How to build the full code-server Proxy URL for absProxyBasePath when using code-server as a devcontainer feature #21227

Buk1m · 2025-12-11T16:01:21Z

Buk1m
Dec 11, 2025

Hi!
I use the awesome coder tool mostly to spin up devcontainers with code-server access for application that require the absproxy.

The issue is I need to pass the absProxyBasePath value to the ghcr.io/coder/devcontainer-features/code-server:1 feature options and know it before the devcontainer is built.

So the full path when using code-server proxy within coder workspace is for example:
https://coder.my.instance.com/@user-name/workspace-name.workspace-folder/apps/**abc123**-code-server/absproxy/3000

I can dynamically build most of it for the absProxyBasePath as

"absProxyBasePath": "/@${localEnv:CODER_WORKSPACE_OWNER_NAME}/${localEnv:CODER_WORKSPACE_NAME}.${localWorkspaceFolderBasename}/apps/abc123-code-server",

But I cannot find in docs or figure out what value is used for the abc123 hash part in abc123-code-server.

Is there a variable that can be used here or exposed through the terraform template? Or maybe there is a better way to get the path for absProxyBasePath?

It seems to be persistent across multiple workspaces and only changed when I used a different repository as base.

Any help will be greatly appreciated!

For context this is my devcontainer.json:

{
  "name": "Devcontainer",
  "image": "node:24-bookworm-slim",
  "workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}",
  "features": {
    "ghcr.io/coder/devcontainer-features/code-server:1": {
      "port": 13337,
      "host": "0.0.0.0",
      "disableTelemetry": true,
      "disableUpdateCheck": true,
      "disableFileUploads": true,
      "disableFileDownloads": true,
      "workspace": "/workspaces/${localWorkspaceFolderBasename}",
      "absProxyBasePath": "/@${localEnv:CODER_WORKSPACE_OWNER_NAME}/${localEnv:CODER_WORKSPACE_NAME}.${localWorkspaceFolderBasename}/apps/edfpe135-code-server",
      "auth": "none",
      "appName": "AppName",
    }
  },
  "containerEnv": {
    "CODER_AGENT_URL": "${localEnv:CODER_AGENT_URL}",
    "CODER_WORKSPACE_AGENT_NAME": "${localEnv:CODER_WORKSPACE_AGENT_NAME}",
    "CODER_WORKSPACE_FOLDER_NAME": "${localWorkspaceFolderBasename}",
    "CODER_WORKSPACE_NAME": "${localEnv:CODER_WORKSPACE_NAME}",
    "CODER_WORKSPACE_OWNER_NAME": "${localEnv:CODER_WORKSPACE_OWNER_NAME}",
    "CODER_AGENT_ID": "${localEnv:CODER_AGENT_ID}",
    "RUNNING_IN_CONTAINER": "true",
    "VSCODE_PROXY_URI": "${localEnv:VSCODE_PROXY_URI}",
    "DEVCONTAINER_ID": "${devcontainerId}"
  }
}

Answered by DanielleMaywood

Dec 11, 2025

I should also add it is possible to ensure a stable subAgent.Name by providing a name in the devcontainer.json via customizations:

{
  "customizations": {
    "coder": {
      "name": "<name-here>"
    }
  }
}

View full answer

DanielleMaywood · 2025-12-11T16:27:33Z

DanielleMaywood
Dec 11, 2025
Collaborator

We should definitely aim to improve this in the future. At the moment we compute a hash for each app as apps have to have a unique name for a workspace (multiple agents using the same name for an app causes issues).

To work around this we compute a hash, you can see this here:

coder/coderd/agentapi/subagent.go

Lines 170 to 177 in 6f86f67

    
           // NOTE(DanielleMaywood): 
        
           // Slugs must be unique PER workspace/template. As of 2025-06-25, 
        
           // there is no database-layer enforcement of this constraint. 
        
           // We can get around this by creating a slug that *should* be 
        
           // unique (at least highly probable). 
        
           slugHash := sha256.Sum256([]byte(subAgent.Name + "/" + app.Slug)) 
        
           slugHashEnc := base32.HexEncoding.WithPadding(base32.NoPadding).EncodeToString(slugHash[:]) 
        
           computedSlug := strings.ToLower(slugHashEnc[:8]) + "-" + app.Slug

We haven't got any backwards compatibility guarantees on this hash but there is currently nothing planned for changing it.

3 replies

mafredri Dec 11, 2025
Collaborator

We should expose this as an environment variable that can be used in the devcontainer.json. perhaps CODER_WORKSPACE_APP_SLUG_PREFIX.

DanielleMaywood Dec 11, 2025
Collaborator

Is this possible though? It differs per slug

mafredri Dec 11, 2025
Collaborator

Good point, I suppose we'd have to introduce one per slug. So essentially, CODER_WORKSPACE_APP_SLUG_CODE_SERVER?

DanielleMaywood · 2025-12-11T16:33:02Z

DanielleMaywood
Dec 11, 2025
Collaborator

I should also add it is possible to ensure a stable subAgent.Name by providing a name in the devcontainer.json via customizations:

{
  "customizations": {
    "coder": {
      "name": "<name-here>"
    }
  }
}

1 reply

matifali Dec 11, 2025
Maintainer

This could really work and allow us to reuse all registry.coder.com modules if we pair it with coder_devcontainer, also exposing the subagent's agent_id in Terraform.

Buk1m · 2025-12-12T10:29:13Z

Buk1m
Dec 12, 2025
Author

Thank you very much for your fast and detailed answers. Using the name customization and your code I'm now able to predict the same slug value that will be in the URL. (assuming app.Slug part as code-server) Go Snippet

I've tried to calculate the slug within a terraform template to provide it as ENV variable and archive fully automatic process, but the lack of a working base32 encode function is an issue (tried few custom providers but all had problems, maybe I will write my own, but I have to learn more about Terraform). My idea was to use a part of workspace id as name to calculate unique slug within terraform and expose both as ENV. I could then use the slug in absProxyBasePath and provide the name to customizations.coder.name.

Suggetsions

Anyway it would be awesome if you could expose the value somehow or maybe give an option to the code-server feature to append the part [slug]-code-server to the absProxyBasePath automatically (just an idea, not sure at which stage you are calculating it).

It would be enough to provide only the part as the absProxyBasePath:

/@${localEnv:CODER_WORKSPACE_OWNER_NAME}/${localEnv:CODER_WORKSPACE_NAME}.${localWorkspaceFolderBasename}/apps/

Otherwise maybe a function to calculate and provide the slug within the terraform template so the coderd can use an predefined value if provided.
I know there is a config for the code-server when it is installed within the main container using the startup_script.

 resource "coder_app" "code-server" {
   agent_id     = coder_agent.main.id
   slug         = "code-server"
   display_name = "code-server"
   url          = "http://localhost:13337/?folder=/home/${local.username}"
   icon         = "/icon/code.svg"
   subdomain    = false
   share        = "owner"
   (...)
 }

I assume it does not affect the devcontainer feature tho, but maybe a similar approach would work.

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