map POSIX errno to actionable IPC error kinds (T-79)

pane.spawn (via PtyException.errno) and editor.open (via
FileSystemException.osError.errorCode) now route ENOENT to
not_found, EACCES/EPERM to user_error with a permissions hint,
EISDIR/ENOTDIR/EEXIST to distinct user-error/conflict, and
EMFILE/ENFILE to tool_error with a "fd limit hit" hint. The
mapping lives in lib/src/ipc/errno_mapping.dart so other handlers
can adopt the same surface as they pick up errno-bearing failures.

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-05T13:12:14Z",
+  "exported_at": "2026-05-06T07:56:04Z",
   "decisions": [
     {
       "id": "D-1",
@@ -2624,10 +2624,10 @@
       "id": "T-77",
       "type": "task",
       "title": "IPC server: per-request timeout, broadcast logging, stale-socket race",
-      "status": "in_progress",
+      "status": "done",
       "priority": "high",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 13:10:57"
+      "updated_at": "2026-05-05 13:12:20"
     },
     {
       "id": "T-78",
@@ -2642,10 +2642,10 @@
       "id": "T-79",
       "type": "task",
       "title": "pane.spawn / editor.open: map errno to actionable error kinds",
-      "status": "backlog",
+      "status": "in_progress",
       "priority": "medium",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 12:58:59"
+      "updated_at": "2026-05-06 07:45:02"
     },
     {
       "id": "T-80",
@@ -3975,6 +3975,20 @@
       "old_value": "backlog",
       "new_value": "in_progress",
       "changed_at": "2026-05-05 13:10:57"
+    },
+    {
+      "ticket_id": "T-77",
+      "field": "status",
+      "old_value": "in_progress",
+      "new_value": "done",
+      "changed_at": "2026-05-05 13:12:20"
+    },
+    {
+      "ticket_id": "T-79",
+      "field": "status",
+      "old_value": "backlog",
+      "new_value": "in_progress",
+      "changed_at": "2026-05-06 07:45:02"
     }
   ]
 }

CHANGELOG.md

@@ -48,6 +48,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   client dropped on response-write failure, and the stale-socket
   retry now probes for a live daemon before unlinking the socket
   (refusing to start if one answers).
+- `pane.spawn` and `editor.open` now map POSIX errno values to
+  actionable IPC error kinds. ENOENT → `not_found`, EACCES/EPERM
+  → `user_error` with a permissions hint, EISDIR/ENOTDIR/EEXIST
+  → distinct user-error/conflict, EMFILE/ENFILE → `tool_error`
+  with a "fd limit hit" hint. Previously every spawn/open failure
+  was an indistinguishable `tool_error`.
 
 ### Security
 

lib/src/daemon/editor_commands.dart

@@ -9,8 +9,11 @@
 /// one-to-one onto these in `bin/clide.dart`.
 library;
 
+import 'dart:io' show FileSystemException;
+
 import '../editor/registry.dart';
 import '../ipc/envelope.dart';
+import '../ipc/errno_mapping.dart';
 import '../ipc/schema_v1.dart';
 import 'dispatcher.dart';
 
@@ -64,6 +67,22 @@ Future<IpcResponse> _open(IpcRequest req, EditorRegistry r) async {
   try {
     final buf = await r.open(path);
     return IpcResponse.ok(id: req.id, data: buf.toJson());
+  } on FileSystemException catch (e) {
+    final errno = e.osError?.errorCode;
+    if (errno != null) {
+      return IpcResponse.err(
+        id: req.id,
+        error: errnoToIpcError(errno: errno, op: 'editor.open', target: path),
+      );
+    }
+    return IpcResponse.err(
+      id: req.id,
+      error: IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: 'editor.open failed: ${e.message}',
+      ),
+    );
   } catch (e) {
     return IpcResponse.err(
       id: req.id,

lib/src/daemon/pane_commands.dart

@@ -12,9 +12,11 @@ library;
 import 'dart:convert';
 
 import '../ipc/envelope.dart';
+import '../ipc/errno_mapping.dart';
 import '../ipc/schema_v1.dart';
 import '../panes/pane.dart';
 import '../panes/registry.dart';
+import '../pty/errors.dart';
 import 'dispatcher.dart';
 
 void registerPaneCommands(DaemonDispatcher d, PaneRegistry registry) {
@@ -84,6 +86,26 @@ Future<IpcResponse> _spawn(IpcRequest req, PaneRegistry registry) async {
       title: args['title'] as String?,
     );
     return IpcResponse.ok(id: req.id, data: pane.toJson());
+  } on PtyException catch (e) {
+    final errno = e.errno;
+    if (errno != null) {
+      return IpcResponse.err(
+        id: req.id,
+        error: errnoToIpcError(
+          errno: errno,
+          op: 'pane.spawn',
+          target: argv.isNotEmpty ? argv.first : null,
+        ),
+      );
+    }
+    return IpcResponse.err(
+      id: req.id,
+      error: IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: 'pane.spawn failed: ${e.message}',
+      ),
+    );
   } catch (e) {
     return IpcResponse.err(
       id: req.id,

