#RFC: Remote Workspace Git Backend

This document is retained as the protocol specification — clients and servers speaking this surface should match the wire shapes defined below. Subsequent revisions should land as separate amendment documents rather than editing this one in place, so the "what was shipped" history stays auditable.

This RFC proposes a protocol and a Rust client that let a3s-code run the built-in git tool against workspaces backed by something other than a local filesystem — most importantly S3 and future container / DFS backends, which cannot host a .git directory directly.

#1. Motivation

The workspace abstraction already lets the built-in tools talk to any filesystem implementation via WorkspaceFileSystem. For bash, grep, glob, and git we use capability gating: if the backend cannot service the operation, the corresponding tool is not registered and the model never sees it.

That works for bash (object storage really cannot run a shell) and is acceptable for grep (we added a degraded LIST + GET + regex path). It is painful for git: many a3s-code workflows expect to query branch / commit state, diff working tree, create branches, and stash changes. Hiding git everywhere except local sessions splits the user experience and prevents cloud workspaces from being first-class.

The proposal is to introduce a remote WorkspaceGit backend that delegates these operations over the network to an external service the host operates. The service owns a real working tree (or a libgit2-backed implementation) and exposes a small HTTP API. The a3s-code client speaks that API and presents it to tools as just another WorkspaceGit provider.

                ┌──────────────────┐
                │   a3s-code       │
   model        │                  │
   ──────►      │  git tool        │       HTTP/JSON
                │      │           │   ┌────────────────┐
                │      ▼           │   │                │
                │  WorkspaceGit ───┼──►│  gitserver     │
                │  (RemoteGit…)    │   │  (libgit2 / sh)│
                │                  │   │                │
                │  WorkspaceFs ────┼──►│  (S3, etc.)    │
                └──────────────────┘   └────────────────┘

#2. Non-Goals

• Hosting a gitserver. This RFC defines a client and a protocol the client speaks. Implementations of the server side (libgit2 wrapper, shell-out, Gitea-API adapter, ...) are out of scope.
• Replacing local git. When WorkspaceFs is LocalWorkspaceBackend, the existing LocalWorkspaceBackend WorkspaceGit impl remains the default. Hosts mix and match per session.
• Push / pull. The WorkspaceGit trait surface is read-only against remotes (list_remotes returns the configured set; no git push / git fetch). A future RFC may add those if a tool needs them.
• Worktrees. Worktrees are a local-filesystem concept; the remote backend will not implement WorkspaceGitWorktreeProvider. See §8.

#3. Protocol Choice — HTTP/JSON

Decision: HTTP/JSON, not gRPC.

The operation set is small (~12 RPCs) and the request/response shapes are flat. Streaming would only meaningfully help log and diff, both of which are bounded by client-side limits already. The mocking and operator debuggability wins of HTTP/JSON outweigh the schema benefits of gRPC at this scale. If the operation surface grows substantially or streaming becomes load-bearing, gRPC can be revisited.

We use POST for every operation. Git operations are imperative, not resource-CRUD, and request bodies make versioning trivial (add optional fields, ignore unknown). GET would force everything into query strings.

#4. Repository Identity

The client knows it is talking about a specific repository; the server needs to route. We encode the repo identity in the URL path:

POST /v1/repos/{repo_id}/git/<operation>

repo_id is an opaque, URL-safe string negotiated out of band between the host that configures the client and the gitserver operator. Typical values:

• users/{user_id}/sessions/{session_id} (1:1 with an S3 workspace prefix)
• A UUID
• A path to a working tree on the gitserver

The client treats repo_id as opaque; the server is responsible for mapping it to an actual working tree / bare repo.

#5. Endpoint Reference

All endpoints are POST. All requests and responses are application/json. Field naming is snake_case to match Rust serde defaults.

