diff --git a/kb_pipeline/platforms/__init__.py b/kb_pipeline/platforms/__init__.py
new file mode 100644
index 0000000..aa8d484
--- /dev/null
+++ b/kb_pipeline/platforms/__init__.py
@@ -0,0 +1,21 @@
+"""Platform Adapters for KB Pipeline
+
+This module provides platform-specific adapters for content extraction
+from various platforms (Bilibili, Zhihu, etc.).
+
+Each adapter implements the PlatformAdapter interface:
+- get_content_type: Identify content type from URL
+- fetch_metadata: Fetch content metadata
+- fetch_content: Fetch content body in Markdown format
+- get_output_filename: Generate output filename
+"""
+
+from .base import PlatformAdapter
+
+# BilibiliAdapter will be imported when implemented
+# from .bilibili import BilibiliAdapter
+
+__all__ = [
+    "PlatformAdapter",
+    # "BilibiliAdapter",  # Uncomment when implemented
+]
diff --git a/kb_pipeline/platforms/base.py b/kb_pipeline/platforms/base.py
new file mode 100644
index 0000000..05f6782
--- /dev/null
+++ b/kb_pipeline/platforms/base.py
@@ -0,0 +1,122 @@
+"""Platform Adapter Base Class
+
+Provides an abstract base class for platform-specific content adapters.
+Each platform (Bilibili, Zhihu, etc.) should implement a concrete adapter
+that handles metadata extraction and content fetching.
+
+Purpose: Define a unified interface for multi-platform content extraction
+"""
+
+from abc import ABC, abstractmethod
+from typing import Dict, Any
+
+
+class PlatformAdapter(ABC):
+    """Abstract base class for platform-specific content adapters
+
+    A platform adapter encapsulates the logic for:
+    - Identifying content type from URL (video, article, answer, etc.)
+    - Fetching metadata for a given content ID
+    - Fetching content body in Markdown format
+    - Generating output filenames
+
+    Subclasses must implement all abstract methods to provide
+    platform-specific functionality.
+
+    Example:
+        class BilibiliAdapter(PlatformAdapter):
+            def get_content_type(self, url: str) -> str:
+                if '/video/' in url:
+                    return 'video'
+                elif '/read/' in url:
+                    return 'article'
+                return 'unknown'
+
+            async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+                # Platform-specific implementation
+                ...
+    """
+
+    @abstractmethod
+    def get_content_type(self, url: str) -> str:
+        """Identify content type from URL
+
+        Args:
+            url: The content URL to analyze
+
+        Returns:
+            Content type string (e.g., "video", "article", "answer", "zhuanlan")
+
+        Example:
+            adapter.get_content_type("https://www.bilibili.com/video/BV1xx...")
+            -> "video"
+        """
+        pass
+
+    @abstractmethod
+    async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+        """Fetch metadata for the given content
+
+        Args:
+            content_id: Platform-specific content identifier (e.g., BV号, CV号)
+            content_type: Type of content ("video", "article", etc.)
+
+        Returns:
+            Dictionary containing metadata fields. Structure varies by platform
+            and content type, but typically includes:
+            - title: Content title
+            - author: Author name
+            - url: Original content URL
+            - Platform-specific fields
+
+        Raises:
+            NetworkError: If network request fails
+            AuthError: If authentication fails or quota exceeded
+        """
+        pass
+
+    @abstractmethod
+    async def fetch_content(self, content_id: str, content_type: str) -> str:
+        """Fetch content body in Markdown format
+
+        Args:
+            content_id: Platform-specific content identifier
+            content_type: Type of content
+
+        Returns:
+            Markdown-formatted content string with YAML frontmatter:
+            ---
+            title: Article Title
+            author: Author Name
+            source_url: https://...
+            platform: bilibili
+            content_type: article
+            fetched_at: 2024-01-15T10:30:00
+            ---
+
+            # Article Content...
+
+        Note:
+            For video content, this may return an empty string or
+            placeholder text, as video content extraction is handled
+            separately (e.g., ASR transcription).
