[BUG] ToolSearch loads MCP tools with names over 64 chars, next request 400s "Tool reference ... not found in available tools" and the session is permanently dead #97616

Summary

When ToolSearch loads an MCP tool whose full name (mcp__<server>__<tool>) is longer than 64 characters, the very next API request fails with:

API Error: 400 Tool reference '<tool name>' not found in available tools

The tool_reference stays in the conversation history, so every following turn fails the same way. The session cannot be continued; only rewind or a new session recovers it.

Same symptom as #22336 (canonical), #22455, #21846, #19882 and #18015, all closed/locked, so filing new. It still reproduces on current builds.

Reproduction (no auth needed)

Uses the public hosted Firecrawl MCP endpoint with a long server name so the full tool name is 68 chars.

.mcp.json:

{ "mcpServers": { "firecrawl-longname-padding-control-xxxxxxxxxx": { "type": "http", "url": "https://mcp.firecrawl.dev/v2/mcp" } } }

.claude/settings.json: { "enabledMcpjsonServers": ["firecrawl-longname-padding-control-xxxxxxxxxx"] }

claude -p "Call the ToolSearch tool exactly once with query 'select:mcp__firecrawl-longname-padding-control-xxxxxxxxxx__firecrawl_scrape' and max_results 1. Do not call the loaded tool. Then reply with the single word DONE." --output-format json

Result: "is_error": true, API Error: 400 Tool reference 'mcp__firecrawl-longname-padding-control-xxxxxxxxxx__firecrawl_scrape' not found in available tools.

Same setup with a shorter server name (full tool name 59 chars) returns DONE.

Boundary and versions (all tested on the same day)

• Cutoff is exactly 64/65 characters, for plugin and non-plugin MCP servers alike.
• In late August 2026, 2.1.246 loaded 65-68 char tool references without error, but those were claude.ai connector tools (UUID-named servers), which I could not retest today. Local/plugin MCP servers fail on every build tested, so the version table is not evidence of a client-side regression.
• Debug log shows ToolSearchTool: selected <name>, then Dynamic tool loading: 1/N deferred tools included, then the 400. So the client believes the tool is included, but the reference is not matched.
• Docs state tool names may be up to 128 characters, so a 65-128 char MCP tool name is valid, yet it cannot be loaded via ToolSearch.

Real-world trigger

Plugin-provided MCP servers get the prefix mcp__plugin_<plugin>_<server>__, which pushes many tools of large servers (e.g. the Meta Ads MCP at https://mcp.facebook.com/ads) past 64. That server's instructions tell the model to load suggested tools via tool discovery, so sessions die "randomly" whenever a long-named tool is suggested.

Expected

Either the API accepts tool_reference names up to the documented tool-name limit, or ToolSearch never returns tools it cannot reference (and warns about them), so the session is not bricked.

Workarounds found

• Declare the same MCP endpoint in project .mcp.json under a short name; Claude Code suppresses the plugin duplicate and the names drop under 64.
• A PreToolUse hook on ToolSearch that denies select: queries naming tools over 64 chars prevents the session from dying (does not cover keyword searches).

Environment

• Claude Code native install, 2.1.283 (also reproduced with 2.1.246, 2.1.281, 2.1.282)
• Platform: Windows