chore(internal): support custom-instructions-path flag in MCP servers

Author: stainless-app[bot]

packages/mcp-server/src/http.ts

@@ -23,7 +23,8 @@ const newServer = async ({
   res: express.Response;
 }): Promise<McpServer | null> => {
   const stainlessApiKey = getStainlessApiKey(req, mcpOptions);
-  const server = await newMcpServer(stainlessApiKey);
+  const customInstructionsPath = mcpOptions.customInstructionsPath;
+  const server = await newMcpServer({ stainlessApiKey, customInstructionsPath });
 
   const authOptions = parseClientAuthHeaders(req, false);
 

packages/mcp-server/src/instructions.ts

@@ -1,5 +1,6 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
+import fs from 'fs/promises';
 import { readEnv } from './util';
 import { getLogger } from './logger';
 
@@ -12,9 +13,15 @@ interface InstructionsCacheEntry {
 
 const instructionsCache = new Map<string, InstructionsCacheEntry>();
 
-export async function getInstructions(stainlessApiKey: string | undefined): Promise<string> {
+export async function getInstructions({
+  stainlessApiKey,
+  customInstructionsPath,
+}: {
+  stainlessApiKey?: string | undefined;
+  customInstructionsPath?: string | undefined;
+}): Promise<string> {
   const now = Date.now();
-  const cacheKey = stainlessApiKey ?? '';
+  const cacheKey = customInstructionsPath ?? stainlessApiKey ?? '';
   const cached = instructionsCache.get(cacheKey);
 
   if (cached && now - cached.fetchedAt <= INSTRUCTIONS_CACHE_TTL_MS) {
@@ -28,12 +35,28 @@ export async function getInstructions(stainlessApiKey: string | undefined): Prom
     }
   }
 
-  const fetchedInstructions = await fetchLatestInstructions(stainlessApiKey);
+  let fetchedInstructions: string;
+
+  if (customInstructionsPath) {
+    fetchedInstructions = await fetchLatestInstructionsFromFile(customInstructionsPath);
+  } else {
+    fetchedInstructions = await fetchLatestInstructionsFromApi(stainlessApiKey);
+  }
+
   instructionsCache.set(cacheKey, { fetchedInstructions, fetchedAt: now });
   return fetchedInstructions;
 }
 
-async function fetchLatestInstructions(stainlessApiKey: string | undefined): Promise<string> {
+async function fetchLatestInstructionsFromFile(path: string): Promise<string> {
+  try {
+    return await fs.readFile(path, 'utf-8');
+  } catch (error) {
+    getLogger().error({ error, path }, 'Error fetching instructions from file');
+    throw error;
+  }
+}
+
+async function fetchLatestInstructionsFromApi(stainlessApiKey: string | undefined): Promise<string> {
   // Setting the stainless API key is optional, but may be required
   // to authenticate requests to the Stainless API.
   const response = await fetch(

packages/mcp-server/src/options.ts

@@ -22,6 +22,7 @@ export type McpOptions = {
   codeAllowedMethods?: string[] | undefined;
   codeBlockedMethods?: string[] | undefined;
   codeExecutionMode: McpCodeExecutionMode;
+  customInstructionsPath?: string | undefined;
 };
 
 export type McpCodeExecutionMode = 'stainless-sandbox' | 'local';
@@ -52,6 +53,10 @@ export function parseCLIOptions(): CLIOptions {
       description:
         "Where to run code execution in code tool; 'stainless-sandbox' will execute code in Stainless-hosted sandboxes whereas 'local' will execute code locally on the MCP server machine.",
     })
+    .option('custom-instructions-path', {
+      type: 'string',
+      description: 'Path to custom instructions for the MCP server',
+    })
     .option('debug', { type: 'boolean', description: 'Enable debug logging' })
     .option('log-format', {
       type: 'string',
@@ -117,6 +122,7 @@ export function parseCLIOptions(): CLIOptions {
     codeAllowedMethods: argv.codeAllowedMethods,
     codeBlockedMethods: argv.codeBlockedMethods,
     codeExecutionMode: argv.codeExecutionMode as McpCodeExecutionMode,
+    customInstructionsPath: argv.customInstructionsPath,
     transport,
     logFormat,
     port: argv.port,

packages/mcp-server/src/server.ts

@@ -16,14 +16,20 @@ import { McpOptions } from './options';
 import { blockedMethodsForCodeTool } from './methods';
 import { HandlerFunction, McpRequestContext, ToolCallResult, McpTool } from './types';
 
-export const newMcpServer = async (stainlessApiKey: string | undefined) =>
+export const newMcpServer = async ({
+  stainlessApiKey,
+  customInstructionsPath,
+}: {
+  stainlessApiKey?: string | undefined;
+  customInstructionsPath?: string | undefined;
+}) =>
   new McpServer(
     {
       name: 'brand_dev_api',
       version: '0.31.0',
     },
     {
-      instructions: await getInstructions(stainlessApiKey),
+      instructions: await getInstructions({ stainlessApiKey, customInstructionsPath }),
       capabilities: { tools: {}, logging: {} },
     },
   );

packages/mcp-server/src/stdio.ts

@@ -4,7 +4,10 @@ import { initMcpServer, newMcpServer } from './server';
 import { getLogger } from './logger';
 
 export const launchStdioServer = async (mcpOptions: McpOptions) => {
-  const server = await newMcpServer(mcpOptions.stainlessApiKey);
+  const server = await newMcpServer({
+    stainlessApiKey: mcpOptions.stainlessApiKey,
+    customInstructionsPath: mcpOptions.customInstructionsPath,
+  });
 
   await initMcpServer({ server, mcpOptions, stainlessApiKey: mcpOptions.stainlessApiKey });