docs(coding-agent): add #1375 changelog entry and extension event docs fixes #1375

Author: badlogic

packages/coding-agent/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - `ContextUsage.tokens` and `ContextUsage.percent` are now `number | null`. After compaction, context token count is unknown until the next LLM response, so these fields return `null`. Extensions that read `ContextUsage` must handle the `null` case. Removed `usageTokens`, `trailingTokens`, and `lastUsageIndex` fields from `ContextUsage` (implementation details that should not have been public) ([#1382](https://github.com/badlogic/pi-mono/pull/1382) by [@ferologics](https://github.com/ferologics))
 
+### Added
+
+- Added extension event forwarding for message and tool execution lifecycles (`message_start`, `message_update`, `message_end`, `tool_execution_start`, `tool_execution_update`, `tool_execution_end`) ([#1375](https://github.com/badlogic/pi-mono/pull/1375) by [@sumeet](https://github.com/sumeet))
+
 ### Fixed
 
 - Fixed context usage percentage in footer showing stale pre-compaction values. After compaction the footer now shows `?/200k` until the next LLM response provides accurate usage ([#1382](https://github.com/badlogic/pi-mono/pull/1382) by [@ferologics](https://github.com/ferologics))

packages/coding-agent/docs/extensions.md

@@ -237,6 +237,7 @@ user sends prompt ────────────────────
   ├─► (skill/template expansion if not handled)            │
   ├─► before_agent_start (can inject message, modify system prompt)
   ├─► agent_start                                          │
+  ├─► message_start / message_update / message_end         │
   │                                                        │
   │   ┌─── turn (repeats while LLM calls tools) ───┐       │
   │   │                                            │       │
@@ -245,7 +246,9 @@ user sends prompt ────────────────────
   │   │                                            │       │
   │   │   LLM responds, may call tools:            │       │
   │   │     ├─► tool_call (can block)              │       │
-  │   │     │   tool executes                      │       │
+  │   │     ├─► tool_execution_start               │       │
+  │   │     ├─► tool_execution_update              │       │
+  │   │     ├─► tool_execution_end                 │       │
   │   │     └─► tool_result (can modify)           │       │
   │   │                                            │       │
   │   └─► turn_end                                 │       │
@@ -434,6 +437,46 @@ pi.on("turn_end", async (event, ctx) => {
 });
 ```
 
+#### message_start / message_update / message_end
+
+Fired for message lifecycle updates.
+
+- `message_start` and `message_end` fire for user, assistant, and toolResult messages.
+- `message_update` fires for assistant streaming updates.
+
+```typescript
+pi.on("message_start", async (event, ctx) => {
+  // event.message
+});
+
+pi.on("message_update", async (event, ctx) => {
+  // event.message
+  // event.assistantMessageEvent (token-by-token stream event)
+});
+
+pi.on("message_end", async (event, ctx) => {
+  // event.message
+});
+```
+
+#### tool_execution_start / tool_execution_update / tool_execution_end
+
+Fired for tool execution lifecycle updates.
+
+```typescript
+pi.on("tool_execution_start", async (event, ctx) => {
+  // event.toolCallId, event.toolName, event.args
+});
+
+pi.on("tool_execution_update", async (event, ctx) => {
+  // event.toolCallId, event.toolName, event.args, event.partialResult
+});
+
+pi.on("tool_execution_end", async (event, ctx) => {
+  // event.toolCallId, event.toolName, event.result, event.isError
+});
+```
+
 #### context
 
 Fired before each LLM call. Modify messages non-destructively. See [session.md](session.md) for message types.