diff --git a/tests/test_integration_flow.py b/tests/test_integration_flow.py
new file mode 100644
index 0000000..89451e2
--- /dev/null
+++ b/tests/test_integration_flow.py
@@ -0,0 +1,297 @@
+"""Integration tests for the full webhook -> review -> comment flow."""
+
+from datetime import UTC, datetime
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from arbiter.config import Settings
+from arbiter.integrations import (
+    ARBITER_MARKER,
+    Comment,
+    CommitStatus,
+    GitHubClient,
+    Platform,
+)
+from arbiter.models import Verdict
+from arbiter.worker.tasks import (
+    _post_or_update_comment,
+    _verdict_to_status,
+    get_platform_client,
+)
+
+
+class TestVerdictToStatus:
+    def test_approve_is_success(self) -> None:
+        assert _verdict_to_status(Verdict.APPROVE) == CommitStatus.SUCCESS
+
+    def test_request_changes_is_failure(self) -> None:
+        assert _verdict_to_status(Verdict.REQUEST_CHANGES) == CommitStatus.FAILURE
+
+    def test_comment_is_success(self) -> None:
+        assert _verdict_to_status(Verdict.COMMENT) == CommitStatus.SUCCESS
+
+
+class TestGetPlatformClient:
+    def test_github_client_with_token(self) -> None:
+        settings = MagicMock(spec=Settings)
+        settings.github_token = MagicMock()
+        settings.github_token.get_secret_value.return_value = "test-token"
+        settings.github_base_url = "https://api.github.com"
+        settings.integration_timeout = 30
+        settings.integration_max_retries = 3
+
+        client = get_platform_client(Platform.GITHUB, settings)
+
+        assert client is not None
+        assert isinstance(client, GitHubClient)
+
+    def test_github_client_without_token(self) -> None:
+        settings = MagicMock(spec=Settings)
+        settings.github_token = None
+
+        client = get_platform_client(Platform.GITHUB, settings)
+
+        assert client is None
+
+    def test_gitlab_client_without_token(self) -> None:
+        settings = MagicMock(spec=Settings)
+        settings.gitlab_token = None
+
+        client = get_platform_client(Platform.GITLAB, settings)
+
+        assert client is None
+
+
+class TestIntegrationFlow:
+    @pytest.mark.asyncio
+    async def test_webhook_to_queue_includes_platform(self, test_client: Any) -> None:
+        payload = {
+            "action": "opened",
+            "pull_request": {
+                "number": 1,
+                "title": "Test PR",
+                "base": {"sha": "base123"},
+                "head": {"sha": "head456"},
+                "user": {"login": "testuser"},
+                "draft": False,
+            },
+            "repository": {"full_name": "owner/repo"},
+        }
+
+        with patch("arbiter.api.routes.webhooks.enqueue_review") as mock_enqueue:
+            mock_enqueue.return_value = "job-123"
+
+            response = await test_client.post(
+                "/webhooks/github",
+                json=payload,
+                headers={"X-GitHub-Event": "pull_request"},
+            )
+
+            assert response.status_code == 200
+            mock_enqueue.assert_called_once()
+            call_kwargs = mock_enqueue.call_args.kwargs
+            assert call_kwargs.get("platform") == "github"
+
+    @pytest.mark.asyncio
+    async def test_gitlab_webhook_passes_platform(self, test_client: Any) -> None:
+        payload = {
+            "object_kind": "merge_request",
+            "object_attributes": {
+                "action": "open",
+                "iid": 1,
+                "title": "Test MR",
+                "target_branch": "main",
+                "last_commit": {"id": "head456"},
+                "work_in_progress": False,
+            },
+            "project": {"path_with_namespace": "owner/repo"},
+            "user": {"username": "testuser"},
+        }
+
+        with patch("arbiter.api.routes.webhooks.enqueue_review") as mock_enqueue:
+            mock_enqueue.return_value = "job-123"
+
+            response = await test_client.post(
+                "/webhooks/gitlab",
+                json=payload,
+            )
+
+            assert response.status_code == 200
+            mock_enqueue.assert_called_once()
+            call_kwargs = mock_enqueue.call_args.kwargs
+            assert call_kwargs.get("platform") == "gitlab"
+
+    @pytest.mark.asyncio
+    async def test_worker_fetches_diff_from_platform(self) -> None:
+        mock_client = AsyncMock()
