diff --git a/pyproject.toml b/pyproject.toml
index 411e2df..9dede81 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -42,7 +42,7 @@ sphinxcontrib-napoleon = "^0.7"
 toml = "^0.10.2"
 
 [tool.pytest.ini_options]
-addopts = "--doctest-modules"
+addopts = "--doctest-modules -vv --showlocals"
 
 [tool.pyright]
 pythonVersion = "3.10"
diff --git a/tests/test_models/test_messages.py b/tests/test_models/test_messages.py
new file mode 100644
index 0000000..6a1571c
--- /dev/null
+++ b/tests/test_models/test_messages.py
@@ -0,0 +1,56 @@
+from textwrap import dedent
+
+from wanga.models.messages import (
+    AssistantMessage,
+    ImageContent,
+    SystemMessage,
+    ToolInvocation,
+    ToolMessage,
+    UserMessage,
+    parse_messages,
+)
+
+
+def test_consistency():
+    messages = [
+        SystemMessage(content="Be a helpful assistant!\n"),
+        UserMessage(name="Alice", content=["Hello, world!"]),
+        AssistantMessage(
+            name="Bob",
+            content="Here's a helpful image!",
+            tool_invocations=[
+                ToolInvocation(
+                    invocation_id="abc123",
+                    name="paste_image",
+                    arguments={"url": "https://example.com/image.png"},
+                )
+            ],
+        ),
+        ToolMessage(
+            invocation_id="abc123",
+            content=[
+                "Here is the image you requested:\n\n",
+                ImageContent(url="https://example.com/image.png"),
+            ],
+        ),
+        UserMessage(name="Alice", content=["Thanks for the image!"]),
+    ]
+
+    chat_str = "\n".join(str(message) for message in messages)
+    parsed_messages = parse_messages(chat_str)
+    assert parsed_messages == messages
+
+    chat_str = r"""
+    [|user|]
+    Hi! Here is the pic<|image url="https://example.com/image.png"|>
+    [|assistant|]
+    It is beautiful!
+    [|user|]
+    I love you!
+    [|assistant|]
+    I love you too!
+    """
+    chat_str = dedent(chat_str.removeprefix("\n"))
+
+    parsed_messages = parse_messages(chat_str)
+    assert "\n".join(str(message) for message in parsed_messages) == chat_str.strip()
diff --git a/wanga/models/messages.py b/wanga/models/messages.py
index 7a2d372..705b300 100644
--- a/wanga/models/messages.py
+++ b/wanga/models/messages.py
@@ -1,61 +1,60 @@
-from pprint import pformat
-from typing import TYPE_CHECKING, TypeAlias
+import json
+import logging
+import re
+from collections.abc import Iterable
+from typing import NamedTuple
 
-from attrs import frozen
-
-from wanga import _PIL_INSTALLED
+from attrs import field, frozen
 
 from ..common import JSON
 
 __all__ = [
-    "ImageURL",
+    "AssistantMessage",
     "ImageContent",
     "Message",
-    "UserMessage",
-    "AssistantMessage",
     "ToolInvocation",
     "ToolMessage",
+    "UserMessage",
 ]
 
 
-@frozen
-class ImageURL:
-    url: str
+class TagPair(NamedTuple):
+    open: str
+    close: str
 
 
-if TYPE_CHECKING or _PIL_INSTALLED:
-    from PIL.Image import Image
+_MESSAGE_TAGS = TagPair(r"[|", r"|]")
+_CONTENT_BLOCK_TAGS = TagPair(r"<|", r"|>")
 
-    ImageContent: TypeAlias = Image | ImageURL
-else:
-    ImageContent: TypeAlias = ImageURL
 
+_logger = logging.getLogger(__name__)
 
-_MESSAGE_TAG_OPEN = "[|"
-_MESSAGE_TAG_CLOSE = "|]"
 