#5.1 Status — WorkspaceGit::status

POST /v1/repos/{repo_id}/git/status
→ 200 {
    "branch":       "main",
    "commit":       "abc123...",
    "is_worktree":  false,
    "is_dirty":     true,
    "dirty_count":  3
  }
→ 404 {"error":{"code":"REPO_NOT_FOUND", ...}}

#5.2 Log — WorkspaceGit::log

POST /v1/repos/{repo_id}/git/log
{"max_count": 10}
→ 200 {
    "commits": [
      {"id":"abc...", "message":"feat: ...", "author":"Alice <a@b>", "date":"2026-05-19T..."}
    ]
  }

#5.3 List Branches — WorkspaceGit::list_branches

POST /v1/repos/{repo_id}/git/branches
→ 200 {"branches":[{"name":"main", "is_current":true}, ...]}

#5.4 Create Branch — WorkspaceGit::create_branch

POST /v1/repos/{repo_id}/git/branches/create
{"name":"feat/x", "base":"main"}
→ 201 {}
→ 409 {"error":{"code":"BRANCH_EXISTS", ...}}
→ 404 {"error":{"code":"BASE_NOT_FOUND", ...}}

#5.5 Checkout — WorkspaceGit::checkout

POST /v1/repos/{repo_id}/git/checkout
{"refspec":"feat/x", "force":false}
→ 200 {"stdout":"Switched to branch 'feat/x'"}
→ 409 {"error":{"code":"WORKING_TREE_DIRTY", ...}}

#5.6 Diff — WorkspaceGit::diff

POST /v1/repos/{repo_id}/git/diff
{"target": null}            // null = working tree vs index
{"target": "main"}           // diff against ref
→ 200 {"diff":"<unified diff text>", "truncated": false}

truncated is true when the server clipped the body — see §9. The client surfaces this in the git diff tool result.

#5.7 List Remotes — WorkspaceGit::list_remotes

POST /v1/repos/{repo_id}/git/remotes
→ 200 {"remotes":[{"name":"origin", "url":"git@github.com:...", "direction":"fetch"}]}

#5.8 Is Repository — WorkspaceGit::is_repository

POST /v1/repos/{repo_id}/git/exists
→ 200 {"is_repository": true}

Distinct from "repo not found" (404): the server may be configured to serve repo IDs that map to non-git directories; is_repository lets the client probe without inferring intent from a 404.

#5.9 List Stashes — WorkspaceGitStashProvider::list_stashes

POST /v1/repos/{repo_id}/git/stashes
→ 200 {"stashes":[{"index":0, "message":"WIP on main: ..."}]}

#5.10 Stash — WorkspaceGitStashProvider::stash

POST /v1/repos/{repo_id}/git/stashes/create
{"message":"wip", "include_untracked":true}
→ 201 {}
→ 409 {"error":{"code":"NOTHING_TO_STASH", ...}}

#6. Authentication

The client supports two transport modes, configurable per-session:

1. Bearer token (default). Authorization: Bearer <token>. Token provisioning is the host's responsibility (e.g. short-lived JWT minted by the same identity layer that gates S3 access).
2. mTLS. Pass both client_cert_pem and client_key_pem paths via the backend config. The client reads both files at construction, concatenates them, and hands the result to reqwest::Identity::from_pem. The rustls-tls backend expects the key in PKCS#8 PEM format. Setting only one of the pair fails at construction with a clear error.

Both are mutually compatible — a deployment that uses both for defence in depth simply sets both.

No-auth mode (for localhost development) is supported by configuring an empty bearer token (and no mTLS); the client emits a tracing::warn! on construction to make this visible.

#7. Error Model

HTTP status code is the transport signal. The error kind is in the JSON body:

{
  "error": {
    "code": "BRANCH_EXISTS",
    "message": "branch 'feat/x' already exists"
  }
}

Client mapping:

The client introduces one typed error for backwards-compatible recovery:

#[derive(Debug, Clone, thiserror::Error)]
#[error("remote git conflict: {code}: {message}")]
pub struct RemoteGitConflict {
    pub code: String,
    pub message: String,
}

Tools that want to recover from BRANCH_EXISTS / WORKING_TREE_DIRTY / NOTHING_TO_STASH downcast anyhow::Error::downcast_ref::<RemoteGitConflict>(), the same pattern edit / patch use for WorkspaceVersionConflict on S3 compare-and-swap writes.

