fix(mcp): remove client-side cache and reduce server cache from 5m to 30s (#2182)

* fix(mcp): remove client-side cache and reduce server cache from 5m to 30s

* ack PR comments

Author: waleedlatif1

apps/sim/lib/mcp/client.ts

@@ -44,11 +44,14 @@ export class McpClient {
 
   /**
    * Creates a new MCP client
+   *
+   * No session ID parameter (we disconnect after each operation).
+   * The SDK handles session management automatically via Mcp-Session-Id header.
+   *
    * @param config - Server configuration
    * @param securityPolicy - Optional security policy
-   * @param sessionId - Optional session ID for session restoration (from previous connection)
    */
-  constructor(config: McpServerConfig, securityPolicy?: McpSecurityPolicy, sessionId?: string) {
+  constructor(config: McpServerConfig, securityPolicy?: McpSecurityPolicy) {
     this.config = config
     this.connectionStatus = { connected: false }
     this.securityPolicy = securityPolicy ?? {
@@ -65,7 +68,6 @@ export class McpClient {
       requestInit: {
         headers: this.config.headers,
       },
-      sessionId,
     })
 
     this.client = new Client(

apps/sim/lib/mcp/service.ts

@@ -42,42 +42,17 @@ interface CacheStats {
 
 class McpService {
   private toolCache = new Map<string, ToolCache>()
-  private readonly cacheTimeout = MCP_CONSTANTS.CACHE_TIMEOUT
-  private readonly maxCacheSize = 1000
+  private readonly cacheTimeout = MCP_CONSTANTS.CACHE_TIMEOUT // 30 seconds
+  private readonly maxCacheSize = MCP_CONSTANTS.MAX_CACHE_SIZE // 1000
   private cleanupInterval: NodeJS.Timeout | null = null
   private cacheHits = 0
   private cacheMisses = 0
   private entriesEvicted = 0
 
-  private sessionCache = new Map<string, string>()
-
   constructor() {
     this.startPeriodicCleanup()
   }
 
-  /**
-   * Get cached session ID for a server
-   */
-  private getCachedSessionId(serverId: string): string | undefined {
-    return this.sessionCache.get(serverId)
-  }
-
-  /**
-   * Cache session ID for a server
-   */
-  private cacheSessionId(serverId: string, sessionId: string): void {
-    this.sessionCache.set(serverId, sessionId)
-    logger.debug(`Cached session ID for server ${serverId}`)
-  }
-
-  /**
-   * Clear cached session ID for a server
-   */
-  private clearCachedSessionId(serverId: string): void {
-    this.sessionCache.delete(serverId)
-    logger.debug(`Cleared cached session ID for server ${serverId}`)
-  }
-
   /**
    * Start periodic cleanup of expired cache entries
    */
@@ -341,49 +316,9 @@ class McpService {
       allowedOrigins: config.url ? [new URL(config.url).origin] : undefined,
     }
 
-    const cachedSessionId = this.getCachedSessionId(config.id)
-
-    const client = new McpClient(config, securityPolicy, cachedSessionId)
-
-    try {
-      await client.connect()
-
-      const newSessionId = client.getSessionId()
-      if (newSessionId) {
-        this.cacheSessionId(config.id, newSessionId)
-      }
-
-      return client
-    } catch (error) {
-      if (cachedSessionId && this.isSessionError(error)) {
-        logger.debug(`Session restoration failed for server ${config.id}, retrying fresh`)
-        this.clearCachedSessionId(config.id)
-
-        const freshClient = new McpClient(config, securityPolicy)
-        await freshClient.connect()
-
-        const freshSessionId = freshClient.getSessionId()
-        if (freshSessionId) {
-          this.cacheSessionId(config.id, freshSessionId)
-        }
-
-        return freshClient
-      }
-
-      throw error
-    }
-  }
-
-  private isSessionError(error: unknown): boolean {
-    if (error instanceof Error) {
-      const message = error.message.toLowerCase()
-      return (
-        message.includes('no valid session') ||
-        message.includes('invalid session') ||
-        message.includes('session expired')
-      )
-    }
-    return false
+    const client = new McpClient(config, securityPolicy)
+    await client.connect()
+    return client
   }
 
   /**
@@ -466,21 +401,29 @@ class McpService {
         })
       )
 
+      let failedCount = 0
       results.forEach((result, index) => {
         if (result.status === 'fulfilled') {
           allTools.push(...result.value)
         } else {
+          failedCount++
           logger.warn(
             `[${requestId}] Failed to discover tools from server ${servers[index].name}:`,
             result.reason
           )
         }
       })
 
-      this.setCacheEntry(cacheKey, allTools)
+      if (failedCount === 0) {
+        this.setCacheEntry(cacheKey, allTools)
+      } else {
+        logger.warn(
+          `[${requestId}] Skipping cache due to ${failedCount} failed server(s) - will retry on next request`
+        )
+      }
 
       logger.info(
-        `[${requestId}] Discovered ${allTools.length} tools from ${servers.length} servers`
+        `[${requestId}] Discovered ${allTools.length} tools from ${servers.length - failedCount}/${servers.length} servers`
       )
       return allTools
     } catch (error) {

apps/sim/lib/mcp/utils.ts

@@ -6,9 +6,10 @@ import type { McpApiResponse } from '@/lib/mcp/types'
  */
 export const MCP_CONSTANTS = {
   EXECUTION_TIMEOUT: 60000,
-  CACHE_TIMEOUT: 5 * 60 * 1000,
+  CACHE_TIMEOUT: 30 * 1000,
   DEFAULT_RETRIES: 3,
   DEFAULT_CONNECTION_TIMEOUT: 30000,
+  MAX_CACHE_SIZE: 1000,
 } as const
 
 /**