lib/src/ipc/errno_mapping.dart

@@ -0,0 +1,103 @@
+/// Map POSIX errno values to IPC error envelopes with actionable
+/// messages. Used by command handlers that wrap syscall-backed work
+/// (PTY spawn, file open) so the client can distinguish "binary not
+/// found" from "permission denied" from "system limit hit" instead
+/// of seeing the same generic `tool_error: foo failed`.
+library;
+
+import 'envelope.dart';
+import 'schema_v1.dart';
+
+/// Selected POSIX errno values we map specially. Others fall through
+/// to a generic toolError. Values match Linux glibc and macOS Darwin
+/// (the two platforms that share the same numbers for these entries).
+abstract class PosixErrno {
+  static const int eperm = 1;
+  static const int enoent = 2;
+  static const int esrch = 3;
+  static const int eio = 5;
+  static const int ebadf = 9;
+  static const int eagain = 11;
+  static const int enomem = 12;
+  static const int eacces = 13;
+  static const int eexist = 17;
+  static const int enotdir = 20;
+  static const int eisdir = 21;
+  static const int emfile = 24;
+  static const int enfile = 23;
+  static const int epipe = 32;
+}
+
+/// Build an [IpcError] from a POSIX [errno] for an operation [op]
+/// (e.g. `pane.spawn`, `editor.open`) on optional [target] (a path,
+/// command name, etc.). The returned error uses `notFound`,
+/// `userError`, or `toolError` based on what's actionable.
+IpcError errnoToIpcError({
+  required int errno,
+  required String op,
+  String? target,
+  String? raw,
+}) {
+  final what = target != null ? ' ($target)' : '';
+  switch (errno) {
+    case PosixErrno.enoent:
+      return IpcError(
+        code: IpcExitCode.notFound,
+        kind: IpcErrorKind.notFound,
+        message: '$op: not found$what',
+      );
+    case PosixErrno.eacces:
+    case PosixErrno.eperm:
+      return IpcError(
+        code: IpcExitCode.userError,
+        kind: IpcErrorKind.userError,
+        message: '$op: permission denied$what',
+        hint: 'check file permissions or run with appropriate access',
+      );
+    case PosixErrno.eisdir:
+      return IpcError(
+        code: IpcExitCode.userError,
+        kind: IpcErrorKind.userError,
+        message: '$op: is a directory$what',
+      );
+    case PosixErrno.enotdir:
+      return IpcError(
+        code: IpcExitCode.userError,
+        kind: IpcErrorKind.userError,
+        message: '$op: not a directory$what',
+      );
+    case PosixErrno.eexist:
+      return IpcError(
+        code: IpcExitCode.conflict,
+        kind: IpcErrorKind.conflict,
+        message: '$op: already exists$what',
+      );
+    case PosixErrno.emfile:
+    case PosixErrno.enfile:
+      return IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: '$op: too many open files',
+        hint: 'system or per-process file descriptor limit reached',
+      );
+    case PosixErrno.enomem:
+      return IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: '$op: out of memory',
+      );
+    case PosixErrno.eagain:
+      return IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: '$op: resource temporarily unavailable',
+        hint: 'retry may succeed',
+      );
+    default:
+      return IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: '$op failed${raw != null ? ': $raw' : ' (errno=$errno)'}',
+      );
+  }
+}

test/ipc/errno_mapping_test.dart

@@ -0,0 +1,55 @@
+import 'package:clide/src/ipc/errno_mapping.dart';
+import 'package:clide/src/ipc/schema_v1.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('errnoToIpcError', () {
+    test('ENOENT → notFound with target', () {
+      final err = errnoToIpcError(errno: PosixErrno.enoent, op: 'pane.spawn', target: 'claude');
+      expect(err.kind, IpcErrorKind.notFound);
+      expect(err.code, IpcExitCode.notFound);
+      expect(err.message, contains('claude'));
+      expect(err.message, contains('not found'));
+    });
+
+    test('EACCES → userError with hint', () {
+      final err = errnoToIpcError(errno: PosixErrno.eacces, op: 'editor.open', target: '/etc/shadow');
+      expect(err.kind, IpcErrorKind.userError);
+      expect(err.code, IpcExitCode.userError);
+      expect(err.message, contains('permission denied'));
+      expect(err.hint, isNotNull);
+    });
+
+    test('EISDIR → userError', () {
+      final err = errnoToIpcError(errno: PosixErrno.eisdir, op: 'editor.open', target: 'src/');
+      expect(err.kind, IpcErrorKind.userError);
+      expect(err.message, contains('is a directory'));
+    });
+
+    test('EEXIST → conflict', () {
+      final err = errnoToIpcError(errno: PosixErrno.eexist, op: 'files.create', target: 'README.md');
+      expect(err.kind, IpcErrorKind.conflict);
+      expect(err.code, IpcExitCode.conflict);
+    });
+
+    test('EMFILE → toolError with hint', () {
+      final err = errnoToIpcError(errno: PosixErrno.emfile, op: 'pane.spawn');
+      expect(err.kind, IpcErrorKind.toolError);
+      expect(err.message, contains('too many open files'));
+      expect(err.hint, isNotNull);
+    });
+
+    test('unknown errno falls through to toolError', () {
+      final err = errnoToIpcError(errno: 999, op: 'pane.spawn');
+      expect(err.kind, IpcErrorKind.toolError);
+      expect(err.code, IpcExitCode.toolError);
+      expect(err.message, contains('errno=999'));
+    });
+
+    test('raw message overrides errno suffix in fallback', () {
+      final err = errnoToIpcError(errno: 999, op: 'pane.spawn', raw: 'kernel exploded');
+      expect(err.message, contains('kernel exploded'));
+      expect(err.message, isNot(contains('errno=999')));
+    });
+  });
+}