Why Agents Fail · Ingénierie IA

Name the failure, find it in the trace, gate it. Detectors, MAST, retry storms, PVE and a trace debugger.

Symptoms, causes, fixes

Symptom	Cause	Fix
Success message on a 400 error	Success hallucination	Re-probe state. Was the file actually created?
One wrong call triggers four API calls	Cascading error	Automated verification gates at every step
Downstream load goes to 10x	Retry storm	Circuit breakers and capped retry budgets per request
Gradual accuracy decay, no crash	Memory poisoning	Append-only log, provenance, unwritable verifier
Three agents give the same wrong answer	Monoculture collapse	Use diverse prompts and/or models
Two agents both think they are the reviewer	Role ambiguity	Explicit role contracts and acceptance tests per task
Agent A thinks done, agent B still runs	State drift	Versioned shared state, state-sync checkpoints
Retrieved page overrides the developer prompt	Indirect prompt injection	Treat all retrieved content as untrusted. Add PVE

Sources

Five recurring modes and their detectors

Command	What it does
`detect_hallucinated_action`	Tool name not in `KNOWN_TOOLS`
`detect_scope_creep`	`write_file` call with no write word in the request
`detect_cascading_errors`	An error, then 2 or more tool calls after it
`detect_context_loss`	A "do not" token appears in tool args
`detect_tool_misuse`	Required args missing for the tool schema
`detect_success_hallucination`	Success claim, write intent, state unchanged
`tag(trace)`	Runs all `DETECTORS`, returns the labels
`python3 code/main.py`	Per-trace labels and aggregate distribution

Sources

MAST root categories

Source: 1642 execution traces, 7 open-source MAS, 41-86.7% failure rate.
Specification Problems (41.77%): role ambiguity, unclear task, implicit success criteria.
Coordination Failures (36.94%): lost messages, unsynchronized shared state, state drift.
Verification Gaps (21.30%): one agent claims success and no one verifies.
Cost: verification gaps are the most expensive per failure. They fail silently.
Code: categorize_incident(symptoms) returns spec, coord, verify or unknown.

Sources

Groupthink family

Monoculture collapse: same base model gives correlated errors. Signal correlated_errors.
Conformity bias: agents align with the loudest peer. Signal agreement_rate_spike.
Deficient ToM: agents cannot model each other. Signal coordination_drop_long_horizon.
Mixed-motive dynamics: compromise-middle satisfies no one. Signal compromise_outputs.
Cascading reliability failures: one error pattern triggers others. Signal retry_amplification.
STRATUS: detection, diagnosis and validation agents. 1.5x mitigation success.

Sources

Retry storm

Payment service fails 10% of requests.
Order agent each retry is a new order-inventory check.
Inventory service sees 2x normal load and starts timing out.
Every order retries. Inventory sees 10x load. Cluster goes down.
Fix a circuit breaker short-circuits with cached or default results.

Sources

phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md

Circuit breaker and storm simulator

breaker = CircuitBreaker()starts BreakerState.CLOSED

breaker.allow()False while OPEN, inside cooldown

breaker.record(ok)opens when error rate > threshold

HALF_OPEN: one success closes, one failure opens again

service = DownstreamService()base_failure_rate 0.1

effective_rate = self.base_failure_rate + (self.load * 0.02)

while attempts_for_req < 4:retry budget per request

...

simulate_retry_storm(200, use_breaker=True, seed=0)

returns total_calls, successes, short_circuits

Sources

phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/code/main.py

Defaults and numbers

Setting	Default	Note
`failure_threshold`	0.5	Error rate that opens the breaker
`window_size`	20	Recent outcomes in the error rate
`open_cooldown_s`	0.5	Time before HALF_OPEN
Production open threshold	5-10%	Error rate on each outbound call
CI trace gate	>=5%	Lesson exercise. Fail the build when traces tag a mode
Audit trace sample	~1000	Real traces per failure-mode audit
Debate N, R	N=3, R=2	Original experiments, due to cost

