Serve flows with LFX

import Tabs from '@theme/Tabs'; import TabItem from '@theme/TabItem'; import LfxPrereqs from '@site/docs/_partial-lfx-run-serve-prereqs.mdx'; import LfxComponentCategories from '@site/docs/_partial-lfx-component-categories.mdx';

lfx serve starts a FastAPI server that exposes your flows as HTTP API endpoints at POST /flows/{flow_id}/run.

A LANGFLOW_API_KEY is required because lfx serve starts a publicly accessible server.

To execute a flow once and stream results to stdout instead, see Run flows with LFX.

Both lfx serve and lfx run accept a .json flow file or a .py Python script.

Start the server

Generate a Langflow API key.

For LFX, generate a secure token locally:
bash
```
uv run python -c "import secrets; print(secrets.token_urlsafe(32))"
```
This is different from creating an API key through the Langflow UI or CLI, which stores the key in the Langflow database. LFX only needs a secure token string to authenticate requests.

Set your environment variables. Use a .env file or export directly:

.env file:

text

LANGFLOW_API_KEY="sk..."
OPENAI_API_KEY="sk-..."

Export:

bash

export LANGFLOW_API_KEY="sk..."
export OPENAI_API_KEY="sk-..."

Start the server with your flow:

With a .env file:

bash

uv run lfx serve simple-agent-flow.json --env-file .env

With exported variables:

bash

uv run lfx serve simple-agent-flow.json

Copy the flow_id from the startup output:

 LFX Server
 Flow loaded: simple-agent-flow.json (c1dab29d-3364-58ef-8fef-99311d32ee42)
 Server:      http://127.0.0.1:8000
 Run flows at: POST /flows/{flow_id}/run
 API key:     x-api-key header or ?x-api-key= query parameter

Call the server

In a new terminal, set your API key and flow ID, then send a test request:

bash

export LANGFLOW_API_KEY="sk..."
export FLOW_ID="c1dab29d-3364-58ef-8fef-99311d32ee42"

python

import os

import requests

flow_id = os.environ["FLOW_ID"]
api_key = os.environ["LANGFLOW_API_KEY"]

url = f"http://localhost:8000/flows/{flow_id}/run"

headers = {
    "Content-Type": "application/json",
    "x-api-key": api_key,
}

payload = {
    "input_value": "Hello, world!",
}

response = requests.post(url, headers=headers, json=payload, timeout=60)
response.raise_for_status()
print(response.text)

</TabItem> <TabItem value="JavaScript" label="JavaScript">

const flowId = process.env.FLOW_ID;
const apiKey = process.env.LANGFLOW_API_KEY;

const options = {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "x-api-key": apiKey,
  },
  body: JSON.stringify({
    input_value: "Hello, world!",
  }),
};

fetch(`http://localhost:8000/flows/${flowId}/run`, options)
  .then(async (response) => {
    if (!response.ok) {
      throw new Error(`HTTP ${response.status}`);
    }
    console.log(await response.text());
  })
  .catch((error) => console.error(error));

</TabItem> <TabItem value="curl" label="curl">

bash

curl -X POST http://localhost:8000/flows/$FLOW_ID/run \
  -H "Content-Type: application/json" \
  -H "x-api-key: $LANGFLOW_API_KEY" \
  -d '{"input_value": "Hello, world!"}'

</TabItem> </Tabs>

Serve multiple flows

To serve multiple flows, pass a directory to lfx serve to load all .json flows at server startup:

bash

uv run lfx serve ./flows/

Start the server

Call the server

Serve multiple flows

See also