## Thinking Path
> - Paperclip is the open source app people use to manage AI agents for
work
> - AI agents run in sandboxed execution environments (Kubernetes pods,
Daytona workspaces, etc.) and need to sync files between the host and
those environments — for workspace setup, asset delivery, and output
retrieval
> - The existing sync path for Kubernetes uses a base64-over-exec chunk
loop: each ~4 MB chunk requires its own `execInPod` round-trip, so large
syncs balloon into many exec calls with corresponding overhead
> - `execInPod` supports piped stdin/stdout, meaning the full transfer
can be done as a single exec that streams a raw `tar` archive over the
data channel — one round-trip regardless of file size, with nothing
base64-encoded and nothing buffered whole in memory on either side
> - PR-1 (#10013, merged) added the
`onEnvironmentSyncIn`/`onEnvironmentSyncOut` opt-in hook API to the
sandbox provider interface and documented the protocol; PR-2 (#10028,
merged) implemented these hooks for the Daytona provider
> - This pull request implements the same two lifecycle hooks in the
Kubernetes sandbox provider, so workspace/asset file sync streams
through one `execInPod` per operation instead of the chunk loop
> - The benefit is significantly fewer exec round-trips for large syncs
and flat memory use on both host and pod, with security properties
preserved: atomic replace, secret-mode enforcement, path confinement,
TOCTOU-safe snapshot, and member-confinement on host-assembled archives
from sandbox-authored tar output
## Linked Issues or Issue Description
This is the third and final PR in a sequential series:
- Refs #10013 — PR-1: opt-in sync hook API + provider docs (merged)
- Refs #10028 — PR-2: native file-sync lifecycle hooks for Daytona
provider (merged)
**Feature:** Native single-exec file-sync lifecycle hooks for the
Kubernetes sandbox provider.
*Motivation:* The existing Kubernetes sync path encodes files as base64
and loops over `execInPod` one chunk at a time (~4 MB per exec). For
large workspaces or asset sets this is slow and resource-intensive. The
Kubernetes `execInPod` API supports piped stdin/stdout, enabling a
raw-`tar` streaming transfer that needs only one exec regardless of file
count or size and never buffers the whole payload in memory.
*Proposed solution:* Implement `onEnvironmentSyncIn` and
`onEnvironmentSyncOut` in the Kubernetes provider using a streaming
`execInPod` with a tar pipeline — for syncIn the host builds the archive
on disk and streams its raw bytes into the pod's stdin (`head -c
<exact-size> | tar -x`, no base64); for syncOut in-pod `tar` writes to
the exec's stdout and the host streams those bytes straight to a file.
Path confinement, atomic replace, secret-mode enforcement, TOCTOU
protection, and a streamed-bytes fail-closed guard are all enforced.