Sources

PVE: Prompt-Validator-Executor

Nodes and connections

Node	Role	Kind
User message	source user_message	client
Retrieved content	web, memory, tool output	store
Main model	proposes a ToolCall	model
Validator	allowlist + marker scan	process
Executor	runs approved call	process
Refusal	try a different approach

From	To	Connection
User message	Main model	intent
Retrieved content	Validator	scanned
Main model	Validator	candidate call
Validator	Executor	allow
Validator	Refusal	reject
Refusal	Main model	retry

Sources

Debate topologies

Factor	Full mesh	Sparse star
Who reads whom	Every debater reads every peer	Spokes read only the hub
N=5, R=3 proposals	15	15
N=5, R=3 critique ops	60	12
Failure risk	Convergence collapse	Hub failure
Mitigation	Required disagreement rounds	Rotate or use multiple hubs
Code	`full_mesh_round`	`sparse_star_round`

Sources

Agent Trace Debugger

Task	Code
Init the workspace	`python3 scripts/project_test.py agent-trace-debugger --init learning-artifacts/agent-trace-debugger`
Grade one stage	`python3 scripts/project_test.py agent-trace-debugger --stage 1 --path learning-artifacts/agent-trace-debugger --strict`
Compare to a baseline	`node cli.ts --input samples/trace.jsonl --baseline samples/before.jsonl --output trace.html --json trace.json`
Import OTLP JSON	`node cli.ts --format otlp --input samples/otlp.json --output otlp-trace.html --json otlp-trace.json`
Public API	`parseTrace`, `validateTree`, `unionDuration`, `analyze`, `render`
Regression fields	`wall_time_delta_ms`, `token_delta`, `error_delta`

Sources

Trace debugger errors

Symptom	Cause	Fix
`invalid JSON line <n>`	Line is not JSON	Fix line n. Blank lines count
`invalid span line <n>`	Bad field, `end < start` or negative tokens	Emit `ok` or `error` and finite times
`duplicate span id`	Two spans share an id	Make ids unique
`span cycle`	Ancestor chain loops	Fix the parent ids
`missing parent`	Parent id not in the trace	Export the parent span
`child outside parent`	Child interval exceeds parent	Normalize cross-machine clocks
`format must be jsonl or otlp`	Bad `--format` value	Use `jsonl` or `otlp`

Sources

Questions

Why is tagging only crashes not enough?: Most agent failures produce valid-looking output.
Why not guardrails at the end only?: The model already touched the world.
Which slow-failure proxies help?: Agreement rate, retry rate, output-length distribution.
Why subtract the union of child intervals?: Parallel children overlap. Their sum overcounts.
Why compare tokens and errors, not only wall time?: The sample keeps 100 ms while tokens rise by 200.

Sources

Original source

---
id: agent-failure-modes
title: Why Agents Fail
code: WA
subtitle: Name the failure, find it in the trace, gate it. Detectors, MAST, retry storms, PVE and a trace debugger.
hue: 0
sources: phases/14-agent-engineering/25-multi-agent-debate, phases/14-agent-engineering/26-failure-modes-agentic, phases/14-agent-engineering/27-prompt-injection-defense, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink, projects/agent-trace-debugger
source_hash: 7b1f84213ea46a0a
---

## Symptoms, causes, fixes
type: troubleshooting
span: 2
from: phases/14-agent-engineering/26-failure-modes-agentic/docs/en.md, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md, phases/14-agent-engineering/27-prompt-injection-defense/docs/en.md, phases/14-agent-engineering/25-multi-agent-debate/docs/en.md

| Symptom | Cause | Fix |
|---|---|---|
| Success message on a 400 error | Success hallucination | Re-probe state. Was the file actually created? |
| One wrong call triggers four API calls | Cascading error | Automated verification gates at every step |
| Downstream load goes to 10x | Retry storm | Circuit breakers and capped retry budgets per request |
| Gradual accuracy decay, no crash | Memory poisoning | Append-only log, provenance, unwritable verifier |
| Three agents give the same wrong answer | Monoculture collapse | Use diverse prompts and/or models |
| Two agents both think they are the reviewer | Role ambiguity | Explicit role contracts and acceptance tests per task |
| Agent A thinks done, agent B still runs | State drift | Versioned shared state, state-sync checkpoints |
| Retrieved page overrides the developer prompt | Indirect prompt injection | Treat all retrieved content as untrusted. Add PVE |

