Add type guards for tool_result event narrowing

- Export isBashToolResult, isReadToolResult, etc. type guards
- Update hooks.md with type guard usage examples
- Document custom tool handling in hooks.md

packages/coding-agent/CHANGELOG.md

@@ -11,6 +11,7 @@
   - `ToolResultEventResult.result` renamed to `ToolResultEventResult.text` (removed), use `content` instead
   - Hook handlers returning `{ result: "..." }` must change to `{ content: [{ type: "text", text: "..." }] }`
   - Built-in tool details types exported: `BashToolDetails`, `ReadToolDetails`, `GrepToolDetails`, `FindToolDetails`, `LsToolDetails`, `TruncationResult`
+  - Type guards exported for narrowing: `isBashToolResult`, `isReadToolResult`, `isEditToolResult`, `isWriteToolResult`, `isGrepToolResult`, `isFindToolResult`, `isLsToolResult`
 
 ### Changed
 

packages/coding-agent/docs/hooks.md

@@ -234,20 +234,26 @@ pi.on("tool_result", async (event, ctx) => {
 });
 ```
 
-The event type is a discriminated union based on `toolName`. TypeScript will narrow `details` to the correct type:
+The event type is a discriminated union based on `toolName`. Use the provided type guards to narrow `details` to the correct type:
 
 ```typescript
-pi.on("tool_result", async (event, ctx) => {
-  if (event.toolName === "bash") {
-    // event.details is BashToolDetails | undefined
-    if (event.details?.truncation?.truncated) {
-      // Access full output from temp file
-      const fullPath = event.details.fullOutputPath;
+import { isBashToolResult, type HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function (pi: HookAPI) {
+  pi.on("tool_result", async (event, ctx) => {
+    if (isBashToolResult(event)) {
+      // event.details is BashToolDetails | undefined
+      if (event.details?.truncation?.truncated) {
+        // Access full output from temp file
+        const fullPath = event.details.fullOutputPath;
+      }
     }
-  }
-});
+  });
+}
 ```
 
+Available type guards: `isBashToolResult`, `isReadToolResult`, `isEditToolResult`, `isWriteToolResult`, `isGrepToolResult`, `isFindToolResult`, `isLsToolResult`.
+
 #### Tool Details Types
 
 Each built-in tool has a typed `details` field. Types are exported from `@mariozechner/pi-coding-agent`:
@@ -272,7 +278,18 @@ Common fields in details:
 - `totalLines`, `totalBytes` - original size
 - `outputLines`, `outputBytes` - truncated size
 
-Custom tools use `CustomToolResultEvent` with `details: unknown`.
+Custom tools use `CustomToolResultEvent` with `details: unknown`. You'll need to cast or validate:
+
+```typescript
+pi.on("tool_result", async (event, ctx) => {
+  if (event.toolName === "my-custom-tool") {
+    // Cast to your tool's details type
+    const details = event.details as MyCustomToolDetails;
+  }
+});
+```
+
+Custom tools define their own details type in their `execute` function return value. The hook receives whatever the tool returned, but since the hook system doesn't know about custom tool types at compile time, it's typed as `unknown`.
 
 **Note:** If you modify `content`, you should also update `details` accordingly. The TUI uses `details` (e.g., truncation info) for rendering, so inconsistent values will cause display issues.
 

packages/coding-agent/src/core/hooks/index.ts

@@ -29,3 +29,12 @@ export type {
 	TurnStartEvent,
 	WriteToolResultEvent,
 } from "./types.js";
+export {
+	isBashToolResult,
+	isEditToolResult,
+	isFindToolResult,
+	isGrepToolResult,
+	isLsToolResult,
+	isReadToolResult,
+	isWriteToolResult,
+} from "./types.js";

packages/coding-agent/src/core/hooks/types.ts

@@ -224,6 +224,29 @@ export type ToolResultEvent =
 	| LsToolResultEvent
 	| CustomToolResultEvent;
 
+// Type guards for narrowing ToolResultEvent to specific tool types
+export function isBashToolResult(e: ToolResultEvent): e is BashToolResultEvent {
+	return e.toolName === "bash";
+}
+export function isReadToolResult(e: ToolResultEvent): e is ReadToolResultEvent {
+	return e.toolName === "read";
+}
+export function isEditToolResult(e: ToolResultEvent): e is EditToolResultEvent {
+	return e.toolName === "edit";
+}
+export function isWriteToolResult(e: ToolResultEvent): e is WriteToolResultEvent {
+	return e.toolName === "write";
+}
+export function isGrepToolResult(e: ToolResultEvent): e is GrepToolResultEvent {
+	return e.toolName === "grep";
+}
+export function isFindToolResult(e: ToolResultEvent): e is FindToolResultEvent {
+	return e.toolName === "find";
+}
+export function isLsToolResult(e: ToolResultEvent): e is LsToolResultEvent {
+	return e.toolName === "ls";
+}
+
 /**
  * Event data for branch event.
  */

packages/coding-agent/src/index.ts

@@ -35,7 +35,6 @@ export type {
 	ToolUIContext,
 } from "./core/custom-tools/index.js";
 export { discoverAndLoadCustomTools, loadCustomTools } from "./core/custom-tools/index.js";
-// Hook system types
 export type {
 	AgentEndEvent,
 	AgentStartEvent,
@@ -62,6 +61,16 @@ export type {
 	TurnStartEvent,
 	WriteToolResultEvent,
 } from "./core/hooks/index.js";
+// Hook system types and type guards
+export {
+	isBashToolResult,
+	isEditToolResult,
+	isFindToolResult,
+	isGrepToolResult,
+	isLsToolResult,
+	isReadToolResult,
+	isWriteToolResult,
+} from "./core/hooks/index.js";
 export { messageTransformer } from "./core/messages.js";
 export {
 	type CompactionEntry,