# X_MCP_IDEMPOTENCY_KEY_SHADOWED

## Means

an idempotent action's input declares idempotencyKey, the MCP argument reserved for the idempotency key

## Typical cause

the tool argument would be both the action's input and the retry key

## Fix

rename the action's idempotencyKey input field — MCP clients pass the Idempotency-Key as the reserved idempotencyKey argument

[MCP and AI](/error-codes#mcp-and-ai)