## What Changed
- **New `src/file-sync.ts`** in
`packages/plugins/sandbox-providers/kubernetes/` — `performSyncIn` and
`performSyncOut` over an injected pod-exec closure, keeping transfer
logic hermetically unit-testable
- **New `execInPodStreaming` in `src/pod-exec.ts`** — a streaming exec
primitive that binds a caller-supplied stdin readable and a stdout
writable to the exec WebSocket data channel, added alongside the
existing `execInPod` (which is unchanged). This lets a transfer stream
raw bytes to/from disk instead of buffering the payload as a single
string
- **Updated `src/plugin.ts`** — registers
`onEnvironmentSyncIn`/`onEnvironmentSyncOut`; resolves the `sandbox-cr`
pod exactly like `onEnvironmentExecute` and delegates; `job` backend
rejects file-sync calls explicitly (out of scope)
- **syncIn path:** host builds the tarball to a temp file → streams its
raw bytes over exec stdin, bounded in-pod by `head -c
<exact-archive-size> | tar -x` (no base64 anywhere) → extract into a
`/proc/self/fd`-pinned reserved `0700` staging dir → `chmod`-before-`mv
-f` atomic replace per file (directory mappings use
`followSymlinks`→`-h`)
- **syncOut path:** in-pod validate + realpath-snapshot each source
(closes the validation→copy TOCTOU window) → single-exec `tar -c`
streamed over exec stdout → host streams that stdout straight to a temp
file through a byte-counting transform → member-confined extraction of
the sandbox-authored archive
- **Security properties:** secret files land at requested mode with no
widened window; every interpolated path is shell-quoted and confined
lexically plus via in-pod `realpath`; the outbound stream is bounded by
a **streamed-bytes disk guard** (`MAX_SYNC_OUTPUT_BYTES`, 8 GiB default,
per-call overridable) that fails the transfer closed — writing no target
file — if an untrusted pod emits more bytes than allowed. Neither host
nor pod buffers the whole payload, so there is no in-memory size cap on
the transfer
- **No changes** to `execInPod`, `wrapCommandWithEnv`, or
`FastUploadInterceptor` (the `environmentExecute` path is untouched)
- **No dependency or lockfile changes**
- **New tests** in `test/unit/file-sync.test.ts` (atomic-replace, `0600`
secret mode, symlink preserve/deref, dir-mapping, exclude,
path-confinement rejection, streamed-output guard fail-closed) and
`test/unit/pod-exec.test.ts` (streaming stdin/stdout, caller-sink error
fail-closed), plus extended `test/unit/plugin.test.ts`
## Follow-up: Legacy Job-Lease Base64 Fallback Fix
Addresses the Greptile 4/5 blocking finding ("Handle existing job
leases", `server/src/services/environment-runtime.ts`).
Job leases provisioned before the `nativeFileSyncUnsupported` metadata
flag existed carry `backend: "job"` but no flag, so `supportsSync()`
treated them as native-capable and routed their sync to the pod-exec
hook — which the job backend rejects (it has no exec channel) instead of
using the byte-identical base64 fallback. The fix adds a
belt-and-suspenders gate on the persisted `backend === "job"` field
alongside the existing `nativeFileSyncUnsupported` flag check, so
pre-existing job leases continue syncing via the base64 fallback after
deployment. No behaviour change for `sandbox-cr` leases.
## Verification
- `pnpm --filter @paperclipai/sandbox-provider-kubernetes test` — 19
files / 182 tests green, including the existing `upload-interceptor` and
`pod-exec` suites
- `tsc --noEmit` in the kubernetes package — 0 errors
- The sync hooks are opt-in; existing `environmentExecute` behaviour is
unaffected and tested by the unchanged existing suites
## Risks
- **Opt-in only:** `onEnvironmentSyncIn`/`onEnvironmentSyncOut` are
registered conditionally; providers that do not register them fall back
to the existing chunk loop. No regression risk on the existing path.
- **Shell-injection surface:** all path interpolation uses
shell-quoting; paths are additionally confined lexically and via in-pod
`realpath` before use.
- **TOCTOU on syncOut:** the in-pod snapshot validates and records file
metadata before the tar call, closing the window between validation and
copy.
- **Archive member confinement:** host-side reassembly rejects any tar
member whose resolved path escapes the target directory, preventing a
malicious in-pod tar from writing outside the intended destination.
- **Untrusted-output volume:** an over-large outbound stream trips the
streamed-bytes disk guard and fails closed (no target written and the
temp sink is swept) rather than filling host disk or memory; the guard
bounds disk unconditionally and bounds memory insofar as WebSocket
write-backpressure holds.
## Model Used
Anthropic Claude Sonnet 4.6 (`claude-sonnet-4-6`) — produced by a
Claude-based AI agent using agentic tool use and multi-step code
generation. 200K context window, extended reasoning, code execution and
verification capabilities.
## 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: Harold Kim <harold@paperclip.ing>
Co-authored-by: Paperclip <noreply@paperclip.ing>
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