diff --git a/mail_intake/__init__.py b/mail_intake/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/adapters/__init__.py b/mail_intake/adapters/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/adapters/base.py b/mail_intake/adapters/base.py
new file mode 100644
index 0000000..e26e17d
--- /dev/null
+++ b/mail_intake/adapters/base.py
@@ -0,0 +1,48 @@
+from abc import ABC, abstractmethod
+from typing import Iterator, Dict, Any
+
+
+class MailIntakeAdapter(ABC):
+    """
+    Base adapter interface for mail providers.
+
+    This interface defines the minimal contract required for
+    read-only mail ingestion. No provider-specific concepts
+    should leak beyond implementations of this class.
+    """
+
+    @abstractmethod
+    def iter_message_refs(self, query: str) -> Iterator[Dict[str, str]]:
+        """
+        Iterate over lightweight message references.
+
+        Must yield dictionaries containing at least:
+        - message_id
+        - thread_id
+
+        Example yield:
+        {
+            "message_id": "...",
+            "thread_id": "..."
+        }
+        """
+        raise NotImplementedError
+
+    @abstractmethod
+    def fetch_message(self, message_id: str) -> Dict[str, Any]:
+        """
+        Fetch a full raw message by message_id.
+
+        Returns the provider-native message payload
+        (e.g., Gmail message JSON).
+        """
+        raise NotImplementedError
+
+    @abstractmethod
+    def fetch_thread(self, thread_id: str) -> Dict[str, Any]:
+        """
+        Fetch a full raw thread by thread_id.
+
+        Returns the provider-native thread payload.
+        """
+        raise NotImplementedError
diff --git a/mail_intake/adapters/gmail.py b/mail_intake/adapters/gmail.py
new file mode 100644
index 0000000..d4c52dc
--- /dev/null
+++ b/mail_intake/adapters/gmail.py
@@ -0,0 +1,105 @@
+from typing import Iterator, Dict, Any
+
+from googleapiclient.discovery import build
+from googleapiclient.errors import HttpError
+
+from mail_intake.adapters.base import MailIntakeAdapter
+from mail_intake.exceptions import MailIntakeAdapterError
+from mail_intake.auth.base import MailIntakeAuthProvider
+
+
+class MailIntakeGmailAdapter(MailIntakeAdapter):
+    """
+    Gmail read-only adapter.
+
+    This class is the ONLY place where:
+    - googleapiclient is imported
+    - Gmail REST semantics are known
+    - .execute() is called
+
+    It must remain thin and dumb by design.
+    """
+
+    def __init__(
+        self,
+        auth_provider: MailIntakeAuthProvider,
+        user_id: str = "me",
+    ):
+        self._auth_provider = auth_provider
+        self._user_id = user_id
+        self._service = None
+
+    @property
+    def service(self):
+        if self._service is None:
+            try:
+                creds = self._auth_provider.get_credentials()
+                self._service = build("gmail", "v1", credentials=creds)
+            except Exception as exc:
+                raise MailIntakeAdapterError(
+                    "Failed to initialize Gmail service"
+                ) from exc
+        return self._service
+
+    def iter_message_refs(self, query: str) -> Iterator[Dict[str, str]]:
+        """
+        Iterate over message references matching the query.
+
+        Yields:
+        {
+            "message_id": "...",
+            "thread_id": "..."
