🤖 feat: add MCP servers section to system prompt

Shows configured servers (name + command) when at least one exists.
Explains they're configured in user's local project's .mux/mcp.jsonc.

Author: ammar-agent

.mux/mcp.jsonc

@@ -0,0 +1,5 @@
+{
+  "servers": {
+    "chrome": "npx -y chrome-devtools-mcp@latest --headless --chromeArg='--no-sandbox'"
+  }
+}

docs/system-prompt.mdx

@@ -59,6 +59,27 @@ You are in a git worktree at ${workspacePath}
 </environment>
 `;
 }
+
+/**
+ * Build MCP servers context XML block.
+ * Only included when at least one MCP server is configured.
+ */
+function buildMCPContext(mcpServers: MCPServerMap): string {
+  const entries = Object.entries(mcpServers);
+  if (entries.length === 0) return "";
+
+  const serverList = entries.map(([name, command]) => `- ${name}: \`${command}\``).join("\n");
+
+  return `
+<mcp>
+MCP (Model Context Protocol) servers provide additional tools. Configured in user's local project's .mux/mcp.jsonc:
+
+${serverList}
+
+Use /mcp add|edit|remove or Settings → Projects to manage servers.
+</mcp>
+`;
+}
 ```
 
 {/* END SYSTEM_PROMPT_DOCS */}

src/node/services/aiService.ts

@@ -982,14 +982,20 @@ export class AIService extends EventEmitter {
         ? metadata.projectPath
         : runtime.getWorkspacePath(metadata.projectPath, metadata.name);
 
+      // Fetch MCP server config for system prompt (before building message)
+      const mcpServers = this.mcpServerManager
+        ? await this.mcpServerManager.listServers(metadata.projectPath)
+        : undefined;
+
       // Build system message from workspace metadata
       const systemMessage = await buildSystemMessage(
         metadata,
         runtime,
         workspacePath,
         mode,
         additionalSystemInstructions,
-        modelString
+        modelString,
+        mcpServers
       );
 
       // Count system message tokens for cost tracking

src/node/services/mcpServerManager.ts

@@ -28,6 +28,14 @@ export class MCPServerManager {
 
   constructor(private readonly configService: MCPConfigService) {}
 
+  /**
+   * List configured MCP servers for a project (name -> command).
+   * Used to show server info in the system prompt.
+   */
+  async listServers(projectPath: string): Promise<MCPServerMap> {
+    return this.configService.listServers(projectPath);
+  }
+
   async getToolsForWorkspace(options: {
     workspaceId: string;
     projectPath: string;

src/node/services/systemMessage.ts

@@ -1,4 +1,5 @@
 import type { WorkspaceMetadata } from "@/common/types/workspace";
+import type { MCPServerMap } from "@/common/types/mcp";
 import {
   readInstructionSet,
   readInstructionSetFromRuntime,
@@ -77,6 +78,27 @@ You are in a git worktree at ${workspacePath}
 </environment>
 `;
 }
+
+/**
+ * Build MCP servers context XML block.
+ * Only included when at least one MCP server is configured.
+ */
+function buildMCPContext(mcpServers: MCPServerMap): string {
+  const entries = Object.entries(mcpServers);
+  if (entries.length === 0) return "";
+
+  const serverList = entries.map(([name, command]) => `- ${name}: \`${command}\``).join("\n");
+
+  return `
+<mcp>
+MCP (Model Context Protocol) servers provide additional tools. Configured in user's local project's .mux/mcp.jsonc:
+
+${serverList}
+
+Use /mcp add|edit|remove or Settings → Projects to manage servers.
+</mcp>
+`;
+}
 // #endregion SYSTEM_PROMPT_DOCS
 
 /**
@@ -183,6 +205,7 @@ async function readInstructionSources(
  * @param mode - Optional mode name (e.g., "plan", "exec")
  * @param additionalSystemInstructions - Optional instructions appended last
  * @param modelString - Active model identifier used for Model-specific sections
+ * @param mcpServers - Optional MCP server configuration (name -> command)
  * @throws Error if metadata or workspacePath invalid
  */
 export async function buildSystemMessage(
@@ -191,7 +214,8 @@ export async function buildSystemMessage(
   workspacePath: string,
   mode?: string,
   additionalSystemInstructions?: string,
-  modelString?: string
+  modelString?: string,
+  mcpServers?: MCPServerMap
 ): Promise<string> {
   if (!metadata) throw new Error("Invalid workspace metadata: metadata is required");
   if (!workspacePath) throw new Error("Invalid workspace path: workspacePath is required");
@@ -237,6 +261,11 @@ export async function buildSystemMessage(
   // Build system message
   let systemMessage = `${PRELUDE.trim()}\n\n${buildEnvironmentContext(workspacePath)}`;
 
+  // Add MCP context if servers are configured
+  if (mcpServers && Object.keys(mcpServers).length > 0) {
+    systemMessage += buildMCPContext(mcpServers);
+  }
+
   if (customInstructions) {
     systemMessage += `\n<custom-instructions>\n${customInstructions}\n</custom-instructions>`;
   }