Elasticsearch MCP: self-signed certificate verify failed (trust the cluster CA)

TL;DR: Your cluster's certificate is self-signed and the MCP server does not trust it. Point the server at the cluster's CA certificate, or for a local test setup allow insecure TLS. The cluster is fine; the trust chain is the problem.

SSL: CERTIFICATE_VERIFY_FAILED: self-signed certificate in certificate chain

Fix it

1. Get the CA certificate from your cluster. For docker-compose Elastic setups it is usually at certs/ca/ca.crt. For Elastic Cloud, the cert is public and this error should not happen.
2. Configure the MCP server to trust it. Options depend on the server version: set the CA bundle path env var, or set the insecure/skip-verify flag for local testing only.
3. For a quick local-dev test only, disabling verification unblocks you:

{
  "env": {
    "ES_URL": "your-cluster",
    "ES_SSL_SKIP_VERIFY": "true"
  }
}

(Flag name varies by server version. Check the server README for the exact variable.)

4. Restart the MCP client.

   Expected: the SSL error is gone. The next failure, if any, will be 401 auth, which is progress.

When to use this

• The error mentions certificate verify failed, self-signed certificate, or unable to get local issuer certificate.
• curl -k YOUR_ES_HOST works but without -k it fails the same way.

When NOT to use this

• The error is 401. TLS is fine; credentials are missing.
• The cluster URL is http://. There is no TLS at all, so this is not a cert problem.

Compatibility

• elastic/mcp-server-elasticsearch against self-hosted Elasticsearch with custom certs.

Why it happens

TLS clients verify the server certificate against trusted CAs. Self-hosted clusters generate their own CA, which no client trusts by default. curl -k and skip-verify flags bypass the check; the correct fix is trusting the actual CA.

Edge cases

• Skip-verify in production is a real security hole (anyone can impersonate the cluster). Use it for local dev only.
• The CA file must be readable by the MCP server process. In Docker, mount it into the container.
• Certificate expiry produces a different error (certificate has expired). Renew instead of disabling verification.

The EFK setup guide (warnier-zhang/warnier-zhang.github.io) documents the self-signed certificate failure against Elasticsearch: certificate verify failed (self signed certificate in certificate chain), with the fix of providing the CA certificate or disabling SSL verification. The elk-setup troubleshooting table (cloud-prakhar/elk-setup) lists curl: (60) SSL certificate problem with the fix of adding --cacert with the CA cert. The same failure surfaces through the Elasticsearch MCP server when ESURL is https with a self-signed cert. Source: https://github.com/warnier-zhang/warnier-zhang.github.io/blob/HEAD/posts/2022-4-6-How-to-setup-EFK-Stack.md