diff --git a/README.md b/README.md
index 7180efd5a..64afd4e06 100644
--- a/README.md
+++ b/README.md
@@ -147,12 +147,14 @@ You can follow these steps to generate a PageIndex tree from a PDF document.
 pip3 install --upgrade -r requirements.txt
 ```
 
-### 2. Set your OpenAI API key
+### 2. Set your API key
 
 Create a `.env` file in the root directory and add your API key:
 
 ```bash
-CHATGPT_API_KEY=your_openai_key_here
+OPENAI_API_KEY=your_openai_key_here
+# or
+CHATGPT_API_KEY=your_openai_key_here  # legacy, still supported
 ```
 
 ### 3. Run PageIndex on your PDF
@@ -189,7 +191,41 @@ python3 run_pageindex.py --md_path /path/to/your/document.md
 > Note: in this function, we use "#" to determine node heading and their levels. For example, "##" is level 2, "###" is level 3, etc. Make sure your markdown file is formatted correctly. If your Markdown file was converted from a PDF or HTML, we don't recommend using this function, since most existing conversion tools cannot preserve the original hierarchy. Instead, use our [PageIndex OCR](https://pageindex.ai/blog/ocr), which is designed to preserve the original hierarchy, to convert the PDF to a markdown file and then use this function.
 </details>
 
-<!-- 
+---
+
+# 🐍 Python API
+
+### Index & Retrieve
+
+```python
+from pageindex import PageIndexClient
+
+client = PageIndexClient(workspace="~/.pageindex")
+
+# Index a document (PDF or Markdown)
+doc_id = client.index("path/to/document.pdf")
+
+# Retrieve
+client.get_document(doc_id)            # metadata: name, type, page count
+client.get_document_structure(doc_id)  # full tree structure
+client.get_page_content(doc_id, pages="5-7")  # page content
+```
+
+### Agent-based QA (OpenAI Agents)
+
+For a complete agent QA example using the [OpenAI Agents SDK](https://github.com/openai/openai-agents-python), see [`examples/openai_agents_demo.py`](examples/openai_agents_demo.py).
+
+```bash
+# Install optional dependency
+pip install openai-agents
+
+# Run the demo
+python examples/openai_agents_demo.py
+```
+
+---
+
+<!--
 # ☁️ Improved Tree Generation with PageIndex OCR
 
 This repo is designed for generating PageIndex tree structure for simple PDFs, but many real-world use cases involve complex PDFs that are hard to parse by classic Python tools. However, extracting high-quality text from PDF documents remains a non-trivial challenge. Most OCR tools only extract page-level content, losing the broader document context and hierarchy.
diff --git a/examples/openai_agents_demo.py b/examples/openai_agents_demo.py
new file mode 100644
index 000000000..a1fcb163e
--- /dev/null
+++ b/examples/openai_agents_demo.py
@@ -0,0 +1,154 @@
+"""
+PageIndex x OpenAI Agents Demo
+
+Demonstrates how to use PageIndexClient with the OpenAI Agents SDK
+to build a document QA agent with 3 tools:
+  - get_document()
+  - get_document_structure()
+  - get_page_content()
+
+Requirements:
+    pip install openai-agents
+
+Steps:
+  1 — Index PDF and inspect tree structure
+  2 — Inspect document metadata
+  3 — Ask a question (agent auto-calls tools)
+  4 — Reload from workspace and verify persistence
+"""
+import os
+import sys
+import asyncio
+import concurrent.futures
+import requests
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from agents import Agent, Runner, function_tool
+from agents.stream_events import RunItemStreamEvent
+
+from pageindex import PageIndexClient
+import pageindex.utils as utils
+
+PDF_URL = "https://arxiv.org/pdf/2501.12948.pdf"
+PDF_PATH = "tests/pdfs/deepseek-r1.pdf"
+WORKSPACE = "~/.pageindex"
+
+AGENT_SYSTEM_PROMPT = """
+You are PageIndex, a document QA assistant.
+TOOL USE:
+- Call get_document() first to confirm status and page/line count.
+- Call get_document_structure() to find relevant page ranges (use node summaries and start_index/end_index).
+- Call get_page_content(pages="5-7") with tight ranges. Never fetch the whole doc.
+- For Markdown, pages = line numbers from the structure (the line_num field). Use line_count from get_document() as the upper bound.
