Event Log

Format

The event log is NDJSON — one JSON object per line, emitted in chronological order. Every object has an "event" field identifying its type and a "ts" field with a Unix timestamp in milliseconds.

Output destination

By default, events are written to stdout, interleaved with your agent’s own output. To separate them, configure file output:

[observability]
log      = "file"
log_file = "nanny.log"

Or pipe stdout to a file at the shell level:

nanny run > nanny.log

Guaranteed events

Every execution emits exactly these two events, in this order:

ExecutionStarted

Always the first event. Emitted immediately before the child process is spawned.

{
  "event": "ExecutionStarted",
  "ts": 1711234567000,
  "limits_set": "[limits]",
  "command": "python agent.py",
  "limits": {
    "steps": 100,
    "tokens": 1000,
    "timeout": 30000
  }
}

ExecutionStopped

Always the last event. Emitted on every exit path — clean exit, timeout, error, or signal.

{
  "event": "ExecutionStopped",
  "ts": 1711234572000,
  "reason": "AgentCompleted",
  "steps": 7,
  "tokens_spent": 70,
  "elapsed_ms": 4823
}

If the process was killed, reason will be one of the stop reasons listed in Limits & Enforcement.

SDK events

When the Rust SDK macros or Python SDK decorators are active, additional events are emitted for each tool call. These appear between ExecutionStarted and ExecutionStopped:

Event	When emitted
`StepCompleted`	After each agent step
`ToolAllowed`	When Nanny permits a tool call
`ToolDenied`	When a tool call is blocked because the tool is not in the allowlist
`RuleDenied`	When a tool call is blocked by a custom rule or a per-tool `max_calls` limit
`ToolFailed`	When a permitted tool fails at runtime (network error, bad args, etc.)
`AgentScopeEntered`	When a `#[nanny::agent("name")]` function is entered
`AgentScopeExited`	When a `#[nanny::agent("name")]` function returns

ToolDenied and RuleDenied are distinct denial events — ToolDenied means the tool was not permitted at all; RuleDenied means the tool was permitted but a rule blocked this specific call. ToolFailed is different from both — the tool was allowed and called, but encountered a runtime error. No tokens are charged on failure.

Using the log

The event log is designed to be piped into standard tools:

# Count total steps
cat nanny.log | grep StepCompleted | wc -l

# Find all denied tool calls (allowlist blocks and rule denials)
cat nanny.log | jq 'select(.event == "ToolDenied" or .event == "RuleDenied")'

# Check why a run stopped
cat nanny.log | jq 'select(.event == "ExecutionStopped") | .reason'

Concepts

SDKs

Guides

CLI

Reference

Format

Output destination

Guaranteed events

ExecutionStarted

ExecutionStopped

SDK events

Using the log

​Format

​Output destination

​Guaranteed events

​ExecutionStarted

​ExecutionStopped

​SDK events

​Using the log

Format

Output destination

Guaranteed events

ExecutionStarted

ExecutionStopped

SDK events

Using the log