runtime contract enforcement (#6)

CONTRIBUTING.md

@@ -21,6 +21,7 @@ pnpm --dir site install
 - `src/` holds production code and unit tests
 - `tests/` holds integration tests
 - `tests/support/` holds shared X11 and daemon helpers for integration coverage
+- `docs/runtime-output.md` is the stable-vs-best-effort runtime output contract for agent-facing CLI work
 
 Keep integration-only helpers out of `src/`.
 

README.md

@@ -100,6 +100,7 @@ deskctl doctor
 - `@wN` refs are short-lived handles assigned by `snapshot` and `list-windows`
 - `--json` output includes a stable `window_id` for programmatic targeting within the current daemon session
 - `list-windows` is a cheap read-only operation and does not capture or write a screenshot
+- the stable runtime JSON/error contract is documented in [docs/runtime-output.md](docs/runtime-output.md)
 
 ## Read and Wait Surface
 
@@ -147,6 +148,16 @@ Successful `get active-window`, `wait window`, and `wait focus` responses return
 
 Wait timeout and selector failures are structured in `--json` mode so agents can recover without string parsing.
 
+## Output Policy
+
+Text mode is compact and follow-up-oriented, but JSON is the parsing contract.
+
+- use `--json` when an agent needs strict parsing
+- rely on `window_id`, selector-related fields, grouped read payloads, and structured error `kind` values for stable automation
+- treat monitor naming, incidental whitespace, and default screenshot file names as best-effort
+
+See [docs/runtime-output.md](docs/runtime-output.md) for the exact stable-vs-best-effort breakdown.
+
 ## Selector Contract
 
 Explicit selector modes:

docs/runtime-output.md

@@ -0,0 +1,178 @@
+# Runtime Output Contract
+
+This document defines the current output contract for `deskctl`.
+
+It is intentionally scoped to the current Linux X11 runtime surface.
+It does not promise stability for future Wayland or window-manager-specific features.
+
+## Goals
+
+- Keep `deskctl` fully non-interactive
+- Make text output actionable for quick terminal and agent loops
+- Make `--json` safe for agent consumption without depending on incidental formatting
+
+## JSON Envelope
+
+Every runtime command uses the same top-level JSON envelope:
+
+```json
+{
+  "success": true,
+  "data": {},
+  "error": null
+}
+```
+
+Stable top-level fields:
+
+- `success`
+- `data`
+- `error`
+
+`success` is always the authoritative success/failure bit.
+When `success` is `false`, the CLI exits non-zero in both text mode and `--json` mode.
+
+## Stable Fields
+
+These fields are stable for agent consumption in the current Phase 1 runtime contract.
+
+### Window Identity
+
+Whenever a runtime response includes a window payload, these fields are stable:
+
+- `ref_id`
+- `window_id`
+- `title`
+- `app_name`
+- `x`
+- `y`
+- `width`
+- `height`
+- `focused`
+- `minimized`
+
+`window_id` is the stable public identifier for a live daemon session.
+`ref_id` is a short-lived convenience handle for the current window snapshot/ref map.
+
+### Grouped Reads
+
+`deskctl get active-window`
+
+- stable: `data.window`
+
+`deskctl get monitors`
+
+- stable: `data.count`
+- stable: `data.monitors`
+- stable per monitor:
+  - `name`
+  - `x`
+  - `y`
+  - `width`
+  - `height`
+  - `width_mm`
+  - `height_mm`
+  - `primary`
+  - `automatic`
+
+`deskctl get version`
+
+- stable: `data.version`
+- stable: `data.backend`
+
+`deskctl get systeminfo`
+
+- stable: `data.backend`
+- stable: `data.display`
+- stable: `data.session_type`
+- stable: `data.session`
+- stable: `data.socket_path`
+- stable: `data.screen`
+- stable: `data.monitor_count`
+- stable: `data.monitors`
+
+### Waits
+
+`deskctl wait window`
+`deskctl wait focus`
+
+- stable: `data.wait`
+- stable: `data.selector`
+- stable: `data.elapsed_ms`
+- stable: `data.window`
+
+### Selector-Driven Action Success
+
+For selector-driven action commands that resolve a window target, these identifiers are stable when present:
+
+- `data.ref_id`
+- `data.window_id`
+- `data.title`
+- `data.selector`
+
+This applies to:
+
+- `click`
+- `dblclick`
+- `focus`
+- `close`
+- `move-window`
+- `resize-window`
+
+The exact human-readable text rendering of those commands is not part of the JSON contract.
+
+### Artifact-Producing Commands
+
+`snapshot`
+`screenshot`
+
+- stable: `data.screenshot`
+
+When the command also returns windows, `data.windows` uses the stable window payload documented above.
+
+## Stable Structured Error Kinds
+
+When a runtime command returns structured JSON failure data, these error kinds are stable:
+
+- `selector_not_found`
+- `selector_ambiguous`
+- `selector_invalid`
+- `timeout`
+- `not_found`
+- `window_not_focused` as `data.last_observation.kind` or equivalent observation payload
+
+Stable structured failure fields include:
+
+- `data.kind`
+- `data.selector` when selector-related
+- `data.mode` when selector-related
+- `data.candidates` for ambiguous selector failures
+- `data.message` for invalid selector failures
+- `data.wait`
+- `data.timeout_ms`
+- `data.poll_ms`
+- `data.last_observation`
+
+## Best-Effort Fields
+
+These values are useful but environment-dependent and should be treated as best-effort:
+
+- exact monitor naming conventions
+- EWMH/window-manager-dependent window ordering details
+- cosmetic text formatting in non-JSON mode
+- screenshot file names when the caller did not provide an explicit path
+- command stderr wording outside the structured `kind` classifications above
+
+## Text Mode Expectations
+
+Text mode is intended to stay compact and follow-up-useful.
+
+The exact whitespace/alignment of text output is not stable.
+The following expectations are stable at the behavioral level:
+
+- important runtime reads print actionable identifiers or geometry
+- selector failures print enough detail to recover without `--json`
+- artifact-producing commands print the artifact path
+- window listings print both `@wN` refs and `window_id` values
+
+If an agent needs strict parsing, it should use `--json`.

