Add batch 1 (YosysHQ_picorv32, alexforencich_verilog-ethernet, The-OpenROAD-Project_OpenROAD, darklife_darkriscv, corundum_corundum)

f61bac1 verified 28 days ago

preview code

Raw History Blame Contribute Delete

31.7 kB

Web Viewer Server API

This document describes the wire protocol between the OpenROAD web viewer's browser frontend and its C++ server. The protocol is intended to be stable enough that an alternate client (custom browser app, headless test driver, third-party visualization) can be implemented against it without reading the JavaScript reference frontend.

The server exposes one HTTP route for static assets and one WebSocket endpoint for all dynamic operations. The WebSocket carries a small binary framing with JSON or PNG payloads.

Connecting

HTTP and WebSocket share one TCP port (default 8080, configured via web_server -port). A single Boost.Beast listener serves both.
HTTP GET / returns index.html; GET /<path> returns the embedded asset at that path (*.js, *.css, etc.). 404 otherwise. No method other than GET is supported.
The WebSocket upgrade is also at the root path. Once upgraded, all request/response traffic uses the binary framing below.

Wire frame format

Every WebSocket message — both client→server and server→client — is a binary frame with the following layout:

Bytes	Field	Type	Description
0..3	`id`	`uint32` big-endian	Request correlator (see Correlation).
4	`type`	`uint8`	Payload type: `0=JSON`, `1=PNG`, `2=Error`.
5..7	reserved	`uint8 × 3`	Must be zero.
8..	`payload`	bytes	UTF-8 JSON, PNG-encoded image, or UTF-8 error string.

Correlation

The client picks a 32-bit id for each request and the server echoes it on the corresponding response. The reference frontend uses a monotonically-increasing counter; any unique value is fine.
Server-push messages (broadcasts that aren't replies to a specific request) use id = 0.

Payload types

0 = JSON — body is a UTF-8 JSON document. Default response shape.
1 = PNG — body is a raw PNG file. Used by tile and heatmap_tile.
2 = Error — body is a UTF-8 plain-text error message. Surfaces in the server log as WEB-0043 (see Error contract below).

Request envelope

Every request from the client is a JSON object with at least:

Field	Type	Required	Description
`id`	`int`	✓	Request id; echoed back unmodified.
`type`	`string`	✓	Selects the handler. See per-type sections.

The rest of the object's fields are handler-specific.

A request with malformed JSON, a non-object root, or missing/wrongly-typed id or type produces a kError response of the form Malformed request (missing or invalid id/type) and a WEB-0043 warning. A well-formed request whose type is not registered produces Unknown request type: <type> and the same warning.

Wire conventions

The server is strict on field types. Every field below has one canonical JSON type; sending any other type for a present field yields a kError response.

Integers travel as JSON numbers parsed as int64.
Booleans travel as JSON true / false.
Doubles are JSON numbers; whole-valued numbers may parse as integers on the wire, which the server tolerates for double-typed fields only.
Strings are JSON strings; arrays are JSON arrays.
Two narrow exceptions documented inline:
- select.zoom accepts int or double (Leaflet's zoomSnap: 0 permits fractional zoom values).
- set_heatmap.value for int-typed settings accepts int or double and rounds (the JS frontend always uses parseFloat).

A "✓" in the Required column means the server reads the field unconditionally. Missing fields throw and produce a kError.

Server-push messages

These arrive with id = 0 and type = JSON. The client subscribes by simply listening on the WebSocket; there's no opt-in.

`type` field	Trigger	Body
`refresh`	Initial spatial-index build finished after a connection / `web_server`.	`{"type":"refresh"}`
`log`	Buffered server log lines being flushed to clients.	`{"type":"log","text":"<lines>"}`
`debug_paused`	The graphics-debug pause hook entered (e.g. placer breakpoint).	`{"type":"debug_paused"}`
`debug_resumed`	Pause hook exited.	`{"type":"debug_resumed"}`
`debug_refresh`	Debug overlay needs a re-render.	`{"type":"debug_refresh"}`
`shutdown`	Server is exiting; client should not auto-reconnect.	`{"type":"shutdown"}`

Tile rendering

`tile`

Render a single 256×256 PNG tile of the layout.