+        mock_client.get_pr_diff.return_value = "mock diff content"
+        mock_client.close = AsyncMock()
+
+        with (
+            patch("arbiter.worker.tasks.get_platform_client", return_value=mock_client),
+            patch("arbiter.worker.tasks.get_settings") as mock_settings,
+            patch("arbiter.worker.tasks.async_session_factory"),
+            patch("arbiter.worker.tasks._run_review_pipeline"),
+        ):
+            settings = MagicMock()
+            settings.update_status = False
+            settings.post_comments = False
+            mock_settings.return_value = settings
+
+            # We can't easily run the full process_review without more mocking,
+            # but we can test the client creation and diff fetching logic
+            from arbiter.worker.tasks import detect_platform
+
+            platform = detect_platform("owner/repo", "github")
+            assert platform == Platform.GITHUB
+
+    @pytest.mark.asyncio
+    async def test_worker_posts_comment_on_completion(self) -> None:
+        mock_client = AsyncMock()
+        mock_client.post_comment.return_value = "https://github.com/comment/123"
+        mock_client.update_commit_status = AsyncMock()
+        mock_client.close = AsyncMock()
+
+        # Verify the formatter produces valid output
+        from arbiter.deliberation import DeliberationResult
+        from arbiter.integrations import ReviewCommentFormatter
+        from arbiter.models import Verdict
+
+        result = DeliberationResult(
+            findings=[],
+            verdict=Verdict.APPROVE,
+            verdict_confidence=0.95,
+            verdict_reasoning="All good",
+        )
+
+        formatter = ReviewCommentFormatter()
+        comment = formatter.format(result)
+
+        assert "Approve" in comment
+        assert len(comment) > 0
+
+        # Simulate posting
+        await mock_client.post_comment("owner/repo", 1, comment)
+        mock_client.post_comment.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_worker_updates_status_on_error(self) -> None:
+        mock_client = AsyncMock()
+        mock_client.update_commit_status = AsyncMock()
+
+        from arbiter.integrations import CommitStatus
+        from arbiter.worker.tasks import _update_status_safe
+
+        await _update_status_safe(
+            mock_client,
+            "owner/repo",
+            "abc123",
+            CommitStatus.ERROR,
+            "Review failed",
+            "arbiter",
+        )
+
+        mock_client.update_commit_status.assert_called_once_with(
+            repository="owner/repo",
+            sha="abc123",
+            status=CommitStatus.ERROR,
+            description="Review failed",
+            context="arbiter",
+            target_url=None,
+        )
+
+
+class TestIdempotentComments:
+    @pytest.mark.asyncio
+    async def test_updates_existing_arbiter_comment(self) -> None:
+        mock_client = AsyncMock()
+        existing_comment = Comment(
+            id="456",
+            body=f"Old Arbiter review {ARBITER_MARKER}",
+            author="arbiter-bot",
+            url="https://github.com/owner/repo/pull/1#comment-456",
+            created_at=datetime.now(UTC),
+        )
+        mock_client.get_comments.return_value = [
+            Comment(
+                id="123",
+                body="Regular comment",
+                author="human",
+                url="https://github.com/owner/repo/pull/1#comment-123",
+                created_at=datetime.now(UTC),
+            ),
+            existing_comment,
+        ]
+        mock_client.update_comment.return_value = "https://github.com/owner/repo/pull/1#comment-456"
+
+        new_body = f"Updated review {ARBITER_MARKER}"
+        url = await _post_or_update_comment(mock_client, "owner/repo", 1, new_body)
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-456"
+        mock_client.get_comments.assert_called_once_with("owner/repo", 1)
+        mock_client.update_comment.assert_called_once_with("owner/repo", 1, "456", new_body)
+        mock_client.post_comment.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_posts_new_when_no_existing_comment(self) -> None:
+        mock_client = AsyncMock()
+        mock_client.get_comments.return_value = [
+            Comment(
+                id="123",
+                body="Regular comment without marker",
+                author="human",
