diff --git a/CHANGELOG.md b/CHANGELOG.md
index 793f50c0..b6012be6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,45 @@
 # Change Log
 
+## [4.0.0](https://github.com/ably/ably-python/tree/v4.0.0) (unreleased)
+
+[Full Changelog](https://github.com/ably/ably-python/compare/v3.1.2...v4.0.0)
+
+> This entry describes work in progress on the `integration/v4` branch. Nothing
+> is published under these names yet; the heading loses "(unreleased)" when
+> 4.0.0 ships.
+
+### Breaking change
+
+Version 4.0.0 applies [PDR-091b](https://ably.atlassian.net/wiki/spaces/product/pages/5362810886)
+and splits the SDK into new distributions. The `ably` package is superseded: it
+receives security and critical-bug fixes only for one year from the 4.0.0
+release date, and is then end-of-life.
+
+- **New distributions.** `ably-pubsub-server` is the public package for servers
+  and other trusted environments. It is built on `ably-pubsub-core`, an internal
+  package that must never be depended on directly; the two are versioned and
+  released in lockstep, with the server pinning the core to the exact same
+  version.
+- **New import namespace.** `ably` becomes `ably_pubsub.server` (and
+  `ably.sync` becomes `ably_pubsub.server.sync`). Both installs can coexist in
+  one environment while migrating, since the import packages differ.
+- **Factory doors replace the constructors.** `AblyRest(...)` becomes
+  `ably_pubsub.server.create_http_client(...)`, `AblyRealtime(...)` becomes
+  `ably_pubsub.server.create_realtime_client(...)`, and `AblyRestSync(...)`
+  becomes `ably_pubsub.server.sync.create_http_client(...)`. Each takes exactly
+  the arguments the constructor it replaces took.
+- **Agent identifier.** Clients now report
+  `ably-pubsub-python/4.0.0 python/<version> ably-pubsub-server`. The
+  `ably-pubsub-server` flag is what declares the server side, which is how the
+  platform exempts these connections from monthly-active-user counting. A new
+  `agents` client option lets a layered SDK add its own entries.
+- **Python floor raised.** `requires-python` is now `>=3.8`; supported versions
+  are 3.8 through 3.14.
+- **`LONG_DESCRIPTION.rst` removed.** Each distribution's PyPI page now renders
+  its own `README.md`.
+
+For detailed migration instructions, please refer to the [Upgrading Guide](UPDATING.md).
+
 ## [3.1.2](https://github.com/ably/ably-python/tree/v3.1.2)
 
 [Full Changelog](https://github.com/ably/ably-python/compare/v3.1.1...v3.1.2)
diff --git a/README.md b/README.md
index 4ee29fd5..0e215068 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,5 @@
 ![Ably Pub/Sub Python Header](images/pythonSDK-github.png)
-[![PyPI version](https://badge.fury.io/py/ably.svg)](https://pypi.org/project/ably/)
+[![PyPI version](https://badge.fury.io/py/ably-pubsub-server.svg)](https://pypi.org/project/ably-pubsub-server/)
 [![License](https://img.shields.io/github/license/ably/ably-python)](https://github.com/ably/ably-python/blob/main/LICENSE)
 
 
@@ -25,6 +25,28 @@ Everything you need to get started with Ably:
 
 ---
 
+## Packages
+
+Python is a server-side language, so this repository publishes one package for
+applications to install, plus the shared implementation it is built on:
+
+| Distribution | For | Import |
+| --- | --- | --- |
+| `ably-pubsub-server` | Servers, backends, jobs — any trusted environment holding an API key | `ably_pubsub.server` |
+| `ably-pubsub-core` | **Internal.** Ably's own packages only — never depend on it directly | — |
+
+Installing the package that names the side your code runs on is more than a
+naming convention: clients created by `ably-pubsub-server` declare themselves as
+server-side on the wire, and server connections are exempt from monthly-active-user
+counting.
+
+Nothing under `ably_pubsub.core` is public API — its module layout and the names
+within it may change in any release. Everything supported is re-exported from
+`ably_pubsub.server`. The two distributions are versioned and released in
+lockstep: `ably-pubsub-server` pins `ably-pubsub-core` to the exact same version.
+
+---
+
 ## Supported platforms
 
 Ably aims to support a wide range of platforms. If you experience any compatibility issues, open an issue in the repository or contact [Ably support](https://ably.com/support).
@@ -33,13 +55,13 @@ The following platforms are supported:
 
 | Platform | Support                  |
 |----------|--------------------------|
-| Python | Python 3.7+ through 3.14 |
+| Python | Python 3.8 through 3.14 |
 
 > [!NOTE]
-> This SDK works across all major operating platforms (Linux, macOS, Windows) as long as Python 3.7+ is available.
+> This SDK works across all major operating platforms (Linux, macOS, Windows) as long as Python 3.8 or greater is available.
 
