MCP SSE server: tools/call fails with -32602 despite valid connection and correct schemas

Bug Description

MCP tool calls to a Home Assistant SSE server consistently fail with MCP error -32602: Invalid request parameters, even though:

• claude mcp list shows the server as Connected
• The Bearer token is valid (HA REST API responds correctly)
• All tools are listed correctly via the server's tools/list
• Direct MCP protocol calls work perfectly (tested via Python)

Environment

• Claude Code version: (current)
• OS: Linux 6.17.0-35-generic
• MCP Server: Home Assistant built-in MCP server v1.26.0
• Transport: SSE (http://<host>:8123/mcp_server/sse)
• Protocol version: 2024-11-05

MCP Server Configuration

Type: sse
URL: http://192.168.2.102:8123/mcp_server/sse
Headers: Authorization: Bearer <token>

Steps to Reproduce

1. Add a Home Assistant MCP server via SSE transport:

``
claude mcp add --transport sse homeassistant http://<ha-host>:8123/mcp_server/sse \
--header "Authorization: Bearer <token>"
``

1. Confirm claude mcp list shows ✔ Connected
2. Call any tool, e.g. GetDateTime (no required parameters)
3. Observe error: MCP error -32602: Invalid request parameters

Direct Protocol Test (Works Fine)

The following Python code calls the same tool via the same endpoint successfully:

import requests, threading, json, time

BASE = "http://192.168.2.102:8123"
HEADERS = {"Authorization": "Bearer <token>"}
messages = []
msg_endpoint = None

def listen_sse():
    global msg_endpoint
    with requests.get(f"{BASE}/mcp_server/sse", headers={**HEADERS, "Accept": "text/event-stream"}, stream=True, timeout=15) as r:
        for line in r.iter_lines(decode_unicode=True):
            if line.startswith("data:"):
                data = line[5:].strip()
                if data.startswith("/"): msg_endpoint = data
                elif data:
                    try: messages.append(json.loads(data))
                    except: pass

t = threading.Thread(target=listen_sse, daemon=True)
t.start()
time.sleep(1)

# Initialize (required before tool calls)
requests.post(f"{BASE}{msg_endpoint}", headers={**HEADERS, "Content-Type": "application/json"},
    json={"jsonrpc": "2.0", "id": 1, "method": "initialize",
          "params": {"protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": {"name": "test", "version": "1"}}})
time.sleep(0.3)

# Call GetDateTime
requests.post(f"{BASE}{msg_endpoint}", headers={**HEADERS, "Content-Type": "application/json"},
    json={"jsonrpc": "2.0", "id": 2, "method": "tools/call", "params": {"name": "GetDateTime", "arguments": {}}})
time.sleep(1)
# Returns: {"result": {"content": [{"type": "text", "text": "{\"success\": true, \"result\": {...}}"}]}}

Hypothesis

Claude Code's MCP client may not be sending the initialize request before tools/call, or may be omitting/malforming the arguments field. The HA MCP server requires a proper initialization handshake before accepting tool calls on an SSE session.

Expected Behavior

Tool calls to a Connected SSE MCP server should succeed.

Actual Behavior

All tool calls return MCP error -32602: Invalid request parameters regardless of tool or parameters.