🔐 fix: MCP OAuth Tool Discovery and Event Emission (#11599)

* fix: MCP OAuth tool discovery and event emission in event-driven mode

- Add discoverServerTools method to MCPManager for tool discovery when OAuth is required
- Fix OAuth event emission to send both ON_RUN_STEP and ON_RUN_STEP_DELTA events
- Fix hasSubscriber flag reset in GenerationJobManager for proper event buffering
- Add ToolDiscoveryOptions and ToolDiscoveryResult types
- Update reinitMCPServer to use new discovery method and propagate OAuth URLs

* refactor: Update ToolService and MCP modules for improved functionality

- Reintroduced Constants in ToolService for better reference management.
- Enhanced loadToolDefinitionsWrapper to handle both response and streamId scenarios.
- Updated MCP module to correct type definitions for oauthStart parameter.
- Improved MCPConnectionFactory to ensure proper disconnection handling during tool discovery.
- Adjusted tests to reflect changes in mock implementations and ensure accurate behavior during OAuth handling.

* fix: Refine OAuth handling in MCPConnectionFactory and related tests

- Updated the OAuth URL assignment logic in reinitMCPServer to prevent overwriting existing URLs.
- Enhanced error logging to provide clearer messages when tool discovery fails.
- Adjusted tests to reflect changes in OAuth handling, ensuring accurate detection of OAuth requirements without generating URLs in discovery mode.

* refactor: Clean up OAuth URL assignment in reinitMCPServer

- Removed redundant OAuth URL assignment logic in the reinitMCPServer function to streamline the tool discovery process.
- Enhanced error logging for tool discovery failures, improving clarity in debugging and monitoring.

* fix: Update response handling in ToolService for event-driven mode

- Changed the condition in loadToolDefinitionsWrapper to check for writableEnded instead of headersSent, ensuring proper event emission when the response is still writable.
- This adjustment enhances the reliability of event handling during tool execution, particularly in streaming scenarios.

api/server/services/Tools/mcp.js

@@ -1,11 +1,14 @@
 const { logger } = require('@librechat/data-schemas');
 const { CacheKeys, Constants } = require('librechat-data-provider');
 const { findToken, createToken, updateToken, deleteTokens } = require('~/models');
-const { getMCPManager, getFlowStateManager } = require('~/config');
 const { updateMCPServerTools } = require('~/server/services/Config');
+const { getMCPManager, getFlowStateManager } = require('~/config');
 const { getLogStores } = require('~/cache');
 
 /**
+ * Reinitializes an MCP server connection and discovers available tools.
+ * When OAuth is required, uses discovery mode to list tools without full authentication
+ * (per MCP spec, tool listing should be possible without auth).
  * @param {Object} params
  * @param {IUser} params.user - The user from the request object.
  * @param {string} params.serverName - The name of the MCP server
@@ -14,7 +17,7 @@ const { getLogStores } = require('~/cache');
  * @param {boolean} [params.forceNew]
  * @param {number} [params.connectionTimeout]
  * @param {FlowStateManager<any>} [params.flowManager]
- * @param {(authURL: string) => Promise<boolean>} [params.oauthStart]
+ * @param {(authURL: string) => Promise<void>} [params.oauthStart]
  * @param {Record<string, Record<string, string>>} [params.userMCPAuthMap]
  */
 async function reinitMCPServer({
@@ -36,10 +39,12 @@ async function reinitMCPServer({
   let tools = null;
   let oauthRequired = false;
   let oauthUrl = null;
+
   try {
     const customUserVars = userMCPAuthMap?.[`${Constants.mcp_prefix}${serverName}`];
     const flowManager = _flowManager ?? getFlowStateManager(getLogStores(CacheKeys.FLOWS));
     const mcpManager = getMCPManager();
+    const tokenMethods = { findToken, updateToken, createToken, deleteTokens };
 
     const oauthStart =
       _oauthStart ??
@@ -57,15 +62,10 @@ async function reinitMCPServer({
         oauthStart,
         serverName,
         flowManager,
+        tokenMethods,
         returnOnOAuth,
         customUserVars,
         connectionTimeout,
-        tokenMethods: {
-          findToken,
-          updateToken,
-          createToken,
-          deleteTokens,
-        },
       });
 
       logger.info(`[MCP Reinitialize] Successfully established connection for ${serverName}`);
@@ -84,9 +84,33 @@ async function reinitMCPServer({
 
       if (isOAuthError || oauthRequired || isOAuthFlowInitiated) {
         logger.info(
-          `[MCP Reinitialize] OAuth required for ${serverName} (isOAuthError: ${isOAuthError}, oauthRequired: ${oauthRequired}, isOAuthFlowInitiated: ${isOAuthFlowInitiated})`,
+          `[MCP Reinitialize] OAuth required for ${serverName}, attempting tool discovery without auth`,
         );
         oauthRequired = true;
+
+        try {
+          const discoveryResult = await mcpManager.discoverServerTools({
+            user,
+            signal,
+            serverName,
+            flowManager,
+            tokenMethods,
+            oauthStart,
+            customUserVars,
+            connectionTimeout,
+          });
+
+          if (discoveryResult.tools && discoveryResult.tools.length > 0) {
+            tools = discoveryResult.tools;
+            logger.info(
+              `[MCP Reinitialize] Discovered ${tools.length} tools for ${serverName} without full auth`,
+            );
+          }
+        } catch (discoveryErr) {
+          logger.debug(
+            `[MCP Reinitialize] Tool discovery failed for ${serverName}: ${discoveryErr?.message ?? String(discoveryErr)}`,
+          );
+        }
       } else {
         logger.error(
           `[MCP Reinitialize] Error initializing MCP server ${serverName} for user:`,
@@ -97,6 +121,9 @@ async function reinitMCPServer({
 
     if (connection && !oauthRequired) {
       tools = await connection.fetchTools();
+    }
+
+    if (tools && tools.length > 0) {
       availableTools = await updateMCPServerTools({
         userId: user.id,
         serverName,
@@ -109,6 +136,9 @@ async function reinitMCPServer({
     );
 
     const getResponseMessage = () => {
+      if (oauthRequired && tools && tools.length > 0) {
+        return `MCP server '${serverName}' tools discovered, OAuth required for execution`;
+      }
       if (oauthRequired) {
         return `MCP server '${serverName}' ready for OAuth authentication`;
       }
@@ -120,19 +150,25 @@ async function reinitMCPServer({
 
     const result = {
       availableTools,
-      success: Boolean((connection && !oauthRequired) || (oauthRequired && oauthUrl)),
+      success: Boolean(
+        (connection && !oauthRequired) ||
+          (oauthRequired && oauthUrl) ||
+          (tools && tools.length > 0),
+      ),
       message: getResponseMessage(),
       oauthRequired,
       serverName,
       oauthUrl,
       tools,
     };
+
     logger.debug(`[MCP Reinitialize] Response for ${serverName}:`, {
       success: result.success,
       oauthRequired: result.oauthRequired,
       oauthUrl: result.oauthUrl ? 'present' : null,
       toolsCount: tools?.length ?? 0,
     });
+
     return result;
   } catch (error) {
     logger.error(