 > [!IMPORTANT]
-> SDK versions < 2.0.0 are [deprecated](https://ably.com/docs/platform/deprecate/protocol-v1).
+> The `ably` package (3.x and earlier) is superseded by `ably-pubsub-server`. See [Migrating from `ably` 3.x](#migrating-from-ably-3x).
 
 ---
 
@@ -48,36 +70,100 @@ The following platforms are supported:
 To get started with your project, install the package:
 
 ```sh
-pip install ably
+pip install ably-pubsub-server
 ```
 
 > [!NOTE]
 Install [Python](https://www.python.org/downloads/) version 3.8 or greater.
 
+Optional extras: `crypto` for channel encryption, `vcdiff` for delta decoding,
+and `oldcrypto` for the legacy `pycrypto` backend.
+
+```sh
+pip install "ably-pubsub-server[crypto]"
+```
+
+---
+
 ## Usage
 
-The following code connects to Ably's realtime messaging service, subscribes to a channel to receive messages, and publishes a test message to that same channel.
+Clients are created through the factory functions in `ably_pubsub.server`. They
+take exactly the arguments the client constructors take, and return the same
+clients.
+
+### Realtime client
+
+Connects to Ably's realtime messaging service, subscribes to a channel to
+receive messages, and publishes a test message to that same channel.
 
 ```python
-# Initialize Ably Realtime client
-async with AblyRealtime('your-ably-api-key', client_id='me') as realtime_client:
-    # Wait for connection to be established
+import asyncio
+from ably_pubsub.server import create_realtime_client
+
+
+async def main():
+    realtime_client = create_realtime_client(key='your-ably-api-key', client_id='me')
     await realtime_client.connection.once_async('connected')
     print('Connected to Ably')
-    
-    # Get a reference to the 'test-channel' channel
+
     channel = realtime_client.channels.get('test-channel')
-    
-    # Subscribe to all messages published to this channel
+
     def on_message(message):
         print(f'Received message: {message.data}')
-    
+
     await channel.subscribe(on_message)
-    
-    # Publish a test message to the channel
     await channel.publish('test-event', 'hello world')
+
+    await realtime_client.close()
+
+asyncio.run(main())
+```
+
+### HTTP client
+
+For publishing, history, presence reads, stats and token issuing, with no
+persistent connection:
+
+```python
+import asyncio
+from ably_pubsub.server import create_http_client
+
+
+async def main():
+    async with create_http_client(key='your-ably-api-key') as client:
+        channel = client.channels.get('test-channel')
+        await channel.publish('test-event', 'hello world')
+
+asyncio.run(main())
 ```
 
+### Synchronous HTTP client
+
+For code that has no event loop. There is no synchronous realtime client.
+
+```python
+from ably_pubsub.server.sync import create_http_client
+
+client = create_http_client(key='your-ably-api-key')
+client.channels.get('test-channel').publish('test-event', 'hello world')
+client.close()
+```
+
+---
+
+## Migrating from `ably` 3.x
+
+Version 4.0.0 moves the SDK to the `ably-pubsub-server` distribution and the
+`ably_pubsub.server` import namespace. For most applications the change is
+confined to the install line, the import, and the constructor call.
+[UPDATING.md](./UPDATING.md) has the full mapping table and worked examples.
+
+The `ably` package receives security and critical-bug fixes only for one year
+from the 4.0.0 release, and is then end-of-life. The two packages can be
+installed side by side while you migrate — they use different import packages.
+
+---
+
 ## Releases
 
 The [CHANGELOG.md](https://github.com/ably/ably-python/blob/main/CHANGELOG.md) contains details of the latest releases for this SDK. You can also view all Ably releases on [changelog.ably.com](https://changelog.ably.com).
@@ -93,8 +179,3 @@ Read the [CONTRIBUTING.md](./CONTRIBUTING.md) guidelines to contribute to Ably.
 ## Support, feedback, and troubleshooting
 
 For help or technical support, visit Ably's [support page](https://ably.com/support) or [GitHub Issues](https://github.com/ably/ably-python/issues) for community-reported bugs and discussions.
-
-### Full Realtime support unavailable
-
-This SDK currently supports only [Ably REST](https://ably.com/docs/rest) and basic realtime message subscriptions. To access full [Ably Realtime](https://ably.com/docs/realtime) features in Python, consider using the [MQTT adapter](https://ably.com/docs/mqtt).
-
diff --git a/UPDATING.md b/UPDATING.md
index 4b4dd719..a89b866b 100644
--- a/UPDATING.md
+++ b/UPDATING.md
@@ -1,5 +1,163 @@
 # Upgrade / Migration Guide
 