+        }
+        """
+        try:
+            request = (
+                self.service.users()
+                .messages()
+                .list(userId=self._user_id, q=query)
+            )
+
+            while request is not None:
+                response = request.execute()
+
+                for msg in response.get("messages", []):
+                    yield {
+                        "message_id": msg["id"],
+                        "thread_id": msg["threadId"],
+                    }
+
+                request = (
+                    self.service.users()
+                    .messages()
+                    .list_next(request, response)
+                )
+
+        except HttpError as exc:
+            raise MailIntakeAdapterError(
+                "Gmail API error while listing messages"
+            ) from exc
+
+    def fetch_message(self, message_id: str) -> Dict[str, Any]:
+        try:
+            return (
+                self.service.users()
+                .messages()
+                .get(userId=self._user_id, id=message_id)
+                .execute()
+            )
+        except HttpError as exc:
+            raise MailIntakeAdapterError(
+                f"Gmail API error while fetching message {message_id}"
+            ) from exc
+
+    def fetch_thread(self, thread_id: str) -> Dict[str, Any]:
+        try:
+            return (
+                self.service.users()
+                .threads()
+                .get(userId=self._user_id, id=thread_id)
+                .execute()
+            )
+        except HttpError as exc:
+            raise MailIntakeAdapterError(
+                f"Gmail API error while fetching thread {thread_id}"
+            ) from exc
diff --git a/mail_intake/auth/__init__.py b/mail_intake/auth/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/auth/base.py b/mail_intake/auth/base.py
new file mode 100644
index 0000000..526ff36
--- /dev/null
+++ b/mail_intake/auth/base.py
@@ -0,0 +1,20 @@
+from abc import ABC, abstractmethod
+
+
+class MailIntakeAuthProvider(ABC):
+    """
+    Abstract authentication provider.
+
+    Mail adapters depend on this interface, not on concrete
+    OAuth or credential implementations.
+    """
+
+    @abstractmethod
+    def get_credentials(self):
+        """
+        Return provider-specific credentials object.
+
+        This method is synchronous by design and must either
+        return valid credentials or raise MailIntakeAuthError.
+        """
+        raise NotImplementedError
diff --git a/mail_intake/auth/google.py b/mail_intake/auth/google.py
new file mode 100644
index 0000000..ec9a4f1
--- /dev/null
+++ b/mail_intake/auth/google.py
@@ -0,0 +1,81 @@
+import os
+import pickle
+from typing import Sequence
+
+import google.auth.exceptions
+from google.auth.transport.requests import Request
+from google_auth_oauthlib.flow import InstalledAppFlow
+
+from mail_intake.auth.base import MailIntakeAuthProvider
+from mail_intake.exceptions import MailIntakeAuthError
+
+
+class MailIntakeGoogleAuth(MailIntakeAuthProvider):
+    """
+    Google OAuth provider for Gmail access.
+
+    Responsibilities:
+    - Load cached credentials from disk
+    - Refresh expired tokens when possible
+    - Trigger interactive login only when strictly required
+
+    This class is synchronous and intentionally state-light.
+    """
+
+    def __init__(
+        self,
+        credentials_path: str,
+        token_path: str,
+        scopes: Sequence[str],
+    ):
+        self.credentials_path = credentials_path
+        self.token_path = token_path
+        self.scopes = list(scopes)
+
+    def get_credentials(self):
+        creds = None
+
+        # Attempt to load cached credentials
+        if os.path.exists(self.token_path):
+            try:
+                with open(self.token_path, "rb") as fh:
+                    creds = pickle.load(fh)
+            except Exception:
+                creds = None
+
+        # Validate / refresh credentials
+        if not creds or not creds.valid:
+            if creds and creds.expired and creds.refresh_token:
+                try:
+                    creds.refresh(Request())
+                except google.auth.exceptions.RefreshError:
+                    creds = None
+
+            # Interactive login if refresh failed or creds missing
+            if not creds:
+                if not os.path.exists(self.credentials_path):
+                    raise MailIntakeAuthError(
+                        f"Google credentials file not found: {self.credentials_path}"
+                    )
+
+                try:
+                    flow = InstalledAppFlow.from_client_secrets_file(
+                        self.credentials_path,
+                        self.scopes,
+                    )
+                    creds = flow.run_local_server(port=0)
+                except Exception as exc:
+                    raise MailIntakeAuthError(
+                        "Failed to complete Google OAuth flow"
+                    ) from exc
+
+            # Persist refreshed / new credentials
+            try:
+                with open(self.token_path, "wb") as fh:
+                    pickle.dump(creds, fh)
+            except Exception as exc:
+                raise MailIntakeAuthError(
+                    f"Failed to write token file: {self.token_path}"
+                ) from exc
+
+        return creds
diff --git a/mail_intake/config.py b/mail_intake/config.py
new file mode 100644
index 0000000..afd63c5
--- /dev/null
+++ b/mail_intake/config.py
@@ -0,0 +1,20 @@
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass(frozen=True)
+class MailIntakeConfig:
+    """
+    Global configuration for mail-intake.
+
+    This configuration is intentionally explicit and immutable.
