# \[Bug\]: MCP Server in Cursor prompts for optional params instead of calling the tool **URL:** \ **Category:** Bug Reports\ **Created:** [January 5, 2026, 8:58pm UTC](https://forum.cursor.com/t/bug-mcp-server-in-cursor-prompts-for-optional-params-instead-of-calling-the-tool/148042 "2026-01-05T20:58:47Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![ericviana](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/ericviana/32/93638_2.png) [@ericviana](https://forum.cursor.com/u/ericviana)\ **Post date:** [January 5, 2026, 8:58pm UTC](https://forum.cursor.com/t/bug-mcp-server-in-cursor-prompts-for-optional-params-instead-of-calling-the-tool/148042/1 "2026-01-05T20:58:47Z")
### Where does the bug appear (feature/product)? Cursor IDE ### Describe the Bug ### MCP Server: Cursor prompts for optional params instead of calling the tool I’m running a custom MCP server (`@blindpay/mcp`: [https://www.npmjs.com/package/@blindpay/mcp](https://www.npmjs.com/package/@blindpay/mcp)) that makes `instance_id` optional when `BLINDPAY_INSTANCE_ID` is set, and auto-injects it at runtime. **Issue:** In Cursor, the AI still asks me for `instance_id` instead of calling the tool without it. **Confirmed:** Works as expected in MCP Inspector, so this looks Cursor-specific. **Expected (when `BLINDPAY_INSTANCE_ID` is set):** - User: “list the receivers for my blindpay instance” - AI calls `GetV1InstancesReceivers` without `instance_id` - Server injects the env value and the request succeeds **Actual in Cursor:** - AI asks for the instance ID rather than calling the tool **Actual in MCP Inspector (works):** - Schema shows `instance_id` is not required - Tool call without it succeeds; server injects the ID and the request succeeds ### Steps to Reproduce 1. Create an MCP server that dynamically removes a parameter from the `required` array in tool schemas when an environment variable is set 2. Configure the MCP server in `~/.cursor/mcp.json` with the environment variable: { “mcpServers”: { “test-server”: { “command”: “node”, “args”: [“./server.js”], “env”: { “DEFAULT\_PARAM”: “value123” } } } }3. Restart Cursor completely 3. Ask the AI to use a tool that has an optional parameter (made optional via env var) **Expected** : AI calls the tool without the optional parameter **Actual** : AI prompts user for the parameter instead of calling the tool 1. Test the same MCP server with MCP Inspector: DEFAULT\_PARAM=“value123” mcp-inspector node ./server.js **Result** : Works correctly - tool can be called without the parameter, and auto-injection fills it in This proves the issue is specific to Cursor’s AI decision-making, not the MCP server implementation. ### Screenshots / Screen Recordings ![532106597-b2f37f42-bf78-4945-91a7-a55296850e0b.png](https://us1.discourse-cdn.com/cursor1/original/3X/5/9/59e0502a831569920228bd2a0cc62a2cf67091ca.png) ### Operating System MacOS ### Current Cursor Version (Menu → About Cursor → Copy) Version: 2.2.44 VSCode Version: 1.105.1 Commit: 20adc1003928b0f1b99305dbaf845656ff81f5d0 Date: 2025-12-24T21:41:47.598Z Electron: 37.7.0 Chromium: 138.0.7204.251 Node.js: 22.20.0 V8: 13.8.258.32-electron.0 OS: Darwin arm64 25.2.0 ### For AI issues: which model did you use? composer-1 and sonnet 4.5 ### Does this stop you from using Cursor No - Cursor works, but with this issue ---
**Author:** ![deanrie](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/deanrie/32/1263_2.png) [@deanrie](https://forum.cursor.com/u/deanrie)\ **Post date:** [January 6, 2026, 5:18am UTC](https://forum.cursor.com/t/bug-mcp-server-in-cursor-prompts-for-optional-params-instead-of-calling-the-tool/148042/2 "2026-01-06T05:18:32Z")
Hey, thanks for the report. I can see the screenshot from MCP Inspector. The `GetV1InstancesPayouts` tool runs successfully without `instance_id` (API response status: 200). That clearly confirms that: - The MCP server correctly handles the optional parameter - The schema correctly shows `instance_id` as not required - The issue is on Cursor’s side. The AI isn’t recognizing that the parameter is now optional I’ll pass this to the team. ---
**Author:** ![system](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/system/32/99069_2.png) [@system](https://forum.cursor.com/u/system)\ **Post date:** [January 28, 2026, 5:18am UTC](https://forum.cursor.com/t/bug-mcp-server-in-cursor-prompts-for-optional-params-instead-of-calling-the-tool/148042/4 "2026-01-28T05:18:53Z")
Closed on January 28, 2026, 5:18am UTC This topic was automatically closed 22 days after the last reply. New replies are no longer allowed. ---
**Author:** ![Colin](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/colin/32/90452_2.png) [@Colin](https://forum.cursor.com/u/Colin)\ **Post date:** [March 25, 2026, 2:05pm UTC](https://forum.cursor.com/t/bug-mcp-server-in-cursor-prompts-for-optional-params-instead-of-calling-the-tool/148042/6 "2026-03-25T14:05:03Z")
Hey @ericviana! I walked through your reproduction steps and can no longer reproduce this. Created a minimal MCP server where a parameter is dynamically made optional via an env var. Cursor correctly recognized it as optional, called the tool without prompting, and the server auto-injected the value. If you’re still seeing this on a recent build, please raise a new thread!