Field	Type	Required	Description
`layer`	`string`	✓	Layer name (`metal1`, `metal2`, …) or one of the synthetic layers `_instances`, `_modules`, `_pins`, or `_overlay`.
`z`	`int`	✓	Leaflet tile zoom level (0 = whole design).
`x`	`int`	✓	Tile column at zoom `z`.
`y`	`int`	✓	Tile row at zoom `z`.
`visible_layers`	`string[]`	✓	Tech layers currently visible on the pins synthetic layer; empty array means hide all pin markers.
visibility flags	`bool`	per-flag default	See TileVisibility flags below. Any flag may be omitted to take the default.
`site_<name>`	`bool`	—	Per-row-site visibility. Only consulted when `rows == true`.

Response: PNG image (frame type 1).

`bounds`

Return the design's bounding box and shape-cache readiness.

No request fields beyond the envelope.

Response (JSON):

{
  "bounds": [[yMin, xMin], [yMax, xMax]],
  "shapes_ready": true,
  "pin_max_size": 200
}

bounds is in DBU. shapes_ready indicates whether the spatial index finished building (the refresh push fires when this flips to true). pin_max_size is the largest BPin dimension in DBU, used by the client to size the pin-marker overlay.

`tech`

Return tech-layer metadata, sites, and block info.

No request fields.

Response (JSON):

{
  "layers": ["metal1", "via1", "metal2", "..."],
  "layer_colors": [[r, g, b], ...],
  "sites": ["FreePDK45_38x28_10R_NP_162NW_34O", "..."],
  "has_liberty": true,
  "dbu_per_micron": 1000,
  "block_name": "top"
}

layer_colors is parallel to layers; each entry is RGB in 0..255. block_name is the empty string when no block is loaded.

Selection and inspection

`select`

Pick the topmost selectable object at a DBU coordinate. Cycles through overlapping objects on repeated calls at the same point.

Field	Type	Required	Description
`dbu_x`	`int`	✓	X coordinate in DBU.
`dbu_y`	`int`	✓	Y coordinate in DBU.
`zoom`	`int` or `double`	✓	Current Leaflet zoom level. Server truncates fractional values.
`visible_layers`	`string[]`	✓	Same semantics as `tile`.
visibility flags	`bool`	per-flag default	Same as `tile`.
`site_<name>`	`bool`	—	Same as `tile`.

Response (JSON): the inspect payload (see inspect below) plus a top-level selected array listing every overlapping candidate:

{
  "selected": [
    {"name": "buf1", "type": "Inst", "bbox": [xMin, yMin, xMax, yMax]},
    ...
  ],
  "can_navigate_back": 0,
  "name": "buf1",
  "type": "Inst",
  "properties": [...],
  "bbox": [xMin, yMin, xMax, yMax]
}

When no object is selectable at the point, selected is [] and only can_navigate_back is set (no name/type/properties).

`inspect`

Re-inspect a previously-selected object using its session-local id from a prior select.

Field	Type	Required	Description
`select_id`	`int`	✓	ID returned in a previous `select` `*_select_id` field.

Response (JSON): same shape as the inspect portion of select's response, without the selected array:

{
  "can_navigate_back": 1,
  "name": "buf1/Z",
  "type": "ITerm",
  "properties": [
    {"name": "Master", "value": "BUF_X16",
     "value_select_id": 12},
    {"name": "Children", "children": [
      {"name": "child", "value": "...", "value_select_id": 13},
      ...
    ]}
  ],
  "bbox": [xMin, yMin, xMax, yMax],
  "has_guides": 1
}

*_select_id fields appear next to any property whose value is itself selectable; the client passes them back via inspect to drill down. has_guides is present (and 1) only when the inspected object is a net with route guides. can_navigate_back is 1 whenever the back-navigation history is non-empty.

If select_id is out of range, the response includes {"error": "invalid select_id"} instead of the metadata.

`inspect_back`

Pop the back-navigation stack and re-emit the inspect payload for the previous object. No request fields.

Response (JSON): same shape as inspect but for the previous object.

`hover`

Highlight an object's shapes (mouseover preview) without selecting it.

Field	Type	Required	Description
`select_id`	`int`	✓	Selectable id; pass `-1` to clear hover (mouseleave).

Response (JSON):

{"ok": 1, "count": 3, "rects": [[xMin, yMin, xMax, yMax], ...]}

count is the number of highlight rects collected; an empty rect array falls back to the object bbox.

