🏷️ feat: Request Placeholders for Custom Endpoint & MCP Headers (#9095)

* feat: Add conversation ID support to custom endpoint headers

- Add LIBRECHAT_CONVERSATION_ID to customUserVars when provided
- Pass conversation ID to header resolution for dynamic headers
- Add comprehensive test coverage

Enables custom endpoints to access conversation context using {{LIBRECHAT_CONVERSATION_ID}} placeholder.

* fix: filter out unresolved placeholders from headers (thanks @MrunmayS)

* feat: add support for request body placeholders in custom endpoint headers

- Add {{LIBRECHAT_BODY_*}} placeholders for conversationId, parentMessageId, messageId
- Update tests to reflect new body placeholder functionality

* refactor resolveHeaders

* style: minor styling cleanup

* fix: type error in unit test

* feat: add body to other endpoints

* feat: add body for mcp tool calls

* chore: remove changes that unnecessarily increase scope after clarification of requirements

* refactor: move http.ts to packages/api and have RequestBody intersect with Express request body

* refactor: processMCPEnv now uses single object argument pattern

* refactor: update processMCPEnv to use 'options' parameter and align types across MCP connection classes

* feat: enhance MCP connection handling with dynamic request headers to pass request body fields

---------

Co-authored-by: Gopal Sharma <gopalsharma@gopal.sharma1>
Co-authored-by: s10gopal <36487439+s10gopal@users.noreply.github.com>
Co-authored-by: Dustin Healy <dustinhealy1@gmail.com>

packages/api/src/mcp/MCPManager.ts

@@ -6,11 +6,13 @@ import type { TokenMethods } from '@librechat/data-schemas';
 import type { FlowStateManager } from '~/flow/manager';
 import type { TUser } from 'librechat-data-provider';
 import type { MCPOAuthTokens } from '~/mcp/oauth';
+import type { RequestBody } from '~/types';
 import type * as t from './types';
 import { UserConnectionManager } from '~/mcp/UserConnectionManager';
 import { ConnectionsRepository } from '~/mcp/ConnectionsRepository';
 import { formatToolContent } from './parsers';
 import { MCPConnection } from './connection';
+import { processMCPEnv } from '~/utils/env';
 import { CONSTANTS } from './enum';
 
 /**
@@ -179,6 +181,7 @@ Please follow these instructions when using tools from the respective MCP server
     toolArguments,
     options,
     tokenMethods,
+    requestBody,
     flowManager,
     oauthStart,
     oauthEnd,
@@ -190,6 +193,7 @@ Please follow these instructions when using tools from the respective MCP server
     provider: t.Provider;
     toolArguments?: Record<string, unknown>;
     options?: RequestOptions;
+    requestBody?: RequestBody;
     tokenMethods?: TokenMethods;
     customUserVars?: Record<string, string>;
     flowManager: FlowStateManager<MCPOAuthTokens | null>;
@@ -214,6 +218,7 @@ Please follow these instructions when using tools from the respective MCP server
           oauthEnd,
           signal: options?.signal,
           customUserVars,
+          requestBody,
         });
       } else {
         /** App-level connection */
@@ -234,6 +239,17 @@ Please follow these instructions when using tools from the respective MCP server
         );
       }
 
+      const rawConfig = this.getRawConfig(serverName) as t.MCPOptions;
+      const currentOptions = processMCPEnv({
+        user,
+        options: rawConfig,
+        customUserVars: customUserVars,
+        body: requestBody,
+      });
+      if ('headers' in currentOptions) {
+        connection.setRequestHeaders(currentOptions.headers || {});
+      }
+
       const result = await connection.client.request(
         {
           method: 'tools/call',