[BUG] MCP Tool Discovery Fails for Streamable-HTTP Transport (AgentCore)

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?

Description
Claude Code fails to discover tools from MCP servers using streamable-http transport (AWS Bedrock AgentCore). Tools don't appear in available tools list despite successful connection and no error messages.

Environment

• Claude Code version: [2.1.63]
• Platform: Linux
• Transport: streamable-http (AgentCore Runtime)

Root Cause
Claude Code doesn't send the required Accept: application/json, text/event-stream header when calling tools/list, causing AgentCore to return HTTP 406 and tool discovery to fail silently.

Evidence

Without Accept header (Claude Code's current behavior):

curl -X POST <agentcore-url> \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/list","params":{},"id":1}'
# Response: 406 - "MCP Accept header must contain: application/json, text/event-stream"

With Accept header (working):

curl -X POST <agentcore-url> \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","method":"tools/list","params":{},"id":1}'
# Response: 200 - Returns tool list successfully

Expected Behavior

• Tools from streamable-http MCP servers are discovered
• Tools appear in Claude Code's available tools list

Actual Behavior

• No tools discovered (silent failure)
• No error messages shown to user
• Connection appears successful but tools are missing

"claude_code_github_issue.md" 67L, 2069B 1,1 Top

What Should Happen?

should list tools for mcp server deployed in agentCore

Error Messages/Logs

see descripttion section

Steps to Reproduce

see description section

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.63

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_