-_CONTENT_BLOCK_TAG_OPEN = "<|"
-_CONTENT_BLOCK_TAG_CLOSE = "|>"
+def _format_header(_tags: TagPair, _name: str, /, **kwargs: str) -> str:
+    param_str = "".join(f' {key}="{value}"' for key, value in kwargs.items())
+    return f"{_tags.open}{_name}{param_str}{_tags.close}"
 
 
-def _pretty_print_message(role: str, name: str | None, content: str | list[str | ImageContent]) -> str:
-    if name:
-        role_str = f"{_MESSAGE_TAG_OPEN}{role} {name}{_MESSAGE_TAG_CLOSE}"
-    else:
-        role_str = f"{_MESSAGE_TAG_OPEN}{role}{_MESSAGE_TAG_CLOSE}"
+@frozen
+class ImageContent:
+    r"""Image embedded in a message. Can either be a URL, or a base64-encoded file."""
 
-    content_strings = []
-    if isinstance(content, str):
-        content = [content]
-    for content_block in content:
-        if isinstance(content_block, ImageURL):
-            content_block = f'{_CONTENT_BLOCK_TAG_OPEN}image url="{content_block.url}"{_CONTENT_BLOCK_TAG_CLOSE}'
-        if not isinstance(content_block, str):
-            content_block = f"{_CONTENT_BLOCK_TAG_OPEN}image{_CONTENT_BLOCK_TAG_CLOSE}"
-        content_strings.append(content_block)
-    joined_content = "\n".join(content_strings)
+    url: str | None = None
+    base64: str | None = None
 
-    return f"{role_str}\n{joined_content}"
+    def __attrs_post_init__(self):
+        if self.url is None and self.base64 is None:
+            raise ValueError("ImageURL must have either a URL or base64 data")
+        if self.url is not None and self.base64 is not None:
+            raise ValueError("ImageURL cannot have both a URL and base64 data")
+
+    def __str__(self) -> str:
+        kwargs = {}
+        if self.url:
+            kwargs["url"] = self.url
+        if self.base64:
+            kwargs["base64"] = self.base64
+        return _format_header(_CONTENT_BLOCK_TAGS, "image", **kwargs)
 
 
 @frozen
@@ -63,56 +62,215 @@ class Message:
     pass
 
 
+@frozen
 class SystemMessage(Message):
-    name: str | None
     content: str
+    name: str | None = None
 
     def __str__(self) -> str:
-        return _pretty_print_message("system", self.name, self.content)
+        kwargs = {}
+        if self.name:
+            kwargs["name"] = self.name
+        header = _format_header(_MESSAGE_TAGS, "system", **kwargs)
+        return f"{header}\n{self.content}"
 
 
 @frozen
 class UserMessage(Message):
-    name: str | None
     content: str | list[str | ImageContent]
+    name: str | None = None
 
     def __str__(self) -> str:
-        return _pretty_print_message("user", self.name, self.content)
+        kwargs = {}
+        if self.name:
+            kwargs["name"] = self.name
+        header = _format_header(_MESSAGE_TAGS, "user", **kwargs)
+        content = "".join(str(item) for item in self.content)
+        return f"{header}\n{content}"
 
 
 @frozen
 class ToolInvocation:
     invocation_id: str
-    tool_name: str
-    tool_args: JSON
+    name: str
+    arguments: JSON
 
     def __str__(self) -> str:
-        pretty_json = pformat(self.tool_args, sort_dicts=True)
-        call_header = (
-            f'{_CONTENT_BLOCK_TAG_OPEN}call {self.tool_name} id="{self.invocation_id}"{_CONTENT_BLOCK_TAG_CLOSE}'
-        )
-        return f"{call_header}\n{pretty_json}"
+        pretty_json = json.dumps(self.arguments, indent=4, sort_keys=True)
+        header = _format_header(_CONTENT_BLOCK_TAGS, "tool", name=self.name, id=self.invocation_id)
+        return f"{header}\n{pretty_json}"
 
