fix(gateway): bounded redacted result preview on tool.completed run events

Slims the salvaged preview helper (drop the try/except around json.dumps —
default=str cannot raise on tool results) and documents the tool.completed
SSE shape. Adds the control test that the multimodal envelope dict is still
classified as a success, so the dict passthrough only widens the failure
detection to real structured results.

The idea of carrying the tool result on tool.completed for /v1/runs
consumers was first proposed in #22362; that PR's executor half
(result=function_result) is already on main, and its wire half is landed
here in redacted, bounded form instead of the raw payload.

Salvages #111821 (@KoNit-K), part of #111815.

Co-authored-by: kidrauhl123 <105764349+kidrauhl123@users.noreply.github.com>

gateway/platforms/api_server_runs.py

@@ -51,18 +51,14 @@ _TOOL_COMPLETED_PREVIEW_MAX_CHARS = 500
 
 
 def _tool_completed_preview(result: Any, redact_sensitive_text: Callable[..., str]) -> str:
-    """Return a bounded, secret-redacted completion summary for public run events."""
+    """Bounded, secret-redacted result summary for the public run stream — redacted BEFORE
+    truncation so a cut never leaves a secret's prefix on the wire."""
     if result is None:
         return ""
-    if not isinstance(result, str):
-        try:
-            result = json.dumps(result, ensure_ascii=False, default=str)
-        except (TypeError, ValueError):
-            result = str(result)
-    preview = redact_sensitive_text(result, force=True)
-    if len(preview) > _TOOL_COMPLETED_PREVIEW_MAX_CHARS:
-        return preview[:_TOOL_COMPLETED_PREVIEW_MAX_CHARS - 3] + "..."
-    return preview
+    text = result if isinstance(result, str) else json.dumps(result, ensure_ascii=False, default=str)
+    preview = redact_sensitive_text(text, force=True)
+    limit = _TOOL_COMPLETED_PREVIEW_MAX_CHARS
+    return preview if len(preview) <= limit else preview[: limit - 3] + "..."
 
 
 def _remember_room_retention(request: "web.Request", claims: dict[str, Any]) -> None:

tests/agent/test_display_tool_failure.py

@@ -73,10 +73,16 @@ class TestDetectToolFailureTerminal:
         assert "Terminal backend degraded:" not in suffix
 
     def test_nonzero_dict_result_is_a_failure(self):
+        """An already-parsed terminal result (a plugin tool_execution middleware may hand back the
+        dict instead of the JSON string) must classify like its JSON form (#111815)."""
         is_failure, suffix = _detect_tool_failure("terminal", {"exit_code": 2})
         assert is_failure is True
         assert suffix == " [exit 2]"
 
+    def test_multimodal_envelope_dict_is_still_a_success(self):
+        envelope = {"_multimodal": True, "content": [{"type": "text", "text": "screenshot taken"}]}
+        assert _detect_tool_failure("browser_exec", envelope) == (False, "")
+
     def test_degraded_backend_without_hint_shows_reason_alone(self):
         result = json.dumps({"output": "", "exit_code": -1, "status": "degraded",
                              "reason": "SSH connection to bob@host timed out", "retry_hint": "",

website/docs/user-guide/features/api-server.md

@@ -475,6 +475,15 @@ Statuses are retained briefly after terminal states (`completed`, `failed`, or `
 
 Server-Sent Events stream of the run's tool-call progress, token deltas, and lifecycle events. Designed for dashboards and thick clients that want to attach/detach without losing state.
 
+Tool lifecycle events carry `tool.started` (`tool`, `preview` of the arguments) and
+`tool.completed` (`tool`, `duration` in seconds, `error`, and a `preview` of the result). The
+`error` flag reflects the tool's own outcome — a non-zero terminal `exit_code`, a structured
+`{"error": ...}` result, a denied approval — whether the result arrives as a JSON string or an
+already-parsed object. The completion `preview` is the result text (structured results are
+JSON-encoded), passed through forced secret redaction and then truncated to 500 characters, so a
+client can tell an approval refusal (`BLOCKED: ...`) from an ordinary failure without receiving
+the unbounded tool payload.
+
 When the agent delegates work to background subagents, the stream also carries
 `subagent.start` and `subagent.complete` lifecycle events, so clients can
 observe delegation outcomes — including timeouts and failures — instead of the