+        """
+        pass
+
+    @abstractmethod
+    def get_output_filename(self, content_id: str, content_type: str, suffix: str) -> str:
+        """Generate output filename for content
+
+        Args:
+            content_id: Platform-specific content identifier
+            content_type: Type of content
+            suffix: Filename suffix (e.g., "metadata.json", "content.md")
+
+        Returns:
+            Filename string without directory path
+
+        Example:
+            adapter.get_output_filename("BV1xx...", "video", "metadata.json")
+            -> "bilibili_BV1xx..._metadata.json"
+        """
+        pass
diff --git a/tests/test_platforms/__init__.py b/tests/test_platforms/__init__.py
new file mode 100644
index 0000000..97bb5ec
--- /dev/null
+++ b/tests/test_platforms/__init__.py
@@ -0,0 +1 @@
+"""Tests for platform adapters"""
diff --git a/tests/test_platforms/conftest.py b/tests/test_platforms/conftest.py
new file mode 100644
index 0000000..fc9b62a
--- /dev/null
+++ b/tests/test_platforms/conftest.py
@@ -0,0 +1,134 @@
+"""Shared fixtures for platform adapter tests"""
+
+import pytest
+from pathlib import Path
+import tempfile
+from unittest.mock import MagicMock
+
+from kb_pipeline.state import WorkflowStateManager
+from kb_pipeline.schemas import WorkflowState
+
+
+@pytest.fixture
+def tmp_content_dir():
+    """Create a temporary directory for content files"""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        yield Path(tmpdir)
+
+
+@pytest.fixture
+def state_manager(tmp_content_dir):
+    """Create a WorkflowStateManager with a temporary directory"""
+    return WorkflowStateManager(tmp_content_dir)
+
+
+@pytest.fixture
+def mock_state_manager():
+    """Create a mock WorkflowStateManager for testing"""
+    return MagicMock(spec=WorkflowStateManager)
+
+
+@pytest.fixture
+def mock_video_metadata():
+    """Mock video metadata response from BilibiliExtractor"""
+    return {
+        'metadata': {
+            'title': 'Test Video Title',
+            'author': 'Test Author',
+            'bvid': 'BV1test12345',
+            'aid': 12345678,
+            'mid': 9876543,
+            'pubtime': '2024-01-15 10:30:00',
+            'duration': '10:25',
+            'url': 'https://www.bilibili.com/video/BV1test12345',
+            'tags': 'test, video, bilibili',
+        },
+        'stats': {
+            'view': 100000,
+            'danmaku': 500,
+            'reply': 200,
+            'favorite': 1000,
+            'coin': 800,
+            'share': 300,
+            'like': 5000,
+        },
+        'subtitle': '',
+        'description': 'This is a test video description.'
+    }
+
+
+@pytest.fixture
+def mock_article_metadata():
+    """Mock article metadata response from BilibiliExtractor"""
+    return {
+        'cvid': 'cv12345678',
+        'title': 'Test Article Title',
+        'author': 'Test Author',
+        'mid': '9876543',
+        'publish_time': '2024-01-15 10:30:00',
+        'view_count': 50000,
+        'like_count': 2000,
+        'coin_count': 500,
+        'share_count': 100,
+        'reply_count': 150,
+        'url': 'https://www.bilibili.com/read/cv12345678',
+        'summary': 'This is a test article summary.',
+        'word_count': 3000,
+    }
+
+
+@pytest.fixture
+def mock_article_content():
+    """Mock article content HTML/Markdown"""
+    return """# Test Article Title
+
+This is the first paragraph of the test article.
+
+## Section 1
+
+Content for section 1.
+
+## Section 2
+
+Content for section 2.
+"""
+
+
+@pytest.fixture
+def mock_zhihu_extractor():
+    """Mock ZhihuPlaywrightExtractor for testing."""
+    from unittest.mock import MagicMock
+    extractor = MagicMock()
+    extractor.extract = MagicMock()
+    return extractor
+
+
+@pytest.fixture
+def mock_zhihu_answer_result():
+    """Sample Zhihu answer extraction result."""
+    return {
+        "type": "answer",
+        "title": "Test Question Title",
+        "author": "Test Author",
+        "content": "This is the answer content.",
+        "content_markdown": "# Answer\n\nThis is the answer content.",
+        "publish_time": "2024-01-15T10:30:00",
+        "url": "https://www.zhihu.com/question/123456/answer/789012",
+        "question_id": "123456",
+        "answer_id": "789012",
+    }
+
+
+@pytest.fixture
+def mock_zhihu_zhuanlan_result():
+    """Sample Zhihu zhuanlan extraction result."""
