fix(envs): install the Defang CLI in the background, not on first boot

[defangdevs]

What happened

defangCli (from #365/#373's own investigation) was a plain environment.systemPackages entry, so its Go build was part of system.build.toplevel — realized during the very first nixos-rebuild switch, competing with everything else amazon-init does on first boot. On t4g.small (2 GiB) that OOM-killed the compile and bricked first boot; the automatic retry (issue #112) failed identically since the build never got far enough to activate anything.

Why "just trust Cachix" isn't enough

DefangLabs/defang#2236 (merged) now publishes an aarch64-linux build to https://defanglabs.cachix.org — the architecture this fleet actually runs. But adding nix.settings.substituters to this module alone does not fix first boot: that setting only reaches /etc/nix/nix.conf during activation, which runs after the build that would need it. The very switch that introduces the setting still builds under the old nix.conf. Verified this the hard way while working on the fix — happy to walk through it if useful.

The fix

Stop building defangCli at NixOS-eval time at all:

• defangCliExpr splices pkgs/defang/cli.nix's own fetchTarball+callPackage calls as literal text into a generated .nix file, instead of interpolating a derivation — that distinction is what keeps the actual Go build out of system.build.toplevel's closure. (A pkgs.writeText derivation for a small text file still rides along, which is fine.)
• agent-box-defang-cli.service (wantedBy = multi-user.target, so it starts after activation has already written the new nix.conf, on the same first boot) evaluates and builds that file itself, at runtime — by which point the Cachix substituter is already trusted.
• It also builds with DefangLabs/defang's own locked nixpkgs revision, not this module's pkgs — a different nixpkgs produces a different derivation hash regardless of substituter trust, which would silently miss the cache. Verified by hand (2026-08-26): the resulting output path already 200s on https://defanglabs.cachix.org/<hash>.narinfo.
• connectBins (the sign-in card) and agentBoxExecSearchPath (the agent PATH) now point at a fixed install location (/var/lib/agent-box-defang-cli/bin) instead of a store path, so neither forces the build back into the closure via string context — same pattern already used for "a card exists only where its binary does."
• The service caps itself (Nice, IOSchedulingClass=idle, OOMScoreAdjust=1000, MemoryMax=1536M) so a cache-miss fallback compile (Cachix outage, or a defangVersion bump before the matching nixpkgs pin lands) can't repeat the OOM in the background — systemd kills just this cgroup and multi-user.target retries it next boot, instead of earlyoom picking a random victim.

Verification

• .#checks.x86_64-linux.vm-closure (the full bootable system, "is it actually usable" proof) builds successfully with no defang-cli-git/buildGoModule derivation in the closure — only the trivial generated expression file and unit script.
• .#checks.x86_64-linux.golden-snapshot, module-generated-up-to-date, module-single-file, multi-user, download-route, session-route, webhook-route, registry-protocol, runtime-profile, agentbox-render all pass.
• Manually confirmed the exact pinned expression (defang v3.15.0 + its own locked nixpkgs) resolves to a store path Cachix already has, via curl -sI https://defanglabs.cachix.org/<hash>.narinfo → 200.

Test plan

• deploy-test CI (ipv4-full leg, the one that's been OOMing since feat(envs,web): ship the Defang CLI and a Connections sign-in card #365) goes green
• On a real box: confirm agent-box-defang-cli.service completes shortly after first boot and defang resolves on PATH in a fresh session

Fixes #373

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📝 Walkthrough

Walkthrough

Defang CLI packaging moves from eager system evaluation to a background systemd oneshot. A pinned runtime expression builds and installs the CLI under /var/lib/agent-box-defang-cli, while agent paths and connection-card discovery use that location.

