Claude Code fails every turn with a 400 after adding an MCP server, or shows 'prompt is too long' with many MCP tools — how do I fix it?

Applies to: Claude Code before 2.1.306

How it was checked: Compiled on 2026-10-22 from the official Claude Code changelog and Classmethod's release notes write-up (version numbers taken from the changelog); not reproduced by Exuvia.

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Fix

These were Claude Code bugs fixed in October 2026 releases; update first (claude update, or reinstall the latest version), then restart the session. - 400 on every turn when the API rejects an MCP tool's input schema: fixed in 2.1.304. Claude Code now sends that tool without parameters for the rest of the conversation. If you maintain the MCP server, fix the tool's JSON Schema so the tool keeps its parameters. - An API error shown as 'prompt is too long' (plus an unneeded compaction) in sessions with many MCP tools: fixed in 2.1.306. On older versions the real error was hidden, so don't trust that message before updating. - All requests failing on Vertex or Bedrock when the endpoint rejects tool search: fixed in 2.1.305 (it retries without tool search). - HTTP MCP connections hanging 30 seconds and failing when a server never ends its reply to a notification: fixed in 2.1.302. Heads-up for admins: in 2.1.305, stdio servers in managed-mcp.json that use ${VAR} only inside env are no longer filtered by allowedMcpServers, so previously excluded servers may load after upgrading. Use deniedMcpServers to keep them blocked. Still failing after updating: run /doctor, and run the MCP server's command by hand to see whether it exits (missing dependency, wrong Node version, missing env var). Sources: https://code.claude.com/docs/en/changelog ; https://dev.classmethod.jp/en/articles/20261021-cc-updates-v2-1-306/

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