diff --git a/packages/opencode/src/mcp/catalog.ts b/packages/opencode/src/mcp/catalog.ts
index de374d26f..1253e5f08 100644
--- a/packages/opencode/src/mcp/catalog.ts
+++ b/packages/opencode/src/mcp/catalog.ts
@@ -64,12 +64,7 @@ export function convertTool(mcpTool: MCPToolDef, client: Client, timeout?: numbe
         },
       )
       if (result.isError)
-        throw new Error(
-          result.content
-            .flatMap((item) => (item.type === "text" ? [item.text] : []))
-            .filter((text) => text.trim())
-            .join("\n\n") || "MCP tool returned an error",
-        )
+        throw new Error(formatToolErrorContent(result.content))
       if (result.structuredContent === undefined || result.structuredContent === null) return result
       return {
         ...result,
@@ -107,6 +102,22 @@ export function fetch<T extends { name: string }>(
 
 export const sanitize = (value: string) => value.replace(/[^a-zA-Z0-9_-]/g, "_")
 
+function formatToolErrorContent(content: unknown) {
+  if (!Array.isArray(content)) return "MCP tool returned an error"
+  return (
+    content
+      .flatMap((item) => (isTextContent(item) ? [item.text] : []))
+      .filter((text) => text.trim())
+      .join("\n\n") || "MCP tool returned an error"
+  )
+}
+
+function isTextContent(value: unknown): value is { type: "text"; text: string } {
+  if (typeof value !== "object" || value === null) return false
+  const item = value as Record<string, unknown>
+  return item.type === "text" && typeof item.text === "string"
+}
+
 export function prompts(client: Client, timeout?: number) {
   if (!client.getServerCapabilities()?.prompts) return Promise.resolve([])
   return paginate(