 
 @frozen
 class AssistantMessage(Message):
-    name: str | None
-    content: str | None
-    tool_invocations: list[ToolInvocation]
+    name: str | None = None
+    content: str | None = None
+    tool_invocations: list[ToolInvocation] = field(factory=list)
 
     def __str__(self) -> str:
+        kwargs = {}
+        if self.name:
+            kwargs["name"] = self.name
+        header = _format_header(_MESSAGE_TAGS, "assistant", **kwargs)
+
         content_blocks = []
         if self.content:
             content_blocks.append(self.content)
         for tool_invocation in self.tool_invocations:
             content_blocks.append(str(tool_invocation))
-        return "\n".join(content_blocks)
+        joined_content = "\n".join(content_blocks)
+
+        return f"{header}\n{joined_content}"
 
 
 @frozen
-class ToolMessage:
+class ToolMessage(Message):
     invocation_id: str
-    tool_result: str | list[str | ImageContent]
+    content: str | list[str | ImageContent]
 
     def __str__(self) -> str:
-        return _pretty_print_message("tool", f"id={self.invocation_id}", self.tool_result)
+        header = _format_header(_MESSAGE_TAGS, "tool", id=self.invocation_id)
+        tool_result = self.content
+        if not isinstance(self.content, list):
+            tool_result = [self.content]
+        content = "".join(str(item) for item in tool_result)
+        return f"{header}\n{content}"
+
+
+class ParsedHeader(NamedTuple):
+    name: str
+    params: dict[str, str]
+
+
+class MessageSyntaxError(ValueError):
+    pass
+
+
+class HeaderRegexes(NamedTuple):
+    tag_regex: re.Pattern
+    full_regex: re.Pattern
+
+    def parse(self, header_str: str) -> ParsedHeader:
+        header_match = self.full_regex.match(header_str)
+        if header_match is None:
+            raise MessageSyntaxError(f"Invalid header: {header_str}")
+        name = header_match.group("name")
+        params_str = header_match.group("params")
+        params = {}
+        for param_match in _PARAM_REGEX.finditer(params_str):
+            params[param_match.group("param_name")] = param_match.group("param_value")
+        return ParsedHeader(name, params)
+
+    def split_text(self, text: str) -> Iterable[ParsedHeader | str]:
+        for text_block in self.tag_regex.split(text):
+            if self.tag_regex.match(text_block):
+                yield self.parse(text_block)
+            elif text_block:
+                yield text_block
+
+
+_URL_SPECIAL_SYMBOLS = re.escape(r"/:!#$%&'*+-.^_`|~")
+_PARAM_KEY_SYMBOLS = r"[a-zA-Z0-9_\-]"
+_PARAM_VALUE_SYMBOLS = f"[a-zA-Z0-9{_URL_SPECIAL_SYMBOLS}]"
+
+
+_PARAM_REGEX_STR = f'(?P<param_name>{_PARAM_KEY_SYMBOLS}+) *= *["](?P<param_value>{_PARAM_VALUE_SYMBOLS}*)["]'
+_PARAM_REGEX = re.compile(_PARAM_REGEX_STR)
+
+_PARAMS_REGEX_STR = f"( +(?P<param>{_PARAM_REGEX_STR}))* *"
+_HEADER_BODY_REGEX_STR = f" *(?P<name>{_PARAM_KEY_SYMBOLS}+)(?P<params>{_PARAMS_REGEX_STR})"
+
+
+def _make_header_regex(tags: TagPair, inner_regex: str) -> HeaderRegexes:
+    open_tag = re.escape(tags.open)
+    close_tag = re.escape(tags.close)
+    tag_regex = re.compile(f"({open_tag}.*{close_tag})")
+    full_regex = re.compile(f"(^{open_tag}{_HEADER_BODY_REGEX_STR}{close_tag}$)")
+    return HeaderRegexes(tag_regex, full_regex)
+
+
+_MESSAGE_HEADER_REGEXES = _make_header_regex(_MESSAGE_TAGS, _HEADER_BODY_REGEX_STR)
+_CONTENT_HEADER_REGEXES = _make_header_regex(_CONTENT_BLOCK_TAGS, _HEADER_BODY_REGEX_STR)
+
+
+def _parse_image_content(header: ParsedHeader) -> ImageContent:
+    assert header.name == "image"
+    try:
+        return ImageContent(**header.params)
+    except ValueError as e:
+        raise MessageSyntaxError(f"Invalid image parameters: {header.params}") from e
