{"record":{"id":"a1913f0c25e5ebc1","repo":"mastra-ai/mastra","slug":"cannot-authenticate-mcp-server-servername-the","errorCode":null,"errorMessage":"Cannot authenticate MCP server ${serverName}: the provider's redirect URL must be a loopback address, got ${redirectUrl.origin}.","messagePattern":"Cannot authenticate MCP server (.+?): the provider's redirect URL must be a loopback address, got (.+?)\\.","errorType":"exception","errorClass":"Error","httpStatus":null,"severity":"error","filePath":"packages/mcp/src/client/configuration.ts","lineNumber":899,"sourceCode":"    let provider: MCPOAuthClientProvider | undefined;\n    let sessionStarted = false;\n    let callbackServer: OAuthCallbackServer | undefined;\n\n    // Installed before the first fallible step so the abort-controller entry,\n    // provider session, and callback server never leak on an early throw.\n    try {\n      const config = this.getServerConfig(serverName);\n      const candidateProvider = config.authProvider;\n      if (!(candidateProvider instanceof MCPOAuthClientProvider)) {\n        throw new Error(\n          `Cannot authenticate MCP server ${serverName}: it is not configured with an MCPOAuthClientProvider.`,\n        );\n      }\n      provider = candidateProvider;\n\n      const redirectUrl = new URL(provider.redirectUrl.toString());\n      if (redirectUrl.protocol !== 'http:' || !isLoopbackHostname(redirectUrl.hostname)) {\n        throw new Error(\n          `Cannot authenticate MCP server ${serverName}: the provider's redirect URL must be a loopback address, got ${redirectUrl.origin}.`,\n        );\n      }\n\n      const state = await provider.beginAuthorizationSession();\n      sessionStarted = true;\n      // A cancel that arrived during beginAuthorizationSession() has no callback\n      // server to close yet, so bail here before binding a port and parking.\n      throwIfAborted();\n\n      callbackServer = await createOAuthCallbackServer({ redirectUrl, state });\n      // A cancel during port binding: bail before we ever wait for a code that\n      // will never arrive. The outer finally closes the freshly-bound server.\n      throwIfAborted();\n      this.authCallbackServersByServer.set(serverName, callbackServer);\n\n      // Point the authorization request at the callback URL that actually\n      // bound, and register every fallback candidate during dynamic client","sourceCodeStart":881,"sourceCodeEnd":917,"githubUrl":"https://github.com/mastra-ai/mastra/blob/75dd419e613fe9c39f846ffc500716141b74fda6/packages/mcp/src/client/configuration.ts#L881-L917","documentation":"Error thrown during runAuthorizationFlow when the provider's redirect URL is not an http:// loopback address. The local OAuth callback server only binds loopback hostnames (localhost/127.0.0.1/etc.) over plain HTTP, so non-loopback or https redirect URLs are rejected before binding.","triggerScenarios":"Configuring MCPOAuthClientProvider with redirectUrl such as https://myapp.example.com/callback or http://10.0.0.5:3000/callback, then invoking the authorization flow.","commonSituations":"Copying a production web-app redirect URL into a local CLI/desktop flow; using https for the local callback; using a LAN hostname or container name instead of localhost.","solutions":["Change the provider redirectUrl to a loopback http URL, e.g. http://localhost:<port>/callback.","Pick a port (or port range) that is free on the local machine.","Register the loopback redirect URL with the OAuth authorization server if it validates exact redirects.","For remote/web deployments, do not use the local callback flow; use a flow appropriate for server-to-server auth."],"exampleFix":"// before\nnew MCPOAuthClientProvider({ redirectUrl: 'https://myapp.example.com/callback' })\n// after\nnew MCPOAuthClientProvider({ redirectUrl: 'http://localhost:3456/callback' })","handlingStrategy":"validation","validationCode":"const redirectUrl = new URL(provider.redirectUrl.toString());\nconst isLoopback = ['localhost', '127.0.0.1', '[::1]'].includes(redirectUrl.hostname) || /^127\\./.test(redirectUrl.hostname);\nif (redirectUrl.protocol !== 'http:' || !isLoopback) {\n  throw new Error(`redirectUrl must be http loopback, got ${redirectUrl.origin}`);\n}","typeGuard":"function isLoopbackHttpRedirect(url: URL): boolean {\n  return url.protocol === 'http:' && (url.hostname === 'localhost' || /^127\\./.test(url.hostname) || url.hostname === '::1');\n}","tryCatchPattern":null,"preventionTips":["Use http://localhost:<port>/callback for local OAuth flows.","Register the exact loopback redirect with the OAuth provider's app settings.","Never reuse production https redirect URLs for local flows.","Validate provider config at construction time in tests."],"tags":["mcp","oauth","configuration","loopback","redirect-url"],"backgroundTag":"invalid-redirect-url","analyzedSha":"75dd419e613fe9c39f846ffc500716141b74fda6","analyzedAt":"2026-08-30T00:15:31.844Z","schemaVersion":2},"datasetVersion":"2026-08-30T03:17:51.788Z"}