Swarm builder

The swarm builder checks that an agent config PR builds before an operator merges it. Each time an agent-configs/<agent> PR opens, reopens or gets a new push, the builder builds the PR's head on the agent base, and the result shows as a swarm-builder commit status on the PR and as a build_agent node on the swarm UI's Jobs page.

Where it runs and how it's enabled: services.md.

What happens on a config PR

1. The forge delivers the pull_request event on the config-pr webhook swarm-controller already registers on agent-configs. An opened, reopened or synchronized action on an open PR queues one build_agent node carrying the agent, the head commit and the PR number. An edit, a label or a close builds nothing. Every build_agent node needs the one builder slot, so builds run one at a time and the rest wait pending. A newer head cancels its PR's pending build; a build already running finishes and writes its status on its own, older commit.

2. The node sets the status to pending, then sends the build as a request on the queue subject $SWARM.builder.build and waits up to 61 minutes for the reply. The builder runs one build at a time and refuses a request that has waited 30 seconds for its turn, such as one behind a build a controller restart orphaned: a build started later could still run when the 61 minutes end. Reading the forge token before a build has 20 seconds, and the rest of the minute covers the reply's trip back.

3. The builder runs nix build of nix/builder/check-flake's check output with every input overridden:

   • hyperhive and nixpkgs: the sources the builder's own host was deployed with (c0re.hyperhiveFlake, c0re.nixpkgsFlake). A hive's meta flake pins hyperhive by forge URL and rev instead.
   • agent: git+https://<forge>/agent-configs/<agent>?ref=refs/pull/<pr>/head&rev=<head>. A build has one hour.

4. The reply is the system closure's store path, or the last lines nix printed. The node writes the status on the head commit:

   Outcome	Status	Description	Node
   built	success	the store path	done
   nix failed or timed out	failure	the last line nix printed	failed
   the builder busy for 30 seconds	failure	why	failed
   no builder listening, or no reply in time	error	why	failed

   The controller records every build that gets a final status, and the status links to that record: see Build records. The full tail is also in the failed node's error on the Jobs page and in the builder's journal line for the build, as the field F_ERROR_TAIL:

   $ swarm-logs query '_time:7d _SYSTEMD_UNIT:="swarm-builder.service" _msg:="agent config build failed" | fields _time, F_AGENT, F_PR, F_REV, F_ERROR_TAIL' --format json
   

   The builder holds the last 64 KiB of each of nix's output streams while the build runs, and the reply is the last 30 lines of that (at most 4000 bytes), so a long build log never grows its memory.

Build records

swarm-controller records each build when it writes the final status: built, failed, or no build ran or came back. The newest 100 are one JSON array under the key recent in the builder-runs KV bucket on the swarm queue, which only the controller reads or writes. It creates the bucket on first use and adds each build with a compare-and-set write, so the records survive a controller restart. Recording the 101st drops the oldest. A record the controller can't decode stays in the bucket, out of the list, and counts toward the 100.

Each record has:

• id: <agent>-<first 12 characters of the commit>-<started_unix>
• repo (agent-configs/<agent>), pr and head_sha
• started_unix and finished_unix
• outcome: built, failed or error, the status's success, failure or error
• out_path for built, error_tail (the last 30 lines nix printed) for failed, and detail (why) for error

GET /api/builder/runs lists them, newest first, and GET /api/builder/runs/<id> returns one, or 404 when it isn't among the newest 100. Both are behind the swarm UI vhost's admins gate, and answer 503 when the controller has no swarm queue or can't open the bucket, 500 when it can't read it.

The final status links to https://<swarm domain>/builds?run=<id>, the swarm UI's builds page, so selecting Details on a red swarm-builder check opens that build's card, with nix's tail, in the browser. The raw record stays at GET /api/builder/runs/<id>. A status has no link when the record wasn't stored (the controller logs the failed write) or when the controller doesn't serve the swarm vhost (SWARM_CONTROLLER_PUBLIC_URL unset). The pending status has no link.

What the check is, and isn't

• It composes the config the way a hive's meta flake does (agent-base.extendModules with the config's nixosModules.default), minus what a hive adds: its CA bundle, claude-code path, ports and OTEL settings, and the follows that point the config's own inputs at the hive's. A green check can therefore still fail on a hive, and the reverse.
• The agent base is the builder's host's, not the hive's. Hives on another hyperhive or nixpkgs pin build something else.
• The builder pushes nothing. The closure stays in the builder host's store until it's collected, so on a host that also runs the hive, the hive's own build can reuse whatever matches.
• It doesn't gate anything. A merge deploys whatever the status says.
• A lost webhook delivery isn't retried: the head isn't built until the next push. The jobs graph is in memory, so a controller restart drops a build in flight and leaves its status pending.

Archetype evaluation

The builder also answers $SWARM.builder.archetypes: it evaluates the meta of every agent archetype its own hyperhive source declares, with no forge token, and answers with the JSON. An evaluation doesn't wait for a build in progress and has two minutes. What the controller does with the answer: archetypes.md.

Credentials

The token belongs to the builder forge account, which swarm-controller creates, puts in the hives team of agent-configs (read on every config repo) and mints a read:repository token for, in a five-minute pass. git gets the token through a credential helper reading the child's environment: it's in no argument, URL or lock, so no line nix prints can carry it into a status. The same git fetches a config's own inputs, so the builder registers the helper for the forge's URL only, and the helper answers only an https request for the forge's host: any other host gets nothing. The builder holds no write credential; swarm-controller writes the status with its own forge token. Details per path: credentials.md.

Isolation

The container has its own network namespace on the bridge, as the CI runner does, so a config's evaluation can't reach a host-loopback service. Evaluation is pure and runs in the container. Builds go to the host's nix daemon as an untrusted client: a config can set no daemon option, and its derivations build in the host's sandbox. A fixed-output derivation still has the network, as it does for any build on that host.