+    No implicit environment reads or global state.
+    """
+
+    provider: str = "gmail"
+    user_id: str = "me"
+    readonly: bool = True
+
+    # Provider-specific paths (optional at this layer)
+    credentials_path: Optional[str] = None
+    token_path: Optional[str] = None
diff --git a/mail_intake/exceptions.py b/mail_intake/exceptions.py
new file mode 100644
index 0000000..43043f4
--- /dev/null
+++ b/mail_intake/exceptions.py
@@ -0,0 +1,19 @@
+class MailIntakeError(Exception):
+    """
+    Base exception for all mail-intake errors.
+
+    Users of the library should catch this type (or subclasses)
+    instead of provider-specific or third-party exceptions.
+    """
+
+
+class MailIntakeAuthError(MailIntakeError):
+    """Authentication and credential-related failures."""
+
+
+class MailIntakeAdapterError(MailIntakeError):
+    """Errors raised by mail provider adapters."""
+
+
+class MailIntakeParsingError(MailIntakeError):
+    """Errors encountered while parsing message content."""
diff --git a/mail_intake/ingestion/__init__.py b/mail_intake/ingestion/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/ingestion/reader.py b/mail_intake/ingestion/reader.py
new file mode 100644
index 0000000..186d296
--- /dev/null
+++ b/mail_intake/ingestion/reader.py
@@ -0,0 +1,99 @@
+from datetime import datetime
+from typing import Iterator, Dict, Any
+
+from mail_intake.adapters.base import MailIntakeAdapter
+from mail_intake.models.message import MailIntakeMessage
+from mail_intake.models.thread import MailIntakeThread
+from mail_intake.parsers.headers import parse_headers, extract_sender
+from mail_intake.parsers.body import extract_body
+from mail_intake.parsers.subject import normalize_subject
+from mail_intake.exceptions import MailIntakeParsingError
+
+
+class MailIntakeReader:
+    """
+    High-level read-only ingestion interface.
+
+    This is the primary entry point users should interact with.
+    It orchestrates:
+    - adapter calls
+    - parsing
+    - normalization
+    - model construction
+
+    No provider-specific logic exists here.
+    """
+
+    def __init__(self, adapter: MailIntakeAdapter):
+        self._adapter = adapter
+
+    def iter_messages(self, query: str) -> Iterator[MailIntakeMessage]:
+        """
+        Iterate over parsed messages matching a provider query.
+        """
+        for ref in self._adapter.iter_message_refs(query):
+            raw = self._adapter.fetch_message(ref["message_id"])
+            yield self._parse_message(raw)
+
+    def iter_threads(self, query: str) -> Iterator[MailIntakeThread]:
+        """
+        Iterate over threads constructed from messages matching a query.
+
+        Messages are grouped by thread_id and yielded as complete threads.
+        """
+        threads: Dict[str, MailIntakeThread] = {}
+
+        for ref in self._adapter.iter_message_refs(query):
+            raw = self._adapter.fetch_message(ref["message_id"])
+            message = self._parse_message(raw)
+
+            thread = threads.get(message.thread_id)
+            if thread is None:
+                thread = MailIntakeThread(
+                    thread_id=message.thread_id,
+                    normalized_subject=normalize_subject(message.subject),
+                )
+                threads[message.thread_id] = thread
+
+            thread.add_message(message)
+
+        return iter(threads.values())
+
+    def _parse_message(self, raw_message: Dict[str, Any]) -> MailIntakeMessage:
+        """
+        Parse a raw provider message into a MailIntakeMessage.
