langchaint

langchaint is an opinionated, provider-neutral Python client for LLM applications. It provides fully typed, asynchronous APIs for generation, streaming, embeddings, tools, retries, and billing. The application owns the agent loop.

Alpha: the API may change without notice.

Why langchaint

• Consistent API. Bind request fields once with LLM.bind(), then call generate_one(), generate_many(), or stream_one() on the resulting BoundLLM.
• Output types determined by binding. Binding response_format=Answer gives generate_one() the return type Response[Answer]. Binding tools as well adds ToolCallTurn[Answer] to that return type.
• Result variants with autocomplete. Match on .kind with editor autocomplete and no class imports.
• Coordinated retries. Share concurrency limits, request-start pacing, and provider-directed pauses across models using one rate-limit quota.
• Complete billing. Successful results and GenerationError values retain provider-reported usage from every recorded attempt, including billed retries.
• Streaming. stream_one() returns an async context manager and async iterator. final() returns the typed result with its usage.
• Agent loops in Python. Provider-neutral messages, typed tools with argument validation, concurrent dispatch, and explicit result variants support async control flow.

Install

langchaint requires Python 3.13 or newer.

Install the extra for each backend you use:

pip install "langchaint[openai]"

Install langchaint[tracing] for OpenTelemetry tracing.

Generate a typed response

import asyncio

from pydantic import BaseModel

from langchaint.openai import OpenAI


class Answer(BaseModel):
    answer: str
    confidence: float


async def main() -> None:
    assistant = (
        OpenAI()
        .model("gpt-5.6-terra")
        .bind(
            system_prompt="Answer clearly and concisely.",
            response_format=Answer,
        )
    )
    response = await assistant.generate_one("Why is the sky blue?")

    print(response.output.answer)
    print(response.usage.cost_in_usd)


asyncio.run(main())

The Pydantic model validates the provider response.

generate_many() returns one result per input in input order. A terminal failure becomes that input's GenerationError, so sibling results remain available.

Coordinate retries across a rate-limit quota

Create one OpenAI for each rate-limit quota:

openai = OpenAI(
    max_concurrent_requests=8,
    max_request_starts_per_second=50.0,
)

fast_model = openai.model("gpt-5.6-luna")
strong_model = openai.model("gpt-5.6-sol")

A rate-limit response pauses request starts across the shared quota. After a transient failure local to one request, langchaint waits and retries that request.

Stream with an explicit lifetime

text_assistant = OpenAI().model("gpt-5.6-terra").bind()

async with text_assistant.stream_one("Explain photosynthesis.") as stream:
    async for item in stream:
        if isinstance(item, str):
            print(item, end="", flush=True)

    response = await stream.final()

final() consumes the remaining stream and returns the assembled result.

Build agent loops

The application controls turn limits, state, approvals, model changes, and persistence.

messages: list[Message] = [UserMessage(content=prompt)]

for _ in range(max_turns):
    result = await bound.generate_one(messages)

    match result.kind:
        case "tool_call_turn":
            messages.append(result.assistant_message)
            outcomes = await bound.tool_manager.dispatch_many(result.tool_calls)
            messages.extend(outcome.tool_message for outcome in outcomes)
        case "response":
            return result.output

raise RuntimeError("model did not finish within max_turns")

ToolManager.dispatch_many() runs tool calls concurrently and preserves their order.

See examples/02_tool_loop.py for a complete typed tool loop.

Account for the complete call

response.usage.cost_in_usd includes every billed retry recorded for the call. GenerationError.usage preserves the recorded cost of failed calls.

More examples

See examples/README.md for complete examples.

License

langchaint uses the MIT License.