opencode serve: bind failures print bare "Unexpected error / ServeError" — the listen errno is never surfaced

Description

Any bind failure in opencode serve is reported as two useless lines. The underlying errno never reaches the user:

$ opencode serve --hostname 100.68.120.26 --port 4096
Warning: OPENCODE_SERVER_PASSWORD is not set; server is unsecured.
Error: Unexpected error

ServeError

There is no port, no hostname, no errno, no log path. The same output appears for a busy port, an unavailable address, and a privileged port — three different user mistakes, one identical message.

Root cause

Server.listen runs Effect.runPromise(listenEffect(opts)) and lets the failure escape untouched:

packages/opencode/src/server/server.ts:73

export async function listen(opts: ListenOptions): Promise<Listener> {
  const listener = await Effect.runPromise(listenEffect(opts))

The failure is Effect's HttpServerError.ServeError, declared in effect/unstable/http/HttpServerError.js:208 as:

export class ServeError extends Data.TaggedError("ServeError") {}   // fields: { cause: unknown }

Data.TaggedError sets name = "ServeError" and leaves message empty. The real listen error is parked in .cause.

errorMessage() never looks at .cause:

packages/tui/src/util/error.ts:125

export function errorMessage(error: unknown): string {
  if (error instanceof Error) {
    if (error.message) return error.message   // "" -> falls through
    if (error.name) return error.name         // -> "ServeError"
  }

So index.ts takes the "no formatted message" branch and prints Unexpected error + the bare name.

Impact

This is not cosmetic — it makes bind failures undiagnosable. #37718 (Desktop WSL sidecar, mirrored networking) is the same two lines; the reporter had to run the port allocation sequence 8 times and read the Desktop sidecar source to guess at a cause that a single errno would have named.

Proposed fix

Two independent changes, both small:

1. Unwrap cause in errorMessage — fixes every tagged error, not just this one (packages/tui/src/util/error.ts):

if (error instanceof Error) {
  if (error.message) return error.message
  // Tagged errors (e.g. Effect's `ServeError`) carry an empty `message` and hide
  // the real failure in `cause`. Printing just the name tells the user nothing.
  if (error.cause !== undefined && error.cause !== null) {
    const inner = errorMessage(error.cause)
    if (inner && inner !== "unknown error") return error.name ? `${error.name}: ${inner}` : inner
  }
  if (error.name) return error.name
}

2. Translate bind errno into an actionable message (packages/opencode/src/server/server.ts):

export async function listen(opts: ListenOptions): Promise<Listener> {
  const listener = await Effect.runPromise(listenEffect(opts)).catch((error) => {
    throw bindError(error, opts)
  })
  ...
}

function bindError(error: unknown, opts: ListenOptions) {
  let cause: unknown = error
  const seen = new Set<unknown>()
  while (cause && typeof cause === "object" && !seen.has(cause)) {
    seen.add(cause)
    const code = (cause as { code?: unknown }).code
    const target = `${opts.hostname}:${opts.port}`
    if (code === "EADDRINUSE")
      return new Error(
        `Address ${target} is already in use. Choose a different --port, use --port 0 to pick a free one, ` +
          `or stop the process holding it (lsof -i :${opts.port}).`,
        { cause: error },
      )
    if (code === "EADDRNOTAVAIL")
      return new Error(
        `Cannot bind to ${target}: hostname "${opts.hostname}" is not an address on this machine.`,
        { cause: error },
      )
    if (code === "EACCES")
      return new Error(`Permission denied binding ${target}. Ports below 1024 require elevated privileges.`, {
        cause: error,
      })
    cause = (cause as { cause?: unknown }).cause
  }
  const detail = cause instanceof Error && cause.message ? `: ${cause.message}` : ""
  return new Error(`Failed to start server on ${opts.hostname}:${opts.port}${detail}`, { cause: error })
}

Verified on dev @ f51665191a — before/after for a busy port:

before:  Error: Unexpected error \n ServeError
after:   Address 127.0.0.1:4096 is already in use. Choose a different --port, use --port 0
         to pick a free one, or stop the process holding it (lsof -i :4096).

tsc --noEmit is clean on both packages. Happy to open a PR.

Note: errno-based dispatch alone is not sufficient under Bun — see the companion issue on node:http errno collapsing. But surfacing cause is correct regardless of runtime, and is a strict improvement over printing nothing.

Separately, index.ts still prefixes the real message with Error: Unexpected error. Once a message exists that preamble is misleading and should be dropped.

Plugins

None.

OpenCode version

1.17.9 (installed) and 1.18.4 (dev @ f51665191a, built from source — both reproduce)

Steps to reproduce

1. Occupy a port: python3 -m http.server 4096 --bind 127.0.0.1
2. opencode serve --hostname 127.0.0.1 --port 4096
3. Output is Error: Unexpected error / ServeError with no indication the port is busy.

Also reproduces with any hostname not assigned to a local interface, e.g. opencode serve --hostname 100.68.120.26 --port 4096 when Tailscale is down.

Operating System

macOS 15.5 (arm64), Bun 1.3.14

Terminal

Ghostty