+        """
+        try:
+            message_id = raw_message["id"]
+            thread_id = raw_message["threadId"]
+
+            # Gmail internalDate is milliseconds since epoch
+            timestamp_ms = int(raw_message.get("internalDate", 0))
+            timestamp = datetime.fromtimestamp(timestamp_ms / 1000)
+
+            payload = raw_message.get("payload", {})
+            raw_headers_list = payload.get("headers", [])
+
+            headers = parse_headers(raw_headers_list)
+            from_email, from_name = extract_sender(headers)
+
+            subject = headers.get("subject", "")
+            body_text = extract_body(payload)
+            snippet = raw_message.get("snippet", "")
+
+            return MailIntakeMessage(
+                message_id=message_id,
+                thread_id=thread_id,
+                timestamp=timestamp,
+                from_email=from_email,
+                from_name=from_name,
+                subject=subject,
+                body_text=body_text,
+                snippet=snippet,
+                raw_headers=headers,
+            )
+
+        except Exception as exc:
+            raise MailIntakeParsingError(
+                f"Failed to parse message {raw_message.get('id')}"
+            ) from exc
diff --git a/mail_intake/models/__init__.py b/mail_intake/models/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/models/message.py b/mail_intake/models/message.py
new file mode 100644
index 0000000..2696d75
--- /dev/null
+++ b/mail_intake/models/message.py
@@ -0,0 +1,26 @@
+from dataclasses import dataclass
+from datetime import datetime
+from typing import Optional, Dict
+
+
+@dataclass(frozen=True)
+class MailIntakeMessage:
+    """
+    Canonical internal representation of a single email message.
+
+    This model is provider-agnostic and safe to persist.
+    No Gmail-specific fields should appear here.
+    """
+
+    message_id: str
+    thread_id: str
+    timestamp: datetime
+
+    from_email: str
+    from_name: Optional[str]
+
+    subject: str
+    body_text: str
+    snippet: str
+
+    raw_headers: Dict[str, str]
diff --git a/mail_intake/models/thread.py b/mail_intake/models/thread.py
new file mode 100644
index 0000000..ddcf757
--- /dev/null
+++ b/mail_intake/models/thread.py
@@ -0,0 +1,35 @@
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import List, Set
+
+from mail_intake.models.message import MailIntakeMessage
+
+
+@dataclass
+class MailIntakeThread:
+    """
+    Canonical internal representation of an email thread.
+
+    Threads are the primary unit of reasoning for correspondence
+    workflows (job applications, interviews, follow-ups, etc.).
+    """
+
+    thread_id: str
+    normalized_subject: str
+
+    participants: Set[str] = field(default_factory=set)
+    messages: List[MailIntakeMessage] = field(default_factory=list)
+
+    last_activity_at: datetime | None = None
+
+    def add_message(self, message: MailIntakeMessage) -> None:
+        """
+        Add a message to the thread and update derived fields.
+        """
+        self.messages.append(message)
+
+        if message.from_email:
+            self.participants.add(message.from_email)
+
+        if self.last_activity_at is None or message.timestamp > self.last_activity_at:
+            self.last_activity_at = message.timestamp
diff --git a/mail_intake/parsers/__init__.py b/mail_intake/parsers/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/mail_intake/parsers/body.py b/mail_intake/parsers/body.py
new file mode 100644
index 0000000..82e14de
--- /dev/null
+++ b/mail_intake/parsers/body.py
@@ -0,0 +1,83 @@
+import base64
+from typing import Dict, Any, Optional
+
+from bs4 import BeautifulSoup
+
+from mail_intake.exceptions import MailIntakeParsingError
+
+
+def _decode_base64(data: str) -> str:
+    """
+    Decode Gmail URL-safe base64 payload into UTF-8 text.
+    """
+    try:
+        padded = data.replace("-", "+").replace("_", "/")
+        decoded = base64.b64decode(padded)
+        return decoded.decode("utf-8", errors="replace")
+    except Exception as exc:
+        raise MailIntakeParsingError("Failed to decode message body") from exc
+
+
+def _extract_from_part(part: Dict[str, Any]) -> Optional[str]:
+    """
+    Extract text content from a single MIME part.
+    """
+    mime_type = part.get("mimeType")
+    body = part.get("body", {})
+    data = body.get("data")
+
+    if not data:
+        return None
+
+    text = _decode_base64(data)
+
+    if mime_type == "text/plain":
+        return text
+
+    if mime_type == "text/html":
+        # soup = BeautifulSoup(text, "lxml")
+        soup = BeautifulSoup(text, "html.parser")
+        return soup.get_text(separator="\n", strip=True)
+
+    return None
+
+
+def extract_body(payload: Dict[str, Any]) -> str:
+    """
+    Extract the best-effort message body from a Gmail payload.