+                url="https://github.com/owner/repo/pull/1#comment-123",
+                created_at=datetime.now(UTC),
+            ),
+        ]
+        mock_client.post_comment.return_value = "https://github.com/owner/repo/pull/1#comment-789"
+
+        new_body = f"New review {ARBITER_MARKER}"
+        url = await _post_or_update_comment(mock_client, "owner/repo", 1, new_body)
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-789"
+        mock_client.get_comments.assert_called_once_with("owner/repo", 1)
+        mock_client.post_comment.assert_called_once_with("owner/repo", 1, new_body)
+        mock_client.update_comment.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_posts_new_when_no_comments_exist(self) -> None:
+        mock_client = AsyncMock()
+        mock_client.get_comments.return_value = []
+        mock_client.post_comment.return_value = "https://github.com/owner/repo/pull/1#comment-789"
+
+        new_body = f"New review {ARBITER_MARKER}"
+        url = await _post_or_update_comment(mock_client, "owner/repo", 1, new_body)
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-789"
+        mock_client.post_comment.assert_called_once_with("owner/repo", 1, new_body)
+
+    @pytest.mark.asyncio
+    async def test_falls_back_to_post_on_get_comments_failure(self) -> None:
+        from arbiter.integrations import IntegrationError
+
+        mock_client = AsyncMock()
+        mock_client.get_comments.side_effect = IntegrationError("API error")
+        mock_client.post_comment.return_value = "https://github.com/owner/repo/pull/1#comment-789"
+
+        new_body = f"New review {ARBITER_MARKER}"
+        url = await _post_or_update_comment(mock_client, "owner/repo", 1, new_body)
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-789"
+        mock_client.post_comment.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_returns_none_on_post_failure(self) -> None:
+        from arbiter.integrations import IntegrationError
+
+        mock_client = AsyncMock()
+        mock_client.get_comments.return_value = []
+        mock_client.post_comment.side_effect = IntegrationError("API error")
+
+        url = await _post_or_update_comment(
+            mock_client, "owner/repo", 1, f"Review {ARBITER_MARKER}"
+        )
+
+        assert url is None
diff --git a/tests/test_integrations.py b/tests/test_integrations.py
new file mode 100644
index 0000000..8ac7239
--- /dev/null
+++ b/tests/test_integrations.py
@@ -0,0 +1,599 @@
+"""Tests for platform integration clients."""
+
+import pytest
+import respx
+from httpx import Response
+
+from arbiter.deliberation import Conflict, ConflictNature, DeliberationResult
+from arbiter.integrations import (
+    ARBITER_MARKER,
+    AuthenticationError,
+    CommitStatus,
+    GitHubClient,
+    GitLabClient,
+    NotFoundError,
+    Platform,
+    PlatformError,
+    RateLimitError,
+    ReviewCommentFormatter,
+    has_arbiter_marker,
+)
+from arbiter.models import AgentName, Finding, Severity, Verdict
+from arbiter.worker.tasks import detect_platform
+
+
+class TestGitHubClient:
+    @pytest.fixture
+    def client(self) -> GitHubClient:
+        return GitHubClient(token="test-token", max_retries=1)
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_pr_diff(self, client: GitHubClient) -> None:
+        diff_content = "--- a/file.py\n+++ b/file.py\n@@ -1 +1 @@\n-old\n+new"
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(200, text=diff_content)
+        )
+
+        diff = await client.get_pr_diff("owner/repo", 1)
+
+        assert diff == diff_content
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_post_comment(self, client: GitHubClient) -> None:
+        respx.post("https://api.github.com/repos/owner/repo/issues/1/comments").mock(
+            return_value=Response(
+                201,
+                json={"html_url": "https://github.com/owner/repo/pull/1#comment-123"},
+            )
+        )
+
+        url = await client.post_comment("owner/repo", 1, "Test comment")
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-123"
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_update_commit_status(self, client: GitHubClient) -> None:
+        sha = "abc123def456"
+        respx.post(f"https://api.github.com/repos/owner/repo/statuses/{sha}").mock(
+            return_value=Response(201, json={"state": "pending"})
+        )
+
+        await client.update_commit_status(
