diff --git a/packages/opencode/src/cli/cmd/github.ts b/packages/opencode/src/cli/cmd/github.ts
index ed1ca2124d..fe8e233dd1 100644
--- a/packages/opencode/src/cli/cmd/github.ts
+++ b/packages/opencode/src/cli/cmd/github.ts
@@ -985,7 +985,8 @@ export const GithubRunCommand = cmd({
               const err = result.info.error
               console.error("Agent error:", err)
               if (err.name === "ContextOverflowError") throw new Error(formatPromptTooLargeError(files))
-              throw new Error(`${err.name}: ${err.data?.message || ""}`)
+              const message = "message" in err.data ? err.data.message : ""
+              throw new Error(`${err.name}: ${message}`)
             }
 
             const text = extractResponseText(result.parts)
@@ -1014,7 +1015,8 @@ export const GithubRunCommand = cmd({
               const err = summary.info.error
               console.error("Summary agent error:", err)
               if (err.name === "ContextOverflowError") throw new Error(formatPromptTooLargeError(files))
-              throw new Error(`${err.name}: ${err.data?.message || ""}`)
+              const message = "message" in err.data ? err.data.message : ""
+              throw new Error(`${err.name}: ${message}`)
             }
 
             const summaryText = extractResponseText(summary.parts)
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index aceecd9b8c..123f7b5401 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -18,6 +18,7 @@ import { ModelID, ProviderID } from "@/provider/schema"
 import { Effect, Schema, Types } from "effect"
 import { zod, ZodOverride } from "@/util/effect-zod"
 import { withStatics } from "@/util/schema"
+import { namedSchemaError } from "@/util/named-schema-error"
 import { EffectLogger } from "@/effect"
 
 /** Error shape thrown by Bun's fetch() when gzip/br decompression fails mid-stream */
@@ -30,38 +31,29 @@ interface FetchDecompressionError extends Error {
 export const SYNTHETIC_ATTACHMENT_PROMPT = "Attached image(s) from tool result:"
 export { isMedia }
 
-export const OutputLengthError = NamedError.create("MessageOutputLengthError", z.object({}))
-export const AbortedError = NamedError.create("MessageAbortedError", z.object({ message: z.string() }))
-export const StructuredOutputError = NamedError.create(
-  "StructuredOutputError",
-  z.object({
-    message: z.string(),
-    retries: z.number(),
-  }),
-)
-export const AuthError = NamedError.create(
-  "ProviderAuthError",
-  z.object({
-    providerID: z.string(),
-    message: z.string(),
-  }),
-)
-export const APIError = NamedError.create(
-  "APIError",
-  z.object({
-    message: z.string(),
-    statusCode: z.number().optional(),
-    isRetryable: z.boolean(),
-    responseHeaders: z.record(z.string(), z.string()).optional(),
-    responseBody: z.string().optional(),
-    metadata: z.record(z.string(), z.string()).optional(),
-  }),
-)
+export const OutputLengthError = namedSchemaError("MessageOutputLengthError", {})
+export const AbortedError = namedSchemaError("MessageAbortedError", { message: Schema.String })
+export const StructuredOutputError = namedSchemaError("StructuredOutputError", {
+  message: Schema.String,
+  retries: Schema.Number,
+})
+export const AuthError = namedSchemaError("ProviderAuthError", {
+  providerID: Schema.String,
+  message: Schema.String,
+})
+export const APIError = namedSchemaError("APIError", {
+  message: Schema.String,
+  statusCode: Schema.optional(Schema.Number),
+  isRetryable: Schema.Boolean,
+  responseHeaders: Schema.optional(Schema.Record(Schema.String, Schema.String)),
+  responseBody: Schema.optional(Schema.String),
+  metadata: Schema.optional(Schema.Record(Schema.String, Schema.String)),
+})
 export type APIError = z.infer<typeof APIError.Schema>
-export const ContextOverflowError = NamedError.create(
-  "ContextOverflowError",
-  z.object({ message: z.string(), responseBody: z.string().optional() }),
-)
+export const ContextOverflowError = namedSchemaError("ContextOverflowError", {
+  message: Schema.String,
+  responseBody: Schema.optional(Schema.String),
+})
 
 export class OutputFormatText extends Schema.Class<OutputFormatText>("OutputFormatText")({
   type: Schema.Literal("text"),
diff --git a/packages/opencode/src/util/named-schema-error.ts b/packages/opencode/src/util/named-schema-error.ts
new file mode 100644
index 0000000000..5fcc93cba3
--- /dev/null
+++ b/packages/opencode/src/util/named-schema-error.ts
@@ -0,0 +1,59 @@
+import { Schema } from "effect"
+import z from "zod"
+import { zod } from "@/util/effect-zod"
+
+/**
+ * Create a Schema-backed NamedError-shaped class.
+ *
+ * Drop-in replacement for `NamedError.create(tag, zodShape)` but backed by
+ * `Schema.Struct` under the hood. The wire shape emitted by the derived
+ * `.Schema` is still `{ name: tag, data: {...fields} }` so the generated
+ * OpenAPI/SDK output is byte-identical to the original NamedError schema.
+ *
+ * Preserves the existing surface:
+ *   - static `Schema` (Zod schema of the wire shape)
+ *   - static `isInstance(x)`
+ *   - instance `toObject()` returning `{ name, data }`
+ *   - `new X({ ...data }, { cause })`
+ */
+export function namedSchemaError<Tag extends string, Fields extends Schema.Struct.Fields>(tag: Tag, fields: Fields) {
+  // Wire shape matches the original NamedError output so the SDK stays stable.
+  const dataSchema = Schema.Struct(fields)
+  const wire = z
+    .object({
+      name: z.literal(tag),
+      data: zod(dataSchema),
+    })
+    .meta({ ref: tag })
+
+  type Data = Schema.Schema.Type<typeof dataSchema>
+
+  class NamedSchemaError extends Error {
+    static readonly Schema = wire
+    static readonly tag = tag
+    public static isInstance(input: unknown): input is NamedSchemaError {
+      return (
+        typeof input === "object" &&
+        input !== null &&
+        "name" in input &&
+        (input as { name: unknown }).name === tag
+      )
+    }
+
+    public override readonly name: Tag = tag
+    public readonly data: Data
+
+    constructor(data: Data, options?: ErrorOptions) {
+      super(tag, options)
+      this.data = data
+    }
+
+    toObject(): { name: Tag; data: Data } {
+      return { name: tag, data: this.data }
+    }
+  }
+
+  Object.defineProperty(NamedSchemaError, "name", { value: tag })
+
+  return NamedSchemaError
+}