| 1 | //! Host-defined tools and the policies that guard them. |
| 2 | |
| 3 | use async_trait::async_trait; |
| 4 | use serde::{Deserialize, Serialize}; |
| 5 | |
| 6 | /// A tool the model may ask the host application to run. |
| 7 | #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)] |
| 8 | pub struct AgentToolDefinition { |
| 9 | pub name: String, |
| 10 | pub description: String, |
| 11 | /// JSON Schema encoded as a string. |
| 12 | pub parameters_schema: String, |
| 13 | pub risk: ToolRisk, |
| 14 | } |
| 15 | |
| 16 | impl AgentToolDefinition { |
| 17 | pub fn read_only( |
| 18 | name: impl Into<String>, |
| 19 | description: impl Into<String>, |
| 20 | parameters_schema: impl Into<String>, |
| 21 | ) -> Self { |
| 22 | Self { |
| 23 | name: name.into(), |
| 24 | description: description.into(), |
| 25 | parameters_schema: parameters_schema.into(), |
| 26 | risk: ToolRisk::ReadOnly, |
| 27 | } |
| 28 | } |
| 29 | |
| 30 | pub fn mutating( |
| 31 | name: impl Into<String>, |
| 32 | description: impl Into<String>, |
| 33 | parameters_schema: impl Into<String>, |
| 34 | ) -> Self { |
| 35 | Self { |
| 36 | name: name.into(), |
| 37 | description: description.into(), |
| 38 | parameters_schema: parameters_schema.into(), |
| 39 | risk: ToolRisk::Mutating, |
| 40 | } |
| 41 | } |
| 42 | |
| 43 | pub(crate) fn validate(&self) -> Result<(), AgentError> { |
| 44 | let mut chars = self.name.chars(); |
| 45 | let valid_first = chars.next().is_some_and(|c| c.is_ascii_alphabetic()); |
| 46 | let valid_rest = chars.all(|c| c.is_ascii_alphanumeric() || c == '_' || c == '-'); |
| 47 | if !valid_first || !valid_rest || self.name.len() > 64 { |
| 48 | return Err(AgentError::InvalidTool { |
| 49 | reason: format!( |
| 50 | "tool name {:?} must start with an ASCII letter and contain only letters, numbers, '_' or '-' (maximum 64 bytes)", |
| 51 | self.name |
| 52 | ), |
| 53 | }); |
| 54 | } |
| 55 | if self.description.trim().is_empty() { |
| 56 | return Err(AgentError::InvalidTool { |
| 57 | reason: format!("tool {:?} needs a description", self.name), |
| 58 | }); |
| 59 | } |
| 60 | let schema: serde_json::Value = |
| 61 | serde_json::from_str(&self.parameters_schema).map_err(|error| { |
| 62 | AgentError::InvalidTool { |
| 63 | reason: format!("tool {:?} has invalid JSON Schema: {error}", self.name), |
| 64 | } |
| 65 | })?; |
| 66 | if schema.get("type").and_then(serde_json::Value::as_str) != Some("object") { |
| 67 | return Err(AgentError::InvalidTool { |
| 68 | reason: format!("tool {:?} schema must have type object", self.name), |
| 69 | }); |
| 70 | } |
| 71 | Ok(()) |
| 72 | } |
| 73 | } |
| 74 | |
| 75 | #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)] |
| 76 | pub enum ToolRisk { |
| 77 | ReadOnly, |
| 78 | Mutating, |
| 79 | } |
| 80 | |
| 81 | #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)] |
| 82 | pub struct ToolCall { |
| 83 | pub id: String, |
| 84 | pub name: String, |
| 85 | pub arguments: String, |
| 86 | } |
| 87 | |
| 88 | #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)] |
| 89 | pub struct ToolExecutionResult { |
| 90 | pub tool_call_id: String, |
| 91 | pub content: String, |
| 92 | pub is_error: bool, |
| 93 | } |
| 94 | |
| 95 | #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)] |
| 96 | pub struct ApprovalRequest { |
| 97 | pub call: ToolCall, |
| 98 | pub risk: ToolRisk, |
| 99 | } |
| 100 | |
| 101 | #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)] |
| 102 | pub enum ApprovalDecision { |
| 103 | AllowOnce, |
| 104 | AllowForSession, |
| 105 | Deny, |
| 106 | } |
| 107 | |
| 108 | #[async_trait] |
| 109 | pub trait ToolExecutor: Send + Sync + 'static { |
| 110 | async fn execute(&self, tool_name: &str, arguments: &str) -> Result<String, String>; |
| 111 | } |
| 112 | |
| 113 | #[async_trait] |
| 114 | pub trait ApprovalHandler: Send + Sync + 'static { |
| 115 | async fn approve(&self, request: ApprovalRequest) -> ApprovalDecision; |
| 116 | } |
| 117 | |
| 118 | #[derive(Debug, Default)] |
| 119 | pub struct RejectingExecutor; |
| 120 | |
| 121 | #[async_trait] |
| 122 | impl ToolExecutor for RejectingExecutor { |
| 123 | async fn execute(&self, tool_name: &str, _: &str) -> Result<String, String> { |
| 124 | Err(format!("no executor is registered for tool {tool_name:?}")) |
| 125 | } |
| 126 | } |
| 127 | |
| 128 | #[derive(Debug, Default)] |
| 129 | pub struct DenyApprovals; |
| 130 | |
| 131 | #[async_trait] |
| 132 | impl ApprovalHandler for DenyApprovals { |
| 133 | async fn approve(&self, _: ApprovalRequest) -> ApprovalDecision { |
| 134 | ApprovalDecision::Deny |
| 135 | } |
| 136 | } |
