)

Stacks on the merged CapTarget kernel (#1436). Part of the codemode
rip-out sequence (itx-next.md §4).

## What

The first-party half of the §1 litmus test: **MCP is a client
implementation, not a transport** — so it's an ordinary
loopback-dialable RPC target, parameterized per server:

```ts
await itx.caps.define({
  invoke: "path-call",
  name: "docs",
  target: {
    type: "rpc",
    worker: { type: "loopback" },
    entrypoint: "McpClient",
    props: {
      serverUrl: "https://docs.example.com/mcp",
      headers: { authorization: 'Bearer getSecret({ key: "DOCS_TOKEN" })' },
    },
  },
});

await itx.docs.listTools();
await itx.docs.someToolName({ ... });
```

- **All transport HTTP rides project egress** via the cap's own itx
handle (the MCP SDK's custom-`fetch` option), so header values may carry
`getSecret()` placeholders — substitution happens in the Project DO, the
credential never exists in the isolate (Law 5).
- **Stateless by design**: connect → call → close per invocation. When
handshake latency matters, the same class becomes a `durable-object` ref
without changing callers (the connection-caching pattern of the old
`OutboundMcpFromOurClientCapability` DO).
- `listTools()` is an ordinary call, not a special discovery protocol —
same convention the codemode provider used.
- Added to `DIALABLE_LOOPBACKS`; result shaping reuses the existing
`outbound-mcp-from-our-client-capability-core` helpers.

In the upcoming codemode-drop PR this replaces `connectToMcpServer` and
`createOutboundMcpFromOurClientToolProviderRegistration`.

## Testing

typecheck / lint / format / unit green. New e2e test (`itx.e2e.test.ts`)
defines an McpClient cap, lists tools, and calls the first one — gated
on `OS_E2E_MCP_SERVER_URL` or `MOCK_PROVIDER_BASE_URL` (skips when no
MCP server is reachable).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Expands dialable loopback surface and routes all MCP HTTP through
project egress; behavior is gated by existing allowlists and attribution
checks but affects outbound networking for defined caps.
> 
> **Overview**
> Adds a first-party **`McpClient`** loopback RPC capability so projects
can define remote MCP servers as ordinary `path-call` caps
(`listTools()` plus dotted tool calls). Each invocation is **connect →
call → close**; transport HTTP is forced through **`itx.fetch`**
(project egress) so header secrets resolve in the Project DO, and tool
listing/execution reuse the existing outbound MCP core helpers.
> 
> **`McpClient`** is allowlisted in **`DIALABLE_LOOPBACKS`** and
exported from the main worker. Several itx entrypoints switch from
**`Reflect.get(this, "ctx")`** to **`this.ctx.props`**. E2E gains a
skipped-when-no-server test that defines an **`McpClient`** cap and
exercises list/call, plus **`createdProjectIds`** cleanup for the AI
bindings test project.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
236953d. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- CLOUDFLARE_PREVIEW -->
## Environment Config Lease
<!-- CLOUDFLARE_PREVIEW_STATE -->
<!--
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"runUrl": "https://github.com/iterate/iterate/actions/runs/27280672268",
      "shortSha": "236953d"
    }
  },
  "environmentConfigLease": {
    "dopplerConfig": "preview_5",
    "leasedUntil": 1781102759817,
    "leaseId": "8f0c5c8c-96b3-43bd-9ed2-1126964f26ec",
    "slug": "preview-5",
    "type": "environment-config-lease"
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-->
<!-- /CLOUDFLARE_PREVIEW_STATE -->
Lease: `preview-5`
Doppler config: `preview_5`
Type: `environment-config-lease`
Leased until: 2026-06-10T14:45:59.817Z

### OS
Status: deployed
Commit: `236953d`
Preview: https://os.iterate-preview-5.com
[Workflow
run](https://github.com/iterate/iterate/actions/runs/27280672268)
Updated: 2026-06-10T13:52:03.377Z
<!-- /CLOUDFLARE_PREVIEW -->

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>