Fix slash commands and hook commands during streaming

- Hook commands now execute immediately during streaming (they manage their own LLM interaction via pi.sendMessage())
- File-based slash commands are expanded and queued via steer/followUp during streaming
- prompt() accepts new streamingBehavior option ('steer' or 'followUp') for explicit queueing during streaming
- steer() and followUp() now expand file-based slash commands and error on hook commands
- RPC prompt command accepts optional streamingBehavior field
- Updated docs: rpc.md, sdk.md, CHANGELOG.md

fixes #420

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

@@ -915,26 +915,13 @@ export class InteractiveMode {
 				return;
 			}
 
-			// Hook commands always run immediately, even during streaming
-			// (if they need to interact with LLM, they use pi.sendMessage which handles queueing)
-			if (text.startsWith("/") && this.session.hookRunner) {
-				const spaceIndex = text.indexOf(" ");
-				const commandName = spaceIndex === -1 ? text.slice(1) : text.slice(1, spaceIndex);
-				const command = this.session.hookRunner.getCommand(commandName);
-				if (command) {
-					this.editor.addToHistory(text);
-					this.editor.setText("");
-					await this.session.prompt(text);
-					return;
-				}
-			}
-
-			// Queue steering message if agent is streaming (interrupts current work)
+			// If streaming, use prompt() with steer behavior
+			// This handles hook commands (execute immediately), slash command expansion, and queueing
 			if (this.session.isStreaming) {
-				await this.session.steer(text);
-				this.updatePendingMessagesDisplay();
 				this.editor.addToHistory(text);
 				this.editor.setText("");
+				await this.session.prompt(text, { streamingBehavior: "steer" });
+				this.updatePendingMessagesDisplay();
 				this.ui.requestRender();
 				return;
 			}
@@ -1461,11 +1448,12 @@ export class InteractiveMode {
 		if (!text) return;
 
 		// Alt+Enter queues a follow-up message (waits until agent finishes)
+		// This handles hook commands (execute immediately), slash command expansion, and queueing
 		if (this.session.isStreaming) {
-			await this.session.followUp(text);
-			this.updatePendingMessagesDisplay();
 			this.editor.addToHistory(text);
 			this.editor.setText("");
+			await this.session.prompt(text, { streamingBehavior: "followUp" });
+			this.updatePendingMessagesDisplay();
 			this.ui.requestRender();
 		}
 		// If not streaming, Alt+Enter acts like regular Enter (trigger onSubmit)

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

@@ -244,10 +244,12 @@ 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()
+				// Hook commands are executed immediately, file slash commands are expanded
+				// If streaming and streamingBehavior specified, queues via steer/followUp
 				session
 					.prompt(command.message, {
 						images: command.images,
+						streamingBehavior: command.streamingBehavior,
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");

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

@@ -17,7 +17,7 @@ import type { CompactionResult } from "../../core/compaction/index.js";
 
 export type RpcCommand =
 	// Prompting
-	| { id?: string; type: "prompt"; message: string; images?: ImageContent[] }
+	| { id?: string; type: "prompt"; message: string; images?: ImageContent[]; streamingBehavior?: "steer" | "followUp" }
 	| { id?: string; type: "steer"; message: string }
 	| { id?: string; type: "follow_up"; message: string }
 	| { id?: string; type: "abort" }