+## Version 3.x (`ably`) to 4.0.0 (`ably-pubsub-server`)
+
+> **Status: draft.** The public API naming is still under review; the class and
+> function names in this section may change before the 4.0.0 GA release.
+
+Version 4.0.0 splits the SDK into new distributions, following
+[PDR-091b](https://ably.atlassian.net/wiki/spaces/product/pages/5362810886). The
+`ably` package is superseded: it receives security and critical-bug fixes only
+for one year from the 4.0.0 release date, and is then end-of-life.
+
+### Why
+
+Under MAU-based pricing the platform must classify every connection as
+device-side or server-side. The new packages declare which side they are on
+automatically, as part of the agent identifier they put on the wire. The old
+`ably` constructors cannot: nothing in them says where the code runs, so once
+MAU pricing is live they are rejected on MAU-enabled accounts because the
+platform cannot classify them.
+
+Python is a server-side language, so there is a single new public distribution,
+`ably-pubsub-server`, whose factory functions are the only recommended entry
+points. It is built on `ably-pubsub-core`, an internal distribution you should
+never depend on directly. The objects the factories return are the same clients
+as today — channels, presence, history, auth and error handling are unchanged.
+For most applications the migration is confined to the install line, the import,
+and the constructor call.
+
+### Mapping
+
+| 3.x (`ably`) | 4.0 (`ably-pubsub-server`) |
+| --- | --- |
+| `pip install ably` | `pip install ably-pubsub-server` |
+| `pip install ably[crypto]` | `pip install ably-pubsub-server[crypto]` |
+| `from ably import AblyRest` | `from ably_pubsub.server import create_http_client` |
+| `AblyRest(key=...)` | `create_http_client(key=...)` |
+| `from ably import AblyRealtime` | `from ably_pubsub.server import create_realtime_client` |
+| `AblyRealtime(key=...)` | `create_realtime_client(key=...)` |
+| `from ably.sync import AblyRestSync` | `from ably_pubsub.server.sync import create_http_client` |
+| `AblyRestSync(key=...)` | `create_http_client(key=...)` (from `ably_pubsub.server.sync`) |
+| `from ably import X` (any name exported by `ably`) | `from ably_pubsub.server import X` |
+| `from ably.types.channeloptions import ChannelOptions` | `from ably_pubsub.server import ChannelOptions` |
+| `from ably.util.crypto import CipherParams` | `from ably_pubsub.server import CipherParams` |
+
+#### Deep imports
+
+In 3.x many types could only be reached by importing the submodule they were
+defined in. In 4.0 all of them are re-exported from `ably_pubsub.server`, so the
+submodule path goes away entirely — **the flat import is the supported one**.
+Nothing under `ably_pubsub.core` is public API.
+
+| 3.x deep import | 4.0 |
+| --- | --- |
+| `from ably.types.message import Message, MessageAnnotations` | `from ably_pubsub.server import Message, MessageAnnotations` |
+| `from ably.types.presence import Presence, PresenceMessage, PresenceAction` | `from ably_pubsub.server import Presence, PresenceMessage, PresenceAction` |
+| `from ably.types.tokenrequest import TokenRequest` | `from ably_pubsub.server import TokenRequest` |
+| `from ably.types.tokendetails import TokenDetails` | `from ably_pubsub.server import TokenDetails` |
+| `from ably.types.channeldetails import ChannelDetails, ChannelStatus, ChannelOccupancy, ChannelMetrics` | `from ably_pubsub.server import ChannelDetails, ChannelStatus, ChannelOccupancy, ChannelMetrics` |
+| `from ably.types.channelstate import ChannelState, ChannelStateChange` | `from ably_pubsub.server import ChannelState, ChannelStateChange` |
+| `from ably.types.connectionstate import ConnectionState, ConnectionEvent, ConnectionStateChange` | `from ably_pubsub.server import ConnectionState, ConnectionEvent, ConnectionStateChange` |
+| `from ably.types.stats import Stats` | `from ably_pubsub.server import Stats` |
+| `from ably.http.paginatedresult import PaginatedResult, HttpPaginatedResponse` | `from ably_pubsub.server import PaginatedResult, HttpPaginatedResponse` |
