lebsral/dspy-async

Run DSPy Modules Asynchronously

Guide the user through running DSPy modules with async/await for non-blocking execution in web frameworks, concurrent processing, and high-throughput applications.

What is async in DSPy

Every DSPy module supports async execution via aforward() and acall(). These return awaitable coroutines instead of blocking the event loop, making DSPy compatible with async web frameworks (FastAPI, Starlette, aiohttp) and enabling concurrent LM calls with asyncio.gather().

When to use async

| Use async when... | Use sync when... | |-------------------|-----------------| | Serving DSPy behind FastAPI/Starlette | Running scripts or notebooks | | Making concurrent LM calls | Processing one input at a time | | Building real-time APIs | Running optimization/evaluation | | Combining with async streaming | Simple CLI tools | | Integrating with async databases/caches | No event loop in your application |

Step 1: Basic async execution

Every DSPy module has an async variant:

import asyncio
import dspy

lm = dspy.LM("openai/gpt-4o-mini")  # or "anthropic/claude-sonnet-4-5-20250929", etc.
dspy.configure(lm=lm)

qa = dspy.ChainOfThought("question -> answer")

async def ask(question: str):
    # aforward() is the async version of forward()
    result = await qa.aforward(question=question)
    return result.answer

# Run it
answer = asyncio.run(ask("What is DSPy?"))
print(answer)

Two async methods:

• module.aforward(**kwargs) -- async version of module.forward()
• module.acall(**kwargs) -- async version of module(**kwargs) (same thing, convenience alias)

Step 2: Concurrent calls with asyncio.gather

Run multiple independent LM calls concurrently:

import asyncio
import dspy

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

summarizer = dspy.ChainOfThought("text -> summary")

async def summarize_batch(texts: list[str]):
    # Launch all summarizations concurrently
    tasks = [
        summarizer.aforward(text=text)
        for text in texts
    ]
    results = await asyncio.gather(*tasks)
    return [r.summary for r in results]

texts = ["Article 1...", "Article 2...", "Article 3..."]
summaries = asyncio.run(summarize_batch(texts))

This is significantly faster than sequential processing because LM calls are I/O-bound -- the network round-trip dominates.

Step 3: FastAPI endpoint

from fastapi import FastAPI
import dspy

app = FastAPI()

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

classifier = dspy.Predict("text -> label, confidence: float")

@app.post("/classify")
async def classify(text: str):
    # Non-blocking -- does not hold up other requests
    result = await classifier.aforward(text=text)
    return {"label": result.label, "confidence": result.confidence}

Why this matters: Without async, each request blocks the FastAPI worker thread. With aforward(), the worker is free to handle other requests while waiting for the LM response.

Step 4: Semaphore-based concurrency limiting

Prevent overwhelming the LM provider with too many concurrent requests:

import asyncio
import dspy

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

processor = dspy.ChainOfThought("input -> output")

# Limit to 10 concurrent LM calls
semaphore = asyncio.Semaphore(10)

async def process_one(input_text: str):
    async with semaphore:
        return await processor.aforward(input=input_text)

async def process_batch(inputs: list[str]):
    tasks = [process_one(text) for text in inputs]
    return await asyncio.gather(*tasks)

# Even with 1000 inputs, only 10 run concurrently
results = asyncio.run(process_batch(["input"] * 1000))

Step 5: Async with streaming

Combine async execution with streaming output:

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import dspy
from dspy.streaming import streamify, StreamListener

app = FastAPI()

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

qa = dspy.ChainOfThought("question -> answer")
listener = StreamListener(signature_field_name="answer")
streaming_qa = streamify(qa, stream_listeners=[listener])

@app.get("/ask")
async def ask(question: str):
    async def generate():
        async for chunk in streaming_qa(question=question):
            if hasattr(chunk, "answer"):
                yield f"data: {chunk.answer}\n\n"
        yield "data: [DONE]\n\n"
    return StreamingResponse(generate(), media_type="text/event-stream")

Step 6: Async custom modules

When writing custom modules, implement aforward for async:

import dspy

class AsyncPipeline(dspy.Module):
    def __init__(self):
        self.classify = dspy.Predict("text -> category")
        self.summarize = dspy.ChainOfThought("text, category -> summary")

    async def aforward(self, text):
        # Run classification (async)
        classification = await self.classify.aforward(text=text)

        # Run summarization with the category (async)
        result = await self.summarize.aforward(
            text=text,
            category=classification.category,
        )
        return dspy.Prediction(
            category=classification.category,
            summary=result.summary,
        )

# Usage
pipeline = AsyncPipeline()
result = asyncio.run(pipeline.aforward(text="..."))

Step 7: Async with ReAct agents

Agents with MCP tools or async tool functions need acall():

import dspy

lm = dspy.LM("openai/gpt-4o-mini")
dspy.configure(lm=lm)

async def async_search(query: str) -> str:
    """Search the web asynchronously."""
    # Your async search implementation
    return "results..."

agent = dspy.ReAct("question -> answer", tools=[async_search])

async def run_agent(question: str):
    # acall() handles async tools automatically
    result = await agent.acall(question=question)
    return result.answer

Gotchas

1. Claude uses module() inside async functions instead of await module.aforward(). Calling a module synchronously inside an async function blocks the event loop. Always use aforward() or acall() in async contexts.
2. Claude nests asyncio.run() inside an existing event loop. You cannot call asyncio.run() from inside an async function -- it raises RuntimeError: This event loop is already running. Use await directly instead.
3. Claude forgets the semaphore for batch processing. Without a concurrency limit, asyncio.gather() with 1000 tasks hits rate limits immediately. Always add a semaphore when processing large batches.
4. Claude defines forward() but not aforward() in custom modules. If your module will be called with await, implement aforward(). DSPy does not auto-wrap forward() into an async version.
5. Claude mixes sync and async in the same pipeline. If one step is async (e.g., MCP tools), the entire call chain must be async. You cannot await inside a sync forward().

Additional resources

• dspy.ai/api/modules (aforward documentation)
• For API details, see reference.md
• For worked examples, see examples.md

Cross-references

Install any skill: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill <name>

• Serving APIs with FastAPI -- see /ai-serving-apis
• Concurrent batch processing -- see /dspy-parallel
• Streaming output with async generators -- see /dspy-streaming
• MCP tools that require async -- see /dspy-mcp
• General utilities (caching, debugging) -- see /dspy-utils
• Install /ai-do if you do not have it -- it routes any AI problem to the right skill and is the fastest way to work: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill ai-do