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Can a stack have several projects? It seems a little odd to see already existing stacks suggested for new projects (my understanding of a project is just an index file and a mapped runtime (i.e. nodejs).

No.  A stack is an instance of one project.

&gt; It seems a little odd to see already existing stacks suggested for new projects
Where are you seeing this?  As written, that indeed seems odd.

&gt;  my understanding of a project is just an index file and a mapped runtime (i.e. nodejs).
That's right - a project is a `Pulumi.yaml` file (which includes what runtime to use, a name for the project, and optionally a pointer to a non-default location for the entrypoint to load using the language runtime).

When running `pulumi up` on a new project (or one without an associated stack) a prompt to select a stack is given. It defaults to an already-associated stack. I always scroll down and type a  new name.

Oh - is the project name the same across these?  The identity of the project is based on project name - and if you have multiple projects with the same "name", you may see this?

No. The project names are all different.
I have a tree like this:
``````

```00-base/
  Pulumi.yaml
01-identity/
  Pulumi.yaml
02-platform/
  Pulumi.yaml```

And just to make sure - are the `name` fields in those `Pulumi.yaml` files each different?

<@UB3BGTV63> same issue is described here: <https://github.com/pulumi/pulumi/issues/2814#issuecomment-512723246>

that happens for gcs and s3 backends (when projects share the same S3/GCS path for state)

same path is used to be able to use StackReference which works well only with pulumi service backend where you can refer to stack by org/project/stack. For GCS/S3 you can refer only by stack name and both stack states should be under the same path

Ahh - yes - if this is a local/s3/GCS backend that could definitely be the issue. 

FWIW - this issue is more directly tracking fixing this: <https://github.com/pulumi/pulumi/issues/2522|https://github.com/pulumi/pulumi/issues/2522>

side note: it's better to make `backend` to be a stack property, not project property.

Agreed - backend should very much be associated with a stack not a project. 