+| `from ably.rest.channel import Channel` | `from ably_pubsub.server import Channel` |
+| `from ably.realtime.channel import RealtimeChannel` | `from ably_pubsub.server import RealtimeChannel` |
+| `from ably.realtime.connection import Connection` | `from ably_pubsub.server import Connection` |
+| `from ably.realtime.presence import RealtimePresence` | `from ably_pubsub.server import RealtimePresence` |
+
+The full supported surface of `ably_pubsub.server`:
+
+```
+AblyAuthException, AblyException, AblyRealtime, AblyRest, AblyVCDiffDecoder,
+Annotation, AnnotationAction, Auth, Capability, Channel, ChannelDetails,
+ChannelMetrics, ChannelMode, ChannelOccupancy, ChannelOptions, ChannelState,
+ChannelStateChange, ChannelStatus, CipherParams, Connection, ConnectionEvent,
+ConnectionState, ConnectionStateChange, DeviceDetails, HttpPaginatedResponse,
+IncompatibleClientIdException, Message, MessageAction, MessageAnnotations,
+MessageOperation, MessageVersion, Options, PaginatedResult, Presence,
+PresenceAction, PresenceMessage, PublishResult, Push, PushChannelSubscription,
+RealtimeChannel, RealtimePresence, SERVER_AGENT_IDENTIFIER, Stats,
+TokenDetails, TokenRequest, UpdateDeleteResult, VCDiffDecoder,
+create_http_client, create_realtime_client
+```
+
+`ably_pubsub.server.sync` re-exports the same set minus the realtime types, with
+the synchronous flavours under their `Sync` names: `AblyRestSync`, `AuthSync`,
+`PushSync`, `ChannelSync`, `PaginatedResultSync` and
+`HttpPaginatedResponseSync`. There is no synchronous realtime client, so
+`AblyRealtime`, `RealtimeChannel`, `RealtimePresence`, `Connection` and the
+channel/connection state types are not there.
+
+Two names you may go looking for and not find, in either version:
+
+- **`TokenParams` is not a class in this SDK.** Token params are plain
+  dictionaries, for example
+  `await auth.request_token(token_params={'ttl': 3600000, 'client_id': 'me'})`.
+- **There is no separate `ErrorInfo` type.** `AblyException` is the equivalent
+  and carries `code` and `status_code`; it is exported from
+  `ably_pubsub.server`.
+
+### Example
+
+```python
+# 3.x
+from ably import AblyRest
+
+client = AblyRest(key='your-api-key')
+
+# 4.0
+from ably_pubsub.server import create_http_client
+
+client = create_http_client(key='your-api-key')
+```
+
+```python
+# 3.x
+from ably import AblyRealtime
+
+client = AblyRealtime(key='your-api-key', client_id='me')
+
+# 4.0
+from ably_pubsub.server import create_realtime_client
+
+client = create_realtime_client(key='your-api-key', client_id='me')
+```
+
+```python
+# 3.x
+from ably.sync import AblyRestSync
+
+client = AblyRestSync(key='your-api-key')
+
+# 4.0
+from ably_pubsub.server.sync import create_http_client
+
+client = create_http_client(key='your-api-key')
+```
+
+The factories take exactly the keyword arguments the old constructors took:
+`create_http_client(key=None, token=None, token_details=None, **options)` and
+`create_realtime_client(key=None, loop=None, **options)`, where `**options` is
+the same client options as before. The only argument that behaves differently is
+`agents`: your entries are preserved, but the package's own `ably-pubsub-server`
+entry is always added, so the wire agent reads
+`ably-pubsub-python/4.0.0 python/3.x ably-pubsub-server`.
+
+### Packaging changes
+
+- **Python floor.** `requires-python` is now `>=3.8` (it was `>=3.7`, though 3.7
+  was already untested). Supported versions are 3.8 through 3.14.
+- **Extras.** The same extras exist under the new name:
+  `ably-pubsub-server[crypto]`, `ably-pubsub-server[vcdiff]` and
+  `ably-pubsub-server[oldcrypto]`.
+- **Two distributions.** `ably-pubsub-server` depends on `ably-pubsub-core`
+  pinned to the exact same version; the two are always released in lockstep. Do
+  not install or import `ably-pubsub-core` directly.
+- **Side-by-side install is safe.** `ably` and `ably-pubsub-server` use
+  different import packages (`ably` and `ably_pubsub`), so both can be installed
+  in one environment while you migrate.
+
 ## Version 2.x to 3.0.0
 