`snap`

Find the nearest design edge to a cursor position (for the ruler / crosshair tool).

Field	Type	Required	Description
`dbu_x`	`int`	✓	Cursor X in DBU.
`dbu_y`	`int`	✓	Cursor Y in DBU.
`radius`	`int`	✓	Search radius in DBU.
`point_threshold`	`int`	✓	If two snap candidates fall within this many DBU of each other, prefer the point endpoint.
`horizontal`	`bool`	✓	Allow snapping to horizontal edges.
`vertical`	`bool`	✓	Allow snapping to vertical edges.
`visible_layers`	`string[]`	✓	Layers to consider for routing snaps.
visibility flags	`bool`	per-flag default	Same as `tile`.

Response (JSON):

{
  "found": true,
  "is_point": false,
  "edge": [[x1, y1], [x2, y2]]
}

is_point is true when the snap collapsed to a point (e.g. corner). On "found": false, edge and is_point are absent.

Schematic

`schematic_cone`

Build a cone of influence around an instance (fanin/fanout) and return it as a Yosys-format netlist for the netlistsvg renderer.

Field	Type	Required	Description
`inst_name`	`string`	✓	Anchor instance name.
`fanin_depth`	`int`	✓	BFS depth upstream (`0` = no fanin).
`fanout_depth`	`int`	✓	BFS depth downstream.

Response (JSON): a Yosys netlist JSON (schema) limited to one module named top:

{
  "modules": {
    "top": {
      "attributes": {},
      "ports":    {"<bterm name>": {"direction": "input", "bits": [n]}},
      "cells":    {"<inst name>":  {"hide_name": 0, "type": "<master>", "attributes": {}, "parameters": {}, "port_directions": {...}, "connections": {...}}},
      "netnames": {"<net name>":   {"hide_name": 0, "bits": [n], "attributes": {}}}
    }
  }
}

Cones are capped at 150 instances; nets with fanout > 30 are skipped to keep the schematic readable.

`schematic_full`

Same shape as schematic_cone but emits the entire block (no caps). No request fields.

`schematic_inspect`

Inspect a single instance by name (used when the schematic widget selects a cell). Equivalent to a select+inspect pair but skips the geometric pick.

Field	Type	Required	Description
`inst_name`	`string`	✓	Instance name to inspect.

Response (JSON): same shape as inspect.

Tcl

`tcl_eval`

Evaluate a Tcl command. Captures info log output and the command's result.

Field	Type	Required	Description
`cmd`	`string`	✓	The Tcl source string.

Response (JSON):

{
  "output":  "captured stdout/log lines",
  "result":  "Tcl_GetStringResult(interp)",
  "is_error": false
}

When the user runs exit or quit from the browser, the server's override turns the result into a shutdown signal:

{
  "output":   "...",
  "result":   "Exiting OpenROAD.",
  "is_error": false,
  "action":   "shutdown"
}

Clients that see action: "shutdown" should disable auto-reconnect.

`tcl_complete`

Tab-completion for the Tcl prompt. Splits the line at the cursor and reports candidate completions plus the replacement range.

Field	Type	Required	Description
`line`	`string`	✓	Full input line.
`cursor_pos`	`int`	✓	Cursor offset into `line` (0-based).

Response (JSON):

{
  "completions":   ["...", "..."],
  "mode":          "commands" | "variables" | "arguments",
  "prefix":        "the matched prefix",
  "replace_start": 4,
  "replace_end":   8
}

Timing and charts

`timing_report`

Return the worst N timing paths.

Field	Type	Required	Default	Description
`is_setup`	`bool`	✓	—	`true` → setup paths, `false` → hold.
`max_paths`	`int`	✓	—	Maximum number of paths to return.
`slack_min`	`double`		`-FLT_MAX`	Lower slack bound (inclusive). Optional filter.
`slack_max`	`double`		`+FLT_MAX`	Upper slack bound (exclusive). Optional filter.

Response (JSON):

{
  "paths": [
    {
      "start_clk": "clk",  "end_clk": "clk",
      "required":  1.0,    "arrival": 1.5,
      "slack":    -0.5,    "skew":    0.0,
      "path_delay": 0.9,   "logic_depth": 4, "fanout": 12,
      "start_pin":  "ff1/CK", "end_pin": "ff2/D",
      "data_nodes":    [{"pin": "...", "fanout": 1, "rise": true,  "clk": false, "time": 0.0, "delay": 0.0, "slew": 0.0, "load": 0.0}, ...],
      "capture_nodes": [{...}]
    },
    ...
  ]
}

`timing_highlight`

Highlight one timing path's shapes on the layout, optionally with a single stage emphasized.

