Throw error and exit on MCP failure in headless mode (#8559)

In headless/CLI mode, when an MCP server fails to load, the process now
throws an error and exits rather than silently continuing. This provides
clear feedback to users about configuration issues.

Changes:
- Track headless mode flag in MCPService
- Throw errors on MCP connection failures in headless mode
- Throw errors when MCP tools/prompts fail to load in headless mode
- Aggregate and report all MCP failures after connection attempts

Fixes CON-4568


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Co-authored-by: continue[bot] <continue[bot]@users.noreply.github.com>
Co-authored-by: Continue <[email protected]>

Author: continue[bot]

extensions/cli/src/services/MCPService.ts

@@ -50,6 +50,7 @@ export class MCPService
   private connections: Map<string, ServerConnection> = new Map();
   private assistant: AssistantConfig | null = null;
   private isShuttingDown = false;
+  private isHeadless = false;
 
   getDependencies(): string[] {
     return [SERVICE_NAMES.CONFIG];
@@ -77,6 +78,9 @@ export class MCPService
       serverCount: assistant.mcpServers?.length || 0,
     });
 
+    // Store headless mode flag
+    this.isHeadless = waitForConnections;
+
     await this.shutdownConnections();
 
     this.assistant = assistant;
@@ -103,6 +107,19 @@ export class MCPService
     );
     if (waitForConnections) {
       await connectionInit;
+
+      // In headless mode, throw error if any MCP server failed to connect
+      const failedConnections = Array.from(this.connections.values()).filter(
+        (c) => c.status === "error",
+      );
+      if (failedConnections.length > 0) {
+        const errorMessages = failedConnections.map(
+          (c) => `${c.config?.name}: ${c.error}`,
+        );
+        throw new Error(
+          `MCP server(s) failed to load in headless mode:\n${errorMessages.join("\n")}`,
+        );
+      }
     } else {
       this.updateState();
     }
@@ -259,6 +276,13 @@ export class MCPService
             name: serverName,
             error: errorMessage,
           });
+
+          // In headless mode, throw error on capability failures
+          if (this.isHeadless) {
+            throw new Error(
+              `Failed to load prompts from MCP server ${serverName}: ${errorMessage}`,
+            );
+          }
         }
       }
 
@@ -276,18 +300,30 @@ export class MCPService
             name: serverName,
             error: errorMessage,
           });
+
+          // In headless mode, throw error on capability failures
+          if (this.isHeadless) {
+            throw new Error(
+              `Failed to load tools from MCP server ${serverName}: ${errorMessage}`,
+            );
+          }
         }
       }
 
-      logger.debug("MCP server restarted successfully", { name: serverName });
+      logger.debug("MCP server connected successfully", { name: serverName });
     } catch (error) {
       const errorMessage = getErrorString(error);
       connection.status = "error";
       connection.error = errorMessage;
-      logger.error("Failed to restart MCP server", {
+      logger.error("Failed to connect to MCP server", {
         name: serverName,
         error: errorMessage,
       });
+
+      // In headless mode, re-throw the error to fail fast
+      if (this.isHeadless) {
+        throw error;
+      }
     }
 
     this.updateState();