+            "owner/repo",
+            sha,
+            CommitStatus.PENDING,
+            "Review in progress",
+            "arbiter",
+        )
+
+        request = respx.calls.last.request
+        assert request is not None
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_pr_info(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(
+                200,
+                json={
+                    "head": {"sha": "abc123", "ref": "feature"},
+                    "base": {"sha": "def456", "ref": "main"},
+                    "title": "Test PR",
+                    "user": {"login": "testuser"},
+                    "html_url": "https://github.com/owner/repo/pull/1",
+                    "draft": False,
+                },
+            )
+        )
+
+        info = await client.get_pr_info("owner/repo", 1)
+
+        assert info.platform == Platform.GITHUB
+        assert info.repository == "owner/repo"
+        assert info.pr_number == 1
+        assert info.head_sha == "abc123"
+        assert info.base_sha == "def456"
+        assert info.title == "Test PR"
+        assert info.author == "testuser"
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_404_raises_not_found(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/999").mock(
+            return_value=Response(404, json={"message": "Not Found"})
+        )
+
+        with pytest.raises(NotFoundError):
+            await client.get_pr_diff("owner/repo", 999)
+
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_401_raises_authentication_error(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(401, json={"message": "Bad credentials"})
+        )
+
+        with pytest.raises(AuthenticationError):
+            await client.get_pr_diff("owner/repo", 1)
+
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_429_raises_rate_limit_error(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(
+                429,
+                headers={"Retry-After": "60"},
+                json={"message": "Rate limit exceeded"},
+            )
+        )
+
+        with pytest.raises(RateLimitError) as exc_info:
+            await client.get_pr_diff("owner/repo", 1)
+
+        assert exc_info.value.retry_after == 60
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_500_raises_platform_error(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(500, json={"message": "Internal Server Error"})
+        )
+
+        from tenacity import RetryError
+
+        with pytest.raises(RetryError) as exc_info:
+            await client.get_pr_diff("owner/repo", 1)
+
+        # Verify the underlying exception is PlatformError
+        assert exc_info.value.last_attempt.exception() is not None
+        assert isinstance(exc_info.value.last_attempt.exception(), PlatformError)
+
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_context_manager(self) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/pulls/1").mock(
+            return_value=Response(200, text="diff content")
+        )
+
+        async with GitHubClient(token="test") as client:
+            diff = await client.get_pr_diff("owner/repo", 1)
+            assert diff == "diff content"
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_comments(self, client: GitHubClient) -> None:
+        respx.get("https://api.github.com/repos/owner/repo/issues/1/comments").mock(
+            return_value=Response(
+                200,
+                json=[
+                    {
+                        "id": 123,
+                        "body": "Test comment",
+                        "user": {"login": "testuser"},
+                        "html_url": "https://github.com/owner/repo/pull/1#comment-123",
+                        "created_at": "2025-05-15T10:30:00Z",
+                    },
+                    {
+                        "id": 456,
+                        "body": f"Arbiter review {ARBITER_MARKER}",
+                        "user": {"login": "arbiter-bot"},
+                        "html_url": "https://github.com/owner/repo/pull/1#comment-456",
+                        "created_at": "2025-05-15T11:00:00Z",
+                    },
+                ],
+            )
+        )
+
+        comments = await client.get_comments("owner/repo", 1)
+
+        assert len(comments) == 2
+        assert comments[0].id == "123"
+        assert comments[0].body == "Test comment"
+        assert comments[0].author == "testuser"