+ANSWERING: Answer based only on tool output. Be concise.
+"""
+
+
+def query_agent(client: PageIndexClient, doc_id: str, prompt: str, verbose: bool = False) -> str:
+    """Run a document QA agent using the OpenAI Agents SDK."""
+
+    @function_tool
+    def get_document() -> str:
+        """Get document metadata: status, page count, name, and description."""
+        return client.get_document(doc_id)
+
+    @function_tool
+    def get_document_structure() -> str:
+        """Get the document's full tree structure (without text) to find relevant sections."""
+        return client.get_document_structure(doc_id)
+
+    @function_tool
+    def get_page_content(pages: str) -> str:
+        """
+        Get the text content of specific pages or line numbers.
+        Use tight ranges: e.g. '5-7' for pages 5 to 7, '3,8' for pages 3 and 8, '12' for page 12.
+        For Markdown documents, use line numbers from the structure's line_num field.
+        """
+        return client.get_page_content(doc_id, pages)
+
+    agent = Agent(
+        name="PageIndex",
+        instructions=AGENT_SYSTEM_PROMPT,
+        tools=[get_document, get_document_structure, get_page_content],
+        model=client.model,
+    )
+
+    async def _run():
+        if not verbose:
+            result = await Runner.run(agent, prompt)
+            return result.final_output
+
+        turn = 0
+        stream = Runner.run_streamed(agent, prompt)
+        async for event in stream.stream_events():
+            if not isinstance(event, RunItemStreamEvent):
+                continue
+            if event.name == "tool_called":
+                turn += 1
+                raw = event.item.raw_item
+                args = getattr(raw, "arguments", "{}")
+                print(f"\n[Turn {turn}] → {raw.name}({args})")
+            elif event.name == "tool_output":
+                output = str(event.item.output)
+                preview = output[:200] + "..." if len(output) > 200 else output
+                print(f"         ← {preview}")
+        return stream.final_output
+
+    try:
+        asyncio.get_running_loop()
+        with concurrent.futures.ThreadPoolExecutor(max_workers=1) as pool:
+            return pool.submit(asyncio.run, _run()).result()
+    except RuntimeError:
+        return asyncio.run(_run())
+
+
+# ── Download PDF if needed ─────────────────────────────────────────────────────
+if not os.path.exists(PDF_PATH):
+    print(f"Downloading {PDF_URL} ...")
+    os.makedirs(os.path.dirname(PDF_PATH), exist_ok=True)
+    with requests.get(PDF_URL, stream=True, timeout=30) as r:
+        r.raise_for_status()
+        with open(PDF_PATH, "wb") as f:
+            for chunk in r.iter_content(chunk_size=8192):
+                if chunk:
+                    f.write(chunk)
+    print("Download complete.\n")
+
+# ── Setup ──────────────────────────────────────────────────────────────────────
+client = PageIndexClient(workspace=WORKSPACE)
+
+# ── Step 1: Index + Tree ───────────────────────────────────────────────────────
+print("=" * 60)
+print("Step 1: Indexing PDF and inspecting tree structure")
+print("=" * 60)
+doc_id = client.index(PDF_PATH)
+print(f"\nDocument ID: {doc_id}")
+print("\nTree Structure:")
+utils.print_tree(client.documents[doc_id]["structure"])
+
+# ── Step 2: Document Metadata ──────────────────────────────────────────────────
+print("\n" + "=" * 60)
+print("Step 2: Document Metadata (get_document)")
+print("=" * 60)
+print(client.get_document(doc_id))
+
+# ── Step 3: Agent Query ────────────────────────────────────────────────────────
+print("\n" + "=" * 60)
+print("Step 3: Agent Query (auto tool-use)")
+print("=" * 60)
+question = "What are the main conclusions of this paper?"
