feat(tiktok): add tiktok.comments verb for public comment scraping

Comments load over a signed /api/comment/list XHR that TikTok serves to
anonymous sessions once the comments panel is opened (unlike profile-video and
general-search feeds), so this is a reliable verb. Given video URLs it returns
CommentItems (text, author, likes, reply counts; replies carry repliesToId),
deduped per video, capped, and degraded to an ErrorItem for empty/withheld
videos or a bad_url ErrorItem for non-video inputs.

Generalizes the browser capture over a pluggable interaction step so the
comments flow (open panel, scroll the panel to paginate) reuses the same
warm+capture scaffolding as listing/user-search. Billed per comment on a new
TIKTOK_COMMENT meter (TIKTOK_MICROS_PER_COMMENT, matching the per-comment
market), surfaced on the chat subagent alongside tiktok.scrape/user_search.

surfsense_backend/tests/unit/capabilities/tiktok/comments/test_executor.py

@@ -0,0 +1,48 @@
+"""``tiktok.comments`` executor: verb input → scraper args → typed comment items.
+
+Boundary mocked: the proprietary comments actor (injected fake). NOT mocked: the
+verb's own payload→args forwarding and the dict→CommentItem wrapping.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from app.capabilities.tiktok.comments.executor import build_comments_executor
+from app.capabilities.tiktok.comments.schemas import CommentsInput, CommentsOutput
+
+pytestmark = pytest.mark.unit
+
+_VIDEO = "https://www.tiktok.com/@bob/video/123"
+
+
+class _FakeComments:
+    """Records the urls + kwargs it was called with; returns canned items."""
+
+    def __init__(self, items: list[dict]):
+        self._items = items
+        self.calls: list[tuple[list[str], int, int | None]] = []
+
+    async def __call__(
+        self, video_urls: list[str], *, per_video: int, limit: int | None = None
+    ) -> list[dict]:
+        self.calls.append((video_urls, per_video, limit))
+        return self._items
+
+
+async def test_forwards_urls_and_limits_and_wraps_items():
+    comments = _FakeComments([{"id": "1", "text": "hi"}])
+    execute = build_comments_executor(comments_fn=comments)
+
+    out = await execute(
+        CommentsInput(video_urls=[_VIDEO], comments_per_video=15, max_items=40)
+    )
+
+    assert isinstance(out, CommentsOutput)
+    assert len(out.items) == 1
+    assert out.items[0].text == "hi"
+
+    (urls, per_video, limit) = comments.calls[0]
+    assert urls == [_VIDEO]
+    assert per_video == 15
+    assert limit == 40

surfsense_backend/tests/unit/capabilities/tiktok/comments/test_schemas.py

@@ -0,0 +1,48 @@
+"""``tiktok.comments`` input guards and billing: a video URL is required, bounded,
+and ErrorItems are surfaced free."""
+
+from __future__ import annotations
+
+import pytest
+from pydantic import ValidationError
+
+from app.capabilities.tiktok.comments.schemas import CommentsInput, CommentsOutput
+from app.capabilities.tiktok.scrape.schemas import MAX_TIKTOK_ITEMS, MAX_TIKTOK_SOURCES
+
+pytestmark = pytest.mark.unit
+
+_VIDEO = "https://www.tiktok.com/@bob/video/123"
+
+
+def test_rejects_input_with_no_url():
+    with pytest.raises(ValidationError):
+        CommentsInput(video_urls=[])
+
+
+def test_defaults_and_bounds():
+    payload = CommentsInput(video_urls=[_VIDEO])
+    assert payload.max_items == 20
+    assert payload.comments_per_video == 20
+    assert payload.estimated_units == 20
+    with pytest.raises(ValidationError):
+        CommentsInput(video_urls=[_VIDEO], max_items=0)
+    with pytest.raises(ValidationError):
+        CommentsInput(video_urls=[_VIDEO], max_items=MAX_TIKTOK_ITEMS + 1)
+
+
+def test_rejects_more_urls_than_the_cap():
+    too_many = [f"{_VIDEO}{i}" for i in range(MAX_TIKTOK_SOURCES + 1)]
+    with pytest.raises(ValidationError):
+        CommentsInput(video_urls=too_many)
+
+
+def test_error_items_are_not_billed():
+    # Real comments count; ErrorItems (bad URL / empty video) are surfaced free.
+    out = CommentsOutput(
+        items=[
+            {"id": "1", "text": "hi"},
+            {"errorCode": "no_comments", "input": "123", "error": "empty"},
+        ]
+    )
+    assert len(out.items) == 2
+    assert out.billable_units == 1

surfsense_backend/tests/unit/capabilities/tiktok/test_registry.py

@@ -9,6 +9,7 @@ from app.capabilities import (
 )
 from app.capabilities.core import BillingUnit
 from app.capabilities.core.store import get_capability
+from app.capabilities.tiktok.comments.schemas import CommentsInput, CommentsOutput
 from app.capabilities.tiktok.scrape.schemas import ScrapeInput, ScrapeOutput
 from app.capabilities.tiktok.user_search.schemas import (
     UserSearchInput,
@@ -34,3 +35,12 @@ def test_tiktok_user_search_is_registered_and_billed_per_profile():
     assert cap.input_schema is UserSearchInput
     assert cap.output_schema is UserSearchOutput
     assert cap.billing_unit is BillingUnit.TIKTOK_USER
+
+
+def test_tiktok_comments_is_registered_and_billed_per_comment():
+    cap = get_capability("tiktok.comments")
+
+    assert cap.name == "tiktok.comments"
+    assert cap.input_schema is CommentsInput
+    assert cap.output_schema is CommentsOutput
+    assert cap.billing_unit is BillingUnit.TIKTOK_COMMENT