Defining Prompts

Prompting is the fundamental input that gives LLMs their expressive power. LlamaIndex uses prompts to build the index, do insertion, perform traversal during querying, and to synthesize the final answer.

LlamaIndex uses a set of default prompt templates that works well out of the box. Users may also provide their own prompt templates to further customize the behavior of the framework.

Defining a custom prompt

Defining a custom prompt is as simple as creating a format string

from llama_index import Prompt

template = (
    "We have provided context information below. \n"
    "Given this information, please answer the question: {query_str}\n"
qa_template = Prompt(template)

Note: you may see references to legacy prompt subclasses such as QuestionAnswerPrompt, RefinePrompt. These have been deprecated (and now are type aliases of Prompt). Now you can directly specify Prompt(template) to construct custom prompts. But you still have to make sure the template string contains the expected parameters (e.g. {context_str} and {query_str}) when replacing a default question answer prompt.

Passing custom prompts into the pipeline

Since LlamaIndex is a multi-step pipeline, it’s important to identify the operation that you want to modify and pass in the custom prompt at the right place. At a high-level, prompts are used in 1) index construction, and 2) query engine execution

Modify prompts used in index construction

Different indices use different types of prompts during construction (some don’t use prompts at all). For instance, TreeIndex uses a SummaryPrompt to hierarchically summarize the nodes, and KeywordTableIndex uses a KeywordExtractPrompt to extract keywords.

There are two equivalent ways to override the prompts:

  1. via the default nodes constructor

index = TreeIndex(nodes, summary_template=<custom_prompt>)
  1. via the documents constructor.

index = TreeIndex.from_documents(docs, summary_template=<custom_prompt>)

For more details on which index uses which prompts, please visit Index class references.

Modify prompts used in query engine

More commonly, prompts are used at query-time (i.e. for executing a query against an index and synthesizing the final response). There are also two equivalent ways to override the prompts:

  1. via the high-level API

query_engine = index.as_query_engine(text_qa_template=<custom_prompt>)
  1. via the low-level composition API

retriever = index.as_retriever()
synth = ResponseSynthesizer.from_args(text_qa_template=<custom_prompt>)
query_engine = RetrieverQueryEngine(retriever, response_synthesizer)

The two approaches above are equivalent, where 1 is essentially syntactic sugar for 2 and hides away the underlying complexity. You might want to use 1 to quickly modify some common parameters, and use 2 to have more granular control.

For more details on which classes use which prompts, please visit Query class references.

Full Example

An example can be found in this notebook.

A corresponding snippet is below. We show how to define a custom prompt for question answer which requires both a context_str and query_str field. The prompt is passed in during query-time.

from llama_index import Prompt, VectorStoreIndex, SimpleDirectoryReader

# load documents
documents = SimpleDirectoryReader('data').load_data()

# define custom Prompt
    "We have provided context information below. \n"
    "Given this information, please answer the question: {query_str}\n"

# Build index 
index = VectorStoreIndex.from_documents(documents)

# Configure query engine
query_engine = index.as_query_engine(text_qa_template=QA_TEMPLATE)

# Execute query
response = query_engine.query("What did the author do growing up?")

Check out the reference documentation for a full set of all prompts.