+print(f"\nQuestion: '{question}'\n")
+answer = query_agent(client, doc_id, question)
+print("Answer:")
+print(answer)
+
+# ── Step 4: Persistence Verification ──────────────────────────────────────────
+print("\n" + "=" * 60)
+print("Step 4: Persistence — reload without re-indexing")
+print("=" * 60)
+client2 = PageIndexClient(workspace=WORKSPACE)
+answer2 = query_agent(client2, doc_id, "What are the main conclusions of this paper?", verbose=True)
+print("Answer from reloaded client:")
+print(answer2)
+print("\nPersistence verified. ✓")
diff --git a/pageindex/__init__.py b/pageindex/__init__.py
index 4606eb396..658003bf5 100644
--- a/pageindex/__init__.py
+++ b/pageindex/__init__.py
@@ -1,2 +1,4 @@
 from .page_index import *
-from .page_index_md import md_to_tree
\ No newline at end of file
+from .page_index_md import md_to_tree
+from .retrieve import get_document, get_document_structure, get_page_content
+from .client import PageIndexClient
diff --git a/pageindex/client.py b/pageindex/client.py
new file mode 100644
index 000000000..18af3c371
--- /dev/null
+++ b/pageindex/client.py
@@ -0,0 +1,118 @@
+import os
+import uuid
+import json
+import asyncio
+from pathlib import Path
+
+from .page_index import page_index
+from .page_index_md import md_to_tree
+from .retrieve import get_document, get_document_structure, get_page_content
+
+
+class PageIndexClient:
+    """
+    A client for indexing and retrieving document content.
+    Flow: index() -> get_document() / get_document_structure() / get_page_content()
+
+    For agent-based QA, see examples/openai_agents_demo.py.
+    """
+    def __init__(self, api_key: str = None, model: str = "gpt-4o-2024-11-20", workspace: str = None):
+        if api_key:
+            os.environ["OPENAI_API_KEY"] = api_key
+        elif not os.getenv("OPENAI_API_KEY") and os.getenv("CHATGPT_API_KEY"):
+            os.environ["OPENAI_API_KEY"] = os.getenv("CHATGPT_API_KEY")
+        self.model = model
+        self.workspace = Path(workspace).expanduser() if workspace else None
+        if self.workspace:
+            self.workspace.mkdir(parents=True, exist_ok=True)
+        self.documents = {}
+        if self.workspace:
+            self._load_workspace()
+
+    def index(self, file_path: str, mode: str = "auto") -> str:
+        """Index a document. Returns a document_id."""
+        if not os.path.exists(file_path):
+            raise FileNotFoundError(f"File not found: {file_path}")
+
+        doc_id = str(uuid.uuid4())
+        ext = os.path.splitext(file_path)[1].lower()
+
+        is_pdf = ext == '.pdf'
+        is_md = ext in ['.md', '.markdown']
+
+        if mode == "pdf" or (mode == "auto" and is_pdf):
+            print(f"Indexing PDF: {file_path}")
+            result = page_index(
+                doc=file_path,
+                model=self.model,
+                if_add_node_summary='yes',
+                if_add_node_text='yes',
+                if_add_node_id='yes',
+                if_add_doc_description='yes'
+            )
+            self.documents[doc_id] = {
+                'id': doc_id,
+                'path': file_path,
+                'type': 'pdf',
+                'structure': result['structure'],
+                'doc_name': result.get('doc_name', ''),
+                'doc_description': result.get('doc_description', '')
+            }
+
+        elif mode == "md" or (mode == "auto" and is_md):
+            print(f"Indexing Markdown: {file_path}")
+            result = asyncio.run(md_to_tree(
+                md_path=file_path,
+                if_thinning=False,
+                if_add_node_summary='yes',
+                summary_token_threshold=200,
+                model=self.model,
+                if_add_doc_description='yes',
+                if_add_node_text='yes',
+                if_add_node_id='yes'
+            ))
+            self.documents[doc_id] = {
+                'id': doc_id,
+                'path': file_path,
+                'type': 'md',
+                'structure': result['structure'],
+                'doc_name': result.get('doc_name', ''),
+                'doc_description': result.get('doc_description', '')
+            }
+        else:
+            raise ValueError(f"Unsupported file format for: {file_path}")
+
+        print(f"Indexing complete. Document ID: {doc_id}")
+        if self.workspace:
+            self._save_doc(doc_id)
+        return doc_id
+
+    def _save_doc(self, doc_id: str):
+        path = self.workspace / f"{doc_id}.json"
+        with open(path, "w", encoding="utf-8") as f:
+            json.dump(self.documents[doc_id], f, ensure_ascii=False, indent=2)
+
+    def _load_workspace(self):
+        loaded = 0
+        for path in self.workspace.glob("*.json"):
+            try:
+                with open(path, "r", encoding="utf-8") as f:
+                    doc = json.load(f)
+                self.documents[path.stem] = doc
+                loaded += 1
+            except (json.JSONDecodeError, OSError) as e:
+                print(f"Warning: skipping corrupt workspace file {path.name}: {e}")
+        if loaded:
+            print(f"Loaded {loaded} document(s) from workspace.")
