## What it does
- Sets out the ways you can extend Microsoft 365 Copilot: give it new knowledge, new skills, or build a purpose-built agent that sits inside Copilot Chat and the app catalogue.
+ Sets out the ways to extend Microsoft 365 Copilot. The first decision is now where people will use the result: inside Microsoft 365 Copilot experiences, or in your own apps and custom agents built on APIs and SDKs. A single solution can use both.
  
  ## Key facts
- - Three broad routes: declarative agents (Copilot's own model, your instructions/knowledge/actions), custom engine agents (your own model and orchestration), and Copilot connectors to ground Copilot in external data.
- - Actions come via API plugins (OpenAPI), Copilot connectors, or MCP servers.
- - Declarative agents are packaged as Teams/M365 app packages and distributed through the same app pipeline.
- - Building needs a Microsoft 365 Copilot licence in the target tenant to test end to end; makers need Teams Toolkit, Copilot Studio, or Agent Builder.
+ - Inside Copilot: build or configure supported agents, connectors, skills and MCP-based tools, usually brought together as a **plugin**, then follow the package, distribution and governance route for that object.
+ - In your own apps and agents: use the Work IQ APIs for Microsoft 365 work context and the Microsoft 365 Copilot APIs for search, retrieval, chat, export, meeting, reporting or administration capabilities.
+ - Declarative agents use Copilot's orchestrator and models. Custom engine agents bring their own orchestration, models, hosting and runtime.
+ - Using an API doesn't select a package or distribution route. Each part of a combined solution keeps its own identity, authentication, deployment, distribution and governance requirements.
+ - Microsoft now splits the guidance by audience: builders, administrators, and ISVs or software publishers each have their own entry point.
+ - Teams message extensions are listed as something to maintain or evaluate, not as the default new build.
  
  ## When to use / skip
- Start here before you commit to a build path. If you only need to answer questions over your own content, a connector plus a declarative agent is usually enough. Reach for a custom engine agent only when you genuinely need your own model or orchestration logic.
+ Start here before you commit to a build path. If you only need answers over your own content, a connector plus a declarative agent is usually enough. Reach for a custom engine agent, or an app calling the APIs, only when you need your own runtime, model or user interface.
  
  ## Configuration decisions
- - Declarative vs custom engine — the single biggest fork.
- - Where knowledge lives: SharePoint/Graph, a connector, or an external API.
- - Whether actions are read-only or write-back, which changes your governance burden.
+ - Experience inside Copilot, or one you host and operate. This decides who runs it and who supports it.
+ - Declarative versus custom engine, the biggest build fork.
+ - Where knowledge lives: SharePoint and Graph, a connector, or an external API.
+ - Read-only or write-back actions, which sets the governance burden.
+ - Whether you're building for your own tenant or for customers, which puts you on the ISV route.
  
  ## Gotchas
- - "Extensibility" spans several docs and product teams; terminology drifts between Copilot Studio and the developer docs. Confirm which surface a feature actually lands in.
+ - The old "three routes" mental model (declarative, custom engine, connectors) misses the apps-and-APIs path entirely. Check older internal decks.
+ - A plugin, its package, any app package, a marketplace offer and a hosted runtime are separate objects, each with its own process.
+ - Terminology drifts between Copilot Studio and the developer docs. Confirm which surface a capability lands in.
  - A licence gap in the test tenant will block validation late in the day.
  
  ## Consultant notes
  
- - Default to declarative. Custom engine is a real engineering commitment — model hosting, evals, the lot — and most business asks don't need it.
+ - Ask "where do people sit when they use this?" first. It settles half the architecture before anyone mentions a model.
+ - Default to declarative. Custom engine means model hosting, evals and an operations commitment most business asks don't justify.
  - Connector and grounding costs are per-item and per-query; price the data volume before you promise anything.
- - Publishing runs through admin approval, so factor governance friction into the timeline from day one.
+ - Publishing runs through admin approval, so put governance friction in the plan from day one.
  
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- *Review if Microsoft restructures the extensibility landing page or adds a new build
+ *Review when Microsoft next restructures the extensibility landing page or adds a build path.*
  

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- *Review if Microsoft restructures the extensibility landing page or adds a new build path.*
+ *Review if Microsoft restructures the extensibility landing page or adds a new build