## Five recurring modes and their detectors
type: command-table
from: phases/14-agent-engineering/26-failure-modes-agentic/code/main.py, phases/14-agent-engineering/26-failure-modes-agentic/docs/en.md

| Command | What it does |
|---|---|
| `detect_hallucinated_action` | Tool name not in `KNOWN_TOOLS` |
| `detect_scope_creep` | `write_file` call with no write word in the request |
| `detect_cascading_errors` | An error, then 2 or more tool calls after it |
| `detect_context_loss` | A "do not" token appears in tool args |
| `detect_tool_misuse` | Required args missing for the tool schema |
| `detect_success_hallucination` | Success claim, write intent, state unchanged |
| `tag(trace)` | Runs all `DETECTORS`, returns the labels |
| `python3 code/main.py` | Per-trace labels and aggregate distribution |

## MAST root categories
type: concepts
from: phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/code/main.py, phases/14-agent-engineering/26-failure-modes-agentic/docs/en.md

- **Source**: 1642 execution traces, 7 open-source MAS, 41-86.7% failure rate.
- **Specification Problems (41.77%)**: role ambiguity, unclear task, implicit success criteria.
- **Coordination Failures (36.94%)**: lost messages, unsynchronized shared state, state drift.
- **Verification Gaps (21.30%)**: one agent claims success and no one verifies.
- **Cost**: verification gaps are the most expensive per failure. They fail silently.
- **Code**: `categorize_incident(symptoms)` returns `spec`, `coord`, `verify` or `unknown`.

## Groupthink family
type: glossary
from: phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/code/main.py

- **Monoculture collapse**: same base model gives correlated errors. Signal `correlated_errors`.
- **Conformity bias**: agents align with the loudest peer. Signal `agreement_rate_spike`.
- **Deficient ToM**: agents cannot model each other. Signal `coordination_drop_long_horizon`.
- **Mixed-motive dynamics**: compromise-middle satisfies no one. Signal `compromise_outputs`.
- **Cascading reliability failures**: one error pattern triggers others. Signal `retry_amplification`.
- **STRATUS**: detection, diagnosis and validation agents. 1.5x mitigation success.

## Retry storm
type: flow
from: phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md

1. **Payment service**: fails 10% of requests.
2. **Order agent**: each retry is a new order-inventory check.
3. **Inventory service**: sees 2x normal load and starts timing out.
4. **Every order**: retries. Inventory sees 10x load. Cluster goes down.
5. **Fix**: a circuit breaker short-circuits with cached or default results.

## Circuit breaker and storm simulator
type: annotated-code
lang: python
from: phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/code/main.py

```python
breaker = CircuitBreaker()           # starts BreakerState.CLOSED
breaker.allow()                      # False while OPEN, inside cooldown
breaker.record(ok)                   # opens when error rate > threshold
# HALF_OPEN: one success closes, one failure opens again
service = DownstreamService()        # base_failure_rate 0.1
effective_rate = self.base_failure_rate + (self.load * 0.02)
while attempts_for_req < 4:          # retry budget per request
    ...
simulate_retry_storm(200, use_breaker=True, seed=0)
# returns total_calls, successes, short_circuits
```

## Defaults and numbers
type: defaults
span: 1
from: phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/code/main.py, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md, phases/14-agent-engineering/26-failure-modes-agentic/docs/en.md, phases/14-agent-engineering/25-multi-agent-debate/docs/en.md

| Setting | Default | Note |
|---|---|---|
| `failure_threshold` | 0.5 | Error rate that opens the breaker |
| `window_size` | 20 | Recent outcomes in the error rate |
| `open_cooldown_s` | 0.5 | Time before HALF_OPEN |
| Production open threshold | 5-10% | Error rate on each outbound call |
| CI trace gate | >=5% | Lesson exercise. Fail the build when traces tag a mode |
| Audit trace sample | ~1000 | Real traces per failure-mode audit |
| Debate N, R | N=3, R=2 | Original experiments, due to cost |