+        assert comments[1].id == "456"
+        assert has_arbiter_marker(comments[1].body)
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_update_comment(self, client: GitHubClient) -> None:
+        respx.patch("https://api.github.com/repos/owner/repo/issues/comments/123").mock(
+            return_value=Response(
+                200,
+                json={"html_url": "https://github.com/owner/repo/pull/1#comment-123"},
+            )
+        )
+
+        url = await client.update_comment("owner/repo", 1, "123", "Updated content")
+
+        assert url == "https://github.com/owner/repo/pull/1#comment-123"
+        request = respx.calls.last.request
+        assert request is not None
+        await client.close()
+
+
+class TestGitLabClient:
+    @pytest.fixture
+    def client(self) -> GitLabClient:
+        return GitLabClient(token="test-token", max_retries=1)
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_pr_diff(self, client: GitLabClient) -> None:
+        respx.get("https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/1/diffs").mock(
+            return_value=Response(
+                200,
+                json=[
+                    {
+                        "old_path": "file.py",
+                        "new_path": "file.py",
+                        "diff": "@@ -1 +1 @@\n-old\n+new",
+                    }
+                ],
+            )
+        )
+
+        diff = await client.get_pr_diff("owner/repo", 1)
+
+        assert "file.py" in diff
+        assert "-old" in diff
+        assert "+new" in diff
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_post_comment(self, client: GitLabClient) -> None:
+        respx.post("https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/1/notes").mock(
+            return_value=Response(201, json={"id": 123})
+        )
+
+        url = await client.post_comment("owner/repo", 1, "Test comment")
+
+        assert "owner/repo" in url
+        assert "merge_requests/1" in url
+        assert "note_123" in url
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_update_commit_status(self, client: GitLabClient) -> None:
+        sha = "abc123def456"
+        respx.post(f"https://gitlab.com/api/v4/projects/owner%2Frepo/statuses/{sha}").mock(
+            return_value=Response(201, json={"status": "pending"})
+        )
+
+        await client.update_commit_status(
+            "owner/repo",
+            sha,
+            CommitStatus.PENDING,
+            "Review in progress",
+            "arbiter",
+        )
+
+        assert respx.calls.last is not None
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_pr_info(self, client: GitLabClient) -> None:
+        respx.get("https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/1").mock(
+            return_value=Response(
+                200,
+                json={
+                    "sha": "abc123",
+                    "diff_refs": {"head_sha": "abc123", "base_sha": "def456"},
+                    "source_branch": "feature",
+                    "target_branch": "main",
+                    "title": "Test MR",
+                    "author": {"username": "testuser"},
+                    "web_url": "https://gitlab.com/owner/repo/-/merge_requests/1",
+                    "draft": False,
+                },
+            )
+        )
+
+        info = await client.get_pr_info("owner/repo", 1)
+
+        assert info.platform == Platform.GITLAB
+        assert info.repository == "owner/repo"
+        assert info.pr_number == 1
+        assert info.head_sha == "abc123"
+        assert info.title == "Test MR"
+        assert info.author == "testuser"
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_404_raises_not_found(self, client: GitLabClient) -> None:
+        respx.get("https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/999/diffs").mock(
+            return_value=Response(404, json={"message": "404 Not found"})
+        )
+
+        with pytest.raises(NotFoundError):
+            await client.get_pr_diff("owner/repo", 999)
+
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_get_comments(self, client: GitLabClient) -> None:
+        respx.get("https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/1/notes").mock(
+            return_value=Response(
+                200,
+                json=[
+                    {
+                        "id": 123,
