DoomLoopDetector

Detects when an agent is calling the same tool over and over with no progress.

A "doom loop" is a tool call pattern that the model keeps repeating with no effective change — same arguments, or a cycle of arguments that loops back. The detector raises an error and the FSM terminates the run with RunStatus::Failed { reason: DoomLoop }.

The full file is src/runtime/doom_loop.rs.

Detection rules

1. Exact duplicate — the same (tool_name, arguments_json, session_id) triple appears max_consecutive_duplicates times in a row (default: 3).
2. Cycle — the same set of (tool_name, args) fingerprints appear in a cyclic permutation of length 2..=max_cycle_length (default: 4).
3. No progress — the same tool has been called more than max_consecutive_same_tool times without a different tool in between (default: 5).

Any rule firing is enough to terminate the run. The detector's state is per-session, reset at the start of each turn.

API

pub struct DoomLoopDetector {
    config: DoomLoopConfig,
    state: Mutex<HashMap<Uuid, DoomLoopState>>,
}

pub struct DoomLoopConfig {
    pub max_consecutive_duplicates: usize,    // default 3
    pub max_cycle_length: usize,              // default 4
    pub max_consecutive_same_tool: usize,    // default 5
}

impl DoomLoopDetector {
    pub fn new(config: DoomLoopConfig) -> Self;
    pub fn observe(&self, session_id: Uuid, call: &ToolCall) -> Result<(), DoomLoopType>;
    pub fn reset(&self, session_id: Uuid);
}

pub enum DoomLoopType {
    Duplicate { count: usize, fingerprint: ToolCallFingerprint },
    Cycle { length: usize, fingerprints: Vec<ToolCallFingerprint> },
    NoProgress { tool: String, count: usize },
}

Relationship to other components

• AgentRuntime — observes the detector after each tool call.
• Turn FSM — the ExecutingTools step is where doom loops manifest.
• ToolRuntime — the executor.

See also

• AgentRuntime — the consumer.
• ToolRuntime — the executor.