# Milo 0.4.0

URL: /milo-cli/releases/0.4.0/
Section: releases
Description: Truthful cross-surface dispatch, MCP Apps, downstream CLI adoption, and host-owned Context.

---

> For a complete page index, fetch /milo-cli/llms.txt.

Feature release focused on making one typed function a truthful, composable
contract for terminal users, Python hosts, and MCP clients.

## Highlights

- **Truthful dispatch everywhere** — Generated schemas are enforced across CLI,
  `call`, `call_raw`, and MCP. Unknown arguments and constraint failures now
  carry stable, repairable diagnostics instead of reaching handlers or being
  silently discarded.
- **MCP Apps end to end** — Typed `ui://` resources, tool links, capability
  negotiation, gateway-safe URI rewriting, resource transport, verifier checks,
  and a dependency-free interactive example now form one tested contract.
- **Downstream CLI adoption** — Public positional and option presentation,
  command surface policy, lazy version reports, terminal renderers, shallow
  help, and selected-command parsing support established CLIs without private
  argparse mutation.
- **Headless Context** — Web servers, workers, and tests can inject their own
  output sink, interaction policy, and approval strategy while terminal defaults
  remain unchanged.
- **Launch-ready onboarding** — The README now leads with a clean-machine
  60-second CLI-to-MCP proof, backed by stricter verifier coverage and honest
  Milo/FastMCP/Typer guidance.

## Added

- Public `validate_arguments()` enforcement and complete lazy exports for
  runtime effect types.
- `Positional`, `Option`, per-command `surfaces`, custom version reports, and
  terminal-only structured-result rendering.
- MCP Apps resource/tool metadata types, negotiated discovery, security
  metadata, gateway routing, and stable structured errors.
- `mcp_apps_in_process`, `mcp_apps_gateway`, and `mcp_apps_transport` verifier
  checks, including subprocess resource reads and clean-stdout validation.
- `OutputSink`, `ConfirmStrategy`, `NullOutputSink`, and host `ctx=` injection
  for programmatic command calls.
- MCP `server/discover` and explicit unsupported-version diagnostics for
  stateless negotiation preparation.

## Changed

- Root and group help use command registration metadata without resolving lazy
  leaf schemas. Selected command execution builds only that command path.
- `CLI.root_option_specs()` is the shared immutable source for built-in and
  user-defined root option metadata.
- The public docs, launch runbook, comparison guide, and framework-neutral
  examples now lead with verified dual CLI/MCP workflows.

## Fixed

- CLI, programmatic, and MCP dispatch now agree on schema constraints, unknown
  arguments, middleware repair, hidden commands, lazy-import failures, and
  protocol-safe hook errors.
- Gateway failures preserve structured child repair data and MCP Apps links
  survive collision-safe namespacing and resource reads.
- Bundled terminal components align by display-cell width and preserve wide
  Unicode; panel border styles render through the documented contract.
- Testing helpers reject incomplete saga traces and expose structured MCP repair
  data; example reducers remain deterministic and replayable.
- Onboarding, generated-project, help, form, platform-path, and version-check
  documentation now matches the executable public surfaces.

## Compatibility Notes

- Python 3.14+ and the pure-Python `kida-templates>=0.9.0,<0.10.0` runtime
  dependency are unchanged.
- No public command flag was removed. Context fields were appended to the frozen
  dataclass, and default stderr, TTY, and terminal confirmation behavior is
  preserved.
- Code that relied on silently ignored programmatic or MCP arguments must remove
  them or update the command signature. This is an intentional contract
  correction: agent calls now receive `M-INP-005` repair data.
- MCP clients cannot invoke hidden commands and cannot inject `ctx`; both now
  fail with structured errors consistent with discovery.

## Downstream Adoption

This is the first released version containing the generic contracts required by
the versioned Chirp CLI canary. Downstream integrations should pin `milo-cli`
0.4.x and advance the supported range only after their CLI, structured output,
MCP, llms.txt, and lazy-startup parity lanes pass.