[BUG] showSetupScreens() bedrock-fallback tier probing adds ~4.3s to startup

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?

Since recent Claude Code patches, startup takes ~5 seconds before the REPL becomes interactive.

Debug logs (--debug) show showSetupScreens() takes 4723ms, with ~4300ms spent between the AWS caller identity resolution and the [bedrock-fallback] unpinnedTiers=3 fallbacks=0 log line.
The bedrock-fallback tier probing appears to make blocking network requests that gate the entire startup path.

What Should Happen?

Startup should be near-instant (sub-1s). Furthermore, according to Claude Code itself, "the bedrock-fallback tier probing should either run in the background (non-blocking), be cached across sessions, or use shorter timeouts".

Prior versions did not have this delay.

Error Messages/Logs

> 2026-04-16T11:43:06.807Z [DEBUG] Fetching AWS caller identity for AWS auth refresh command
> 2026-04-16T11:43:07.299Z [DEBUG] Fetched AWS caller identity, skipping AWS auth refresh command
> 2026-04-16T11:43:11.586Z [DEBUG] [bedrock-fallback] unpinnedTiers=3 fallbacks=0
> 2026-04-16T11:43:11.587Z [DEBUG] [STARTUP] showSetupScreens() completed in 4723ms

Steps to Reproduce

1. Configure Claude Code to use an AWS Bedrock-routed model (e.g. eu.anthropic.claude-opus-4-6-v1[1m])
2. Launch Claude Code with --debug
3. Observe ~5s delay before the REPL becomes interactive
4. In the debug log, search for showSetupScreens() — it reports ~4700ms
5. The gap between Fetched AWS caller identity and [bedrock-fallback] unpinnedTiers=3 is ~4.3s

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

~2 weeks ago

Claude Code Version

2.1.110 (Claude Code)

Platform

AWS Bedrock

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

• Terminal Ghostty v1.2.3

• Model: eu.anthropic.claude-opus-4-6-v1[1m] (EU Bedrock region)
• The showSetupScreens() function accounts for 95% of total startup time (4723ms of ~4940ms)
• Breakdown: AWS caller identity takes ~493ms (acceptable), then bedrock-fallback probing takes ~4287ms across 3 unpinned tiers with 0 fallbacks used — suggesting the probing is wasted work in this case
• Everything after showSetupScreens() is fast: MCP connections, LSP init, file indexing all complete in background after REPL mount
• Node v24.3.0, Linux 6.17.0-22-generic (Ubuntu 25.x)