[BUG] Explore subagents fail with 'Prompt is too long' when MCP servers are configured

Description

Launching Explore subagents via the Agent tool fails immediately with "Prompt is too long" when multiple MCP servers are configured.

Reproduction

1. Configure several MCP servers (in my case: Figma, PostHog, Linear, Slack, Chrome DevTools)
2. Launch an Explore subagent via the Agent tool
3. Agent returns "Prompt is too long" without doing any work

Observed behavior

Both Explore agents I launched in parallel returned Prompt is too long immediately.

Expected behavior

Explore agents should work regardless of how many MCP servers are configured.

Possible cause

The primary agent uses deferred tool loading (ToolSearch + <available-deferred-tools>) so MCP tool definitions aren't all expanded in the prompt. Subagents may not be using this mechanism and instead receiving all MCP tool definitions expanded, which could push the prompt over the limit.

Environment

• Platform: macOS (Darwin 25.2.0)
• Model: claude-opus-4-6

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