Field	Type	Required when	Description
`path_index`	`int`	always	Index into the most recent `timing_report.paths`. `-1` clears.
`is_setup`	`bool`	`path_index >= 0`	Which side of the report `path_index` indexes into.
`pin_name`	`string`	optional, `>= 0` only	If set, emphasize this pin's net within the path.

Response (JSON): {"ok": true}. The actual update is the layer overlay redraw on the next tile request.

`slack_histogram`

Histogram of endpoint slacks for one side of the timing report.

Field	Type	Required	Description
`is_setup`	`bool`	✓	Setup vs hold endpoints.
`path_group`	`string`		If non-empty, restrict to this path group. Default: all.
`clock_name`	`string`		If non-empty, restrict to this clock. Default: all.

Response (JSON):

{
  "bins": [{"lower": -1.0, "upper": 0.0, "count": 12, "negative": true}, ...],
  "unconstrained_count": 5,
  "total_endpoints":    1234,
  "time_unit":          "ns"
}

`chart_filters`

List the path groups and clocks available for the slack histogram filter dropdowns. No request fields.

Response (JSON):

{"path_groups": ["**default**", ...], "clocks": ["clk1", "clk2", ...]}

`clock_tree`

Compute the clock tree (CTS view).

No request fields.

Response (JSON):

{
  "clocks": [
    {
      "name": "clk",
      "min_arrival": 0.0,
      "max_arrival": 0.5,
      "time_unit":   "ns",
      "nodes": [
        {"id": 0, "parent_id": -1, "name": "...", "pin_name": "...",
         "type": "root" | "buffer" | "inverter" | "clock_gate" | "register" | "macro" | "unknown",
         "arrival": 0.0, "delay": 0.0, "fanout": 0, "level": 0,
         "dbu_x": 0, "dbu_y": 0},
        ...
      ]
    }
  ]
}

`clock_tree_highlight`

Highlight a single clock-tree node's instance in the layout.

Field	Type	Required	Description
`inst_name`	`string`	✓	Empty string clears the highlight.

Response (JSON): {"ok": true}.

Module hierarchy

`module_hierarchy`

Return the module hierarchy tree (left sidebar).

No request fields.

Response (JSON):

{
  "nodes": [
    {
      "id": 0, "parent_id": -1,
      "inst_name":   "<top>", "module_name": "...",
      "insts":     1234, "macros":      0, "modules":     5,
      "area":     1.23,
      "local_insts": 100, "local_macros": 0, "local_modules": 5,
      "node_kind":   1,           // present when not kModule (kLeafGroup=1, kTypeGroup=2, kInstance=3)
      "odb_id":      42,           // present only on kModule nodes
      "color":       [r, g, b]     // present only on kModule nodes
    },
    ...
  ]
}

`set_module_colors`

Update the per-module color override map (for the _modules tile layer).

Field	Type	Required	Description
`colors`	`string`	✓	Custom delimited form: `<id>:<r>,<g>,<b>,<a>;<id>:<r>,<g>,<b>,<a>;...`. Empty = clear all.

Response (JSON): {"ok": 1, "count": <updated module count>}.

`set_focus_nets`

Add or remove a net from the focus-nets set (route-tracing overlay).

Field	Type	Required	Description
`action`	`string`	✓	`"add"`, `"remove"`, or `"clear"`.
`net_name`	`string`	✓	Net name for `add`/`remove`. Pass `""` for `clear`; the value is unused but the field must be present.

Response (JSON): {"ok": 1, "count": <focus-net count>}.

`set_route_guides`

Same shape as set_focus_nets but updates the route-guides overlay.

Heat maps

`heatmaps`

Return metadata for every registered heat map.

No request fields.