+
+    def get_document(self, doc_id: str) -> str:
+        """Return document metadata JSON."""
+        return get_document(self.documents, doc_id)
+
+    def get_document_structure(self, doc_id: str) -> str:
+        """Return document tree structure JSON (without text fields)."""
+        return get_document_structure(self.documents, doc_id)
+
+    def get_page_content(self, doc_id: str, pages: str) -> str:
+        """Return page content for the given pages string (e.g. '5-7', '3,8', '12')."""
+        return get_page_content(self.documents, doc_id, pages)
diff --git a/pageindex/retrieve.py b/pageindex/retrieve.py
new file mode 100644
index 000000000..53c9b1961
--- /dev/null
+++ b/pageindex/retrieve.py
@@ -0,0 +1,137 @@
+import json
+import PyPDF2
+
+try:
+    from .utils import get_number_of_pages, remove_fields
+except ImportError:
+    from utils import get_number_of_pages, remove_fields
+
+
+# ── Helpers ──────────────────────────────────────────────────────────────────
+
+def _parse_pages(pages: str) -> list[int]:
+    """Parse a pages string like '5-7', '3,8', or '12' into a sorted list of ints."""
+    result = []
+    for part in pages.split(','):
+        part = part.strip()
+        if '-' in part:
+            start, end = part.split('-', 1)
+            result.extend(range(int(start.strip()), int(end.strip()) + 1))
+        else:
+            result.append(int(part))
+    return sorted(set(result))
+
+
+def _count_pages(doc_info: dict) -> int:
+    """Return total page count for a document."""
+    if doc_info.get('type') == 'pdf':
+        return get_number_of_pages(doc_info['path'])
+    # For MD, find max line_num across all nodes
+    max_line = 0
+    def _traverse(nodes):
+        nonlocal max_line
+        for node in nodes:
+            ln = node.get('line_num', 0)
+            if ln and ln > max_line:
+                max_line = ln
+            if node.get('nodes'):
+                _traverse(node['nodes'])
+    _traverse(doc_info.get('structure', []))
+    return max_line
+
+
+def _get_pdf_page_content(doc_info: dict, page_nums: list[int]) -> list[dict]:
+    """Extract text for specific PDF pages (1-indexed), opening the PDF once."""
+    path = doc_info['path']
+    with open(path, 'rb') as f:
+        pdf_reader = PyPDF2.PdfReader(f)
+        total = len(pdf_reader.pages)
+        valid_pages = [p for p in page_nums if 1 <= p <= total]
+        return [
+            {'page': p, 'content': pdf_reader.pages[p - 1].extract_text() or ''}
+            for p in valid_pages
+        ]
+
+
+def _get_md_page_content(doc_info: dict, page_nums: list[int]) -> list[dict]:
+    """
+    For Markdown documents, 'pages' are line numbers.
+    Find nodes whose line_num falls within the requested set and return their text.
+    """
+    page_set = set(page_nums)
+    results = []
+    seen = set()
+
+    def _traverse(nodes):
+        for node in nodes:
+            ln = node.get('line_num')
+            if ln and ln in page_set and ln not in seen:
+                seen.add(ln)
+                results.append({'page': ln, 'content': node.get('text', '')})
+            if node.get('nodes'):
+                _traverse(node['nodes'])
+
+    _traverse(doc_info.get('structure', []))
+    results.sort(key=lambda x: x['page'])
+    return results
+
+
+# ── Tool functions ────────────────────────────────────────────────────────────
+
+def get_document(documents: dict, doc_id: str) -> str:
+    """Return JSON with document metadata: doc_id, doc_name, doc_description, type, status, page_count (PDF) or line_count (Markdown)."""
+    doc_info = documents.get(doc_id)
+    if not doc_info:
+        return json.dumps({'error': f'Document {doc_id} not found'})
+    result = {
+        'doc_id': doc_id,
+        'doc_name': doc_info.get('doc_name', ''),
+        'doc_description': doc_info.get('doc_description', ''),
+        'type': doc_info.get('type', ''),
+        'status': 'completed',
+    }
+    if doc_info.get('type') == 'pdf':
+        result['page_count'] = _count_pages(doc_info)
+    else:
+        result['line_count'] = _count_pages(doc_info)
+    return json.dumps(result)
+
+
+def get_document_structure(documents: dict, doc_id: str) -> str:
+    """Return tree structure JSON with text fields removed (saves tokens)."""