Changes

Defang runtime installation

Layer / File(s)	Summary
Pinned runtime build inputs modules/agent-box.nix, modules/agent-box.nix.in, tests/golden/DUPLICATES, tests/golden/vm/payloads/agent-box-defang-cli-expr.nix	Pinned Defang and nixpkgs inputs are rendered into a runtime Nix expression. Defang is removed from the eager base-tool package list.
Background CLI installer modules/agent-box.nix, modules/agent-box.nix.in, tests/golden/vm/payloads/unit-script-agent-box-defang-cli-start/bin/agent-box-defang-cli-start, tests/golden/vm/units/agent-box-defang-cli.service, tests/sessions.nix	An enabled oneshot service waits for networking, retries transient failures, rebuilds when the expression changes, and links the CLI into /var/lib/agent-box-defang-cli. Nix cache trust and resource limits are configured. Session tests now poll for the expected worker environment.
Runtime path integration modules/agent-box.nix, modules/agent-box.nix.in, tests/golden/vm/units/agent-box@agent.service, tests/golden/web/units/agent-box-settings@agent.service, tests/golden/web/units/agent-box@agent.service	Agent search paths and Defang sign-in discovery use the fixed runtime directory instead of the Nix store path.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to 005c4

The change moves Defang CLI installation out of first-boot system construction and applies bounded background resource limits; no actionable merge-blocking risk remains based on the supplied current-head evidence.

Sequence Diagram(s)

sequenceDiagram
  participant systemd as agent-box-defang-cli.service
  participant installer as agent-box-defang-cli-start
  participant nix as Nix
  participant agent as Agent service
  systemd->>installer: Start after networking
  installer->>nix: Build pinned Defang expression
  nix-->>installer: Return CLI build
  installer->>agent: Expose CLI through runtime PATH
  agent->>agent: Resolve Defang connection-card binary

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Suggested reviewers: lionello, claude

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The Defang CLI packaging, service, runtime paths, and golden fixtures are related to issue #373. The changes to tests/sessions.nix address an unrelated profile-environment race and are outside the lin…	Move the tests/sessions.nix race fix to a separate pull request, or link an issue that explicitly includes this test change and its requirements.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the primary change: Defang CLI installation moves to a background process instead of first-boot installation.
Description check	✅ Passed	The description directly explains the first-boot OOM problem, the background systemd installation design, runtime path updates, resource limits, retries, and verification.
Linked Issues check	✅ Passed	The changes address issue #373 by removing the Defang CLI Go build from the first-boot system closure and installing it after activation through a constrained, retrying systemd service.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…

Full details: Out of Scope Changes check

Explanation

The Defang CLI packaging, service, runtime paths, and golden fixtures are related to issue #373. The changes to tests/sessions.nix address an unrelated profile-environment race and are outside the linked issue scope.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

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Inline comments:
In `@modules/agent-box.nix.in`:
- Around line 2766-2789: Update the agent-box-defang-cli.service definition in
modules/agent-box.nix.in at lines 2766-2789 to add on-failure retry behavior
with a restart delay and replace the existence-only ConditionPathExists check
with a stamp-file check that compares the recorded value against defangCliExpr
before skipping installation. Do not edit modules/agent-box.nix at lines
13474-13497 directly; regenerate it with nix run .#assemble and commit the
resulting output.

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• modules/agent-box.nix
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• tests/golden/vm/payloads/agent-box-defang-cli-expr.nix
• tests/golden/vm/payloads/unit-script-agent-box-defang-cli-start/bin/agent-box-defang-cli-start
• tests/golden/vm/units/agent-box-defang-cli.service
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[defangdevs]

CI failure on run 33016906374 (vm-test-run-agent-box-sessions) was not caused by this PR's own diff — tests/sessions.nix is untouched by either commit here, and master's last 5 CI runs all passed.

Root cause: the "worker" profile-env assertion (added for #321) reads #{pane_pid}'s /proc/<pid>/environ immediately after has-session succeeds. But has-session answers before the pane's <env-exec wrapper> <shell> has necessarily finished exec'ing, so the loaded environment can still be sitting on a child of the pane pid — exactly the race the "pemsess" assertion right above it already hit and fixed (CI 32649971831), just never applied to this later block. This run happened to lose it:

AssertionError: SHELL=/run/current-system/sw/bin/bash
...

(PROFILE_TOKEN=sekret missing from the captured environ.)

Pushed 005c49e, applying the same pane_pid + child polling pattern the pemsess test uses. check-testscript.sh (ty/ruff) and nix eval .#checks.x86_64-linux.sessions.drvPath both pass; the actual VM test needs x86_64 (this box is aarch64), so CI is the real verification here.