# Web Function URLs | Modal Docs

- **URL:** https://modal.com/docs/guide/webhook-urls
- **Summary:** This guide documents the behavior of URLs for Web Functions on Modal: automatic generation, configuration, programmatic retrieval, and more.

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Web Function URLs
=================

This guide documents the behavior of URLs for [Web Functions](https://modal.com/docs/guide/webhooks)
 on Modal: automatic generation, configuration, programmatic retrieval, and more.

Determine the Web Function URL from code 

Modal Functions with the [`fastapi_endpoint`](https://modal.com/docs/sdk/py/latest/modal.fastapi_endpoint)
, [`asgi_app`](https://modal.com/docs/sdk/py/latest/modal.asgi_app)
, [`wsgi_app`](https://modal.com/docs/sdk/py/latest/modal.wsgi_app)
, or [`web_server`](https://modal.com/docs/sdk/py/latest/modal.web_server)
 decorator are made available over the Internet when they are [`serve`d](https://modal.com/docs/cli/latest/serve)
 or [`deploy`ed](https://modal.com/docs/cli/latest/deploy)
 and so they have a URL.

This URL is displayed in the `modal` CLI output and is available in the Modal [dashboard](https://modal.com/apps)
 for the Function.

To determine a Function’s URL programmatically, check its [`get_web_url()`](https://modal.com/docs/sdk/py/latest/modal.Function#get_web_url)
 property:

    @app.function(image=modal.Image.debian_slim().pip_install("fastapi[standard]"))
    @modal.fastapi_endpoint(docs=True)
    def show_url() -> str:
        return show_url.get_web_url()

For deployed Functions, this also works from other Python code! You just need to do a [`from_name`](https://modal.com/docs/sdk/py/latest/modal.Function#from_name)
 based on the name of the Function and its [App](https://modal.com/docs/guide/apps)
:

    import requests
    
    remote_function = modal.Function.from_name("app", "show_url")
    remote_function.get_web_url() == requests.get(handle.get_web_url()).json()

Auto-generated URLs 

By default, Modal Functions will be served from the `modal.run` domain. The full URL will be constructed from a number of pieces of information to uniquely identify the endpoint.

At a high-level, Web Function URLs for deployed Apps have the following structure: `https://<source>--<label>.modal.run`.

The `source` component represents the Workspace and Environment where the App is deployed. If your Workspace has only a single Environment, the `source` will just be the Workspace name. Multiple Environments are disambiguated by an [“Environment suffix”](https://modal.com/docs/guide/environments#environment-web-suffixes)
, so the full source would be `<workspace>-<suffix>`. However, one Environment per Workspace is allowed to have a null suffix, in which case the source would just be `<workspace>`.

The `label` component represents the specific App and Function that the URL routes to. By default, these are concatenated with a hyphen, so the label would be `<app>-<function>`.

These components are normalized to contain only lowercase letters, numerals, and dashes.

To put this all together, consider the following example. If a member of the `ECorp` Workspace uses the `main` Environment (which has `prod` as its web suffix) to deploy the `text_to_speech` App with a webhook for the `flask-app` Function, the URL will have the following components:

*   _Source_:
    *   _Workspace name slug_: `ECorp` → `ecorp`
    *   _Environment web suffix slug_: `main` → `prod`
*   _Label_:
    *   _App name slug_: `text_to_speech` → `text-to-speech`
    *   _Function name slug_: `flask_app` → `flask-app`

The full URL will be `https://ecorp-prod--text-to-speech-flask-app.modal.run`.

User-specified labels 

It’s also possible to customize the `label` used for each Function by passing a parameter to the relevant Web Function decorator:

    import modal
    
    image = modal.Image.debian_slim().pip_install("fastapi")
    app = modal.App(name="text_to_speech", image=image)
    
    
    @app.function()
    @modal.fastapi_endpoint(label="speechify")
    def web_endpoint_handler():
        ...

Building on the example above, this code would produce the following URL: `https://ecorp-prod--speechify.modal.run`.

User-specified labels are not automatically normalized, but labels with invalid characters will be rejected.