+
+
+def _parse_tool_invocation(header: ParsedHeader, arg_text: str) -> ToolInvocation:
+    assert header.name == "tool"
+    try:
+        kwargs = dict(header.params)
+        kwargs["invocation_id"] = kwargs.pop("id")
+        return ToolInvocation(**kwargs, arguments=json.loads(arg_text))
+    except (ValueError, KeyError) as e:
+        raise MessageSyntaxError(f"Invalid tool invocation parameters: {header}\n{arg_text}") from e
+
+
+def _map_headers_to_content(blocks: Iterable[str | ParsedHeader]) -> Iterable[str | ImageContent]:
+    for block in blocks:
+        if isinstance(block, str):
+            yield block
+        else:
+            yield _parse_image_content(block)
+
+
+def _parse_message(header: ParsedHeader, message_str: str) -> Message:
+    message_blocks = list(_CONTENT_HEADER_REGEXES.split_text(message_str))
+    match header.name:
+        case "system":
+            if len(message_blocks) > 1:
+                raise MessageSyntaxError(f"System message cannot contain anything other than text: {message_str}")
+            return SystemMessage(name=header.params.get("name"), content=message_str)
+        case "user":
+            content = list(_map_headers_to_content(message_blocks))
+            return UserMessage(name=header.params.get("name"), content=content)
+        case "assistant":
+            if not message_blocks:
+                raise MessageSyntaxError(f"No content in assistant message: {message_str}")
+            if isinstance(message_blocks[0], ParsedHeader):
+                content = None
+            else:
+                content = message_blocks.pop(0)
+                assert isinstance(content, str)
+            tool_invocations = []
+            for block_header, arg_str in zip(message_blocks[::2], message_blocks[1::2]):  # type: ignore
+                if not isinstance(block_header, ParsedHeader):
+                    raise MessageSyntaxError(f"Invalid tool invocation header: {block_header}")
+                if not isinstance(arg_str, str):
+                    raise MessageSyntaxError(f"No arguments specified to tool invocation {block_header}")
+                tool_invocations.append(_parse_tool_invocation(block_header, arg_str))
+            if tool_invocations and content is not None:
+                content = content.removesuffix("\n")
+            return AssistantMessage(name=header.params.get("name"), content=content, tool_invocations=tool_invocations)
+        case "tool":
+            content = list(_map_headers_to_content(message_blocks))
+            return ToolMessage(invocation_id=header.params["id"], content=content)
+        case _:
+            raise MessageSyntaxError(f"Invalid message type: {header.name}")
+
+
+def parse_messages(chat_str: str) -> list[Message]:
+    blocks = list(_MESSAGE_HEADER_REGEXES.split_text(chat_str))
+    if not blocks:
+        return []
+    if not isinstance(blocks[0], ParsedHeader):
+        blocks = [ParsedHeader("user", {})] + blocks
+    messages = []
+    for header, message_str in zip(blocks[::2], blocks[1::2]):
+        if not isinstance(header, ParsedHeader):
+            raise MessageSyntaxError(f"Invalid message header: {header}")
+        if not isinstance(message_str, str):
+            raise MessageSyntaxError(f"No content for message: {header}")
+        message_str = message_str.removeprefix("\n").removesuffix("\n")
+        if not message_str.strip():
+            _logger.warning(
+                f"Message doesn't contain non-whitespace symbols: {header}."
+                "Check newlines at the begginning and the end of the prompt."
+            )
+        messages.append(_parse_message(header, message_str))
+    return messages