Auth & the scoped principal

One gate, both audiences#

June has exactly one authorization gate, and both your UI and an agent pass through it: a defineAction()'s run(input, ctx).

export const deletePost = defineAction({
  id: "deletePost",
  input: { id: "string" },
  run: async (input, ctx) => {
    const post = await db.posts.find(input.id);
    if (post.authorId !== ctx.user?.id) throw new Error("forbidden");
    return db.posts.delete(input.id);
  },
});

There is no "expose to agents" step and no second permission system. The action is a server action and an MCP tool; the check you write once protects both.

ctx is the principal#

ctx answers who is calling — ctx.user, ctx.session, and the request (url, params). That is identity, and identity is all ctx carries: data lives on the ambient db instead (see the data model), so the only reason to reach for ctx is authorization.

An agent is just another caller#

When an agent calls a tool at /mcp, it carries the caller's credential, so the SAME run(input, ctx) sees the SAME ctx it would for a UI request — and runs the SAME check. The agent is a scoped principal: it can do exactly what that user can do, no more. Nothing is re-declared; no tool gets a parallel ACL.

UI button ─┐
           ├─ run(input, ctx)   ← one gate, one ctx, one check
agent /mcp ─┘

Better Auth by default#

Authentication itself — sessions, providers, the login flow — is Better Auth, June's blessed default. A first-class integration (wiring a Better Auth session straight into ctx as the principal, and into /mcp scoping) is coming soon. Until then you can wire it by hand, and bring-your-own auth works today — June only needs you to populate ctx.user / ctx.session; the gate doesn't care who issued the session.

Why it matters#

The expensive bug in an agent-ready app is an authorization gap between what the UI enforces and what the tool endpoint allows. June closes it by construction: there is only one gate, so there is nothing to keep in sync.

Status: Changing — the model (one gate, ctx-as-principal) is settled; the Better Auth integration is in progress. See Stability.