---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/40001#article
headline: "40001: Assignment Instruction response is not valid JSON. Ensure it is not escaped"
url: https://www.twilio.com/docs/api/errors/40001
inLanguage: en
dateModified: 2026-06-08T11:38:30.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 40001: Assignment Instruction response is not valid JSON. Ensure it is not escaped

Log Type: APPLICATION

Log Level: WARNING

## Description

This error occurs when TaskRouter receives an Assignment Instruction response that is not valid JSON. A common cause is returning a JSON string that has been escaped instead of returning a raw JSON object in the HTTP response body.

TaskRouter expects your Assignment Callback or related instruction-generating endpoint to respond with properly formatted JSON. If the payload is double-encoded, escaped, malformed, or returned with unexpected formatting, TaskRouter cannot parse it and raises error `40001`.

#### Example

##### Valid JSON (Not Escaped)

```json
{
  "instruction": "call",
  "from": "+15558675309",
  "url": "http://example.com/agent_answer",
  "status_callback_url":
    "http://example.com/agent_answer_status_callback"
}
```

##### Invalid JSON (Escaped)

```json
{\r\n  \"instruction\": \"call\",\r\n  \"from\": \"+15558675309\",\r\n  \"url\": \"http:\/\/example.com\/agent_answer\",   \r\n  \"status_callback_url\":\r\n    \"http:\/\/example.com\/agent_answer_status_callback\"\r\n}
```

### Possible causes

* Your application returns JSON as an escaped string instead of a JSON object.
* The response body is being serialized twice before being sent.
* Your web framework is wrapping the JSON payload in quotes.
* The callback handler is returning text or HTML instead of JSON.
* The response contains malformed JSON syntax, such as missing braces, commas, or quotes.
* The endpoint is setting an unexpected response format or content type for the Assignment Instruction response.

### Possible solutions

* Return a raw JSON object in the HTTP response body, not a stringified or escaped JSON value.
* Check your server code for double serialization, such as calling JSON encoding on data that has already been encoded.
* Verify that your Assignment Callback response is valid JSON by testing the endpoint directly.
* Confirm your application returns the expected `Content-Type` header, such as `application/json`, when sending Assignment Instructions.
* Review framework-specific response helpers to ensure they serialize the payload only once.
* Inspect logs or captured HTTP responses to confirm the response body matches the expected JSON structure.

### Related resources

* `/docs/taskrouter`
* `/docs/taskrouter/api/task-assignment`
* `/docs/taskrouter/quickstart`