diff --git a/.changeset/empty-input-guards.md b/.changeset/empty-input-guards.md
new file mode 100644
index 0000000..e0c5ea0
--- /dev/null
+++ b/.changeset/empty-input-guards.md
@@ -0,0 +1,5 @@
+---
+'@planningo/duul': patch
+---
+
+Guard against empty MCP tool input. Sonnet subagents occasionally emit `request_plan_review` / `request_code_review` / `request_execution_partition` with an empty `{}` input, hitting MCP `-32602` in a retry loop they can't recover from. Now each tool traps empty/short input at handler entry and returns a tool-level error with a concrete retry example, instead of letting zod throw the opaque MCP `-32602`. Also strengthens zod `.describe()` strings and MCP tool descriptions to make the required-vs-optional distinction explicit, and adds a "Tool input rules" section to the `duul-planner` agent prompt.