You must pass the response to the tool via the Result variable. { "toolCallId": "VAPI_TOOL_ID", "result": "{\"availableSlots\":[{\"humanReadable\":\"Friday, September 19 at 9:00 AM\",\"isoDateTime\":\"2025-09-19T09:00:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 9:30 AM\",\"isoDateTime\":\"2025-09-19T09:30:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 10:00 AM\",\"isoDateTime\":\"2025-09-19T10:00:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 10:30 AM\",\"isoDateTime\":\"2025-09-19T10:30:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 11:00 AM\",\"isoDateTime\":\"2025-09-19T11:00:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 11:30 AM\",\"isoDateTime\":\"2025-09-19T11:30:00-04:00\"},{\"humanReadable\":\"Friday, September 19 at 12:00 PM\",\"isoDateTime\":\"2025-09-19T12:00:00-04:00\"}]}" }