Apple shipped a Safari feature that turns a plain-language description into a working browser extension, moving the author of an extension from developer to end user. Anthropic named "Claude Mods," a deeper way to reshape its coding tool. Underneath both, the Model Context Protocol adopted a standard way to share reusable skills across tools, and the draft WebMCP standard added lifecycle events and a site kill switch. HubSpot and Salesforce, meanwhile, wired installed apps straight into their customers' agents.
Platforms: Apple lets an end user author a Safari extension by describing it
Apple shipped a Safari feature that builds an extension from a plain-language description. With the release of iOS 27, iPadOS 27, and macOS 27 on September 14, a person can say in ordinary words what they want an extension to do and get back a working, installable one that syncs across their devices, no code involved. Apple previewed it at its June developer conference; this is the general release. It runs through Apple Intelligence, so it is limited to the devices that support it, rather than the whole install base.
Why it matters: authoring an extension no longer requires the coding skill or the developer account that gatekeep the extension stores. Someone can generate a personal extension for their own device that never passes through a store review, a distribution path no prior Safari extension had.
Customization: Anthropic names "Claude Mods" for its deeper Claude Code layer
Anthropic described "Claude Mods," small programs that load inside Claude Code and reshape how it works. In a public design thread, the company (through engineer Alice Poteat) laid out mods that can inspect, refuse, or rewrite what the tool does, draw their own interface elements, and hide sensitive text, framed as browser extensions for Claude Code. Mods sit alongside the tool's existing customization pieces, its instruction bundles and its outside-tool connectors. The capability is in preview and off by default, with a general release Anthropic says is weeks out.
Because a mod runs with the tool's own reach, it can also read files, launch programs, and use the network. A community review of 31 early mods found that fourteen could start programs and thirteen could observe every action the tool took.
Why it matters: it turns a coding assistant into something a person can reshape at a deep level, not just configure. Because a mod inherits the tool's full access, who publishes one matters as much as what it claims to do.
Standards: MCP shares skills across tools, WebMCP adds a site kill switch
The Model Context Protocol adopted a standard way to share reusable skills across tools. The project made packaged "skills" an official extension, letting any MCP-compatible host pull those reusable bundles of instructions, rather than live tools, from any MCP server. The proposal reached final status on September 13, and on September 18 the project updated its documentation to mark the extension published. Support in its official software kits for TypeScript, Python, C#, and Go is still in open pull requests rather than shipped, and there is no blog announcement yet.
Alongside it, WebMCP, the draft browser standard for letting a web page offer tools to a person's own agent, added structure and a safety control. The editors merged events that tell a page when an agent starts or cancels a tool, a way to mark developer-only tools so a general-purpose agent skips them, and documented guidance for a site to switch WebMCP off entirely as a defense against injected scripts registering tools of their own. WebMCP is a Community Group draft that Chrome, Microsoft Edge, and Brave already run as an origin trial, an experimental phase a site opts into, not a finished, default-on browser feature.
Why it matters: a skill packaged once and usable across any conforming tool lets a person carry their own customizations between apps. The WebMCP changes give the browser, rather than the page, control over which tools an agent may see, and whether a site permits them at all.
Enterprise: HubSpot and Salesforce feed a customer's agent from installed apps
HubSpot made an installed app's tools show up automatically in a customer's agent. In its Fall 2026 release, the company added a way for an app to expose its own remote MCP tools; once a customer installs the app, those tools appear in HubSpot's agent builder with no further setup, though marketplace apps still pass HubSpot's review first.
At its Dreamforce conference, Salesforce introduced AIforce and a "Headless Toolkit", an open set of connectors, interfaces, and skills that exposes Salesforce data, logic, and permissions to outside AI tools, and lets an administrator compose a custom interface by describing it. The headline consumer features are in beta, and the toolkit builds on the marketplace and agent-framework groundwork Salesforce laid at its April developer conference rather than replacing it.
Why it matters: both vendors are moving the unit of extension from an app a person opens to a tool a person's agent can call, which changes what a marketplace listing has to offer: what it contributes to the customer's agent, not what it does as a standalone screen.
Also worth knowing
- AG-UI, CopilotKit's protocol for wiring an app's front end to the agent that drives it, merged a 1.0 draft specification with one schema that all its software kits are generated from. It is aimed at builders more than end users.
- Atlassian gave Trello apps modern sign-in and eased a Forge permission requirement, adding granular, short-lived authorization for Trello and dropping the need for an app's permissions to settle at install time, plus a way to copy a Jira workflow through the API rather than the interface.
- Shopify began rolling out a refreshed admin that restyles apps built on its extension points automatically, while apps that render their own interface must opt in to the new design system to match.
- The chartered W3C WebExtensions Working Group published its September 17 minutes, keeping an ad-measurement control's name despite Chrome deprecating it and adding a single label to track agent and machine-learning extension issues across the group.
On the radar
- September 24 and October 8: the W3C WebExtensions Community Group holds its next public calls.
- October 15: the chartered W3C WebExtensions Working Group meets next.
- Wasmtime v49: the Bytecode Alliance's widely embedded WebAssembly runtime has a release candidate out and a general release expected shortly.
- December 31, 2026: Atlassian retires its Cloud Fortified App program, having stopped new submissions on September 1, with Atlassian Enterprise Certified as the successor.
This Week in Extensibility is curated by Todd Schiller. Research, drafting, and fact checking are AI-assisted.