Typed Error Migration

This note expands the ERR, RENDER, and HTTP tracks from todo.md. It is the current reference for expected failures, typed service errors, and HTTP error boundaries.

For the migration architecture and queue, see error-boundaries-plan.md.

Goal

• Expected service failures live on the Effect error channel.
• Service interfaces expose those failures in their return types.
• Domain errors are authored with Schema.TaggedErrorClass.
• Effect.die(...) is reserved for defects: bugs, impossible states, violated invariants, and final unknown-boundary fallbacks.
• HTTP status codes and public wire bodies are handled at HTTP route boundaries, not inside service modules.
• User-facing boundaries render useful structured error details instead of opaque Error: SomeName strings.

Service Error Shape

export class SessionBusyError extends Schema.TaggedErrorClass<SessionBusyError>()("SessionBusyError", {
  sessionID: SessionID,
  message: Schema.String,
}) {}

export type Error = Storage.Error | SessionBusyError

export interface Interface {
  readonly get: (id: SessionID) => Effect.Effect<Info, Error>
}

Rules:

• Use Schema.TaggedErrorClass for expected domain failures.
• Export a domain-level Error union from each service module.
• Put expected errors in service method signatures.
• Use yield* new DomainError(...) for direct early failures in Effect.gen / Effect.fn.
• Use Schema.Defect for unknown cause fields when preserving the cause is useful for logs or callers.
• Use Effect.try(...), Effect.tryPromise(...), Effect.mapError, Effect.catchTag, and Effect.catchTags to translate external failures into domain errors.
• Do not use throw, Effect.die(...), or catchDefect for expected user, IO, validation, missing-resource, auth, provider, worktree, or busy-state failures.

HTTP Boundary Shape

Service modules stay transport-agnostic. They should not import HTTP status codes, HttpApiError, HttpServerResponse, or route-specific error schemas.

HTTP handlers translate service errors into public endpoint errors:

const get = Effect.fn("SessionHttpApi.get")(function* (ctx: { params: { sessionID: SessionID } }) {
  return yield* session
    .get(ctx.params.sessionID)
    .pipe(Effect.catchTag("StorageNotFoundError", () => notFound("Session not found")))
})

Endpoint definitions declare which public errors can be emitted. Public HTTP error schemas carry their response status with httpApiStatus or the equivalent HttpApi schema annotation.

Effect's own HttpApi examples follow this pattern:

export class Unauthorized extends Schema.TaggedErrorClass<Unauthorized>()(
  "Unauthorized",
  { message: Schema.String },
  { httpApiStatus: 401 },
) {}

export class Authorization extends HttpApiMiddleware.Service<
  Authorization,
  {
    provides: CurrentUser
  }
>()("app/Authorization", {
  security: { bearer: HttpApiSecurity.bearer },
  error: Unauthorized,
}) {}

Endpoint-level errors use the same idea:

export class ConfigApiError extends Schema.ErrorClass<ConfigApiError>("ConfigApiError")(
  {
    name: Schema.Union(Schema.Literal("ConfigInvalidError"), Schema.Literal("ConfigJsonError")),
    data: Schema.Struct({ message: Schema.optional(Schema.String), path: Schema.String }),
  },
  { httpApiStatus: 400 },
) {}

HttpApiEndpoint.get("get", "/config", {
  success: Config.Info,
  error: ConfigApiError,
})

The service error and HTTP error may be the same class only when the wire shape is intentionally public. Use separate HTTP error schemas when the service error contains internals, low-level causes, retry hints, or data that should not be exposed to API clients.

Do not map every domain error into one universal HTTP error class. Prefer a small public error vocabulary by route group: shared shapes like ApiNotFoundError, route-specific shapes like ConfigApiError, and built-in empty HttpApiError.* only when an empty/no-content body is the intended SDK contract.

Mapping Guidance

• Keep one-off translations inline in the handler.
• Extract tiny shared helpers when the same translation repeats across a route group.
• Do not create one giant unknown -> status mapper.
• Do not grow generic HTTP middleware into a registry of domain errors.
• Preserve existing public { name, data } bodies until a deliberate breaking API change.
• Use built-in HttpApiError.* only when its generated body and SDK surface are intentionally the public contract.
• Prefer Schema.ErrorClass for public HTTP error bodies whose wire shape is not the same as the internal domain error shape.
• Prefer Schema.TaggedErrorClass for service/domain errors and middleware errors that are naturally tagged by _tag.
• If preserving a legacy { name, data } body, model that shape explicitly in the public API error schema instead of relying on NamedError.toObject() in generic middleware.

User-Facing Rendering

HTTP serialization and user rendering are separate boundaries. The server should send structured public errors; CLI and TUI code should format those structures through one shared formatter.

For SDK calls using { throwOnError: true }, the generated client may wrap the decoded response body in an Error. The original body should remain available under error.cause.body; FormatError is the right place to unwrap and render that body. TUI aggregation helpers should call FormatError first, then fall back to generic Error.message / string rendering.

When several parallel startup requests fail from the same underlying issue, group identical rendered messages and list the affected request names once. For example:

Configuration is invalid at /path/to/opencode.json
↳ Expected object, got "not-object" provider.bad.options
Affected startup requests: config.providers, provider.list, app.agents, config.get

Middleware Guidance

HTTP middleware should be cross-cutting: auth, context, schema decode formatting, routing, and final unknown-defect fallback.

The current compatibility middleware still knows about some legacy domain errors. As route groups declare expected errors and handlers map them, that middleware should shrink. It should not gain new name checks.

Unknown 500 responses should log full details server-side with Cause.pretty(cause) and return a safe public body.

The config startup regression in #27056 is the failure mode this rule is meant to avoid: a user-authored invalid opencode.json crossed the HttpApi boundary as a defect, so middleware replaced a useful ConfigInvalidError with a safe generic UnknownError. The compatibility fix is to preserve config parse and validation errors as client-visible 400s. The target architecture is better: config loading should fail on the typed error channel, config HTTP handlers should map those errors to declared ConfigApiError responses, and the generic middleware should never see them.

Migration Order

Prefer small vertical slices:

1. Fix rendering at one user-visible boundary.
2. Convert one service domain to Schema.TaggedErrorClass errors.
3. Map those errors at the affected HTTP handlers.
4. Remove the corresponding name-based middleware branch if possible.
5. Add or update focused tests for both service error tags and HTTP wire bodies.

Good early domains are storage not-found, worktree errors, and provider auth validation errors because they currently drive HTTP behavior.

Config parse and validation errors are also a good early slice because they are startup-blocking and must be rendered clearly in both CLI and TUI flows.

Checklist For A PR

• Expected failures are typed errors, not defects.
• Service method signatures expose the expected error union.
• HTTP handlers translate domain errors at the boundary.
• Public HTTP error bodies preserve existing wire contracts.
• Generic middleware gets smaller or stays unchanged.
• Focused tests cover the service error and any public HTTP response.