Lesson 36 — the agent failure mode heatmap. Agents fail in predictable shapes. Knowing the shapes lets you instrument for them before the failure ships.
The failure that did not look like a failure
An agent finishes its task. The trace looks clean. The output looks right. A week later, you discover the agent has been confidently citing facts that do not exist, calling tools with parameters that were syntactically valid but semantically wrong, and ignoring a constraint it had been told to honour. The trace was clean because nothing crashed; the failure was upstream of the crash.
Agent failures cluster into a small number of repeating patterns. Instrumenting for them is cheaper than discovering them in production.
The rule: Treat agent failure as a typology, not an accident. Each failure mode has a signature in the trace. Detect the signature, route to mitigation, escalate when escalation is the right move.
The recurring modes
- Tool hallucination — the agent calls a tool with invented parameters that look plausible.
- Confident wrong answer — the agent ignores its uncertainty and ships a wrong result.
- Loop without progress — the agent retries the same approach with slight variations.
- Wandering off-task — the agent starts solving a related but different problem.
- Constraint forgetting — the agent ignores a constraint it agreed to earlier in the trace.
- Premature termination — the agent decides the task is done before it actually is.
- Prompt-injection compromise — a tool result contains adversarial content that hijacks the agent’s plan.
Try it yourself
The failure heatmap below shows the empirical frequency of each failure mode across task types from past engagements. Hover over a cell for representative traces. Note where the dark cells cluster: which task types are dominated by which failure modes.
agent_failure_heatmap.json missing
What you just saw
Tool-call-heavy tasks fail mostly via tool hallucination and loop-without-progress. Open-ended research tasks fail mostly via wandering off-task. Multi-step planning tasks fail mostly via constraint forgetting. Tasks where the agent consumes external content are the prompt-injection cases.
The mitigations are mode-specific. Tool hallucination is mitigated by schema validation. Constraint forgetting is mitigated by re-anchoring the constraints at each step. Prompt injection is mitigated by treating tool output as untrusted (see Lesson 17 on the broader pattern).
Three architect-grade arguments
- Instrument for the modes, not for the symptoms. Detect “loop without progress” by counting near-duplicate steps. Detect “constraint forgetting” by re-validating constraints periodically.
- Map task types to the modes they hit. The heatmap is your routing prior. New task type, run a small eval first, see which cell lights up.
- The safety case is the heatmap. A regulator asking “have you assessed failure modes” wants the heatmap, not the prose. Build it; ship it.
Next lesson: the Layer 10 evaluation gate — agent safety audit as a deliverable artifact.