Migrate UI cancel error to tagged error (#27112)

packages/opencode/test/cli/error.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, test } from "bun:test"
 import { AccountTransportError } from "../../src/account/schema"
 import { FormatError } from "../../src/cli/error"
+import { UI } from "../../src/cli/ui"
 
 describe("cli.error", () => {
   test("formats account transport errors clearly", () => {
@@ -15,4 +16,8 @@ describe("cli.error", () => {
     expect(formatted).toContain("This failed before the server returned an HTTP response.")
     expect(formatted).toContain("Check your network, proxy, or VPN configuration and try again.")
   })
+
+  test("formats cancelled UI errors as empty output", () => {
+    expect(FormatError(new UI.CancelledError())).toBe("")
+  })
 })

packages/opencode/test/util/error.test.ts

@@ -1,8 +1,6 @@
 import { describe, expect, test } from "bun:test"
-import { Schema } from "effect"
 import { NamedError } from "@opencode-ai/core/util/error"
 import { errorData, errorFormat, errorMessage } from "../../src/util/error"
-import { UI } from "../../src/cli/ui"
 import { MessageError } from "../../src/session/message-error"
 
 describe("util.error", () => {
@@ -60,9 +58,7 @@ describe("util.error", () => {
     expect(error.toObject()).toEqual({ name: "ProviderAuthError", data: { providerID: "anthropic", message: "boom" } })
   })
 
-  test("void named errors accept JSON without data", () => {
-    const serialized = JSON.parse(JSON.stringify(new UI.CancelledError(undefined).toObject()))
-
-    expect(Schema.decodeUnknownOption(UI.CancelledError.Schema)(serialized)._tag).toBe("Some")
+  test("named errors without fields serialize data", () => {
+    expect(new MessageError.OutputLengthError({}).toObject()).toEqual({ name: "MessageOutputLengthError", data: {} })
   })
 })