bug: referencing agent tuple in module throws confusing error

[stirby]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

This is likely a case where we need to improve docs/error message.

There's a strange indexing error that gets thrown when the coder_agent is assigned a count and must be referenced with a tuple, but a module referencing it does contain a count attribute.

With the following agent and module definition...

resource "coder_agent" "dev" {
  count              = data.coder_workspace.me.start_count
  arch               = "amd64"
  auth               = "token"
  ...
}

module "claude-code" {
  source              = "registry.coder.com/modules/claude-code/coder"
  version             = "1.0.31"

  agent_id            = coder_agent.dev[0].id

  folder              = "/home/coder"
  install_claude_code = true
  claude_code_version = "latest"
}

Terraform throws the following error:

Error: Invalid index
on main.tf line 339, in module "claude-code":
  339:   agent_id                = coder_agent.dev[0].id
    ├────────────────
    │ coder_agent.dev is empty tuple
The given key does not identify an element in this collection value: the collection has no elements.

---

Unless you specify count in the module, then no error is thrown:

module "claude-code" {
  count               = data.coder_workspace.me.start_count

  source              = "registry.coder.com/modules/claude-code/coder"
  version             = "1.0.31"

  agent_id            = coder_agent.dev[0].id

  folder              = "/home/coder"
  install_claude_code = true
  claude_code_version = "latest"
}

Relevant Log Output

Expected Behavior

I did not realize that we had to define the count, so this may be a discoverability issue.

Steps to Reproduce

1. Define coder_agent with count = data.workspace.<me>.start_count
2. Define module that requires `agent_id = coder_agent.[0].id
3. Build
4. Add count = data.workspace.<me>.start_count to module to resolve.

Environment

• Host OS: linux
• Coder version: v2.21.0

[matifali]

@stirby this is expected terraform behavior as count required for all resources and modules making use of a count.

[stirby]

I am moving this to internal as I agree with the expected behavior.

But @Emyrk wanted to investigate why that was the resulting error.

[github-actions]

❌ Failed to start the workspace. Please check the action logs for details.

[Emyrk]

@matifali Wait, if you reference a block that uses count, the calling block needs to have count as well?

Can you link to the TF docs on that?

[matifali]

@Emyrk I could not find any explicit docs on this, but this is my understanding from count in Terraform.
I guess that without a count, the dependent module block can not know if the coder_agent is created or not (i.e., the count can be "0" for coder_agent), which causes this error the collection has no elements.. As Terraform may be trying to create both the module and the coder_agent in parallel, and coder_agent may not have been created yet.

I am not sure if I was able to explain this or made it more confusing.