feat(hooks): add empty-message-sanitizer to prevent API errors from empty chat messages

Add new hook that uses the `experimental.chat.messages.transform` hook to prevent 'non-empty content' API errors by injecting placeholder text into empty messages BEFORE they're sent to the API.

This is a preventive fix - unlike session-recovery which fixes errors after they occur, this hook prevents the error from happening by sanitizing messages before API transmission.

Files:
- src/hooks/empty-message-sanitizer/index.ts (new hook implementation)
- src/hooks/index.ts (export hook function)
- src/config/schema.ts (add hook to HookName type)
- src/index.ts (wire up hook to plugin)

🤖 Generated with assistance of OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

Author: code-yeongyu

src/config/schema.ts

@@ -62,6 +62,7 @@ export const HookNameSchema = z.enum([
   "agent-usage-reminder",
   "non-interactive-env",
   "interactive-bash-session",
+  "empty-message-sanitizer",
 ])
 
 export const AgentOverrideConfigSchema = z.object({

src/hooks/empty-message-sanitizer/index.ts

@@ -0,0 +1,91 @@
+import type { Message, Part } from "@opencode-ai/sdk"
+
+const PLACEHOLDER_TEXT = "[user interrupted]"
+
+interface MessageWithParts {
+  info: Message
+  parts: Part[]
+}
+
+type MessagesTransformHook = {
+  "experimental.chat.messages.transform"?: (
+    input: Record<string, never>,
+    output: { messages: MessageWithParts[] }
+  ) => Promise<void>
+}
+
+function hasTextContent(part: Part): boolean {
+  if (part.type === "text") {
+    const text = (part as unknown as { text?: string }).text
+    return Boolean(text && text.trim().length > 0)
+  }
+  return false
+}
+
+function isToolPart(part: Part): boolean {
+  const type = part.type as string
+  return type === "tool" || type === "tool_use" || type === "tool_result"
+}
+
+function hasValidContent(parts: Part[]): boolean {
+  return parts.some((part) => hasTextContent(part) || isToolPart(part))
+}
+
+export function createEmptyMessageSanitizerHook(): MessagesTransformHook {
+  return {
+    "experimental.chat.messages.transform": async (_input, output) => {
+      const { messages } = output
+
+      for (const message of messages) {
+        if (message.info.role === "user") continue
+
+        const parts = message.parts
+
+        if (!hasValidContent(parts) && parts.length > 0) {
+          let injected = false
+
+          for (const part of parts) {
+            if (part.type === "text") {
+              const textPart = part as unknown as { text?: string; synthetic?: boolean }
+              if (!textPart.text || !textPart.text.trim()) {
+                textPart.text = PLACEHOLDER_TEXT
+                textPart.synthetic = true
+                injected = true
+                break
+              }
+            }
+          }
+
+          if (!injected) {
+            const insertIndex = parts.findIndex((p) => isToolPart(p))
+
+            const newPart = {
+              id: `synthetic_${Date.now()}`,
+              messageID: message.info.id,
+              sessionID: (message.info as unknown as { sessionID?: string }).sessionID ?? "",
+              type: "text" as const,
+              text: PLACEHOLDER_TEXT,
+              synthetic: true,
+            }
+
+            if (insertIndex === -1) {
+              parts.push(newPart as Part)
+            } else {
+              parts.splice(insertIndex, 0, newPart as Part)
+            }
+          }
+        }
+
+        for (const part of parts) {
+          if (part.type === "text") {
+            const textPart = part as unknown as { text?: string; synthetic?: boolean }
+            if (textPart.text !== undefined && textPart.text.trim() === "") {
+              textPart.text = PLACEHOLDER_TEXT
+              textPart.synthetic = true
+            }
+          }
+        }
+      }
+    },
+  }
+}

src/hooks/index.ts

@@ -19,3 +19,4 @@ export { createAgentUsageReminderHook } from "./agent-usage-reminder";
 export { createKeywordDetectorHook } from "./keyword-detector";
 export { createNonInteractiveEnvHook } from "./non-interactive-env";
 export { createInteractiveBashSessionHook } from "./interactive-bash-session";
+export { createEmptyMessageSanitizerHook } from "./empty-message-sanitizer";

src/index.ts

@@ -20,6 +20,7 @@ import {
   createAgentUsageReminderHook,
   createNonInteractiveEnvHook,
   createInteractiveBashSessionHook,
+  createEmptyMessageSanitizerHook,
 } from "./hooks";
 import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import {
@@ -246,6 +247,9 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const interactiveBashSession = isHookEnabled("interactive-bash-session")
     ? createInteractiveBashSessionHook(ctx)
     : null;
+  const emptyMessageSanitizer = isHookEnabled("empty-message-sanitizer")
+    ? createEmptyMessageSanitizerHook()
+    : null;
 
   updateTerminalTitle({ sessionId: "main" });
 
@@ -281,6 +285,14 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await keywordDetector?.["chat.message"]?.(input, output);
     },
 
+    "experimental.chat.messages.transform": async (
+      input: Record<string, never>,
+      output: { messages: Array<{ info: unknown; parts: unknown[] }> }
+    ) => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      await emptyMessageSanitizer?.["experimental.chat.messages.transform"]?.(input, output as any);
+    },
+
     config: async (config) => {
       const builtinAgents = createBuiltinAgents(
         pluginConfig.disabled_agents,