+
+    Priority:
+    1. text/plain
+    2. text/html (stripped to text)
+    3. empty string (if nothing usable found)
+    """
+    if not payload:
+        return ""
+
+    # Multipart message
+    if "parts" in payload:
+        text_plain = None
+        text_html = None
+
+        for part in payload.get("parts", []):
+            content = _extract_from_part(part)
+            if not content:
+                continue
+
+            if part.get("mimeType") == "text/plain" and text_plain is None:
+                text_plain = content
+            elif part.get("mimeType") == "text/html" and text_html is None:
+                text_html = content
+
+        if text_plain:
+            return text_plain
+        if text_html:
+            return text_html
+
+    # Single-part message
+    body = payload.get("body", {})
+    data = body.get("data")
+    if data:
+        return _decode_base64(data)
+
+    return ""
diff --git a/mail_intake/parsers/headers.py b/mail_intake/parsers/headers.py
new file mode 100644
index 0000000..7c0c929
--- /dev/null
+++ b/mail_intake/parsers/headers.py
@@ -0,0 +1,58 @@
+from typing import Dict, List, Tuple, Optional
+
+
+def parse_headers(raw_headers: List[Dict[str, str]]) -> Dict[str, str]:
+    """
+    Convert a list of Gmail-style headers into a normalized dict.
+
+    Input:
+    [
+        {"name": "From", "value": "John Doe <john@example.com>"},
+        {"name": "Subject", "value": "Re: Interview Update"},
+        ...
+    ]
+
+    Output:
+    {
+        "from": "...",
+        "subject": "...",
+        ...
+    }
+    """
+    headers: Dict[str, str] = {}
+
+    for header in raw_headers or []:
+        name = header.get("name")
+        value = header.get("value")
+
+        if not name or value is None:
+            continue
+
+        headers[name.lower()] = value.strip()
+
+    return headers
+
+
+def extract_sender(headers: Dict[str, str]) -> Tuple[str, Optional[str]]:
+    """
+    Extract sender email and optional display name from headers.
+
+    Returns:
+        (email, name)
+
+    If name cannot be determined, name will be None.
+    """
+    from_header = headers.get("from")
+    if not from_header:
+        return "", None
+
+    # Common forms:
+    #   Name <email@domain>
+    #   email@domain
+    if "<" in from_header and ">" in from_header:
+        name_part, email_part = from_header.split("<", 1)
+        email = email_part.rstrip(">").strip()
+        name = name_part.strip().strip('"') or None
+        return email, name
+
+    return from_header.strip(), None
diff --git a/mail_intake/parsers/subject.py b/mail_intake/parsers/subject.py
new file mode 100644
index 0000000..df5041d
--- /dev/null
+++ b/mail_intake/parsers/subject.py
@@ -0,0 +1,33 @@
+import re
+
+
+_PREFIX_RE = re.compile(r"^(re|fw|fwd)\s*:\s*", re.IGNORECASE)
+
+
+def normalize_subject(subject: str) -> str:
+    """
+    Normalize an email subject for thread-level comparison.
+
+    Operations:
+    - Strip common prefixes (Re:, Fwd:, FW:)
+    - Collapse whitespace
+    - Preserve original casing (no lowercasing)
+
+    This function is intentionally conservative.
+    """
+    if not subject:
+        return ""
+
+    normalized = subject.strip()
+
+    # Strip prefixes repeatedly (e.g., Re: Fwd: Re:)
+    while True:
+        new_value = _PREFIX_RE.sub("", normalized)
+        if new_value == normalized:
+            break
+        normalized = new_value.strip()
+
+    # Normalize whitespace
+    normalized = " ".join(normalized.split())
+
+    return normalized
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..f94d90d
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,94 @@
+[build-system]
+requires = ["setuptools>=68", "wheel"]
+build-backend = "setuptools.build_meta"
+
+
+[project]
+name = "mail-intake"
+version = "0.0.1"
+description = "Structured mail ingestion and correspondence parsing with provider adapters (Gmail-first)."