+    return {
+        "type": "zhuanlan",
+        "title": "Test Article Title",
+        "author": "Test Author",
+        "content": "This is the article content.",
+        "content_markdown": "# Article\n\nThis is the article content.",
+        "publish_time": "2024-01-15T10:30:00",
+        "url": "https://zhuanlan.zhihu.com/p/123456",
+        "article_id": "123456",
+    }
diff --git a/tests/test_platforms/test_base.py b/tests/test_platforms/test_base.py
new file mode 100644
index 0000000..02467fc
--- /dev/null
+++ b/tests/test_platforms/test_base.py
@@ -0,0 +1,122 @@
+"""Tests for PlatformAdapter base class
+
+This module tests the interface contracts for the PlatformAdapter abstract base class:
+1. PlatformAdapter is an abstract class and cannot be instantiated directly
+2. Subclasses must implement all abstract methods
+"""
+
+import pytest
+from abc import ABC
+from typing import Dict, Any
+
+from kb_pipeline.platforms.base import PlatformAdapter
+
+
+class TestPlatformAdapterInterface:
+    """Test the PlatformAdapter interface contracts"""
+
+    def test_platform_adapter_is_abstract(self):
+        """Test 1: PlatformAdapter is an abstract class and cannot be instantiated directly"""
+        # Verify it's an ABC subclass
+        assert issubclass(PlatformAdapter, ABC)
+
+        # Verify it cannot be instantiated directly
+        with pytest.raises(TypeError) as exc_info:
+            PlatformAdapter()
+
+        # Error should mention abstract methods
+        assert "abstract" in str(exc_info.value).lower()
+
+    def test_subclass_must_implement_all_methods(self):
+        """Test 2: Subclass must implement all abstract methods"""
+        # Create an incomplete implementation
+        class IncompleteAdapter(PlatformAdapter):
+            def get_content_type(self, url: str) -> str:
+                return "video"
+
+            # Missing: fetch_metadata, fetch_content, get_output_filename
+            # This should fail to instantiate
+
+        with pytest.raises(TypeError) as exc_info:
+            IncompleteAdapter()
+
+        # Verify error mentions missing methods
+        error_msg = str(exc_info.value).lower()
+        assert "abstract" in error_msg or "implement" in error_msg
+
+    def test_complete_implementation_can_be_instantiated(self):
+        """Test that a complete implementation can be instantiated"""
+        class CompleteAdapter(PlatformAdapter):
+            def get_content_type(self, url: str) -> str:
+                return "video"
+
+            async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+                return {"title": "test"}
+
+            async def fetch_content(self, content_id: str, content_type: str) -> str:
+                return "# Test Content"
+
+            def get_output_filename(self, content_id: str, content_type: str, suffix: str) -> str:
+                return f"test_{content_id}_{suffix}"
+
+        # Should instantiate without error
+        adapter = CompleteAdapter()
+        assert adapter is not None
+        assert adapter.get_content_type("https://example.com/video/123") == "video"
+
+
+class TestPlatformAdapterMethods:
+    """Test the abstract method signatures"""
+
+    def test_get_content_type_signature(self):
+        """Test get_content_type method signature"""
+        class TestAdapter(PlatformAdapter):
+            def get_content_type(self, url: str) -> str:
+                return "video"
+
+            async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+                return {}
+
+            async def fetch_content(self, content_id: str, content_type: str) -> str:
+                return ""
+
+            def get_output_filename(self, content_id: str, content_type: str, suffix: str) -> str:
+                return ""
+
+        adapter = TestAdapter()
+        # Method should accept URL string and return content type string
+        result = adapter.get_content_type("https://www.bilibili.com/video/BV1test")
+        assert isinstance(result, str)
+
+    def test_fetch_metadata_is_async(self):
+        """Test fetch_metadata is an async method"""
+        import inspect
+        assert inspect.iscoroutinefunction(PlatformAdapter.fetch_metadata)
+
+    def test_fetch_content_is_async(self):
+        """Test fetch_content is an async method"""
+        import inspect
+        assert inspect.iscoroutinefunction(PlatformAdapter.fetch_content)
+
+    def test_get_output_filename_signature(self):
+        """Test get_output_filename method signature"""
+        class TestAdapter(PlatformAdapter):
+            def get_content_type(self, url: str) -> str:
+                return ""
+
+            async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+                return {}
+
+            async def fetch_content(self, content_id: str, content_type: str) -> str:
+                return ""
+
+            def get_output_filename(self, content_id: str, content_type: str, suffix: str) -> str:
+                return f"{content_type}_{content_id}_{suffix}"
+
+        adapter = TestAdapter()
+        # Method should accept content_id, content_type, suffix and return filename string
+        result = adapter.get_output_filename("BV1test", "video", "metadata.json")
+        assert isinstance(result, str)
+        assert "BV1test" in result
+        assert "video" in result
+        assert "metadata.json" in result