diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3e806207..7dfcc1b3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.493] — 2026-06-18 — Release RC (ignore malformed background-process wakeup events)
+
+### Fixed
+
+- **Malformed background-process wakeup events no longer appear as empty `unknown completed` prompts.** WebUI now ignores empty or unknown completion-queue payloads and renders watch-pattern overflow summaries as explicit watch notifications instead of fake process completions. Thanks @ai-ag2026.
+
 ## [v0.51.492] — 2026-06-18 — Release RB (large context window preserved on session reload)
 
 ### Fixed
diff --git a/api/background_process.py b/api/background_process.py
index 46801949..ab82083f 100644
--- a/api/background_process.py
+++ b/api/background_process.py
@@ -350,17 +350,28 @@ def _truncate(text: str, limit: int) -> str:
     return s[:limit] + "\n…(truncated)"
 
 
-def format_wakeup_prompt(evt: dict) -> str:
+def format_wakeup_prompt(evt: object) -> str | None:
     """Build the synthetic [IMPORTANT: …] message the agent will see.
 
     Mirrors ``cli._format_process_notification`` so wakeup payloads look the
     same in CLI and WebUI sessions.
     """
+    if not isinstance(evt, dict) or not evt:
+        return None
+
     evt_type = evt.get("type", "completion")
-    sid = evt.get("session_id", "unknown")
-    cmd = evt.get("command", "unknown")
+    sid = str(evt.get("session_id") or "").strip()
+    cmd = str(evt.get("command") or "").strip()
+    # The current server-side wakeup drain drops global watch-overflow events
+    # before this formatter because they intentionally carry no session_key.
+    # Keep this branch defensive so any future routable overflow summary is not
+    # mis-rendered as a fake process completion.
+    if evt_type in {"watch_overflow_tripped", "watch_overflow_released"}:
+        msg = str(evt.get("message") or "").strip()
+        return f"[IMPORTANT: {msg}]" if msg else None
     if evt_type == "watch_disabled":
-        return f"[IMPORTANT: {evt.get('message', '')}]"
+        msg = str(evt.get("message") or "").strip()
+        return f"[IMPORTANT: {msg}]" if msg else None
     if evt_type == "watch_match":
         pat = evt.get("pattern", "?")
         out = _truncate(evt.get("output", ""), 4000)
@@ -373,6 +384,12 @@ def format_wakeup_prompt(evt: dict) -> str:
         if sup:
             body += f"\n({sup} earlier matches were suppressed by rate limit)"
         return body + "]"
+    if evt_type != "completion":
+        return None
+
+    if not (sid or cmd or "exit_code" in evt or evt.get("output")):
+        return None
+
     # Default: completion event
     exit_code = evt.get("exit_code", "?")
     out = _truncate(evt.get("output", ""), 4000)
@@ -880,7 +897,8 @@ def _process_one(evt: dict) -> None:
         # `{session_id, task_id, completed_at, summary?, event_id}`, so
         # we derive the prompt directly from the evt here (same source the
         # prior _build_payload used).
-        wakeup_prompt = format_wakeup_prompt(evt).strip()
+        wakeup_prompt_raw = format_wakeup_prompt(evt)
+        wakeup_prompt = wakeup_prompt_raw.strip() if wakeup_prompt_raw else ""
         if wakeup_prompt:
             if _session_has_active_turn(session_id):
                 # Defer-path fix: persist the prompt so a turn-teardown
diff --git a/tests/test_background_process_wakeup_format.py b/tests/test_background_process_wakeup_format.py
new file mode 100644
index 00000000..16e2bcf1
--- /dev/null
+++ b/tests/test_background_process_wakeup_format.py
@@ -0,0 +1,78 @@
+from api.background_process import format_wakeup_prompt
+
+
+def test_format_wakeup_prompt_skips_empty_event():
+    assert format_wakeup_prompt({}) is None
+
+
+def test_format_wakeup_prompt_skips_non_dict_event():
+    assert format_wakeup_prompt(None) is None
+    assert format_wakeup_prompt(42) is None
+
+
+def test_format_wakeup_prompt_skips_unknown_event_type():
+    evt = {"type": "unknown_event", "message": "do not inject me"}
+    assert format_wakeup_prompt(evt) is None
+
+
+def test_format_wakeup_prompt_handles_watch_disabled():
+    evt = {
+        "type": "watch_disabled",
+        "session_id": "proc_abc",
+        "session_key": "webui-session",
+        "command": "tail -f log",
+        "message": "Watch patterns disabled for process proc_abc.",
+    }
+    assert (
+        format_wakeup_prompt(evt)
+        == "[IMPORTANT: Watch patterns disabled for process proc_abc.]"
+    )
+
+
+def test_format_wakeup_prompt_skips_blank_watch_disabled():
+    assert format_wakeup_prompt({"type": "watch_disabled"}) is None
+    assert format_wakeup_prompt({"type": "watch_disabled", "message": ""}) is None
+
+
+def test_format_wakeup_prompt_handles_watch_overflow_tripped():
+    evt = {
+        "type": "watch_overflow_tripped",
+        "session_id": "",
+        "session_key": "",
+        "command": "",
+        "message": "Watch-pattern overflow: suppressing further watch_match events.",
+    }
+    assert (
+        format_wakeup_prompt(evt)
+        == "[IMPORTANT: Watch-pattern overflow: suppressing further watch_match events.]"
+    )
+
+
+def test_format_wakeup_prompt_handles_watch_overflow_released():
+    evt = {
+        "type": "watch_overflow_released",
+        "session_id": "",
+        "session_key": "",
+        "command": "",
+        "suppressed": 3,
+        "message": "Watch-pattern notifications resumed. 3 match event(s) were suppressed.",
+    }
+    assert (
+        format_wakeup_prompt(evt)
+        == "[IMPORTANT: Watch-pattern notifications resumed. 3 match event(s) were suppressed.]"
+    )
+
+
+def test_format_wakeup_prompt_keeps_normal_completion():
+    evt = {
+        "type": "completion",
+        "session_id": "proc_abc",
+        "command": "sleep 1",
+        "exit_code": 0,
+        "output": "done",
+    }
+    result = format_wakeup_prompt(evt)
+    assert result is not None
+    assert "Background process proc_abc completed" in result
+    assert "Command: sleep 1" in result
+    assert "Output:\ndone" in result