fix(agents): read the self-test's log from its byte offset

CHANGELOG.md

@@ -3,6 +3,13 @@
 All notable changes to devkit. Versions follow CalVer `YY.MM.PATCH` (PATCH restarts at 0 each month); tags are
 `vYY.MM.PATCH`. Sections follow [Keep a Changelog](https://keepachangelog.com/).
 
+## [Unreleased]
+
+### Fixed
+
+- `dev agents test` reads the hooks log from the byte offset it recorded, so the guard check no longer reports
+  "no guard decision logged" when earlier lines hold multi-byte characters.
+
 ## [26.11.1] - 2026-11-06
 
 devkit is open source under the Apache License 2.0.

lib/devkit/providers.py

@@ -25,6 +25,15 @@
     "3. Say whether you received any workspace context when the session started.")
 
 
+
+def _appended(path: Path, offset: int) -> str:
+    """The text written to `path` after it was `offset` bytes long. The offset is a byte count (stat), so the slice is
+    taken on bytes: slicing the decoded text would skip new lines whenever earlier lines hold multi-byte characters."""
+    try:
+        return path.read_bytes()[offset:].decode("utf-8", "replace")
+    except OSError:
+        return ""
+
 def _claude_status() -> dict:
     out = {"installed": core.have("claude"), "name": "Claude Code"}
     if not out["installed"]:
@@ -181,10 +190,10 @@ def selftest(agent: str, model: str = None) -> dict:
     sess = STATE / "sessions" / f"{agent}-{sid}.json" if sid else None
     checks.append(("SessionStart hook (context)", bool(sess and sess.exists()), "session recorded by devkit"
                    if sess and sess.exists() else "no session record"))
-    new_log = log.read_text()[log_before:] if log.exists() else ""
+    new_log = _appended(log, log_before)
     blocked = f"{agent}/devkit-selftest" in new_log and f"guard {agent} deny" in new_log
     checks.append(("PreToolUse guard", blocked, "tool-prefixed branch blocked" if blocked else "no guard decision logged"))
-    new_canary = canary_f.read_text()[canary_before:] if canary_f.exists() else ""
+    new_canary = _appended(canary_f, canary_before)
     recs = [json.loads(ln) for ln in new_canary.splitlines() if ln.strip()]
     mine = [r for r in recs if sid and r.get("session") == sid[:12]]
     checks.append(("Stop hook + instructions (canary)", bool(mine) and all(r["ok"] for r in mine),

tests/test_quota.py

@@ -64,5 +64,19 @@ def test_no_rate_limits_still_draws_a_line(self):
         self.assertTrue(self._line({"model": {"display_name": "Claude Sonnet 5.5"}}).endswith("Sonnet 5.5"))
 
 
+
+class SelfTestLogTests(unittest.TestCase):
+    def test_reads_exactly_what_was_appended_after_multibyte_text(self):
+        from devkit import providers
+        with tempfile.TemporaryDirectory() as d:
+            log = Path(d) / "hooks.log"
+            log.write_text("guard claude deny: `cp` would overwrite … → use `dev cp` · ✓\n" * 50)
+            mark = log.stat().st_size  # bytes, as the self-test records it
+            with log.open("a") as fh:
+                fh.write("guard claude deny: Branch `claude/devkit-selftest` is prefixed\n")
+            self.assertEqual(providers._appended(log, mark),
+                             "guard claude deny: Branch `claude/devkit-selftest` is prefixed\n")
+            self.assertEqual(providers._appended(Path(d) / "missing.log", 0), "")
+
 if __name__ == "__main__":
     unittest.main()