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CLAUDE.md

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+# Agent Guide
+
+## Project Overview
+
+patterm is a Go terminal orchestration TUI. It runs as one foreground process that owns:
+
+- the host TUI chrome
+- child PTYs
+- headless `libghostty-vt` emulators for rendered-grid snapshots
+- the in-process MCP server
+- per-project scratchpads and command-preset trust state
+
+There is no daemon or attach/detach model. Closing the patterm process tears down every child process it spawned.
+
+Core packages:
+
+- `cmd/patterm`: main binary, including `mcp-stdio` and `debug-harness` subcommands.
+- `internal/app`: TUI/session state, palette, child lifecycle, rendering, MCP tool host.
+- `internal/pty`: thin wrapper around `github.com/creack/pty`.
+- `internal/vt`: `libghostty-vt` backed emulator wrapper.
+- `internal/mcp`: JSON-RPC MCP socket server and tool surface.
+- `internal/preset`: XDG-loaded agent/process presets.
+- `internal/trust`: per-project command-preset trust grants.
+- `internal/harness`: black-box PTY/MCP test harness.
+
+## Build Prerequisites
+
+The normal build depends on the vendored `libghostty-vt` static library and headers under `third_party/libghostty-vt/install`.
+
+If that install tree is missing, run:
+
+```sh
+make deps
+```
+
+This fetches the pinned Ghostty commit and builds `libghostty-vt` with Zig. The Makefile currently requires `zig` on `PATH` with a compatible version.
+
+Common checks:
+
+```sh
+go test ./...
+go build -o ./bin/patterm ./cmd/patterm
+```
+
+## Harness Testing
+
+The harness is the preferred way to test patterm end-to-end without a human watching a terminal.
+
+It starts a real patterm child process in a PTY, feeds its output through the same `internal/vt` emulator used for child panes, and talks to the child patterm process over its per-PID MCP Unix socket. Scenarios are JSON files under:
+
+```text
+internal/harness/scenarios/
+```
+
+Useful harness commands:
+
+```sh
+go test ./internal/harness/...
+go test -race ./internal/harness/...
+go test -count=10 ./internal/harness/...
+```
+
+Run one scenario through the CLI:
+
+```sh
+go build -o ./bin/patterm ./cmd/patterm
+./bin/patterm debug-harness --scenario internal/harness/scenarios/spawn_process_via_palette.json
+```
+
+Harness scenarios create hermetic XDG/config/data/runtime directories, write scenario-local presets and fake scripts, and set `PATTERM_HARNESS=1`. They must not read or write the user's real patterm config.
+
+Failure artifacts are written under:
+
+```text
+internal/harness/.artifacts/
+```
+
+That directory is gitignored. Artifacts include rendered grid text, raw PTY bytes, serialized VT state, MCP snapshots, the resolved environment, and an annotated scenario.
+
+### Harness Environment Notes
+
+The harness and patterm MCP server create Unix sockets and spawn PTYs. In restricted sandboxes this can fail with errors such as:
+
+```text
+listen unix ... setsockopt: operation not permitted
+```
+
+When that happens, rerun the harness tests or `debug-harness` command in an environment that permits Unix sockets and PTYs.
+
+When testing a specific binary, set:
+
+```sh
+PATTERM_BIN=/absolute/path/to/patterm go test ./internal/harness/...
+```
+
+Without `PATTERM_BIN`, harness tests build the current checkout once into a temp location and test that binary.
+
+## Development Notes
+
+- Prefer existing package boundaries. MCP protocol shapes live in `internal/mcp`; runtime behavior usually belongs in `internal/app`.
+- Keep terminal rendering changes covered by focused app tests or a harness scenario.
+- Do not let child PTY output own host chrome. The app owns tab bar, sidebar, status line, and palette.
+- Command-preset trust must be seeded before starting patterm in tests because the app opens the trust store during startup.
+- If a scenario needs external CLIs such as `claude`, `codex`, or `opencode`, gate it behind an explicit opt-in environment variable. CI scenarios should use fake scripts/presets.