> This PR was authored by Mux on behalf of Mike.
## Summary
Adds support for multiple peer root workspace agents sharing the same
`auth_instance_id`, so AWS, Azure, and GCP instance-identity auth can
issue the correct session token for a selected agent instead of assuming
a
single root agent per instance.
## Problem
When a Terraform template attaches two or more `coder_agent` resources
(with `auth = "aws-instance-identity"`) to a single compute instance,
every agent shares the same cloud instance ID. The existing singular
lookup picks whichever agent was created most recently, silently
ignoring
the others.
## Solution
Introduce an optional pre-auth agent selector (`CODER_AGENT_NAME`) and
make the server-side lookup ambiguity-aware.
**Database layer:**
- `GetWorkspaceAgentsByInstanceID` (`:many`): returns all matching root
agents for an instance ID.
- `GetWorkspaceAgentByInstanceIDAndName` (`:one`): returns the named
root
agent for disambiguation.
**SDK and CLI:**
- `agent_name` field added to AWS, Azure, and GCP request structs
(`omitempty` for backward compatibility).
- `CODER_AGENT_NAME` env var and `--agent-name` flag wired into the
agent
bootstrap before instance-identity auth runs.
**Server handler (`handleAuthInstanceID`):**
- When `agent_name` is present: direct lookup by (instance ID, name).
- When absent: legacy lookup, then resource-scoped ambiguity check.
Returns 409 with available agent names if multiple root agents match.
- Whitespace-only names are trimmed and treated as unspecified.
- Sub-agents remain excluded (`parent_id IS NULL` filter).
**Verification template:**
- `examples/templates/aws-multi-agent/` provisions one EC2 instance with
two agents (`main` and `dev`), both using instance-identity auth with
`CODER_AGENT_NAME` set in the cloud-init user data.
## Backward compatibility
Existing single-agent deployments work unchanged. The `agent_name` field
is optional with `omitempty`, and the unnamed path preserves today's
behavior when only one root agent matches.
Fixes https://github.com/coder/internal/issues/933
Refactors CLI tests that check the `--auth` flag parsing for various public clouds into a unit test that just creates the agent Client and asserts on the type.
Testing that the agent client actually authenticates correctly with these auth types is well covered by Coderd tests, so we don't need to retread that ground here, and the deleted tests were flaky on Windows.
Refactors Agent instance identity to be a SessionTokenProvider.
Refactors the CLI to create Agent clients via a centralized function, rather than add-hoc via individual command handlers and their flags.
This allows commands besides `coder agent`, but which still use the agent identity, to support instance identity authentication.
Fixes#19111 by unifying all API requests to go thru the SessionTokenProvider for auth credentials.
Michael Suchacz