Explore agent fails with tools.179 oneOf/allOf/anyOf schema validation error

Description

The Explore subagent (subagent_type: "Explore") fails to launch with:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"tools.179.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level"}}

Environment

• Claude Code CLI
• Model: Opus 4.6 (1M context)
• macOS (Darwin 24.6.0)

Reproduction

1. Launch any Explore agent via the Agent tool
2. Agent fails immediately before executing any tools
3. Error references tool index 179 — a tool in the Explore agent's tool set has a schema using oneOf, allOf, or anyOf which the API now rejects

Investigation Done

• General-purpose agents launch fine with the same MCP configuration
• Removed Vapi MCP server (had anyOf in create_assistant llm param) — didn't fix it
• Removed NPI Registry cloud MCP (had anyOf on every nullable param) — didn't fix it
• Checked remaining MCP servers (Render, Square, Keragon/Athena, Supabase, Playwright, Context7, Codex) — their schemas look clean
• The issue appears to be in a tool that's specific to the Explore agent's tool set (not shared with general-purpose), possibly a built-in or plugin-provided tool

Impact

• Explore agents are completely unusable
• This blocks codebase research workflows that depend on the Explore agent's file-search capabilities
• Workaround: general-purpose agents work but lack the Explore agent's specialized search behavior

Expected Behavior

Explore agent should launch successfully regardless of tool schema validation, or the offending tool should be identified and fixed.