GitHub Copilot / GitHub Copilot

GitHub Copilot CLI /mcp re-auth fails with MCPOAuthError EADDRINUSE

Fix OAuth re-authentication failure in GitHub Copilot CLI MCP server when eager-startup OAuth flow conflicts with user-initiated re-auth Includes evidence for GitHub Copilot troubleshooting demand.

Source-backed Last updated May 23, 2026 1 source Needs local verification

Category: GitHub Copilot
Error signature: Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878
Quick fix: Verify the account session, API key, provider settings, and environment where the failing tool is running.
Updated: May 23, 2026
Verification status: Source-backed
Evidence: 1 public source URL

Before you change production

This page includes public source URLs in the imported troubleshooting record. Compare those references with your version and environment before applying changes.

Reproduce the smallest failing action and save non-secret logs before changing configuration.
Check versions for GitHub Copilot, related SDKs, package managers, CI runners, and hosting providers.
Change one setting or dependency at a time, then rerun the same failing command or request.
Avoid destructive commands, credential rotation, billing changes, or security relaxations without a rollback plan.

What this error means

Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878 is a GitHub Copilot failure pattern reported for developers trying to fix oauth re-authentication failure in github copilot cli mcp server when eager-startup oauth flow conflicts with user-initiated re-auth. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub Issue #3462 on github/copilot-cli (created 2026-05-22): When HTTP MCP server configured with auth.redirectPort, eager OAuth startup binds the callback port, then /mcp re-auth tries to bind same port → EADDRINUSE. Only workaround: wait 5 min timeout or remove server from mcp-config.json. Regression introduced by fix for #3418. Maps to GitHub Copilot category per approved mapping table.

Common causes

GitHub Issue #3462 on github/copilot-cli (created 2026-05-22): When HTTP MCP server configured with auth.redirectPort, eager OAuth startup binds the callback port, then /mcp re-auth tries to bind same port → EADDRINUSE. Only workaround: wait 5 min timeout or remove server from mcp-config.json. Regression introduced by fix for #3418. Maps to GitHub Copilot category per approved mapping table.

Quick fixes

Confirm the exact error signature matches Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878.
Check the GitHub Copilot account, local tool state, and provider configuration involved in the failing workflow.
Verify the account session, API key, provider settings, and environment where the failing tool is running.

Platform/tool-specific checks

Verify the command, editor, extension, or API client that produced the error.
Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

Capture the exact error message and the command, editor action, or request that triggered it.
Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
Review the source evidence below and compare it with your environment.
Apply one change at a time and rerun the smallest failing action.
Keep the working fix documented for the team or deployment environment.

How to prevent it

Keep provider/tool configuration documented.
Record non-secret diagnostics such as tool version, provider name, model name, and command path.
Add a lightweight check before CI or production workflows depend on the tool.

Diagnostic flow for this page

Match Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878 exactly before applying the quick fix.
Compare the failing environment with GitHub Copilot versions, account scope, provider settings, and deployment context.
Check the listed common causes in order, starting with the cause that best matches your logs.
Use the evidence status below to decide whether to confirm against public sources or official documentation.
Apply one reversible change, rerun the smallest failing action, and keep rollback notes.

FAQ

What does Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878 mean in GitHub Copilot?

Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878 indicates the workflow described by "GitHub Copilot CLI /mcp re-auth fails with MCPOAuthError EADDRINUSE" failed in a GitHub Copilot context. Start by confirming the exact signature and the component that emitted it.

What should I check first for Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878?

Check whether this cause matches your environment: GitHub Issue #3462 on github/copilot-cli (created 2026-05-22): When HTTP MCP server configured with auth.redirectPort, eager OAuth startup binds the callback port, then /mcp re-auth tries to bind same port → EADDRINUSE. Only workaround: wait 5 min timeout or remove server from mcp-config.json. Regression introduced by fix for #3418. Maps to GitHub Copilot category per approved mapping table.

Is the Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878 guidance source-backed?

The "GitHub Copilot CLI /mcp re-auth fails with MCPOAuthError EADDRINUSE" page is labeled source-backed. It has 1 public source URL from github.com, so use that evidence status when deciding how much additional verification you need.

When should I avoid the quick fix for Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878?

For "GitHub Copilot CLI /mcp re-auth fails with MCPOAuthError EADDRINUSE", avoid applying "Verify the account session, API key, provider settings, and environment where the failing tool is running." when the error text differs, the affected version or provider is different, or the change would touch credentials, billing, DNS, production deploy settings, or irreversible data.

What should I collect before debugging Authentication failed: MCPOAuthError: listen EADDRINUSE: address already in use 127.0.0.1:19878?

For "GitHub Copilot CLI /mcp re-auth fails with MCPOAuthError EADDRINUSE", collect the failing GitHub Copilot command or request, non-secret configuration, versions, timestamps, request IDs when available, and the smallest reproduction that still triggers the signature.