### Answer --- I don't have specific documentation about the difference in behavior between Rigid Mode and Flex Mode for MCP tool calls in Conversation Flow agents. The available sources describe how to add MCP tools and configure response variables, but don't cover this specific variable-passing issue between modes. [(1) ]() There are community reports of similar MCP-related issues with variables not being extracted or passed correctly. [(2) ]() [(3) ]() One user noted that MCP tools return stringified responses differently than custom functions, which may affect variable handling. [(3) ]() Suggestions based on available information: 1. Update your prompt to explicitly instruct when to invoke the MCP tool with required parameters. [(1) ]() 2. Compare with custom function behavior — some users found that switching to a custom function (HTTP API) with the same endpoint worked correctly when MCP did not. [(2) ]() 3. Check response variable syntax — ensure your response variable configuration matches Retell's MCP specification exactly. Check out these potentially useful sources: - [MCP Documentation]() - [Update Conversation Flow API]() Would you like me to provide a general answer based on my knowledge, or would you prefer to reach out to Retell support directly for this specific Rigid vs Flex mode behavior?

​ To learn more, see the sources I considered: [[1] MCP](https://docs.retellai.com/build/single-multi-prompt/mcp) [[2] Conversation Flow Agent - MCP Response Variables not working](https://discord.com/channels/1186424376300679209/1406261286270144564) [[3] mcp tool variables not extracted](https://discord.com/channels/1186424376300679209/1426340769924382791) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.