# RFC: Resolve filesystem paths against the caller's session cwd

Status: implemented

## Problem

The ACP bridge gives every session its own workspace: `session/new` records the editor's project directory as `SessionHeader.cwd`, and `dsh-tool-bash` defaults each bash call's `workdir` to the calling agent's `session.header.cwd` (see [the per-session cwd RFC work in `packages/ui/acp`](../../../../packages/ui/acp) and `resolveWorkdir` in `dsh-tool-bash`). So a bash command in session A runs in A's project, and in session B runs in B's — one server process, N workspaces.

Filesystem resolution used one plugin-load cwd while bash used the session project directory. Relative paths therefore disagreed whenever the editor project differed from the server launch directory; snapshots hid the bug by making those paths identical.

## Decision

Thread the caller's session cwd into path resolution, exactly as `dsh-tool-bash` already does for `workdir`. The **caller** (the tool) supplies the cwd; the provider does not read a session or agent.

- `FileSystem.resolve` accepts `resolve(path: string, opts?: { cwd?: string; signal?: AbortSignal }): Promise<FsTarget>`. `opts.cwd` is the base a RELATIVE `path` resolves against; an absolute `path` ignores it; omitting `opts.cwd` uses the backend's own default. `opts.signal` cancels resolution when the backend performs I/O. The options object keeps both caller-owned resolution controls together without positional growth.
- `dsh-fs-local.resolve` uses `resolveLocalTarget(opts?.cwd ?? this.config.cwd, path)`. `config.cwd` stays the default for a caller that supplies none (non-ACP / no-session use, and the single-session stdio demo where `process.cwd()` IS the workspace).
- `dsh-tool-fs`'s `read`/`write`/`edit` derive the session cwd through a shared `sessionCwd(exec)` helper (`exec.agent?.session.header.cwd`, mirroring bash's `resolveWorkdir`) and pass it to `resolve`. A non-agent / headerless caller yields `undefined`, so the backend applies its default.

## Alternatives considered

### Why the caller supplies the cwd (not the provider)

The provider seam must not depend on `dsh-agent` / `dsh-session` — it is a text-storage backend that a sandboxed or remote implementation also satisfies, and those have no notion of an "agent session". The tool already receives the `ToolExecution` (`exec`), which carries the agent, so the tool is the right place to project `exec → cwd` and hand the provider a plain string. This is the "explicit > implicit at package seams" convention: the base directory arrives as an explicit argument the provider acts on, not smuggled in by having the provider reach into a session it should not know about. It also matches `dsh-tool-bash` one-to-one, so the two model-facing file surfaces resolve paths identically.

The default lives in ONE place — the provider's `config.cwd`. `sessionCwd` returns `undefined` rather than `process.cwd()` when there is no session, so the tool never manufactures a base the provider would otherwise choose.

## Consequences

- In the ACP demo the fs tools and bash now agree on each session's workspace; an editor can open any project folder and both tool families act on it.
- No change to `FsTarget` identity: `targetKey` is still the realpath of the resolved absolute path, so observed-state keying and symlink identity are unaffected — a correct per-session cwd produces the same key bash targets.
- Backward compatible: every existing `resolve(path)` call (all in tests) keeps working; the new argument is optional.
- The single-session stdio demo is unaffected: it supplies no session cwd (its agent's session has no `cwd`), so resolution falls back to `config.cwd = process.cwd()`, which is the workspace.