> 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.
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