Events

Pi emits its activity as EventFormat::Pi. It runs as pi --mode json --print, which writes a line-delimited JSON stream on standard output where each non-empty line is one complete JSON object carrying a top-level type. The harness layer parses that stream and maps it onto the normalized harness events.

Raw event stream

The stream mixes lifecycle markers, completed assistant messages, and tool executions. The session id is captured from the session record. A tool execution is reported across tool_execution_start, tool_execution_update, and tool_execution_end, and is reconstructed by pairing the start, which carries the arguments, with the end, which carries the result.

Pi event	Handling
session	Captures the session id (also tried as sessionId/session_id). No event.
agent_start, agent_end, agent_settled, turn_start, turn_end, message_start, message_update, tool_execution_update	Lifecycle markers and partial deltas. No event.
message_end	A completed message. An assistant-role message becomes an agent message, and the record’s message.usage becomes a usage event.
tool_execution_start	Records the tool’s toolCallId, name, and arguments for later resolution, and counts the invocation in the run’s tool tally. No event.
tool_execution_end	Resolves the recorded start by toolCallId and maps it by tool name.
any other type	Becomes an unknown event.
a line that is not JSON	Becomes a warning event.

Assistant messages

A message_end record whose message role is assistant becomes an agent message. Its content is read as either a string or an array of typed parts, whose text parts form the message. Its thinking parts form a reasoning event emitted ahead of the agent message. A non-assistant message emits no event, as does an assistant message carrying neither text nor thinking.

Tool executions

tool_execution_start carries the tool name (toolName, also tried as tool_name/tool/name) and its arguments (args, also tried as input/arguments/toolInput). tool_execution_end carries only the result, so the two are paired by toolCallId and the operation the agent requested is classified with its observed outcome.

Success is read from the end record’s isError boolean, then from a non-null error field, then from the presence of a result, and finally from a status/state string. An end with no recorded start, and a record whose tool name is unrecognized, become unknown events so the stream stays lossless.

Tool names are matched case-insensitively:

Pi tool	Event
read	read
write, edit	write
search, grep, glob	search
list	list
bash, shell	command, or a file operation when the command line is recognizably one
any other tool	unknown