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+# JSON Event Stream Mode
+
+```bash
+pi --mode json "Your prompt"
+```
+
+Outputs all session events as JSON lines to stdout. Useful for integrating pi into other tools or custom UIs.
+
+## Event Types
+
+Wire events use `JsonAgentSessionEvent`. It matches
+[`AgentSessionEvent`](https://github.com/earendil-works/pi-mono/blob/main/packages/coding-agent/src/core/agent-session.ts)
+except that streaming message updates omit cumulative snapshots:
+
+```typescript
+type WithoutPartial<T> = T extends { partial: unknown } ? Omit<T, "partial"> : T;
+
+type JsonAssistantMessageEvent<T> = T extends { type: "toolcall_start"; partial: unknown }
+  ? WithoutPartial<T> & { id: string; toolName: string }
+  : WithoutPartial<T>;
+
+type JsonAgentSessionEvent =
+  | Exclude<AgentSessionEvent, { type: "message_update" }>
+  | {
+      type: "message_update";
+      usage: Usage;
+      assistantMessageEvent: JsonAssistantMessageEvent<AssistantMessageEvent>;
+    };
+```
+
+`queue_update` emits the full pending steering and follow-up queues whenever they change. `compaction_start` and `compaction_end` cover both manual and automatic compaction.
+
+Other base events come from
+[`AgentEvent`](https://github.com/earendil-works/pi-mono/blob/main/packages/agent/src/types.ts):
+
+```typescript
+type AgentEvent =
+  // Agent lifecycle
+  | { type: "agent_start" }
+  | { type: "agent_end"; messages: AgentMessage[] }
+  // Turn lifecycle
+  | { type: "turn_start" }
+  | { type: "turn_end"; message: AgentMessage; toolResults: ToolResultMessage[] }
+  // Message lifecycle
+  | { type: "message_start"; message: AgentMessage }
+  | { type: "message_update"; message: AgentMessage; assistantMessageEvent: AssistantMessageEvent }
+  | { type: "message_end"; message: AgentMessage }
+  // Tool execution
+  | { type: "tool_execution_start"; toolCallId: string; toolName: string; args: any }
+  | { type: "tool_execution_update"; toolCallId: string; toolName: string; args: any; partialResult: any }
+  | { type: "tool_execution_end"; toolCallId: string; toolName: string; result: any; isError: boolean };
+```
+
+## Message Types
+
+Base messages from [`packages/ai/src/types.ts`](https://github.com/earendil-works/pi-mono/blob/main/packages/ai/src/types.ts#L134):
+- `UserMessage` (line 134)
+- `AssistantMessage` (line 140)
+- `ToolResultMessage` (line 152)
+
+Extended messages from [`packages/coding-agent/src/core/messages.ts`](https://github.com/earendil-works/pi-mono/blob/main/packages/coding-agent/src/core/messages.ts#L29):
+- `BashExecutionMessage` (line 29)
+- `CustomMessage` (line 46)
+- `BranchSummaryMessage` (line 55)
+- `CompactionSummaryMessage` (line 62)
+
+## Output Format
+
+Each line is a JSON object. The first line is the session header:
+
+```json
+{"type":"session","version":3,"id":"uuid","timestamp":"...","cwd":"/path"}
+```
+
+Followed by events as they occur:
+
+```json
+{"type":"agent_start"}
+{"type":"turn_start"}
+{"type":"message_start","message":{"role":"assistant","content":[],...}}
+{"type":"message_update","usage":{...},"assistantMessageEvent":{"type":"text_delta","contentIndex":0,"delta":"Hello"}}
+{"type":"message_end","message":{...}}
+{"type":"turn_end","message":{...},"toolResults":[]}
+{"type":"agent_end","messages":[...]}
+```
+
+`message_update` records are delta-only. They omit both the cumulative `message` field and
+`assistantMessageEvent.partial` to keep stream size linear. The top-level `usage` field contains
+the latest cumulative provider-reported usage and may remain zero when a provider only reports
+usage at completion. Use `contentIndex` and `delta` to assemble live text, thinking, or tool-call
+arguments if needed. A `toolcall_start` event also includes the constant-sized `id` and `toolName`
+fields. `message_end` contains the final authoritative message.
+
+## Example
+
+```bash
+pi --mode json "List files" 2>/dev/null | jq -c 'select(.type == "message_end")'
+```