fix(todos): tighten the todo tool description

crates/tinyagents-graph/src/todos/test.rs

@@ -365,23 +365,23 @@ mod tool_tests {
         let tool = TodoTool::new(store());
         let description = Tool::description(&tool);
         assert!(
-            description.contains("only after its work has actually run"),
+            description.contains("only after its work has run"),
             "an item is completed after its result exists: {description}"
         );
         assert!(
             description.contains("bookkeeping, not work"),
             "writing the list is not the work: {description}"
         );
         assert!(
-            description.contains("immediately carry out the next step"),
+            description.contains("do the next step"),
             "the model advances after bookkeeping: {description}"
         );
         assert!(
-            description.contains("unless this update completes the final item"),
+            description.contains("unless the list is finished"),
             "a terminal completion does not require a nonexistent next step: {description}"
         );
         assert!(
-            description.contains("next model turn"),
+            description.contains("next turn"),
             "sequential providers may advance on their next turn: {description}"
         );
     }

crates/tinyagents-graph/src/todos/tool.rs

@@ -23,15 +23,15 @@ use tinytools::{Tool, ToolPolicy, ToolResult, ToolRunContext, ToolSideEffects};
 
 const TODO_TOOL_NAME: &str = "todo";
 
-const TODO_DESCRIPTION: &str = "Your todo list for this thread. Pass the complete list every \
-    time; it replaces what was there. Use it for work with 3+ steps: write the steps up front, \
-    keep exactly one `in_progress`, mark each `completed` only after its work has actually run \
-    and its result is in this conversation. Writing the list is bookkeeping, not work: after \
-    updating it, immediately carry out the next step, unless this update completes the final item. \
-    Providers that cannot issue parallel tool calls may make that call in the next model turn, and \
-    one update per response is enough. Omit \
-    `todos` to read the current list. The list is bound \
-    automatically to the current thread — do not pass a thread id.";
+/// Paid on every request of every agent that carries the tool, so each rule
+/// is stated once, tersely. The rules themselves are load-bearing: see the
+/// `description_states_…` test for the failure each one prevents.
+const TODO_DESCRIPTION: &str = "This thread's todo list, for work with 3+ steps. Pass the \
+    complete list each time (it replaces the old one); omit `todos` to read it. Keep one item \
+    `in_progress`; mark an item `completed` only after its work has run and its result is in \
+    the conversation. Writing the list is bookkeeping, not work: in the same response (or your \
+    next turn, if you cannot call tools in parallel) do the next step, unless the list is \
+    finished.";
 
 /// The `todo` harness [`Tool`], backed by a [`Store`](tinyagents_harness::store::Store).
 pub struct TodoTool {