## PVE: Prompt-Validator-Executor
type: architecture
from: phases/14-agent-engineering/27-prompt-injection-defense/docs/en.md, phases/14-agent-engineering/27-prompt-injection-defense/code/main.py

```arch
node | user | 1,1 | User message | source user_message | client
node | ret | 1,2 | Retrieved content | web, memory, tool output | store
node | main | 2,1 | Main model | proposes a ToolCall | model
node | val | 3,1 | Validator | allowlist + marker scan | process
node | exec | 4,1 | Executor | runs approved call | process
node | ref | 3,2 | Refusal | try a different approach |
edge | user | main | intent
edge | ret | val | scanned
edge | main | val | candidate call
edge | val | exec | allow
edge | val | ref | reject
edge | ref | main | retry
```

## Debate topologies
type: comparison
from: phases/14-agent-engineering/25-multi-agent-debate/docs/en.md, phases/14-agent-engineering/25-multi-agent-debate/code/main.py

| Factor | Full mesh | Sparse star |
|---|---|---|
| Who reads whom | Every debater reads every peer | Spokes read only the hub |
| N=5, R=3 proposals | 15 | 15 |
| N=5, R=3 critique ops | 60 | 12 |
| Failure risk | Convergence collapse | Hub failure |
| Mitigation | Required disagreement rounds | Rotate or use multiple hubs |
| Code | `full_mesh_round` | `sparse_star_round` |

## Agent Trace Debugger
type: task-table
span: 2
from: projects/agent-trace-debugger/README.md, projects/agent-trace-debugger/API.md, projects/agent-trace-debugger/solution/cli.ts

| Task | Code |
|---|---|
| Init the workspace | `python3 scripts/project_test.py agent-trace-debugger --init learning-artifacts/agent-trace-debugger` |
| Grade one stage | `python3 scripts/project_test.py agent-trace-debugger --stage 1 --path learning-artifacts/agent-trace-debugger --strict` |
| Compare to a baseline | `node cli.ts --input samples/trace.jsonl --baseline samples/before.jsonl --output trace.html --json trace.json` |
| Import OTLP JSON | `node cli.ts --format otlp --input samples/otlp.json --output otlp-trace.html --json otlp-trace.json` |
| Public API | `parseTrace`, `validateTree`, `unionDuration`, `analyze`, `render` |
| Regression fields | `wall_time_delta_ms`, `token_delta`, `error_delta` |

## Trace debugger errors
type: troubleshooting
span: 1
from: projects/agent-trace-debugger/solution/main.ts, projects/agent-trace-debugger/solution/cli.ts, projects/agent-trace-debugger/stages/01-parse/docs/en.md, projects/agent-trace-debugger/README.md

| Symptom | Cause | Fix |
|---|---|---|
| `invalid JSON line <n>` | Line is not JSON | Fix line n. Blank lines count |
| `invalid span line <n>` | Bad field, `end < start` or negative tokens | Emit `ok` or `error` and finite times |
| `duplicate span id` | Two spans share an id | Make ids unique |
| `span cycle` | Ancestor chain loops | Fix the parent ids |
| `missing parent` | Parent id not in the trace | Export the parent span |
| `child outside parent` | Child interval exceeds parent | Normalize cross-machine clocks |
| `format must be jsonl or otlp` | Bad `--format` value | Use `jsonl` or `otlp` |

## Questions
type: qa
from: phases/14-agent-engineering/26-failure-modes-agentic/docs/en.md, phases/14-agent-engineering/27-prompt-injection-defense/docs/en.md, phases/16-multi-agent-and-swarms/23-failure-modes-mast-groupthink/docs/en.md, projects/agent-trace-debugger/project.json, projects/agent-trace-debugger/README.md

- **Why is tagging only crashes not enough?** Most agent failures produce valid-looking output.
- **Why not guardrails at the end only?** The model already touched the world.
- **Which slow-failure proxies help?** Agreement rate, retry rate, output-length distribution.
- **Why subtract the union of child intervals?** Parallel children overlap. Their sum overcounts.
- **Why compare tokens and errors, not only wall time?** The sample keeps 100 ms while tokens rise by 200.