Constant WORK_HEARTBEAT_TOOL 

pub(crate) const WORK_HEARTBEAT_TOOL: &str = r#"{
  "name": "WorkHeartbeat",
  "description": "Extend your lease on a claimed Muxqueue item while you are still working on it. Call this periodically during long work; without it the lease expires and another agent may take the item. Requires the 'attempt' number from your WorkClaim response — a heartbeat from a superseded claim is rejected (HTTP 409) rather than silently extending someone else's.",
  "inputSchema": {
    "type": "object",
    "properties": {
      "id":       { "type": "string",  "description": "The item id from WorkClaim" },
      "attempt":  { "type": "integer", "description": "The 'attempt' number returned by the WorkClaim that gave you this item" },
      "lease_ms": { "type": "integer", "description": "New lease length in ms. Default 120000." }
    },
    "required": ["id", "attempt"]
  }
}"#;