+    doc_info = documents.get(doc_id)
+    if not doc_info:
+        return json.dumps({'error': f'Document {doc_id} not found'})
+    structure = doc_info.get('structure', [])
+    structure_no_text = remove_fields(structure, fields=['text'])
+    return json.dumps(structure_no_text, ensure_ascii=False)
+
+
+def get_page_content(documents: dict, doc_id: str, pages: str) -> str:
+    """
+    Retrieve page content for a document.
+
+    pages format: '5-7', '3,8', or '12'
+    For PDF: pages are physical page numbers (1-indexed).
+    For Markdown: pages are line numbers corresponding to node headers.
+
+    Returns JSON list of {'page': int, 'content': str}.
+    """
+    doc_info = documents.get(doc_id)
+    if not doc_info:
+        return json.dumps({'error': f'Document {doc_id} not found'})
+
+    try:
+        page_nums = _parse_pages(pages)
+    except (ValueError, AttributeError) as e:
+        return json.dumps({'error': f'Invalid pages format: {pages!r}. Use "5-7", "3,8", or "12". Error: {e}'})
+
+    try:
+        if doc_info.get('type') == 'pdf':
+            content = _get_pdf_page_content(doc_info, page_nums)
+        else:
+            content = _get_md_page_content(doc_info, page_nums)
+    except Exception as e:
+        return json.dumps({'error': f'Failed to read page content: {e}'})
+
+    return json.dumps(content, ensure_ascii=False)
diff --git a/pageindex/utils.py b/pageindex/utils.py
index dc7acd888..0c05d9566 100644
--- a/pageindex/utils.py
+++ b/pageindex/utils.py
@@ -2,6 +2,7 @@
 import openai
 import logging
 import os
+import textwrap
 from datetime import datetime
 import time
 import json
@@ -86,6 +87,21 @@ def ChatGPT_API(model, prompt, api_key=CHATGPT_API_KEY, chat_history=None):
                 return "Error"
             
 
+def ChatGPT_API_stream(model, prompt, api_key=CHATGPT_API_KEY):
+    """Return a generator that yields token chunks (str) one at a time."""
+    client = openai.OpenAI(api_key=api_key)
+    with client.chat.completions.create(
+        model=model,
+        messages=[{"role": "user", "content": prompt}],
+        temperature=0,
+        stream=True,
+    ) as stream:
+        for chunk in stream:
+            delta = chunk.choices[0].delta.content
+            if delta:
+                yield delta
+
+
 async def ChatGPT_API_async(model, prompt, api_key=CHATGPT_API_KEY):
     max_retries = 10
     messages = [{"role": "user", "content": prompt}]
@@ -709,4 +725,29 @@ def load(self, user_opt=None) -> config:
 
         self._validate_keys(user_dict)
         merged = {**self._default_dict, **user_dict}
-        return config(**merged)
\ No newline at end of file
+        return config(**merged)
+
+def create_node_mapping(tree):
+    """Create a flat dict mapping node_id to node for quick lookup."""
+    mapping = {}
+    def _traverse(nodes):
+        for node in nodes:
+            if node.get('node_id'):
+                mapping[node['node_id']] = node
+            if node.get('nodes'):
+                _traverse(node['nodes'])
+    _traverse(tree)
+    return mapping
+
+def print_tree(tree, indent=0):
+    for node in tree:
+        summary = node.get('summary') or node.get('prefix_summary', '')
+        summary_str = f"  —  {summary[:60]}..." if summary else ""
+        print('  ' * indent + f"[{node.get('node_id', '?')}] {node.get('title', '')}{summary_str}")
+        if node.get('nodes'):
+            print_tree(node['nodes'], indent + 1)
+
+def print_wrapped(text, width=100):
+    for line in text.splitlines():
+        print(textwrap.fill(line, width=width))
+
diff --git a/requirements.txt b/requirements.txt
index 463db58f1..3a9569e5a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 openai==1.101.0
+# openai-agents  # optional: required for examples/openai_agents_demo.py
 pymupdf==1.26.4
 PyPDF2==3.0.1
 python-dotenv==1.1.0