## Thinking Path
> - Paperclip provides a control plane for companies that run AI agents.
> - Sandboxed agents need a safe execution path for persistent command
streams.
> - The existing callback transport does not provide a bounded, generic
duplex route.
> - The host must control capability access, route identity, protocol
limits, and close behavior.
> - This pull request adds an opt-in duplex command-stream foundation
across the sandbox layers.
> - The feature stays inert because no current provider declares the
capability.
## Linked Issues or Issue Description
**Subsystem affected**
Cross-cutting (multiple of the above)
**Problem or motivation**
Sandbox command execution needs a persistent host-to-sandbox stream. The
current callback bridge uses a file transport and does not provide this
generic route.
**Proposed solution**
Add a fail-closed provider capability, generic worker protocol messages,
a host-owned bounded route, cross-layer service mediation, and a
versioned newline-delimited frame codec.
**Alternatives considered**
Keep the file transport and add feature-specific commands. This does not
provide one reusable duplex contract or host-owned route bounds.
**Roadmap alignment**
This work supports the completed Cloud / Sandbox agents roadmap area and
the safe autonomy goal in the product definition.
**Additional context**
The change passed a two-stage security review. The final code review
verdict was approve after fixes for active-stream bounds and
service-layer capability mediation.
## What Changed
- Add the opt-in `duplexCommandStream` provider capability with
fail-closed narrowing.
- Add duplex open, write, stop, and close requests and data and exit
notifications to the plugin worker protocol.
- Add a host-owned route with bounds for chunk size, cumulative bytes,
lifetime, protocol errors, pending requests, and pre-bind buffering.
- Add close acknowledgement handling with worker retirement when the
close remains unconfirmed.
- Wire `openDuplexChannel` through the execution target, runtime
service, and plugin worker.
- Add a versioned frame codec with shared wire-compatibility vectors and
split UTF-8 handling.
## Verification
- `server/src/__tests__/plugin-worker-manager-duplex.test.ts` passes 18
tests.
- `server/src/__tests__/environment-execution-target-duplex.test.ts`
passes 11 tests.
- `packages/adapter-utils/src/duplex-frame-codec.test.ts` passes 38
tests.
- `server/src/__tests__/sandbox-capability-contract.test.ts` passes 15
tests.
- Setup-token pseudo-terminal regression tests pass 47 tests.
- Server TypeScript check passes.
- Continuous integration will run the full required test, typecheck,
build, and policy checks.
## Risks
- Providers that opt into the capability must implement the complete
worker protocol.
- Route limit defaults can close a stream when a workload exceeds the
configured bounds.
- The capability remains disabled for current providers, so current
production behavior does not change.
## Model Used
OpenAI GPT-5 (`gpt-5`), with tool use and code execution. The model
reviewed and prepared this pull request from the supplied implementation
and verification record.
## 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
- [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: Paperclip <noreply@paperclip.ing>
