N8N MCP AUTHENTICATION FAILURE — REDACTED EVIDENCE CHECKLIST
Agent Rescue Desk
https://agent-rescue-desk.netlify.app/n8n-mcp-authentication-failed-403

PURPOSE
Capture the exact boundary of an n8n MCP authentication failure without
copying credentials, tokens, cookies, personal data, or customer data.

Before sharing this worksheet:

[ ] I removed bearer tokens, API keys, OAuth codes, cookies, and session IDs.
[ ] I replaced workflow, account, tenant, hostname, and path identifiers where
    they are not necessary to reproduce the failure.
[ ] I kept timestamps, HTTP statuses, non-secret version numbers, and request
    IDs because they help correlate the failure.

1. MCP SURFACE

Choose exactly one:

[ ] n8n MCP Server Trigger node — n8n is the server for one workflow.
[ ] n8n instance-level MCP — enabled under Settings > Instance-level MCP.
[ ] n8n MCP Client Tool — n8n is connecting to an external MCP server.

n8n version:
Cloud or self-hosted:
Deployment topology:
First known failing timestamp and timezone:
Last known working timestamp and timezone:

2. ENDPOINT AND TRANSPORT

Endpoint shape with sensitive identifiers redacted:
[ ] Test URL
[ ] Production URL
[ ] Instance-level server URL copied from Connection details
[ ] External MCP server URL

Transport selected:
[ ] Streamable HTTP
[ ] SSE
[ ] stdio through a local gateway
[ ] Other:

Initial hostname:
Final hostname after redirects:
Redirect statuses and Location paths:

3. AUTHENTICATION CONTRACT

Authentication type:
[ ] Bearer auth
[ ] Header auth
[ ] OAuth2
[ ] Personal MCP access token
[ ] Other:

Header name only:
Scheme or prefix only (for example, Bearer):
Credential created or rotated at:
Client configuration updated at:

Do not paste the credential here.

4. LOWEST-LEVEL RESPONSE

Tested from:
[ ] Same runtime as the failing client
[ ] Official MCP Inspector
[ ] Another direct MCP client:

Exact HTTP status:
Response Content-Type:
WWW-Authenticate header with sensitive values removed:
Redacted response body:
Request or correlation ID:
Timestamp and timezone:

5. MCP PROTOCOL BOUNDARY

[ ] TCP, TLS, and HTTP endpoint reachable
[ ] No unexpected redirect
[ ] Initialize request returned a valid MCP response
[ ] Tool discovery returned a valid MCP response
[ ] Expected tool appeared
[ ] One low-risk direct tool call succeeded
[ ] Full agent or workflow path succeeded
[ ] Final user-visible outcome succeeded

First unchecked item:
Exact evidence at that boundary:

6. SERVER-PATH CHECKS

For an MCP Server Trigger:

[ ] The workflow is published when using the production URL.
[ ] The selected URL and client transport match.
[ ] The trigger and client use the same authentication contract.
[ ] Only the expected tool nodes are attached to the trigger.

For instance-level MCP:

[ ] Instance-level MCP is enabled.
[ ] The intended workflow is enabled for MCP access.
[ ] The client uses the URL copied from Connection details.
[ ] OAuth2 and personal access-token instructions were not mixed.

For self-hosted n8n:

[ ] All /mcp* traffic reaches one dedicated webhook replica when multiple
    webhook replicas are otherwise in use.
[ ] Reverse-proxy buffering is disabled for the MCP endpoint.
[ ] The proxy preserves the intended host, path, and authentication header.

7. MINIMUM REPRODUCTION

Smallest isolated workflow or server:
One redacted test input:
Expected initialize result:
Expected discovered tool:
Expected low-risk tool result:
Actual result:

8. SUPPORT OR RESCUE HANDOFF

One-sentence target outcome:

The smallest change that would prove recovery:

Known customer-side blockers:

Do not attach credentials. Share them only through an agreed secure channel
after a case is accepted and only at the minimum scope required.

OFFICIAL REFERENCES

n8n MCP Server Trigger:
https://docs.n8n.io/integrations/builtin/core-nodes/n8n-nodes-langchain.mcptrigger/

n8n instance-level MCP:
https://docs.n8n.io/connect/connect-to-n8n-mcp-server/

MCP Inspector:
https://modelcontextprotocol.io/docs/tools/inspector