Error codes documented in the public schema (extensible):

#8. Optional Traits

WorkspaceGit is implemented in full. WorkspaceGitStashProvider is implemented. WorkspaceGitWorktreeProvider is deliberately not implemented by the remote backend. Worktrees are a local-filesystem concept that map poorly onto a remote service:

• "Create a worktree at path X" — the client has no path concept; the server's path layout is opaque.
• The local tool flow that uses worktrees (parallel agent runs against isolated copies) is better served on cloud workspaces by spinning up separate sessions with separate repo_ids, not by emulating a filesystem feature.

Tools that depend on WorkspaceGitWorktreeProvider see None from services.git_worktree() and emit a clear "worktrees unavailable on remote git workspaces" error.

#9. Size and Cost Bounds

Following the S3 backend's pattern, the remote git client enforces client-side ceilings so the model cannot trigger unbounded responses:

The server is expected to honour these too — the client passes the relevant caps in the request where applicable (max_log_entries), and surfaces truncated: bool returned by the server on diff so the tool can hint "large diff truncated; narrow your target".

#10. Rust Client Design

// crates/code/core/src/workspace/remote_git.rs   (feature = "remote-git")

#[derive(Debug, Clone)]
pub struct RemoteGitBackendConfig {
    pub base_url:        String,           // https://git.example.invalid
    pub repo_id:         String,           // path-segment safe
    pub bearer_token:    Option<String>,
    pub client_cert_pem: Option<PathBuf>,  // mTLS
    pub client_key_pem:  Option<PathBuf>,
    pub request_timeout: Option<Duration>, // default 30s
    pub max_diff_bytes:  Option<u64>,      // default 1 MiB
    pub max_log_entries: Option<usize>,    // default 200
}

#[derive(Debug, Clone)]
pub struct RemoteGitBackend {
    http:           reqwest::Client,
    base_url:       String,
    repo_id:        String,
    max_diff_bytes: u64,
    max_log_entries: usize,
}

#[async_trait]
impl WorkspaceGit for RemoteGitBackend { /* see §5 */ }

#[async_trait]
impl WorkspaceGitStashProvider for RemoteGitBackend { /* see §5 */ }

Composition factory mirrors the S3 path:

impl WorkspaceServices {
    /// Attach a remote git provider to an existing filesystem backend.
    pub fn with_remote_git(
        self: Arc<Self>,
        cfg: RemoteGitBackendConfig,
    ) -> Arc<Self> { ... }
}

Or as a top-level convenience for an S3 + remote-git workspace:

pub fn s3_with_remote_git(
    s3:  S3BackendConfig,
    git: RemoteGitBackendConfig,
) -> Arc<WorkspaceServices> { ... }

Wiring follows the existing builder pattern:

let backend = Arc::new(RemoteGitBackend::new(cfg));
let git:   Arc<dyn WorkspaceGit>              = backend.clone();
let stash: Arc<dyn WorkspaceGitStashProvider> = backend;

WorkspaceServices::builder(workspace_ref, fs)
    .file_system_ext(fs_ext)        // S3 ETag CAS
    .git(git)
    .git_stash(stash)
    // no git_worktree — see §8
    .operation_timeout(Duration::from_secs(60))
    .build()

Capability gating then registers the git tool automatically.

#11. Per-Call Observability

Every HTTP call emits a tracing::debug! event with the same field shape S3WorkspaceBackend uses for its per-call metering (see emit_s3_call_event in crates/code/core/src/workspace/s3.rs):

Hosts that already meter S3 cost can extend the same subscriber to meter gitserver cost; no new dependency or hook surface is needed.

#12. Composition Examples

#S3 workspace + remote git

let ws = WorkspaceServices::s3_with_remote_git(
    S3BackendConfig::new("workspace", "sessions/example", access_key, secret_key)
        .endpoint("https://s3.example.invalid")
        .force_path_style(true)
        .enable_search(true),
    RemoteGitBackendConfig::new(
        "https://git.example.invalid",
        "sessions/example",
    )
    .bearer_token(token),
);

let session = agent
    .session_builder("s3://workspace/sessions/example")
    .options(SessionOptions::new().with_workspace_backend(ws))
    .build()
    .await?;

Tools registered for this session: read, write, edit, patch, ls, grep, glob, git. (bash remains hidden — object storage cannot host a shell.)