Response (JSON):

{
  "active":  "Pin",                    // empty when none active
  "heatmaps": [
    {
      "name":               "Pin",
      "title":              "Pin Density",
      "active":             true,
      "settings_group":     "Density",
      "has_data":           true,
      "can_adjust_grid":    true,
      "show_numbers":       false,
      "show_legend":        true,
      "supports_numbers":   true,
      "units":              "/um²",
      "display_range_increment": 1.0,
      "display_min":        0.0,    "display_max":       100.0,
      "display_min_limit":  0.0,    "display_max_limit": 100.0,
      "draw_below_min":     true,   "draw_above_max":    true,
      "log_scale":          false,  "reverse_log":       false,
      "grid_x":             10.0,   "grid_y":           10.0,
      "grid_min":           1.0,    "grid_max":         1000.0,
      "alpha":              150,    "alpha_min":        0,    "alpha_max": 255,
      "bounds":             [xMin, yMin, xMax, yMax],
      "options":            [...],   // source-specific extra settings
      "legend":             [{"value": "5.0", "color": [r, g, b, a]}, ...]
    },
    ...
  ]
}

`set_active_heatmap`

Activate a heat map (or none).

Field	Type	Required	Description
`name`	`string`	✓	Empty string deactivates the current.

Response (JSON): same shape as heatmaps.

`set_heatmap`

Update one setting on one heat map.

Field	Type	Required	Description
`name`	`string`	✓	Heat-map name (e.g. `"Pin"`).
`option`	`string`	✓	Setting key (e.g. `"DisplayMin"`, `"Alpha"`, `"ShowNumbers"`); also `"rebuild"` to force a re-population.
`value`	`bool`, `int`, `double`, or `string`	✓ when `option != "rebuild"`	Type must match the setting's variant slot, with one tolerance: int-typed settings accept JSON doubles and round (the JS frontend always uses `parseFloat`).

Response (JSON): same shape as heatmaps.

`heatmap_tile`

Render one tile for the active heat map (or a specified one).

Field	Type	Required	Description
`name`	`string`	✓	Heat-map name; empty string ⇒ use the active heat map.
`z`	`int`	✓	Tile zoom.
`x`	`int`	✓	Tile column.
`y`	`int`	✓	Tile row.

Response: PNG (frame type 1).

DRC

`drc_categories`

List top-level DRC categories.

No request fields.

Response (JSON):

{
  "categories": [
    {"name": "DRC", "count": 42, "description": "...", "source": "..."},
    ...
  ]
}

description and source are present only when the category provides them.

`drc_markers`

Drill into one DRC category and return its subcategories + markers.

Field	Type	Required	Description
`category`	`string`	✓	Top-level category name. Empty string clears the active category and returns `{"subcategories":[]}`.

Response (JSON):

{
  "name":        "DRC",
  "total_count": 42,
  "subcategories": [
    {
      "name":  "Subcat",
      "count": 3,
      "subcategories": [...],
      "markers": [
        {
          "id":     1,
          "index":  1,
          "name":   "...",
          "visited": false, "visible": true, "waived": false,
          "bbox":   [xMin, yMin, xMax, yMax],
          "layer":  "metal1",         // optional
          "comment": "rule X",        // optional
          "sources": [{"type": "Net" | "Inst" | "ITerm" | "BTerm" | "Object", "name": "..."}, ...]
        },
        ...
      ]
    },
    ...
  ]
}

When the named category isn't found, the response is {"error": "Category not found: <name>"}.

`drc_load_report`

Load DRC markers from a .rpt, .drc, or .json file on disk.

Field	Type	Required	Description
`path`	`string`	✓	Server-side filesystem path. Format inferred from extension.

Response (JSON): {"ok": 1, "category": "DRC", "count": 42} on success or {"ok": 0, "error": "..."} on no-violations / failure.

`drc_update_marker`

Toggle one marker's visited or visible flag.

Field	Type	Required	Description
`marker_id`	`int`	✓	Marker id from `drc_markers` `markers[].id`.
`field`	`string`	✓	`"visited"` or `"visible"`.
`value`	`bool`	✓	New value.

Response (JSON): {"ok": 1, "id": <id>, "field": <field>, "value": <bool>}.

`drc_update_category_visibility`

Bulk-set every marker in a category to a single visible value.

Field	Type	Required	Description
`category`	`string`	✓	Top-level name.
`visible`	`bool`	✓	New visibility.

Response (JSON): {"ok": 1, "category": "...", "visible": <bool>, "count": <updated>}.

`drc_highlight`

Center the layout viewport on a marker and set the highlight rect.

