fix(server): return diagnosable body for schema rejections (#26631)

packages/opencode/src/server/routes/instance/httpapi/middleware/schema-error.ts

@@ -0,0 +1,36 @@
+import { Effect } from "effect"
+import { HttpServerResponse } from "effect/unstable/http"
+import { HttpApiMiddleware } from "effect/unstable/httpapi"
+import * as Log from "@opencode-ai/core/util/log"
+
+const log = Log.create({ service: "server" })
+
+// Effect's Issue formatter recursively dumps the rejected `actual` value with
+// no truncation, so a 5KB invalid array produces a ~360KB string. Cap to keep
+// 4xx responses small and avoid mirroring entire request payloads (which may
+// contain secrets) into the response body and log file.
+const REASON_LIMIT = 1024
+function truncateReason(reason: string) {
+  if (reason.length <= REASON_LIMIT) return reason
+  return reason.slice(0, REASON_LIMIT) + `… (${reason.length - REASON_LIMIT} more chars)`
+}
+
+// Default Respondable returns an empty 400 body. Match the NamedError shape
+// used by other 4xx/5xx so the SDK's `wrapClientError` extracts `.data.message`.
+export class SchemaErrorMiddleware extends HttpApiMiddleware.Service<SchemaErrorMiddleware>()(
+  "@opencode/HttpApiSchemaError",
+) {}
+
+export const schemaErrorLayer = HttpApiMiddleware.layerSchemaErrorTransform(
+  SchemaErrorMiddleware,
+  (error) => {
+    const reason = truncateReason(error.cause.message)
+    log.warn("schema rejection", { kind: error.kind, reason })
+    return Effect.succeed(
+      HttpServerResponse.jsonUnsafe(
+        { name: "BadRequest", data: { message: reason, kind: error.kind } },
+        { status: 400 },
+      ),
+    )
+  },
+)