#Local filesystem + remote git (mixed)

Useful when CI runs against a local checkout but the host wants to route git operations through a sandboxed service (e.g. for audit / rate limiting).

let local = WorkspaceServices::local("/workspaces/repo");
let ws    = local.with_remote_git(remote_cfg);   // overrides local git provider

#13. Open Questions

These should be resolved before Phase 4.2 (implementation) starts.

1. Diff format pinning. The local backend returns raw git diff stdout. Should the remote API guarantee a specific diff dialect (POSIX diff -u? libgit2's variant?) or pass through whatever the server produces? Recommendation: pass through, document that the server should produce unified diff.

2. Concurrent operations. If two clients hit the same repo_id simultaneously (one running checkout, one running diff), is the server expected to serialise? Recommendation: server-side serialisation per repo_id. Document the expectation; do not retry on conflict in the client.

3. Long-running operations. checkout against a large tree could exceed request_timeout. Should the protocol support a polling / async-job mode? Recommendation: defer. Set realistic timeouts; the working-tree sizes we target (per-session sandboxes) are small.

4. Hooks. The server may have pre-commit / pre-push hooks installed. Do we surface a "hook output" channel in responses? Recommendation: include hook stderr in the checkout / stash response when non-empty; document that hook failures map to HTTP 422 with a HOOK_FAILED code.

5. Schema versioning. First-cut endpoints are under /v1/. When should we bump to /v2/? Recommendation: only on incompatible request/response shape changes; additive fields stay under /v1/ (clients ignore unknown fields).

6. Reference RFC. Should a reference implementation (e.g. a minimal libgit2-backed gitserver written in Rust) live in this repo, or stay out-of-tree? Recommendation: out-of-tree. The client and protocol are sufficient; reference servers belong with operators.

#14. Out of Scope (Future RFCs)

• Push / fetch from upstreams. Needed for "agent commits and pushes changes back to origin" workflows. Will need credential delegation.
• Sparse / partial checkout. Useful for huge monorepos; the current surface assumes the entire repo is materialised on the server.
• Streaming log and diff. Required if response bodies routinely exceed max_diff_bytes. Would motivate revisiting the gRPC decision.
• Hooks management. Listing / configuring server-side hooks from the client.

#15. Implementation Notes (shipped in v2.6.x)

The original draft sketched an 8-step implementation plan; what actually shipped (Phase 4.2 plus Phase 5.x follow-ups) is:

1. RemoteGitBackend / RemoteGitBackendConfig / RemoteGitConflict live in crates/code/core/src/workspace/remote_git.rs. No remote-git feature was added — reqwest was already a hard dependency for the LLM clients, so the module compiles unconditionally.
2. WorkspaceGit and WorkspaceGitStashProvider implemented in full. WorkspaceGitWorktreeProvider deliberately omitted (see §8).
3. WorkspaceServices::with_remote_git is the public attachment point. The internal helper with_git_provider (added in v2.6.x follow-up) handles the actual struct-literal copy so future fields on WorkspaceServices cannot be silently dropped by the decorator.
4. mTLS is supported in addition to bearer token (client_cert_pem
   • client_key_pem), shipped in a Phase 5.2 follow-up that also updated §6 above.
5. diff is hardened against unbounded responses: HTTP body is streamed with a hard cap of max_diff_bytes * 4 (Phase 6.2). The soft max_diff_bytes display truncation continues to apply post-decode.
6. Test surface: 25+ wiremock-backed unit tests in remote_git.rs, one end-to-end test driving the git built-in tool through the remote backend, plus the workspace-wide conformance suite added in Phase 6.3.
7. README and CHANGELOG updated; rustls-tls backend selected to match the AWS SDK.
8. SDK exposure landed in Phase 5.1 (Node + Python).