Core Concepts

Trace Schema (JSONL)

Specification of the minimal JSONL event schema, supported event types, field alias tolerance, and sample traces.

Last updated: 2026-10-31•v0.1.0

Last Signal standardizes agent telemetry around a minimal, stream-friendly JSON Lines (JSONL) schema. Each line in a trace represents a discrete, timestamped lifecycle event.

Supported Event Types

Event Type	Description	Required Context
agent_start	Agent session initialized	trace_id, timestamp
model_call	Agent issued prompt to LLM	operation, metadata (model, tokens)
tool_call	Agent dispatched tool execution	operation, attempt
tool_result	Tool returned result or exception	operation, status, error_message
retry	Agent triggered retry attempt	operation, attempt
agent_end	Agent gracefully finished loop	status
process_error	Fatal unhandled exception or crash	error_message, metadata

Schema Field Definitions

Field	Type	Description
timestamp	float / string	Epoch timestamp (seconds or milliseconds) or ISO-8601 string.
trace_id	string	Unique execution trace identifier (e.g. ls_9f8e210a).
event_type	string	One of the supported event types above.
operation	string	Name of the tool, action, or model being executed.
status	string	Execution status: 'started', 'completed', 'failed', 'aborted'.
error_message	string / null	Human-readable error description or exception string.
attempt	integer	Current retry attempt index (1-indexed).
metadata	object	Arbitrary key-value dictionary for payloads, durations, or tokens.

Field Alias Tolerance

Different agent frameworks (LangChain, AutoGen, CrewAI, custom loops) use varying field nomenclature. To ensure zero-friction integration, Last Signal's trace reader implements lenient alias tolerance:

tool → operation: If tool is present, Last Signal maps it automatically to operation.
err → error_message: If err is present, it maps to error_message.
error → error_message: Automatically normalized.
type → event_type: Automatically normalized if event_type is absent.

Interactive JSONL Trace Sample

sample_failure_trace.jsonl(6 events)

1{"timestamp":1730412300.1,"trace_id":"ls_demo_881","event_type":"agent_start"}

2{"timestamp":1730412301.2,"trace_id":"ls_demo_881","event_type":"tool_call","operation":"web_search","attempt":1}

3{"timestamp":1730412301.9,"trace_id":"ls_demo_881","event_type":"tool_result","operation":"web_search","status":"failed","error_message":"HTTP 429 Too Many Requests"}

4{"timestamp":1730412302.5,"trace_id":"ls_demo_881","event_type":"retry","operation":"web_search","attempt":1}

5{"timestamp":1730412303,"trace_id":"ls_demo_881","event_type":"tool_result","operation":"web_search","status":"failed","error_message":"HTTP 429 Too Many Requests"}

6{"timestamp":1730412303.8,"trace_id":"ls_demo_881","event_type":"process_error","error_message":"RateLimitExhaustionError: Exceeded provider quota"}