Sandboxed-Agent Git Relay (host-side push/PR with a GitHub App)

The agent in the sandbox can git commit (commits need no secret). Everything that needs a credential — push and PR creation — is done by a tiny relay on the host, reading the same .git through the bind mount. The sandbox never holds a token, a key, or even a forwarded socket; it cannot push at all.

[sandbox: Claude]  commit on claude/<topic>           (bind-mounted worktree)
        │ (filesystem — no network, no creds; merge signal = "Relay-Merge: yes"
        │  trailer in the HEAD commit message, also just a commit)
[host: systemd timer, 60s] relay.mjs:
        scan refs/heads/claude/* → diff vs origin/main? → mint 1h App token
        → push exact refspec → ensure PR (idempotent)
        → if HEAD has the Relay-Merge trailer: squash-merge once checks pass
        REFUSE: non-claude/* refs, force push (diverged), main
[GitHub] ruleset on main (PR + required check, bypass_actors: []) = server-side backstop
         (also the merge gate: an early merge attempt gets 405 → retried next tick)

This is the pattern Anthropic documents for agent deployments ("a proxy outside the agent's security boundary injects credentials; the agent never sees them") — applied to git with zero in-sandbox moving parts.

When to use this skill

• Autonomous agent working in an egress-restricted devcontainer must produce PRs unattended
• The project rule is "no plaintext credentials inside the sandbox" (cf. ADR-style secrets strategy)
• NOT when a human pushes anyway (just keep push on the host), and NOT for fork-based workflows

Why a GitHub App (and not the alternatives) — verified 2026-06

Setup

1. Create the App (human, browser): github.com/settings/apps/new → name like <project>-relay; Homepage URL = the repo; uncheck Webhook Active; Repository permissions: Contents: Read and write, Pull requests: Read and write — and deliberately no Workflows permission: withholding it means the agent pipeline cannot modify its own CI gate (pushes touching .github/workflows/** get a clean REFUSE; land those with a human-credentialed push — see "Relay policy decisions" and references/github-app-setup.md); "Only on this account". Then Generate a private key (.pem downloads) and Install App → only the target repo.
2. Store the key on the host, outside every container mount: ~/.config/<project>-relay/app.pem, dir 700 / file 600. Sanity: openssl rsa -in app.pem -check -noout.
3. Record IDs in config.env next to the key — see references/github-app-setup.md for the config template, the JWT smoke test, and the App ID vs Installation ID trap (the number in the install URL is the installation id; the App ID is on the App settings page; GET /apps/<slug> 404s for private apps, and /user/installations needs an App user token — so verify by minting a JWT and calling GET /app + GET /app/installations).
4. Install the relay: references/relay-mjs.md → ~/.config/<project>-relay/relay.mjs. ⚠️ Keep it outside the repo — the repo is writable from the sandbox, so in-repo policy code could be edited by the thing it polices.
5. systemd user units: references/systemd-units.md (60s timer; set AccuracySec=10s or the default 1-minute coalescing turns your 60s timer into ~2min ticks). Validate manually first: set -a; . config.env; set +a; node relay.mjs → expect an idle log line.
6. Server-side backstop (defense in depth, also the merge gate): ruleset on main with bypass_actors: [] and delete_branch_on_merge=true — see the cloudflare-workers-builds-keyless-deploy skill's references/ruleset.md (works standalone).
7. Sandbox side: allow git add/commit/checkout/switch in .claude/settings.json, keep Bash(git push:*) in deny (deny rules hold even in bypassPermissions mode); set a distinguishable git identity in the container (git config --global user.name "Claude (<project> sandbox)"); convention: all agent work on claude/<topic> branches.
8. Sandbox reads PR/CI status with unauthenticated curl, not gh: gh refuses to run without a token even against public repos, and putting GH_TOKEN in the sandbox would break the whole premise. For a public repo, unauthenticated REST is enough (60 req/h/IP): curl -s https://api.github.com/repos/<owner>/<repo>/commits/<branch>/check-runs for CI results, .../pulls?head=<owner>:<branch> for PR state. Add the matching Bash(curl -s https://api.github.com/*) allow entry; the egress firewall already covers GitHub's IP ranges.

Relay policy decisions (encoded in relay.mjs)

• Prefix-only: scans refs/heads/claude/ and pushes refs/heads/X:refs/heads/X exact refspecs. main is structurally unreachable.
• No force, ever: if the remote branch is not an ancestor of the local tip, log REFUSE and skip — divergence is for a human to resolve. ⚠️ The usual trigger is the agent --amending or rebasing a commit the relay already pushed — the rewritten SHA is no longer a descendant of the pushed tip, so every tick REFUSEs and the PR is stuck (the relay won't force). Recover forward, never force: git reset --hard <pushed-sha>, then add a fast-forward follow-up commit (--allow-empty if it only needs to re-carry a Relay-Merge: yes trailer). Don't be lured into amending by a git log --format='%(trailers:…)' self-check showing the trailer empty: the relay matches the signal with a multiline regex on the full body (/^Relay-Merge: yes$/m), so a blank line before it or its position among other trailers is irrelevant — git's stricter trailer parser is not what detects it.
• Skip when no diff vs origin/main (three-dot diff): empty new branches produce no PR. ⚠️ This three-dot diff does NOT detect a squash merge — filtering squash residue is isTipAlreadyMerged's job (see "The squash-merge re-merge loop" below), not this check's.
• Token hygiene: minted per tick only when needed, down-scoped (repositories: [repo], contents+PR write), passed to git via env + an inline credential helper — never argv, never disk. Prepend -c credential.helper= (empty) first: without that reset, a globally configured gh credential helper silently wins and pushes as the user, not the App.
• Idempotent PR: GET /pulls?head=owner:branch before POST; reruns log "PR exists".
• Merge only on explicit signal: see next section. No trailer → the relay never merges; the human does.
• No workflows permission — on purpose: the token is contents+pull_requests only, so GitHub rejects any push touching .github/workflows/** (refusing to allow a GitHub App to create or update workflow ... without 'workflows' permission). This is a feature, not a gap: the agent pipeline cannot modify its own CI gate — the same gate that enforces CI-green-before-merge for the Relay-Merge path. The relay detects that one rejection and logs REFUSE <branch>: workflow-file change needs a human push (returning cleanly, not crashing the tick with exitCode=1 on every run). Land CI changes with a human-credentialed push (a user/gh token carrying the workflow scope): git push origin claude/<branch> → next tick the relay sees remote == local, skips the push, and opens the PR normally. Grant the App workflows: write only if you deliberately want agent-authored workflow edits — it hands the autonomous loop the power to weaken its own gate.