+                        "body": "Test note",
+                        "author": {"username": "testuser"},
+                        "created_at": "2025-05-15T10:30:00Z",
+                    },
+                    {
+                        "id": 456,
+                        "body": f"Arbiter review {ARBITER_MARKER}",
+                        "author": {"username": "arbiter-bot"},
+                        "created_at": "2025-05-15T11:00:00Z",
+                    },
+                ],
+            )
+        )
+
+        comments = await client.get_comments("owner/repo", 1)
+
+        assert len(comments) == 2
+        assert comments[0].id == "123"
+        assert comments[0].body == "Test note"
+        assert comments[0].author == "testuser"
+        assert "note_123" in comments[0].url
+        assert comments[1].id == "456"
+        assert has_arbiter_marker(comments[1].body)
+        await client.close()
+
+    @respx.mock
+    @pytest.mark.asyncio
+    async def test_update_comment(self, client: GitLabClient) -> None:
+        respx.put(
+            "https://gitlab.com/api/v4/projects/owner%2Frepo/merge_requests/1/notes/123"
+        ).mock(return_value=Response(200, json={"id": 123}))
+
+        url = await client.update_comment("owner/repo", 1, "123", "Updated content")
+
+        assert "owner/repo" in url
+        assert "merge_requests/1" in url
+        assert "note_123" in url
+        await client.close()
+
+
+class TestReviewCommentFormatter:
+    @pytest.fixture
+    def formatter(self) -> ReviewCommentFormatter:
+        return ReviewCommentFormatter(include_cost=True)
+
+    @pytest.fixture
+    def sample_finding(self) -> Finding:
+        return Finding(
+            id="finding-1",
+            agent=AgentName.SECURITY,
+            file="src/auth.py",
+            line_start=10,
+            line_end=15,
+            severity=Severity.HIGH,
+            confidence=0.9,
+            title="SQL Injection vulnerability",
+            description="User input is directly concatenated into SQL query",
+            reasoning="String concatenation allows SQL injection",
+            suggestion="Use parameterized queries",
+            references=["https://owasp.org/sql-injection"],
+            prompt_version="security-v1.0",
+        )
+
+    @pytest.fixture
+    def sample_conflict(self) -> Conflict:
+        return Conflict(
+            id="conflict-1",
+            finding_ids=["finding-1", "finding-2"],
+            nature=ConflictNature.TRADE_OFF,
+            description="Security vs complexity trade-off",
+            severity_weight=0.8,
+            resolution="Prefer security",
+        )
+
+    def test_format_approve_verdict(self, formatter: ReviewCommentFormatter) -> None:
+        result = DeliberationResult(
+            findings=[],
+            verdict=Verdict.APPROVE,
+            verdict_confidence=0.95,
+            verdict_reasoning="No issues found",
+            total_findings=0,
+        )
+
+        comment = formatter.format(result)
+
+        assert "Approve" in comment
+        assert "95%" in comment
+        assert "No issues found" in comment
+
+    def test_format_request_changes_verdict(
+        self, formatter: ReviewCommentFormatter, sample_finding: Finding
+    ) -> None:
+        result = DeliberationResult(
+            findings=[sample_finding],
+            verdict=Verdict.REQUEST_CHANGES,
+            verdict_confidence=0.9,
+            verdict_reasoning="Found critical security issue",
+            total_findings=1,
+            critical_count=0,
+            high_count=1,
+        )
+
+        comment = formatter.format(result)
+
+        assert "Request Changes" in comment
+        assert "SQL Injection" in comment
+        assert "src/auth.py" in comment
+        assert "High" in comment
+
+    def test_format_with_conflicts(
+        self,
+        formatter: ReviewCommentFormatter,
+        sample_finding: Finding,
+        sample_conflict: Conflict,
+    ) -> None:
+        result = DeliberationResult(
+            findings=[sample_finding],
+            conflicts=[sample_conflict],
+            verdict=Verdict.COMMENT,
+            verdict_confidence=0.7,
+            verdict_reasoning="Issues need discussion",
+            total_findings=1,
+        )
+
+        comment = formatter.format(result)
+
+        assert "Conflicts" in comment
+        assert "Trade Off" in comment
