diff --git a/lmcp.lua b/lmcp.lua
index 2a8ac73..772687c 100644
--- a/lmcp.lua
+++ b/lmcp.lua
@@ -163,10 +163,29 @@ end
 --     structuredContent (issue #13; spec-strict clients get first-class
 --     structured access)
 function lmcp:tool(name, description, params_schema, handler, opts)
+    -- Normalise empty inputSchema.properties → nil. JSON Schema allows
+    -- omitting `properties` on a `type: "object"` schema (means "any
+    -- object, no constraints"). Without this, an empty Lua properties
+    -- table goes through json.lua's is_array → emitted as `[]` →
+    -- spec-strict clients (Zod et al.) reject with
+    -- `expected: record, received: array`. The same gotcha already
+    -- bit `ping` in v1.0.0-rc1 (project_json_empty_table_gotcha
+    -- memory). v1.1.1 fix.
+    local schema = params_schema or { type = "object" }
+    if type(schema.properties) == "table" and next(schema.properties) == nil then
+        -- Clone the schema and drop the empty `properties` key. Avoids
+        -- mutating the caller's table (in case they re-use it across
+        -- registrations).
+        local clean = {}
+        for k, v in pairs(schema) do
+            if k ~= "properties" then clean[k] = v end
+        end
+        schema = clean
+    end
     self.tools[name] = {
         name = name,
         description = description,
-        inputSchema = params_schema or { type = "object", properties = {} },
+        inputSchema = schema,
         handler = handler,
         annotations  = opts and opts.annotations  or nil,
         outputSchema = opts and opts.outputSchema or nil,