+readme = "README.md"
+requires-python = ">=3.10"
+license = { text = "MIT" }
+
+authors = [
+    { name = "Aetos Skia", email = "dev@aetoskia.com" }
+]
+maintainers = [
+    { name = "Aetos Skia", email = "dev@aetoskia.com" }
+]
+
+
+keywords = [
+    "email",
+    "gmail",
+    "mail",
+    "ingestion",
+    "automation",
+    "job-search",
+    "correspondence",
+]
+
+classifiers = [
+    "Development Status :: 3 - Alpha",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Topic :: Communications :: Email",
+    "Topic :: Software Development :: Libraries",
+]
+
+dependencies = [
+    # Gmail API stack
+    "google-api-python-client>=2.120.0",
+    "google-auth>=2.28.0",
+    "google-auth-oauthlib>=1.2.0",
+
+    # Parsing
+    "beautifulsoup4>=4.12.0",
+    "lxml>=5.1.0",
+]
+
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=8.0.0",
+    "pytest-cov>=4.1.0",
+    "ruff>=0.3.0",
+    "mypy>=1.8.0",
+    "types-beautifulsoup4",
+]
+
+docs = [
+    "mkdocs>=1.5.0",
+    "mkdocs-material>=9.5.0",
+]
+
+
+[project.urls]
+Homepage = "https://git.aetoskia.com/aetos/mail-intake"
+Documentation = "https://git.aetoskia.com/aetos/mail-intake#readme"
+Repository = "https://git.aetoskia.com/aetos/mail-intake.git"
+Issues = "https://git.aetoskia.com/aetos/mail-intake/issues"
+Versions = "https://git.aetoskia.com/aetos/mail-intake/tags"
+
+
+[tool.setuptools]
+package-dir = { "" = "src" }
+
+[tool.setuptools.packages.find]
+where = ["src"]
+include = ["mail_intake*"]
+
+
+[tool.ruff]
+line-length = 100
+target-version = "py310"
+
+[tool.mypy]
+python_version = "3.10"
+strict = true
+ignore_missing_imports = true
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..ea791d9
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,10 @@
+beautifulsoup4==4.12.0
+
+pytest==7.4.0
+pytest-asyncio==0.21.0
+pytest-cov==4.1.0
+
+types-beautifulsoup4
+
+# Optional, useful locally
+ipython
diff --git a/tests/unit/test_models.py b/tests/unit/test_models.py
new file mode 100644
index 0000000..36be62a
--- /dev/null
+++ b/tests/unit/test_models.py
@@ -0,0 +1,91 @@
+from datetime import datetime, timedelta
+
+from mail_intake.models.message import MailIntakeMessage
+from mail_intake.models.thread import MailIntakeThread
+
+
+def test_message_is_immutable():
+    msg = MailIntakeMessage(
+        message_id="m1",
+        thread_id="t1",
+        timestamp=datetime.utcnow(),
+        from_email="alice@example.com",
+        from_name="Alice",
+        subject="Hello",
+        body_text="Body",
+        snippet="Snippet",
+        raw_headers={"from": "Alice <alice@example.com>"},
+    )
+
+    try:
+        msg.subject = "Changed"
+        assert False, "Message should be immutable"
+    except Exception:
+        assert True
+
+
+def test_thread_add_message_updates_participants_and_timestamp():
+    t0 = datetime.utcnow()
+    t1 = t0 + timedelta(minutes=5)
+
+    msg1 = MailIntakeMessage(
+        message_id="m1",
+        thread_id="t1",
+        timestamp=t0,
+        from_email="alice@example.com",
+        from_name="Alice",
+        subject="Hello",
+        body_text="Body",
+        snippet="Snippet",
+        raw_headers={},
+    )
+
+    msg2 = MailIntakeMessage(
+        message_id="m2",
+        thread_id="t1",
+        timestamp=t1,
+        from_email="bob@example.com",
+        from_name="Bob",
+        subject="Re: Hello",
+        body_text="Reply",
+        snippet="Reply",
+        raw_headers={},
+    )
+
+    thread = MailIntakeThread(
+        thread_id="t1",
+        normalized_subject="Hello",
+    )
+
+    thread.add_message(msg1)
+    assert thread.last_activity_at == t0
+    assert "alice@example.com" in thread.participants
+
+    thread.add_message(msg2)
+    assert thread.last_activity_at == t1
+    assert "bob@example.com" in thread.participants
+    assert len(thread.messages) == 2
+
+
+def test_thread_handles_messages_without_sender():
+    msg = MailIntakeMessage(
+        message_id="m1",