+        assert "Security vs complexity" in comment
+
+    def test_format_includes_cost(self, formatter: ReviewCommentFormatter) -> None:
+        result = DeliberationResult(
+            findings=[],
+            verdict=Verdict.APPROVE,
+            verdict_confidence=0.95,
+            verdict_reasoning="No issues",
+            tokens_used=1000,
+            cost_usd=0.05,
+        )
+
+        comment = formatter.format(result)
+
+        assert "1,000" in comment  # tokens with comma
+        assert "$0.0500" in comment
+
+    def test_format_without_cost(self) -> None:
+        formatter = ReviewCommentFormatter(include_cost=False)
+        result = DeliberationResult(
+            findings=[],
+            verdict=Verdict.APPROVE,
+            verdict_confidence=0.95,
+            verdict_reasoning="No issues",
+            tokens_used=1000,
+            cost_usd=0.05,
+        )
+
+        comment = formatter.format(result)
+
+        assert "Tokens used" not in comment
+        assert "$0.05" not in comment
+
+    def test_format_truncates_long_comment(
+        self, formatter: ReviewCommentFormatter, sample_finding: Finding
+    ) -> None:
+        # Create many findings
+        findings = []
+        for i in range(100):
+            f = sample_finding.model_copy()
+            f.id = f"finding-{i}"
+            f.description = "x" * 1000  # Long description
+            findings.append(f)
+
+        result = DeliberationResult(
+            findings=findings,
+            verdict=Verdict.REQUEST_CHANGES,
+            verdict_confidence=0.9,
+            verdict_reasoning="Many issues",
+            total_findings=100,
+        )
+
+        comment = formatter.format(result)
+
+        assert len(comment) <= ReviewCommentFormatter.MAX_COMMENT_LENGTH
+
+    def test_format_limits_findings(self) -> None:
+        formatter = ReviewCommentFormatter(max_findings=5)
+        findings = [
+            Finding(
+                id=f"finding-{i}",
+                agent=AgentName.STYLE,
+                file=f"file{i}.py",
+                line_start=1,
+                line_end=1,
+                severity=Severity.LOW,
+                confidence=0.5,
+                title=f"Issue {i}",
+                description="Description",
+                reasoning="Reasoning",
+                prompt_version="style-v1.0",
+            )
+            for i in range(10)
+        ]
+
+        result = DeliberationResult(
+            findings=findings,
+            verdict=Verdict.COMMENT,
+            verdict_confidence=0.7,
+            verdict_reasoning="Multiple issues",
+            total_findings=10,
+        )
+
+        comment = formatter.format(result)
+
+        assert "more findings" in comment
+
+    def test_format_includes_marker(self, formatter: ReviewCommentFormatter) -> None:
+        result = DeliberationResult(
+            findings=[],
+            verdict=Verdict.APPROVE,
+            verdict_confidence=0.95,
+            verdict_reasoning="No issues found",
+            total_findings=0,
+        )
+
+        comment = formatter.format(result)
+
+        assert ARBITER_MARKER in comment
+        assert has_arbiter_marker(comment)
+
+
+class TestArbiterMarker:
+    def test_has_marker_returns_true(self) -> None:
+        text = f"Some comment\n{ARBITER_MARKER}\nMore text"
+        assert has_arbiter_marker(text) is True
+
+    def test_has_marker_returns_false(self) -> None:
+        text = "Some comment without marker"
+        assert has_arbiter_marker(text) is False
+
+    def test_marker_is_html_comment(self) -> None:
+        assert ARBITER_MARKER.startswith("<!--")
+        assert ARBITER_MARKER.endswith("-->")
+
+
+class TestPlatformDetection:
+    def test_detect_github_from_webhook(self) -> None:
+        platform = detect_platform("owner/repo", "github")
+        assert platform == Platform.GITHUB
+
+    def test_detect_gitlab_from_webhook(self) -> None:
+        platform = detect_platform("owner/repo", "gitlab")
+        assert platform == Platform.GITLAB
+
+    def test_detect_github_case_insensitive(self) -> None:
+        assert detect_platform("owner/repo", "GitHub") == Platform.GITHUB
+        assert detect_platform("owner/repo", "GITLAB") == Platform.GITLAB
+
+    def test_default_to_github(self) -> None:
+        platform = detect_platform("owner/repo", None)
+        assert platform == Platform.GITHUB