feat(ai-harness): handoffs between agents and to a person, the active agent in the log, and host.transfer between harnesses

.changeset/ai-harness-cli.md

@@ -4,9 +4,9 @@
 
 Add `@tanstack/ai-harness-cli`: `runCli(harness)` runs a harness from the terminal.
 
-- Line mode reads one message or `/command` per line, hides a secret answer while the user types, and sends `@path` files. When a turn waits, each next line answers one wait: `y` or `n` for an approval (`once`, `always`, or `reject` for an approval of `permissions()`), and an answer read with the schema for a question. Pass `ui` to show your own screen on a session view instead.
+- Line mode reads one message or `/command` per line, hides a secret answer while the user types, and sends `@path` files. In a terminal, `/cancel` and a message typed while a turn runs act at once: the message joins the running turn. When a turn waits, each next line answers one wait: `y` or `n` for an approval (`once`, `always`, or `reject` for an approval of `permissions()`), and an answer read with the schema for a question. Pass `ui` to show your own screen on a session view instead.
 - `-p "prompt"` runs one prompt for scripts and CI, and `--output ndjson` prints every AG-UI event. `-p` exits with code 3 (`EXIT.needsAction`) at the first wait of the turn: an approval, a question, or a sign-in. The exit code for this case changed from 2 to 3. A script that checks for exit code 2 must check for 3.
 - `--serve` serves the session protocol over HTTP behind a bearer token.
-- `--acp`, `--mcp`, and `--dashboard` load `@tanstack/ai-acp`, `@tanstack/ai-mcp`, and `@tanstack/ai-dashboard` only when you use the flag. A missing package, or one without harness support, fails that flag with a clear message.
+- `--acp` and `--mcp` load `@tanstack/ai-acp` and `@tanstack/ai-mcp` only when you use the flag. A missing package, or one without harness support, fails that flag with a clear message. Any other error while the package loads is reported, not hidden. The `--dashboard` flag was removed.
 - In every mode, the first SIGINT or SIGTERM cancels the running turn. A second one, or one while no turn runs, stops the CLI, and `runCli` resolves to 130 (SIGINT) or 143 (SIGTERM). The CLI closes a host that it made with `recoverable: true`, or releases its session handle on your own host. The `signals` option takes another signal source, for example an `EventEmitter` in a test.
-- `host` and `principal` options let the CLI use your durable host and run as your user. Every mode except `--dashboard` opens its sessions as that principal. On a host with `access`, `--serve` reads the user of each request with `identify`, and `--mcp` does not start, because stdio has no login.
+- `host` and `principal` options let the CLI use your durable host and run as your user. Every mode opens its sessions as that principal. On a host with `access`, `--serve` reads the user of each request with `identify`, and `--mcp` does not start, because stdio has no login.

.changeset/ai-harness-coding-tools.md

@@ -5,9 +5,16 @@
 Add coding tools in the new Node entry `@tanstack/ai-harness/plugins/coding`:
 
 - `workspaceTools({ root })` gives `read_file` (text pages, images, PDFs), `write_file`, `edit_file` or `patch` (`editStyle`), `list_files`, `grep`, `bash` (with `background`), `webfetch`, and `websearch` (with a `SearchProvider`). With `permissions()`, edits, `bash`, and `webfetch` ask first, and the rules also apply to the real path of a link. `outside: 'ask'` makes a path outside `root` need an approval. `permissions()` decides outside paths and the real paths of links before the call runs, so a tool never asks while it runs. The `cwd` thread setting gives each thread its own working folder.
-- A `WorkspaceBackend` runs the tools. `hostBackend` is the default. On Windows, commands do not run a program from the working folder by its name.
-- `snapshots()` adds `/undo` and `/redo`, which restore only the files of the turn (git 2.26 or newer), file restore for `session.revert()`, and a diff for each model step.
-- `formatter()` formats each written file with the formatter of the project. With `permissions()`, the rules decide the formatter with the call that writes, before that call runs: one approval covers both, and the formatter runs after the write.
+- `workspaceTools({ workspace: { bind, reattach? } })` gives each thread its own workspace. The root of the workspace is the boundary of every tool. The session log keeps where the workspace is (`harness.workspace.bound`). On another host, `reattach` gives the workspace back (for example a resumed sandbox), or `'pinned'` or `'gone'`. A gone workspace sends `tanstack/workspace-tools:stranded`. With this plugin first, `snapshots()` and `formatter()` use the workspace of each thread, and their `root` is optional.
+- `createHarnessHost({ ownership: { hostId } })` gives a host a stable id. Then a thread that uses a folder on that host is pinned to it, and other hosts refuse its tool calls. Without `hostId`, threads are not pinned. A host with `hostId` sends a heartbeat to the log store, and its pins lapse when no heartbeat came for `ttlMs`: then another host reattaches its threads. `host.unpin({ harness, threadId })` unpins a thread from a host that does not come back, once its ownership expired.
+- `workspaceRoot(base, ...parts)` gives a folder in `base` for a tenant and a thread id. It encodes each part, so a thread id from a client cannot leave `base` or reach another thread.
+- Commands, `rg`, `git`, and the formatters get only a short list of variables of the server process, plus the `env` of the backend call. Git runs without the hooks and the `fsmonitor` of the project.
+- The tools resolve links with `realpath` under the file lock, and fail when a backend cannot resolve a path. A write fails when the file changed after the tool read it in the same chat run, so a lead and its subagents do not lose each other's edits. The file locks are shared by the sessions of a host, by the sandbox (or host) and the real path of the file.
+- Background jobs: at most 8 per session (`maxJobs`), the newest 1 MiB of output per job, and the events `tanstack/workspace-tools:job-started`, `job-output`, and `job-ended`. A running command that `session.background()` moves counts as a job, and its process id goes to the log. A job that an earlier host left running is stopped at the next tool call, with a note to the model. When the backend cannot check the job (for example with no `ps`), the log keeps it, and the note says that it can still run. The wake turn of a job runs as the user who started it. The `bash` spill file has `[redacted]` in place of each secret of the session.
+- `read_file` sends images and PDFs up to 256 KB. `webfetch` gives the model the first 256 KB of a page.
+- A `WorkspaceBackend` runs the tools. `hostBackend` is the default. `realpath` is required. `spawn` can give a `jobId`, `exec` can give one with `onStart`, and `killJob`, `binding`, and `close` are optional. `killJob` rejects when it cannot check that the process is the job. On Windows, commands do not run a program from the working folder by its name.
+- `snapshots()` adds `/undo` and `/redo`, which restore only the files of the turn (git 2.26 or newer), file restore for `session.revert()`, and a diff for each model step. Each thread of each tenant has its own shadow repository, which never stores `.env` files. A step keeps only the files of its own run. The steps and the undo point are in the session log, so another host with the same repository can undo. A revert uses the last 500 steps. `host.sessions.delete` removes the repository of the thread, and `purgeSnapshots()` does it from your own code. A failed snapshot sends `tanstack/snapshots:failed`.
+- `formatter()` formats each written file with the formatter of the project. With `permissions()`, the rules decide the formatter with the call that writes, before that call runs: one approval covers both, and the formatter runs after the write. An `always` answer is tied to a hash of the formatter config files. A write of a formatter config below the root, or under `node_modules`, asks each time, and an `always` answer saves no rule for that formatter run.
 - `WorkspaceHooks` lets other plugins run code after a read or a write.
 
 `@vscode/ripgrep` and `turndown` are optional peer dependencies.