skills/SKILL.md

@@ -90,6 +90,14 @@ deskctl daemon status         # Check daemon status
 - `--session NAME` : Session name for multiple daemon instances (default: "default")
 - `--socket PATH` : Custom Unix socket path
 
+## Output Contract
+
+- Prefer `--json` when an agent needs strict parsing.
+- Use `window_id` for stable targeting inside a live daemon session.
+- Use `ref_id` / `@wN` for quick short-lived follow-up actions after `snapshot` or `list-windows`.
+- Structured JSON failures expose machine-usable `kind` values for selector and wait failures.
+- The exact text formatting is intentionally compact but not the parsing contract. See `docs/runtime-output.md` for the stable field policy.
+
 ## Window Refs
 
 After `snapshot` or `list-windows`, windows are assigned short refs:

src/core/types.rs

@@ -88,9 +88,21 @@ impl std::fmt::Display for WindowInfo {
 
 #[allow(dead_code)]
 fn truncate(s: &str, max: usize) -> String {
-    if s.len() <= max {
+    if s.chars().count() <= max {
         s.to_string()
     } else {
-        format!("{}...", &s[..max - 3])
+        let truncated: String = s.chars().take(max.saturating_sub(3)).collect();
+        format!("{truncated}...")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::truncate;
+
+    #[test]
+    fn truncate_is_char_safe() {
+        let input = format!("fire{}fox", '\u{00E9}');
+        assert_eq!(truncate(&input, 7), "fire...");
     }
 }

src/daemon/handler.rs

@@ -81,9 +81,13 @@ async fn handle_click(request: &Request, state: &Arc<Mutex<DaemonState>>) -> Res
         ResolveResult::Match(entry) => {
             let (x, y) = entry.center();
             match state.backend.click(x, y) {
-                Ok(()) => Response::ok(
-                    serde_json::json!({"clicked": {"x": x, "y": y, "selector": selector, "window_id": entry.window_id}}),
-                ),
+                Ok(()) => Response::ok(serde_json::json!({
+                    "clicked": {"x": x, "y": y},
+                    "selector": selector,
+                    "ref_id": entry.ref_id,
+                    "window_id": entry.window_id,
+                    "title": entry.title,
+                })),
                 Err(error) => Response::err(format!("Click failed: {error}")),
             }
         }
@@ -117,9 +121,13 @@ async fn handle_dblclick(request: &Request, state: &Arc<Mutex<DaemonState>>) ->
         ResolveResult::Match(entry) => {
             let (x, y) = entry.center();
             match state.backend.dblclick(x, y) {
-                Ok(()) => Response::ok(
-                    serde_json::json!({"double_clicked": {"x": x, "y": y, "selector": selector, "window_id": entry.window_id}}),
-                ),
+                Ok(()) => Response::ok(serde_json::json!({
+                    "double_clicked": {"x": x, "y": y},
+                    "selector": selector,
+                    "ref_id": entry.ref_id,
+                    "window_id": entry.window_id,
+                    "title": entry.title,
+                })),
                 Err(error) => Response::err(format!("Double-click failed: {error}")),
             }
         }
@@ -267,7 +275,10 @@ async fn handle_window_action(
         Ok(()) => Response::ok(serde_json::json!({
             "action": action,
             "window": entry.title,
+            "title": entry.title,
+            "ref_id": entry.ref_id,
             "window_id": entry.window_id,
+            "selector": selector,
         })),
         Err(error) => Response::err(format!("{action} failed: {error}")),
     }
@@ -296,7 +307,10 @@ async fn handle_move_window(request: &Request, state: &Arc<Mutex<DaemonState>>)
     match state.backend.move_window(entry.backend_window_id, x, y) {
         Ok(()) => Response::ok(serde_json::json!({
             "moved": entry.title,
+            "title": entry.title,
+            "ref_id": entry.ref_id,
             "window_id": entry.window_id,
+            "selector": selector,
             "x": x,
             "y": y,
         })),
@@ -338,7 +352,10 @@ async fn handle_resize_window(request: &Request, state: &Arc<Mutex<DaemonState>>
     {
         Ok(()) => Response::ok(serde_json::json!({
             "resized": entry.title,
+            "title": entry.title,
+            "ref_id": entry.ref_id,
             "window_id": entry.window_id,
+            "selector": selector,
             "width": width,
             "height": height,
         })),