diff --git a/src/shared/index.ts b/src/shared/index.ts
index 07103d094..17ebfd0c9 100644
--- a/src/shared/index.ts
+++ b/src/shared/index.ts
@@ -84,3 +84,4 @@ export * from "./task-system-enabled"
 export * from "./parse-tools-config"
 export { parseModelString } from "./model-string-parser"
 export { EXCLUDED_DIRS } from "./excluded-dirs"
+export * from "./replace-tool-args"
diff --git a/src/shared/replace-tool-args.ts b/src/shared/replace-tool-args.ts
new file mode 100644
index 000000000..0a0566ad3
--- /dev/null
+++ b/src/shared/replace-tool-args.ts
@@ -0,0 +1,16 @@
+/**
+ * Safely replace tool arguments without mutating frozen objects.
+ *
+ * opencode >=1.14 may freeze `output.args` via Immer before plugin hooks run.
+ * Direct property assignment (`output.args.key = value`) or `Object.assign(output.args, patch)`
+ * throws `TypeError: Attempted to assign to readonly property` on a frozen object.
+ *
+ * This helper replaces `output.args` with a shallow clone containing the patch,
+ * which works regardless of whether the original args object is frozen.
+ */
+export function replaceToolArgs(
+	output: { args: Record<string, unknown> },
+	patch: Record<string, unknown>,
+): void {
+	output.args = { ...output.args, ...patch }
+}