+        thread_id="t1",
+        timestamp=datetime.utcnow(),
+        from_email="",
+        from_name=None,
+        subject="System Message",
+        body_text="Body",
+        snippet="Snippet",
+        raw_headers={},
+    )
+
+    thread = MailIntakeThread(
+        thread_id="t1",
+        normalized_subject="System Message",
+    )
+
+    thread.add_message(msg)
+
+    assert len(thread.participants) == 0
+    assert thread.last_activity_at is not None
diff --git a/tests/unit/test_parsers.py b/tests/unit/test_parsers.py
new file mode 100644
index 0000000..6c75491
--- /dev/null
+++ b/tests/unit/test_parsers.py
@@ -0,0 +1,128 @@
+import base64
+
+from mail_intake.parsers.subject import normalize_subject
+from mail_intake.parsers.headers import parse_headers, extract_sender
+from mail_intake.parsers.body import extract_body
+
+
+def _b64(text: str) -> str:
+    return base64.b64encode(text.encode("utf-8")).decode("utf-8")
+
+
+# --------------------
+# Subject parsing
+# --------------------
+
+def test_normalize_subject_strips_common_prefixes():
+    assert normalize_subject("Re: Interview Update") == "Interview Update"
+    assert normalize_subject("Fwd: Re: Offer Letter") == "Offer Letter"
+    assert normalize_subject("FW: Re: FW: Status") == "Status"
+
+
+def test_normalize_subject_preserves_content_and_case():
+    subject = "Interview Update – Backend Role"
+    assert normalize_subject(subject) == subject
+
+
+def test_normalize_subject_empty_and_none_safe():
+    assert normalize_subject("") == ""
+
+
+# --------------------
+# Header parsing
+# --------------------
+
+def test_parse_headers_lowercases_keys():
+    raw_headers = [
+        {"name": "From", "value": "Alice <alice@example.com>"},
+        {"name": "Subject", "value": "Hello"},
+    ]
+
+    headers = parse_headers(raw_headers)
+
+    assert headers["from"] == "Alice <alice@example.com>"
+    assert headers["subject"] == "Hello"
+
+
+def test_parse_headers_ignores_invalid_entries():
+    raw_headers = [
+        {"name": "From", "value": "Bob <bob@example.com>"},
+        {"name": None, "value": "X"},
+        {"name": "X-Test", "value": None},
+    ]
+
+    headers = parse_headers(raw_headers)
+    assert "from" in headers
+    assert "x-test" not in headers
+
+
+def test_extract_sender_with_name_and_email():
+    headers = {"from": "Alice Smith <alice@example.com>"}
+    email, name = extract_sender(headers)
+
+    assert email == "alice@example.com"
+    assert name == "Alice Smith"
+
+
+def test_extract_sender_email_only():
+    headers = {"from": "bob@example.com"}
+    email, name = extract_sender(headers)
+
+    assert email == "bob@example.com"
+    assert name is None
+
+
+def test_extract_sender_missing_from():
+    email, name = extract_sender({})
+    assert email == ""
+    assert name is None
+
+
+# --------------------
+# Body parsing
+# --------------------
+
+def test_extract_body_prefers_text_plain():
+    payload = {
+        "parts": [
+            {
+                "mimeType": "text/html",
+                "body": {"data": _b64("<p>Hello <b>HTML</b></p>")},
+            },
+            {
+                "mimeType": "text/plain",
+                "body": {"data": _b64("Hello TEXT")},
+            },
+        ]
+    }
+
+    body = extract_body(payload)
+    assert body == "Hello TEXT"
+
+
+def test_extract_body_falls_back_to_html():
+    payload = {
+        "parts": [
+            {
+                "mimeType": "text/html",
+                "body": {"data": _b64("<p>Hello <b>World</b></p>")},
+            }
+        ]
+    }
+
+    body = extract_body(payload)
+    assert "Hello" in body
+    assert "World" in body
+
+
+def test_extract_body_single_part():
+    payload = {
+        "body": {"data": _b64("Single part body")}
+    }
+
+    body = extract_body(payload)
+    assert body == "Single part body"
+
+
+def test_extract_body_empty_payload():
+    assert extract_body({}) == ""