed" even after full OAuth reauthorization

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment

• Claude Code desktop app (Code tab)
• macOS 26.5.1 (Build 25F80)
• Connector: Gmail (via claude.ai Connectors), account on a custom Google Workspace domain

Problem
Every call to any Gmail connector tool fails with a generic error:

Precondition check failed

This happens for both read-only tools (list_labels, search_threads) and
write tools (send_message, create_draft), immediately, with no other
detail in the error. The connector itself is reported as "connected"
(status: connected, tool_count: 29) via session connector status the
whole time.

Steps to reproduce

1. Connect Gmail in Settings → Connectors, using an info@ account on a

custom Google Workspace domain

1. Call any Gmail tool (e.g. list labels) → fails with "Precondition

check failed"

Troubleshooting already tried (none fixed it)

1. Disconnected and reconnected the Gmail connector from Claude's

Settings → Connectors. Reconnecting did NOT show Google's consent
screen (silently reused a cached grant). Error persisted.

1. Started a brand-new chat session to rule out session-level caching.

Same error occurred immediately on the first call.

1. Checked the Google Workspace Admin Console → Security → API Controls

→ App access control, and found/removed a restriction on the app.
Retried → same error, unchanged.

1. Went to myaccount.google.com/permissions, fully revoked "Claude for

Gmail" access, then reconnected from Claude Settings. This time
Google's real OAuth consent screen appeared, requesting:

• Read email messages and settings
• Manage drafts and send email
• Read, compose, and send emails from the Gmail account

Granted all three scopes explicitly. Retried the same tool calls →
still "Precondition check failed", completely unchanged.

Expected behavior
Gmail tool calls should succeed after a valid, freshly-granted OAuth
authorization with all requested scopes.

Actual behavior
100% of Gmail connector calls fail identically before and after full
reauthorization, suggesting the issue is not account/permission-related
but a bug or outage in the connector itself.

What Should Happen?

Gmail connector tool calls should succeed after a valid, freshly-granted OAuth authorization with all requested scopes. Read-only calls (e.g. list_labels) should return the label list, and write calls (e.g. send_message, create_draft) should send/create the email as requested.

Error Messages/Logs

Steps to Reproduce

1. In Claude desktop app, go to Settings → Connectors and connect Gmail using an info@ account on a custom Google Workspace domain
2. In a chat, ask Claude to do anything with Gmail (e.g. "list my Gmail labels" or "send a test email")
3. The tool call fails immediately with: "Precondition check failed" — no further detail
4. This happens for every Gmail tool call, both read-only (list_labels, search_threads) and write (send_message, create_draft), 100% of the time
5. Fully disconnect and reconnect the connector in Settings → Connectors → Gmail. Reconnecting sometimes silently reuses a cached Google consent (no permission screen shown). Error persists identically.
6. Revoke access entirely at myaccount.google.com/permissions ("Claude for Gmail"), then reconnect from Claude Settings. This time Google's full OAuth consent screen appears; grant all requested scopes (read, manage drafts/send, read/compose/send).
7. Retry the same Gmail tool calls — still fails identically with "Precondition check failed"

Claude Model

Sonnet (default)

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

Claude 2.110.0 (dfb2ba) 2026-09-15T05:50:55.000Z

Platform

Other

Operating System

macOS

Terminal/Shell

Other

Additional Information

Reproduced via the desktop app's Connectors settings, not via CLI/terminal.

Related: this appears to be the same family of issue as #87808 (closed as "invalid/not related to Claude Code" without investigation — same symptom: ALL Gmail calls fail despite fully confirmed OAuth grant, just a different error string), and the same underlying Google connector OAuth/token pattern discussed in #72375. Given the recurring pattern across multiple users and connectors (Gmail, Calendar), this looks like a server-side token-refresh bug in Claude's connector infrastructure rather than a user-side auth misconfiguration — please don't auto-close as unrelated.