## Thinking Path
> - Paperclip is the open source app people use to manage AI agents for
work
> - Agent adapters (Claude, Codex, Gemini) default to the ACP engine
lane, which needs a live bidirectional stdio session with the agent
process
> - Sandbox execution targets only exposed one-shot command execution,
so every ACP-capable adapter refused remote targets and fell back to the
CLI lane with a "supports only the local Paperclip host" warning
> - Running agents in sandboxes is a core deployment mode, and losing
ACP there means losing streaming updates, structured events, and
default-lane parity with local runs
> - This pull request adds a provider-agnostic process-session bridge
that relays the ACP stdio session into the sandbox over the existing
sandbox runner contract, and updates the adapters to use it
> - The benefit is that the default ACP lane now behaves the same on the
local host and in any sandbox provider, with CLI fallback reserved for
targets that genuinely cannot host a bidirectional session
## Linked Issues or Issue Description
No existing public issue covers this; inline description following the
feature request template:
**Problem or motivation**
Configuring an ACP-capable adapter (e.g. Claude) with a sandbox
environment made every run fall back to the CLI lane with the warning
"Claude ACP currently supports only the local Paperclip host, but this
run targets a remote environment." The ACP engine only knew how to spawn
a local subprocess, while sandbox providers only expose one-shot command
execution — so there was no way to hold the bidirectional stdio session
ACP requires.
**Proposed solution**
Add a process-session bridge in `adapter-utils`: a local ACPX-spawnable
proxy script connects to a token-authenticated loopback TCP server,
which relays JSON-framed stdin/stdout/stderr events to and from a small
relay script executed inside the sandbox via the provider's ordinary
runner. Claude/Codex/Gemini adapters now treat sandbox targets with a
runner as ACP-capable, resolve agent commands against the remote target,
and fall back to CLI only when the sandbox exposes no bidirectional
path. The sandbox callback bridge injects a run-scoped API endpoint and
bridge token so the agent inside the sandbox can reach Paperclip
(including work-product handoffs) without ever receiving the host run
JWT.
**Alternatives considered**
A provider-specific lane was prototyped first: Daytona minting SSH
access metadata at lease time, converted into an SSH execution target.
It was dropped because it only worked for providers able to advertise
SSH, added per-provider surface area, and left every other sandbox
provider on the CLI fallback. The merged design rides the one-shot
runner contract all providers already implement; a regression test pins
that sandbox targets stay on the bridge lane even when lease metadata
advertises SSH access.
**Roadmap alignment**
Directly advances the "Cloud / Sandbox agents" roadmap item — agents
running in remote and sandboxed environments keep the same control-plane
behavior as local ones. No overlap with other planned core work.
## What Changed
- `packages/adapter-utils/src/execution-target.ts`: new
`startAdapterExecutionTargetProcessSessionBridge()` plus helpers —
writes a token-authenticated local proxy script (spawnable by ACPX) and
a remote relay script synced into the sandbox, with a loopback TCP
server streaming JSON-framed stdio between them; events emitted before
the ACP client attaches are buffered so none are lost.
- `packages/adapter-utils/src/acpx-engine/execute.ts`: the ACP engine
can execute against remote sandbox targets through the bridge instead of
requiring a local subprocess, including remote cwd/env shaping.
- `packages/adapter-utils/src/sandbox-callback-bridge.ts`:
sandbox-scoped API bridging extended to allow work-product handoffs; the
sandbox payload env carries a bridge token, never the host run JWT.
- `packages/adapters/claude-local`, `codex-local`, `gemini-local`
(`src/server/acp.ts`): default-lane selection no longer rejects all
remote targets; command resolution is remote-aware
(`ensureAdapterExecutionTargetCommandResolvable`,
`resolveAdapterExecutionTargetCwd`); the fallback reason is now scoped
to sandboxes that expose only one-shot execution.
- `server/src/__tests__/environment-execution-target.test.ts`: pins that
sandbox targets resolve to the bridge lane, including when lease
metadata advertises SSH access.
- Non-sandbox remote targets (e.g. SSH) keep the CLI lane: the ACP
engine's remote transport is sandbox-only, so default-lane selection
falls back for those targets across all three adapters, and tests
covering CLI-specific remote behavior pin `engine: "cli"` explicitly.
- The bridge authenticates loopback connections before they can own the
session or receive buffered output (token required, idle unauthenticated
peers dropped), and remote event writes are serialized so the exit event
always lands after stdout/stderr have drained.
- Daytona plugin: formatting-only residue from the earlier iteration; no
functional change.
## Verification
- `vitest run` over the touched suites —
`packages/adapter-utils/src/acpx-engine/execute.test.ts`,
`packages/adapter-utils/src/execution-target-sandbox.test.ts`,
`packages/adapter-utils/src/sandbox-callback-bridge.test.ts`, the three
adapter `acp.test.ts` files, and
`server/src/__tests__/environment-execution-target.test.ts` — 102 tests
pass.
- End to end: with a Claude agent configured on a Daytona sandbox
environment, the primary-model test now selects the default ACP lane (no
fallback warning), and the full round trip (wake → sandbox execution →
API bridge → comment post) was exercised twice from inside a live
sandbox.
## Risks
- Behavioral shift: adapters that previously always fell back to CLI on
sandbox targets now default to ACP there; `engine=cli` still pins the
CLI lane explicitly.
- The bridge relays stdio as JSON lines over loopback TCP guarded by a
per-session random token; the remote relay runs inside the sandbox under
the provider's runner. Providers with slow one-shot execution will see
higher session startup latency — the CLI fallback remains for genuinely
incapable targets.
- No schema or migration changes.
## Model Used
- Claude Fable 5 (`claude-fable-5`, Anthropic) — extended thinking
enabled, agentic tool use via the Claude Agent SDK harness;
implementation iterated with local Vitest verification.
## Checklist
- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes (no
shipped docs describe the old local-only ACP limitation)
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge
---------
Co-authored-by: Cody <noreply@paperclip.ing>
Co-authored-by: Cody <cody@paperclip.local>
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