| 137 | |
| 138 | #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)] |
| 139 | pub struct AgentConfig { |
| 140 | pub max_tool_rounds: u8, |
| 141 | pub max_tool_output_chars: usize, |
| 142 | } |
| 143 | |
| 144 | impl Default for AgentConfig { |
| 145 | fn default() -> Self { |
| 146 | Self { |
| 147 | max_tool_rounds: 8, |
| 148 | max_tool_output_chars: 10_000, |
| 149 | } |
| 150 | } |
| 151 | } |
| 152 | |
| 153 | impl AgentConfig { |
| 154 | pub(crate) fn validated(self) -> Self { |
| 155 | Self { |
| 156 | max_tool_rounds: self.max_tool_rounds.clamp(1, 24), |
| 157 | max_tool_output_chars: self.max_tool_output_chars.max(1), |
| 158 | } |
| 159 | } |
| 160 | |
| 161 | /// Whether another tool round may run, given how many already have. |
| 162 | /// |
| 163 | /// Asked twice per round, and the pair is what makes the loop terminate: |
| 164 | /// once to decide whether to offer tools to the model again, and once at |
| 165 | /// the top of the next round to decide whether to run it at all. If only |
| 166 | /// the first check existed, a model that requested tools it was never |
| 167 | /// offered would keep the loop alive indefinitely. |
| 168 | pub(crate) fn allows_round(&self, rounds: u8) -> bool { |
| 169 | rounds < self.max_tool_rounds |
| 170 | } |
| 171 | } |
| 172 | |
| 173 | #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)] |
| 174 | pub struct AgentReply { |
| 175 | pub text: String, |
| 176 | pub duration_seconds: f64, |
| 177 | pub duration: String, |
| 178 | pub tool_rounds: u8, |
| 179 | } |
| 180 | |
| 181 | #[derive(Debug, thiserror::Error, Clone, PartialEq, Eq)] |
| 182 | pub enum AgentError { |
| 183 | #[error("invalid tool: {reason}")] |
| 184 | InvalidTool { reason: String }, |
| 185 | #[error("a tool named {name:?} is already registered")] |
| 186 | DuplicateTool { name: String }, |
| 187 | #[error("the loaded model does not support tool calling")] |
| 188 | UnsupportedModel, |
| 189 | #[error("the agent turn was cancelled")] |
| 190 | Cancelled, |
| 191 | #[error("the model returned an empty final reply")] |
| 192 | EmptyReply, |
| 193 | #[error("the agent stopped after {rounds} tool rounds without a final reply")] |
| 194 | ToolRoundsExhausted { rounds: u8 }, |
| 195 | #[error("inference failed: {reason}")] |
| 196 | Inference { reason: String }, |
| 197 | } |
| 198 | |
| 199 | pub(crate) fn truncate_output(value: String, max_chars: usize) -> String { |
| 200 | if value.chars().count() <= max_chars { |
| 201 | return value; |
| 202 | } |
| 203 | let mut truncated: String = value.chars().take(max_chars).collect(); |
| 204 | truncated.push_str("\n[tool output truncated by Ed]"); |
| 205 | truncated |
| 206 | } |
| 207 | |
| 208 | #[cfg(test)] |
| 209 | mod tests { |
| 210 | use super::*; |
| 211 | |
| 212 | #[test] |
| 213 | fn validates_tool_names_and_object_schemas() { |
| 214 | AgentToolDefinition::read_only("clock", "Read the clock", r#"{"type":"object"}"#) |
| 215 | .validate() |
| 216 | .unwrap(); |
| 217 | assert!( |
| 218 | AgentToolDefinition::read_only("bad name", "Read", r#"{"type":"object"}"#) |
| 219 | .validate() |
| 220 | .is_err() |
| 221 | ); |
| 222 | assert!( |
| 223 | AgentToolDefinition::read_only("clock", "Read", r#"{"type":"array"}"#) |
| 224 | .validate() |
| 225 | .is_err() |
| 226 | ); |
| 227 | } |
| 228 | |
| 229 | #[test] |
| 230 | fn a_validated_budget_always_allows_at_least_one_round() { |
| 231 | // A zero budget would mean registering tools quietly disabled them: |
| 232 | // the model would be offered tools it could never be allowed to use. |
| 233 | let config = AgentConfig { |
| 234 | max_tool_rounds: 0, |
| 235 | max_tool_output_chars: 0, |
| 236 | } |
| 237 | .validated(); |
| 238 | |
| 239 | assert!(config.allows_round(0)); |
| 240 | assert!(!config.allows_round(1)); |
| 241 | assert_eq!(config.max_tool_output_chars, 1); |
| 242 | } |
| 243 | |
| 244 | #[test] |
| 245 | fn the_round_budget_closes_at_the_limit_and_stays_closed() { |
| 246 | let config = AgentConfig { |
| 247 | max_tool_rounds: 3, |
| 248 | max_tool_output_chars: 100, |
| 249 | } |
| 250 | .validated(); |
| 251 | |
| 252 | assert!(config.allows_round(2)); |
| 253 | assert!(!config.allows_round(3)); |
| 254 | // Past the limit too: `rounds` is only ever compared, never trusted to |
| 255 | // land exactly on the boundary. |
| 256 | assert!(!config.allows_round(u8::MAX)); |
| 257 | } |
| 258 | |
| 259 | #[test] |
| 260 | fn truncation_is_character_safe_and_explicit() { |
| 261 | assert_eq!( |
| 262 | truncate_output("héllo".into(), 2), |
| 263 | "hé\n[tool output truncated by Ed]" |
| 264 | ); |
| 265 | } |
| 266 | } |