1  ---
  2  title: "Validators"
  3  id: validators-api
  4  description: "Validators validate LLM outputs"
  5  slug: "/validators-api"
  6  ---
  7  
  8  
  9  ## json_schema
 10  
 11  ### is_valid_json
 12  
 13  ```python
 14  is_valid_json(s: str) -> bool
 15  ```
 16  
 17  Check if the provided string is a valid JSON.
 18  
 19  **Parameters:**
 20  
 21  - **s** (<code>str</code>) – The string to be checked.
 22  
 23  **Returns:**
 24  
 25  - <code>bool</code> – `True` if the string is a valid JSON; otherwise, `False`.
 26  
 27  ### JsonSchemaValidator
 28  
 29  Validates JSON content of `ChatMessage` against a specified [JSON Schema](https://json-schema.org/).
 30  
 31  If JSON content of a message conforms to the provided schema, the message is passed along the "validated" output.
 32  If the JSON content does not conform to the schema, the message is passed along the "validation_error" output.
 33  In the latter case, the error message is constructed using the provided `error_template` or a default template.
 34  These error ChatMessages can be used by LLMs in Haystack 2.x recovery loops.
 35  
 36  Usage example:
 37  
 38  ```python
 39  from haystack import Pipeline
 40  from haystack.components.generators.chat import OpenAIChatGenerator
 41  from haystack.components.joiners import BranchJoiner
 42  from haystack.components.validators import JsonSchemaValidator
 43  from haystack import component
 44  from haystack.dataclasses import ChatMessage
 45  
 46  
 47  @component
 48  class MessageProducer:
 49  
 50      @component.output_types(messages=list[ChatMessage])
 51      def run(self, messages: list[ChatMessage]) -> dict:
 52          return {"messages": messages}
 53  
 54  
 55  p = Pipeline()
 56  p.add_component("llm", OpenAIChatGenerator(generation_kwargs={"response_format": {"type": "json_object"}}))
 57  p.add_component("schema_validator", JsonSchemaValidator())
 58  p.add_component("joiner_for_llm", BranchJoiner(list[ChatMessage]))
 59  p.add_component("message_producer", MessageProducer())
 60  
 61  p.connect("message_producer.messages", "joiner_for_llm")
 62  p.connect("joiner_for_llm", "llm")
 63  p.connect("llm.replies", "schema_validator.messages")
 64  p.connect("schema_validator.validation_error", "joiner_for_llm")
 65  
 66  result = p.run(data={
 67      "message_producer": {
 68          "messages":[ChatMessage.from_user("Generate JSON for person with name 'John' and age 30")]},
 69          "schema_validator": {
 70              "json_schema": {
 71                  "type": "object",
 72                  "properties": {"name": {"type": "string"},
 73                  "age": {"type": "integer"}
 74              }
 75          }
 76      }
 77  })
 78  print(result)
 79  # >> {'schema_validator': {'validated': [ChatMessage(_role=<ChatRole.ASSISTANT: 'assistant'>,
 80  # _content=[TextContent(text="\n{\n  "name": "John",\n  "age": 30\n}")],
 81  # _name=None, _meta={'index': 0, 'finish_reason': 'stop', 'usage': {'completion_tokens': 17, 'prompt_tokens': 20,
 82  # 'total_tokens': 37}})]}}
 83  ```
 84  
 85  #### __init__
 86  
 87  ```python
 88  __init__(
 89      json_schema: dict[str, Any] | None = None, error_template: str | None = None
 90  ) -> None
 91  ```
 92  
 93  Initialize the JsonSchemaValidator component.
 94  
 95  **Parameters:**
 96  
 97  - **json_schema** (<code>dict\[str, Any\] | None</code>) – A dictionary representing the [JSON schema](https://json-schema.org/) against which
 98    the messages' content is validated.
 99  - **error_template** (<code>str | None</code>) – A custom template string for formatting the error message in case of validation failure.
100  
101  #### run
102  
103  ```python
104  run(
105      messages: list[ChatMessage],
106      json_schema: dict[str, Any] | None = None,
107      error_template: str | None = None,
108  ) -> dict[str, list[ChatMessage]]
109  ```
110  
111  Validates the last of the provided messages against the specified json schema.
112  
113  If it does, the message is passed along the "validated" output. If it does not, the message is passed along
114  the "validation_error" output.
115  
116  **Parameters:**
117  
118  - **messages** (<code>list\[ChatMessage\]</code>) – A list of ChatMessage instances to be validated. The last message in this list is the one
119    that is validated.
120  - **json_schema** (<code>dict\[str, Any\] | None</code>) – A dictionary representing the [JSON schema](https://json-schema.org/)
121    against which the messages' content is validated. If not provided, the schema from the component init
122    is used.
123  - **error_template** (<code>str | None</code>) – A custom template string for formatting the error message in case of validation. If not
124    provided, the `error_template` from the component init is used.
125  
126  **Returns:**
127  
128  - <code>dict\[str, list\[ChatMessage\]\]</code> – A dictionary with the following keys:
129  - "validated": A list of messages if the last message is valid.
130  - "validation_error": A list of messages if the last message is invalid.
131  
132  **Raises:**
133  
134  - <code>ValueError</code> – If no JSON schema is provided or if the message content is not a dictionary or a list of
135    dictionaries.