Field	Type	Required	Description
`marker_id`	`int`	✓	Marker id; pass `-1` to clear the highlight.

Response (JSON): on hit:

{"ok": 1, "bbox": [xMin, yMin, xMax, yMax], "name": "...", "visited": true, "layer": "metal1"}

layer is omitted when the marker has no tech layer. On miss (or explicit clear with -1): {"ok": 0}.

Files

`list_dir`

Server-side directory listing (used by the file-open dialog).

Field	Type	Required	Description
`path`	`string`	✓	Absolute path. Empty string ⇒ current working directory of the server.

Response (JSON):

{
  "path":   "/canonical/absolute",
  "parent": "/canonical",
  "entries": [
    {"name": "subdir",    "is_dir": true},
    {"name": "report.rpt","is_dir": false, "size": 12345},
    ...
  ]
}

Hidden entries (.-prefixed) and unreadable entries are skipped. Directories are listed before files; both groups sort alphabetically.

Debug graphics

`debug_continue`

Resume execution after a debug-pause hook (gui::Gui::pause() etc.). Inline-dispatched (does not enter the worker thread pool). No request fields.

Response (JSON): {"ok":1}.

`debug_charts`

Snapshot the registered debug charts. Inline-dispatched. No request fields.

Response (JSON):

{
  "charts": [
    {
      "name":     "...",
      "x_label":  "iteration",
      "y_labels": ["loss", "violations"],
      "x_format": "{}",  "y_formats": ["{:.2e}", "{}"],
      "points":   [{"x": 0, "ys": [0.5, 12]}, ...]
    },
    ...
  ]
}

TileVisibility flags

These flags appear together (typically prefixed with vf in the JS client) on every request that takes a viewport: tile, select, and snap. All are bool. Each handler defaults a missing flag to its struct default — the column below — but the JS reference frontend always sends the full set.

Field	Default	Notes
`stdcells`, `macros`	`true`	Instance categories (Liberty-aware when STA is loaded).
`pad_input`, `pad_output`, `pad_inout`, `pad_power`, `pad_spacer`, `pad_areaio`, `pad_other`	`true`	Pad sub-types.
`phys_fill`, `phys_endcap`, `phys_welltap`, `phys_tie`, `phys_antenna`, `phys_cover`, `phys_bump`, `phys_other`	`true`	Physical-only cell sub-types.
`std_bufinv`, `std_bufinv_timing`, `std_clock_bufinv`, `std_clock_gate`, `std_level_shift`, `std_sequential`, `std_combinational`	`true`	Std-cell sub-types (need Liberty/STA).
`net_signal`, `net_power`, `net_ground`, `net_clock`, `net_reset`, `net_tieoff`, `net_scan`, `net_analog`	`true`	By `dbSigType`.
`routing`, `routing_segments`, `routing_vias`, `special_nets`, `srouting_segments`, `srouting_vias`	`true`	Wires & vias.
`pins`, `pin_markers`, `pin_names`	`true`	BTerm shapes & labels.
`inst_names`, `inst_pins`, `inst_pin_names`	`true`	ITerm shapes & labels.
`blockages`, `placement_blockages`, `routing_obstructions`	`true`	dbBlockage / dbObstruction.
`rows`	`false`	Enables the row-outline overlay; gates `site_<name>` lookup.
`tracks_pref`, `tracks_non_pref`	`false`	Routing-track overlay.
`debug`, `debug_renderers`, `debug_live`	`false`	Debug graphics overlay.
`site_<name>` (multi-key)	`false`	Per-row-site visibility. Only consulted when `rows == true`.

Error contract

The server treats wire-protocol violations as recoverable: the offending request gets a kError response, the rest of the session continues.

Malformed message (invalid JSON, non-object root, missing/wrong-typed id or type): kError body Malformed request (missing or invalid id/type).
Unknown type: kError body Unknown request type: <type>.
Handler exception (boost::json at() on a missing field, type conversion failure, internal runtime_error, …): kError body server error: <exception what()>.

Every kError response logs a server-side warning:

[WARNING WEB-0043] request id=<id> type=<type> failed: <body>

type is the original type string from the request when it was parseable, otherwise unknown. The full request payload is not logged; clients should reproduce locally if a payload is needed for diagnosis.

The reference JS client surfaces kError responses as a Promise rejection through WebSocketManager.request(...). Custom clients should handle frame type 2 similarly.