Tavily MCP: Not Acceptable, client must accept application/json and text/event-stream

Fix Tavily MCP Not Acceptable: Client must accept both application/json and text/event-stream

TL;DR

Switch from the type: http config to the stdio config (npx -y tavily-mcp@latest with your key in env). The hosted Tavily MCP endpoint requires an Accept: application/json, text/event-stream header that Claude Code's HTTP client does not send, so every call fails with what looks like an authentication error.

What the failure looks like:

{"error":"Search failed","detail":"Authentication required"}

and at the raw HTTP level:

Not Acceptable: Client must accept both application/json and text/event-stream

Steps

1. Confirm the header is the problem

Run a raw request without the special Accept header:

curl -X POST "https://mcp.tavily.com/mcp/?tavilyApiKey [your value] Tavily API key]" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/list","id":1}'

Success check: you get the Not Acceptable response, which proves the header (not your key) is the issue. With the right header it works:

curl -X POST "https://mcp.tavily.com/mcp/?tavilyApiKey [your value] Tavily API key]" \
  -H "Accept: application/json, text/event-stream" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/list","id":1}'

2. Replace the HTTP config with stdio

In your MCP config, drop the type: http entry and use:

{
  "mcpServers": {
    "tavily": {
      "command": "npx",
      "args": ["-y", "tavily-mcp@latest"],
      "env": { "TAVILY_API_KEY": "[your Tavily API key]" }
    }
  }
}

Success check: restart the client; tavily-search and tavily-extract appear and searches return results.

3. Validate the key separately if it still fails

If the stdio route also fails, verify the key itself:

curl -s "https://api.tavily.com/search" -H "Content-Type: application/json" \
  -d '{"api_key":"[your Tavily API key]","query":"test","max_results":1}' | head -c 300

Success check: valid JSON with results means the key is fine and the problem is the transport.

When this applies

• You configured Tavily with "type": "http" and `"url": "https://mcp.tavily.com/mcp/?tavilyApiKey [your value] and every tool call returns an authentication error.
• Raw curl shows the Not Acceptable message about text/event-stream.

When it does not apply

• The stdio npx tavily-mcp server fails to start at all (check Node and npx first).
• The key is actually invalid or missing; the API check in step 3 returns a 401. Regenerate the key instead.

Tool compatibility

• tavily-mcp npm package (latest)
• Tavily hosted MCP at mcp.tavily.com
• Claude Code (the type: http client is the one affected), also relevant to any client that cannot set custom Accept headers

Why it happens

The Tavily MCP server streams responses over SSE, so it requires clients to send Accept: application/json, text/event-stream. Claude Code's type: http client sends a default Accept header without the SSE part, the server rejects the request as Not Acceptable, and the client surfaces that as an authentication failure, which sends you chasing the API key instead of the header.

Edge cases

• Do not paste the key with quotes or trailing spaces in the URL parameter; the type: http setup passes it verbatim in the query string.
• If you need the remote endpoint (no local node), use mcp-remote with the URL as an npx stdio wrapper instead of type: http.
• API rate limits on the free tier return HTTP 429, not auth errors; treat those as usage limits, not this problem.

A user configured the Tavily hosted MCP server with Claude Code type http pointing at https://mcp.tavily.com/mcp/ with a tavilyApiKey query parameter. Every tool call failed with {"error":"Search failed","detail":"Authentication required"}. The issue thread diagnosed that the Tavily MCP server requires clients to send the header Accept: application/json, text/event-stream, which Claude Code's type http client does not include by default, and that curl without the header returns Not Acceptable: Client must accept both application/json and text/event-stream. The verified workaround is the stdio-based config (npx -y tavily-mcp@latest with TAVILYAPIKEY in env) or mcp-remote. Source: https://github.com/tavily-ai/tavily-mcp/issues/121