 The 3.0.0 version of ably-python introduces several breaking changes to improve the realtime experience and align the API with the Ably specification. These include:
diff --git a/core/README.md b/core/README.md
index 6c3a8697..6b4f15f4 100644
--- a/core/README.md
+++ b/core/README.md
@@ -2,21 +2,18 @@
 
 **This is an internal implementation package. Do not depend on it directly.**
 
-`ably-pubsub-core` holds the shared implementation of Ably's Pub/Sub SDK for
-Python: the HTTP and realtime clients, channels, presence, authentication,
-encryption and the message types. It is published so that the packages
-applications *do* install can depend on one shared implementation, pinned to an
-exact version.
-
-Install the package that names the side your code runs on instead:
-
-| Where your code runs | Install | Import |
-| --- | --- | --- |
-| A server or other trusted environment | `ably-pubsub-server` | `ably_pubsub.server` |
-
-Nothing under `ably_pubsub.core` is public API. Its module layout, and the
-names within it, may change in any release — including patch releases — without
-a deprecation cycle. The supported surface is what `ably_pubsub.server`
+`ably-pubsub-core` holds the shared implementation of [Ably](https://ably.com)'s
+Pub/Sub SDK for Python: the HTTP and realtime clients, channels, presence,
+authentication, encryption and the message types. It is published only so that
+the packages applications *do* install can share one implementation.
+
+Install [`ably-pubsub-server`](https://pypi.org/project/ably-pubsub-server/)
+instead — it is the public package for servers and other trusted environments,
+and it depends on this one at an exact pinned version, released in lockstep.
+
+Nothing under `ably_pubsub.core` is public API. Its module layout, and the names
+within it, may change in any release — including patch releases — without a
+deprecation cycle. The supported surface is what `ably_pubsub.server`
 re-exports.
 
 `ably_pubsub` is a [PEP 420](https://peps.python.org/pep-0420/) namespace
diff --git a/server/README.md b/server/README.md
index a67f195e..07046be2 100644
--- a/server/README.md
+++ b/server/README.md
@@ -7,9 +7,20 @@ Ably is the platform that powers synchronized digital experiences in realtime.
 Pub/Sub is its foundational product: publish and subscribe messaging over
 channels, presence, history, and token issuing.
 
-Choosing the package by side matters beyond naming: connections made by this
-package declare themselves as server-side on the wire, and server connections
-are exempt from monthly-active-user counting.
+## What this package is for
+
+Install this package when your code runs somewhere you control and trust: a
+backend service, an API, a worker or a scheduled job — anywhere that can hold an
+API key. It is not for browsers, mobile apps, or anything else shipped to an end
+user.
+
+Choosing the package by side matters beyond naming. Clients created here stamp
+`ably-pubsub-server` into the agent identifier they put on the wire — the wire
+shape is `ably-pubsub-python/4.0.0 python/3.x ably-pubsub-server` — and that
+flag is how the platform classifies the connection as server-side and exempts it
+from monthly-active-user counting.
+
+Requires Python 3.8 or greater.
 
 ## Installation
 
@@ -17,8 +28,8 @@ are exempt from monthly-active-user counting.
 pip install ably-pubsub-server
 ```
 
-Optional extras: `crypto` (channel encryption, via pycryptodome) and `vcdiff`
-(delta decoding).
+Optional extras: `crypto` (channel encryption, via pycryptodome), `vcdiff`
+(delta decoding) and `oldcrypto` (the legacy pycrypto backend).
 
 ```shell
 pip install "ably-pubsub-server[crypto]"
@@ -58,6 +69,7 @@ from ably_pubsub.server.sync import create_http_client
 
 client = create_http_client(key='your-api-key')
 client.channels.get('some-channel').publish('greeting', 'hello')
+client.close()
 ```
 
 ### Realtime client
@@ -82,12 +94,24 @@ asyncio.run(main())
 
 There is no synchronous realtime client.
 
+Both factories take exactly the arguments the underlying client constructors
+take: `create_http_client(key=None, token=None, token_details=None, **options)`
+and `create_realtime_client(key=None, loop=None, **options)`.
+
+## Documentation
+
+- [Ably Pub/Sub docs](https://ably.com/docs/basics)
+- [Getting started with Pub/Sub using Python](https://ably.com/docs/getting-started/python)
+- [Ably Pub/Sub examples](https://ably.com/examples?product=pubsub)
+
 ## Relationship to other packages
 
 - `ably-pubsub-core` — the shared implementation this package is built on. It is
   an internal package; do not import `ably_pubsub.core` directly. Everything
   supported is re-exported from `ably_pubsub.server`.
-- `ably` — the 3.x package this one replaces. See
+- `ably` — the 3.x package this one replaces. It receives security and
+  critical-bug fixes only for one year from the 4.0.0 release, and is then
+  end-of-life. The migration is usually three lines: see
   [UPDATING.md](https://github.com/ably/ably-python/blob/main/UPDATING.md).
 
 ## Contributing