Optional: agent-initiated merge (the Relay-Merge: yes trailer)

By default the human merges every PR. To let the agent close the loop unattended (merge → deploy → verify), the relay also implements merge delegation — same philosophy as push: the signal comes from inside the boundary (a commit), the execution and the policy live outside it.

• Signal: the agent puts Relay-Merge: yes as a trailer in the HEAD commit message of the claude/* branch. A trailer on an older commit is void — pushing more commits requires re-signaling. This makes merge a deliberate, auditable, per-PR decision recorded in history (the trailer survives in the squash commit).
• Execution: the relay squash-merges the open PR, passing the local tip sha so GitHub rejects (409) if the branch moved in between (TOCTOU guard), then deletes the remote and local branch.
• Safety: CI-green is enforced server-side by the main ruleset — the relay doesn't need to (and doesn't) check CI itself. Signaling before CI finishes is safe: GitHub answers 405 ("Required status check ... is expected"), logged as merge pending, retried next tick. No extra App permission is needed (contents:write covers the merge endpoint).
• What you give up, knowingly: trailer'd PRs reach main (and any main-triggered deploy) with no human review. Mitigations: (a) the agent's own convention — don't add the trailer for risky or uncertain changes; (b) grow the ruleset's required checks (tests!) as the project matures. Record the governance change in an ADR; the revert path is "delete tryMerge" — ruleset, App, and sandbox config are untouched.

The squash-merge re-merge loop (the one that bites — fix is mandatory)

squash merge does not make the branch's commits ancestors of main; only their content lands, as one brand-new commit. So the three-dot diff origin/main...branch stays non-empty after a squash merge, and hasDiffAgainstMain alone judges the branch "still has unmerged work" forever. Combine that with tryMerge's post-merge local-branch delete failing whenever the sandbox still has the branch checked out, and the relay re-pushes → re-PRs → re-squash-merges the same branch every tick, minting an empty squash commit onto main (and firing the deploy) each time. This actually happened: 9 empty commits + 9 redundant deploys before it was caught (2026-06-13).

The guard — isTipAlreadyMerged(branch, sha), checked before push: ask GitHub whether a closed PR with merged_at set and head.sha === the current local tip already exists. If so, the content is already on main → skip push/PR/merge and delete the local branch (best-effort). Do not rely on tryMerge's cleanup alone: it cannot remove a checked-out branch, which is exactly the condition that ignites the loop. The three-dot hasDiffAgainstMain check stays (it cheaply filters truly-empty branches without an API call), but it is not the squash filter — this is.

Operational corollaries: (a) after signaling Relay-Merge, have the agent git switch off the branch so the local ref can be reaped next tick; (b) squash residue already on main (empty commits) can't be removed without rewriting protected history — leave it, it's inert. The guard makes the loop impossible regardless of (a).

Operations

systemctl --user list-timers <project>-relay.timer     # next tick
journalctl --user -u <project>-relay.service -f        # logs (one line per action)
systemctl --user start <project>-relay.service         # manual one-shot

Log vocabulary: pushed / PR created / PR exists / REFUSE / merge pending #N / merged PR #N / idle.

The human's recurring job is reviewing and merging PRs — governance, not secret handling. With the Relay-Merge trailer enabled, that becomes per-PR opt-out: no trailer → human merges, as before.

E2E acceptance test

1. In the container: git checkout -b claude/e2e-test, commit a doc tweak, git checkout main.
2. Within ~70s the journal shows pushed claude/e2e-test + PR created: <url>; PR author is app/<project>-relay.
3. CI runs on the PR with no approval prompt and goes green.
4. Merge (human) → branch auto-deleted; relay's next tick skips the residue via the isTipAlreadyMerged guard (a squash leaves the three-dot diff non-empty, so "no diff vs main" is not what saves you — see the squash-merge re-merge loop above); deploy pipeline (e.g. Workers Builds) fires off main.

Merge-delegation variant (proves the trailer path end-to-end):

1. In the container, on the open branch: git commit --allow-empty -m "chore: request merge" -m "Relay-Merge: yes".
2. Journal shows pushed (new tip) → merge pending #N (405 while CI re-runs — this confirms the server-side gate) → next tick merged PR #N, branch deleted both sides.
3. The whole loop — commit → push → PR → CI → merge → deploy — ran with zero human steps and zero sandbox credentials.