Move hook command execution to AgentSession.prompt()

Hook commands registered via pi.registerCommand() are now handled in
AgentSession.prompt() alongside file-based slash commands. This:

- Removes duplicate tryHandleHookCommand from interactive-mode and rpc-mode
- All modes (interactive, RPC, print) share the same command handling logic
- AgentSession._tryExecuteHookCommand() builds CommandContext using:
  - UI context from hookRunner (set by mode)
  - sessionManager, modelRegistry from AgentSession
  - sendMessage via sendHookMessage
  - exec via exported execCommand
- Handler returning string uses it as prompt, undefined returns early

Also:
- Export execCommand from hooks/runner.ts
- Add getUIContext() and getHasUI() to HookRunner
- Make HookRunner.emitError() public for error reporting

packages/coding-agent/docs/session-tree-plan.md

@@ -133,7 +133,7 @@ Behavior:
 **Renamed:**
 - `renderCustomMessage()` → `registerCustomMessageRenderer()`
 
-**New: `sendMessage()`**
+**New: `sendMessage()` ✅**
 
 Replaces `send()`. Always creates CustomMessageEntry, never user messages.
 
@@ -143,34 +143,57 @@ type HookMessage<T = unknown> = Pick<CustomMessageEntry<T>, 'customType' | 'cont
 sendMessage(message: HookMessage, triggerTurn?: boolean): void;
 ```
 
-Behavior:
-- If streaming → queue, append after turn ends (never triggers turn)
-- If idle AND `triggerTurn: true` → append and trigger turn
-- If idle AND `triggerTurn: false` (default) → just append, no turn
-- TUI updates if `display: true`
+Implementation:
+- Uses agent's queue mechanism with `_hookData` marker on AppMessage
+- `message_end` handler routes based on marker presence
+- `AgentSession.sendHookMessage()` handles three cases:
+  - Streaming: queues via `agent.queueMessage()`, loop processes and emits `message_end`
+  - Not streaming + triggerTurn: direct append + `agent.continue()`
+  - Not streaming + no trigger: direct append only
+- TUI updates via event (streaming) or explicit rebuild (non-streaming)
 
-For hook state (CustomEntry), use `sessionManager.appendCustomEntry()` directly.
+**New: `appendEntry()` ✅**
 
-**New: `registerCommand()`**
+For hook state persistence (NOT in LLM context):
+
+```typescript
+appendEntry(customType: string, data?: unknown): void;
+```
+
+Calls `sessionManager.appendCustomEntry()` directly.
+
+**New: `registerCommand()` (types ✅, wiring TODO)**
 
 ```typescript
 interface CommandContext {
   args: string;                    // Everything after /commandname
-  session: LimitedAgentSession;    // No prompt(), use sendMessage()
   ui: HookUIContext;
+  hasUI: boolean;
+  cwd: string;
+  sessionManager: SessionManager;
+  modelRegistry: ModelRegistry;
+  sendMessage: HookAPI['sendMessage'];
   exec(command: string, args: string[], options?: ExecOptions): Promise<ExecResult>;
 }
 