Ephemeral Apps 

To support development workflows, webhooks for ephemeral Apps (i.e., Apps created with `modal serve`) will have a `-dev` suffix appended to their URL label (regardless of whether the label is auto-generated or user-specified). This prevents development work from interfering with deployed versions of the same App.

If an ephemeral App is serving a Web Function while another ephemeral App is created seeking the same label, the new Function will _steal_ the running Function’s label.

This ensures that the latest iteration of the ephemeral Function is serving requests and that older ones stop receiving web traffic.

Truncation 

If a generated subdomain label is longer than 63 characters, it will be truncated.

For example, the following subdomain label is too long, 67 characters: `ecorp--text-to-speech-really-really-realllly-long-function-name-dev`.

The truncation happens by calculating a SHA-256 hash of the overlong label, then taking the first 6 characters of this hash. The overlong subdomain label is truncated to 56 characters, and then joined by a dash to the hash prefix. In the above example, the resulting URL would be `ecorp--text-to-speech-really-really-rea-1b964b-dev.modal.run`.

The combination of the label hashing and truncation provides a unique list of 63 characters, complying with both DNS system limits and uniqueness requirements.

Custom domains 

For more customization, you can use your own domain names with Web Functions. If your [plan](https://modal.com/pricing)
 supports custom domains, visit the [Custom Domains tab](https://modal.com/settings/custom-domains)
 in your Workspace settings to add a domain name to your Workspace.

You can use three kinds of domains with Modal:

*   **Apex:** root domain names like `example.com`
*   **Subdomain:** single subdomain entries such as `my-app.example.com`, `api.example.com`, etc.
*   **Wildcard domain:** either in a subdomain like `*.example.com`, or in a deeper level like `*.modal.example.com`

You’ll be asked to update your domain DNS records with your domain name registrar and then validate the configuration in Modal. Once the records have been properly updated and propagated, your custom domain will be ready to use.

You can assign any Modal Web Function to any registered domain in your Workspace with the `custom_domains` argument.

    import modal
    
    app = modal.App("custom-domains-example")
    
    
    @app.function()
    @modal.fastapi_endpoint(custom_domains=["api.example.com"])
    def hello(message: str):
        return {"message": f"hello {message}"}

You can then run `modal deploy` to put your Web Functions online, live.

    $ curl -s https://api.example.com?message=world
    {"message": "hello world"}

Note that Modal automatically generates and renews TLS certificates for your custom domains. Since we do this when your domain is first accessed, there may be an additional 1-2s latency on the first request. Additional requests use a cached certificate.

You can also register multiple domain names and associate them with the same Web Function.

    import modal
    
    app = modal.App("custom-domains-example-2")
    
    
    @app.function()
    @modal.fastapi_endpoint(custom_domains=["api.example.com", "api.example.net"])
    def hello(message: str):
        return {"message": f"hello {message}"}

For **Wildcard** domains, Modal will automatically resolve arbitrary custom endpoints (and issue TLS certificates). For example, if you add the wildcard domain `*.example.com`, then you can create any custom domains under `example.com`:

    import random
    import modal
    
    app = modal.App("custom-domains-example-2")
    
    random_domain_name = random.choice(range(10))
    
    
    @app.function()
    @modal.fastapi_endpoint(custom_domains=[f"{random_domain_name}.example.com"])
    def hello(message: str):
        return {"message": f"hello {message}"}

Custom domains can also be used with [ASGI](https://modal.com/docs/sdk/py/latest/modal.asgi_app#modalasgi_app)
 or [WSGI](https://modal.com/docs/sdk/py/latest/modal.wsgi_app)
 apps using the same `custom_domains` argument.

[Web Function URLs](https://modal.com/docs/guide/webhook-urls#web-function-urls)
[Determine the Web Function URL from code](https://modal.com/docs/guide/webhook-urls#determine-the-web-function-url-from-code)
[Auto-generated URLs](https://modal.com/docs/guide/webhook-urls#auto-generated-urls)
[User-specified labels](https://modal.com/docs/guide/webhook-urls#user-specified-labels)
[Ephemeral Apps](https://modal.com/docs/guide/webhook-urls#ephemeral-apps)
[Truncation](https://modal.com/docs/guide/webhook-urls#truncation)
[Custom domains](https://modal.com/docs/guide/webhook-urls#custom-domains)