fix(server): return structured validation errors (#26457)

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

@@ -6,13 +6,41 @@ import { NamedError } from "@opencode-ai/core/util/error"
 import * as Log from "@opencode-ai/core/util/log"
 import { Cause, Effect } from "effect"
 import { HttpRouter, HttpServerError, HttpServerRespondable, HttpServerResponse } from "effect/unstable/http"
+import { HttpApiError } from "effect/unstable/httpapi"
+import { HttpApiSchemaError } from "effect/unstable/httpapi/HttpApiError"
 
 const log = Log.create({ service: "server" })
 
+function badRequestResponse() {
+  return HttpServerResponse.jsonUnsafe(
+    {
+      data: {},
+      errors: [],
+      success: false,
+    },
+    { status: 400 },
+  )
+}
+
+function normalizeEmptyBadRequest(response: HttpServerResponse.HttpServerResponse) {
+  if (response.status !== 400 || response.body._tag !== "Empty") return response
+  return badRequestResponse()
+}
+
 // Keep typed HttpApi failures on their declared error path; this boundary only replaces defect-only empty 500s.
 export const errorLayer = HttpRouter.middleware<{ handles: unknown }>()((effect) =>
   effect.pipe(
+    Effect.catch((error) => {
+      if (error instanceof HttpApiError.BadRequest) return Effect.succeed(badRequestResponse())
+      return Effect.fail(error)
+    }),
+    Effect.map(normalizeEmptyBadRequest),
     Effect.catchCause((cause) => {
+      const schemaError = cause.reasons
+        .filter(Cause.isDieReason)
+        .find((reason) => HttpApiSchemaError.is(reason.defect))
+      if (schemaError) return Effect.succeed(badRequestResponse())
+
       const defect = cause.reasons.filter(Cause.isDieReason).find((reason) => {
         if (HttpServerResponse.isHttpServerResponse(reason.defect)) return false
         if (HttpServerError.isHttpServerError(reason.defect)) return false
@@ -35,7 +63,7 @@ export const errorLayer = HttpRouter.middleware<{ handles: unknown }>()((effect)
               return 500
             }),
           }),
-        )
+)
       }
       if (error instanceof Session.BusyError) {
         return Effect.succeed(