 registerCommand(name: string, options: {
   description?: string;
-  handler: (ctx: CommandContext) => Promise<string | void>;
+  handler: (ctx: CommandContext) => Promise<string | undefined>;
 }): void;
 ```
 
 Handler return:
-- `void` - command completed
+- `undefined` - command completed
 - `string` - text to send as prompt (like file-based slash commands)
 
+Wiring (all in AgentSession.prompt()):
+- [x] Add hook commands to autocomplete in interactive-mode
+- [x] `_tryExecuteHookCommand()` in AgentSession handles command execution
+- [x] Build CommandContext with ui (from hookRunner), exec, sessionManager, etc.
+- [x] If handler returns string, use as prompt text
+- [x] If handler returns undefined, return early (no LLM call)
+- [x] Works for all modes (interactive, RPC, print) via shared AgentSession
+
 **New: `ui.custom()`**
 
 For arbitrary hook UI with keyboard focus:

packages/coding-agent/src/core/agent-session.ts

@@ -27,7 +27,16 @@ import {
 } from "./compaction.js";
 import type { LoadedCustomTool, SessionEvent as ToolSessionEvent } from "./custom-tools/index.js";
 import { exportSessionToHtml } from "./export-html.js";
-import type { HookMessage, HookRunner, SessionEventResult, TurnEndEvent, TurnStartEvent } from "./hooks/index.js";
+import {
+	type CommandContext,
+	type ExecOptions,
+	execCommand,
+	type HookMessage,
+	type HookRunner,
+	type SessionEventResult,
+	type TurnEndEvent,
+	type TurnStartEvent,
+} from "./hooks/index.js";
 import type { BashExecutionMessage } from "./messages.js";
 import type { ModelRegistry } from "./model-registry.js";
 import type { CompactionEntry, SessionManager } from "./session-manager.js";
@@ -441,6 +450,7 @@ export class AgentSession {
 	/**
 	 * Send a prompt to the agent.
 	 * - Validates model and API key before sending
+	 * - Handles hook commands (registered via pi.registerCommand)
 	 * - Expands file-based slash commands by default
 	 * @throws Error if no model selected or no API key available
 	 */
@@ -450,6 +460,20 @@ export class AgentSession {
 
 		const expandCommands = options?.expandSlashCommands ?? true;
 
+		// Handle hook commands first (if enabled and text is a slash command)
+		if (expandCommands && text.startsWith("/")) {
+			const result = await this._tryExecuteHookCommand(text);
+			if (result.handled) {
+				if (result.prompt) {
+					// Hook returned text to use as prompt
+					text = result.prompt;
+				} else {
+					// Hook command executed, no prompt to send
+					return;
+				}
+			}
+		}
+
 		// Validate model
 		if (!this.model) {
 			throw new Error(
@@ -474,13 +498,65 @@ export class AgentSession {
 			await this._checkCompaction(lastAssistant, false);
 		}
 
-		// Expand slash commands if requested
+		// Expand file-based slash commands if requested
 		const expandedText = expandCommands ? expandSlashCommand(text, [...this._fileCommands]) : text;
 
 		await this.agent.prompt(expandedText, options?.attachments);
 		await this.waitForRetry();
 	}
 
+	/**
+	 * Try to execute a hook command. Returns whether it was handled and optional prompt text.
+	 */
+	private async _tryExecuteHookCommand(text: string): Promise<{ handled: boolean; prompt?: string }> {
+		if (!this._hookRunner) return { handled: false };
+
+		// Parse command name and args
+		const spaceIndex = text.indexOf(" ");
+		const commandName = spaceIndex === -1 ? text.slice(1) : text.slice(1, spaceIndex);
+		const args = spaceIndex === -1 ? "" : text.slice(spaceIndex + 1);
+
+		const command = this._hookRunner.getCommand(commandName);
+		if (!command) return { handled: false };
+
+		// Get UI context from hook runner (set by mode)
+		const uiContext = this._hookRunner.getUIContext();
+		if (!uiContext) return { handled: false };
+
+		// Build command context
+		const cwd = process.cwd();
+		const ctx: CommandContext = {
+			args,
+			ui: uiContext,
+			hasUI: this._hookRunner.getHasUI(),
+			cwd,
+			sessionManager: this.sessionManager,
+			modelRegistry: this._modelRegistry,
+			sendMessage: (message, triggerTurn) => {
+				this.sendHookMessage(message, triggerTurn).catch(() => {
+					// Error handling is done in sendHookMessage
+				});
+			},
+			exec: (cmd: string, cmdArgs: string[], options?: ExecOptions) => execCommand(cmd, cmdArgs, cwd, options),
+		};
+
+		try {
+			const result = await command.handler(ctx);
+			if (typeof result === "string") {
+				return { handled: true, prompt: result };
+			}
+			return { handled: true };
+		} catch (err) {
+			// Emit error via hook runner
+			this._hookRunner.emitError({
+				hookPath: `command:${commandName}`,
+				event: "command",
+				error: err instanceof Error ? err.message : String(err),
+			});
+			return { handled: true };
+		}
+	}
+
 	/**
 	 * Queue a message to be sent after the current response completes.
 	 * Use when agent is currently streaming.

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

@@ -6,16 +6,18 @@ export {
 	loadHooks,
 	type SendMessageHandler,
 } from "./loader.js";
-export { type HookErrorListener, HookRunner } from "./runner.js";
+export { execCommand, type HookErrorListener, HookRunner } from "./runner.js";
 export { wrapToolsWithHooks, wrapToolWithHooks } from "./tool-wrapper.js";
 export type {
 	AgentEndEvent,
 	AgentStartEvent,
 	BashToolResultEvent,
+	CommandContext,
 	CustomMessageRenderer,
 	CustomMessageRenderOptions,
 	CustomToolResultEvent,
 	EditToolResultEvent,
+	ExecOptions,
 	ExecResult,
 	FindToolResultEvent,
 	GrepToolResultEvent,

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

@@ -34,7 +34,12 @@ export type HookErrorListener = (error: HookError) => void;
  * Execute a command and return stdout/stderr/code.
  * Supports cancellation via AbortSignal and timeout.
  */
-async function exec(command: string, args: string[], cwd: string, options?: ExecOptions): Promise<ExecResult> {
+export async function execCommand(
+	command: string,
+	args: string[],
+	cwd: string,
+	options?: ExecOptions,
+): Promise<ExecResult> {
 	return new Promise((resolve) => {
 		const proc = spawn(command, args, { cwd, shell: false });
 
@@ -150,6 +155,20 @@ export class HookRunner {
 		this.hasUI = hasUI;
 	}
 
+	/**
+	 * Get the UI context (set by mode).
+	 */
+	getUIContext(): HookUIContext | null {
+		return this.uiContext;
+	}
+
+	/**
+	 * Get whether UI is available.
+	 */
+	getHasUI(): boolean {
+		return this.hasUI;
+	}
+
 	/**
 	 * Get the paths of all loaded hooks.
 	 */
@@ -196,7 +215,10 @@ export class HookRunner {
 	/**
 	 * Emit an error to all listeners.
 	 */
-	private emitError(error: HookError): void {
+	/**
+	 * Emit an error to all error listeners.
+	 */
+	emitError(error: HookError): void {
 		for (const listener of this.errorListeners) {
 			listener(error);
 		}
@@ -261,7 +283,8 @@ export class HookRunner {
 	 */
 	private createContext(): HookEventContext {
 		return {
-			exec: (command: string, args: string[], options?: ExecOptions) => exec(command, args, this.cwd, options),
+			exec: (command: string, args: string[], options?: ExecOptions) =>
+				execCommand(command, args, this.cwd, options),
 			ui: this.uiContext,
 			hasUI: this.hasUI,
 			cwd: this.cwd,

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -179,9 +179,15 @@ export class InteractiveMode {
 			description: cmd.description,
 		}));
 
+		// Convert hook commands to SlashCommand format
+		const hookCommands: SlashCommand[] = (this.session.hookRunner?.getRegisteredCommands() ?? []).map((cmd) => ({
+			name: cmd.name,
+			description: cmd.description ?? "(hook command)",
+		}));
+
 		// Setup autocomplete
 		const autocompleteProvider = new CombinedAutocompleteProvider(
-			[...slashCommands, ...fileSlashCommands],
+			[...slashCommands, ...fileSlashCommands, ...hookCommands],
 			process.cwd(),
 			fdPath,
 		);

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -182,10 +182,10 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 
 			case "prompt": {
 				// Don't await - events will stream
+				// Hook commands and file slash commands are handled in session.prompt()
 				session
 					.prompt(command.message, {
 						attachments: command.attachments,
-						expandSlashCommands: false,
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");