feat(coding-agent): add input event for extension input interception (#761)

* feat(coding-agent): add input event for extension input interception

Extensions can now intercept, transform, or handle user input before the
agent processes it. Three result types: continue (pass through), transform
(modify text/images), handled (respond without LLM). Handlers chain
transforms and short-circuit on handled. Source field identifies origin.

* fix: make source public, use if/else over ternary

* fix: remove response field, extension handles own UI

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

@@ -39,6 +39,7 @@ import { exportSessionToHtml, type ToolHtmlRenderer } from "./export-html/index.
 import { createToolHtmlRenderer } from "./export-html/tool-renderer.js";
 import type {
 	ExtensionRunner,
+	InputSource,
 	SessionBeforeCompactResult,
 	SessionBeforeForkResult,
 	SessionBeforeSwitchResult,
@@ -101,6 +102,8 @@ export interface PromptOptions {
 	images?: ImageContent[];
 	/** When streaming, how to queue the message: "steer" (interrupt) or "followUp" (wait). Required if streaming. */
 	streamingBehavior?: "steer" | "followUp";
+	/** Source of input for extension input event handlers. Defaults to "interactive". */
+	source?: InputSource;
 }
 
 /** Result from cycleModel() */
@@ -566,8 +569,28 @@ export class AgentSession {
 			}
 		}
 
+		// Emit input event for extension interception (before template expansion)
+		let currentText = text;
+		let currentImages = options?.images;
+		if (this._extensionRunner?.hasHandlers("input")) {
+			const inputResult = await this._extensionRunner.emitInput(
+				currentText,
+				currentImages,
+				options?.source ?? "interactive",
+			);
+			if (inputResult.action === "handled") {
+				return;
+			}
+			if (inputResult.action === "transform") {
+				currentText = inputResult.text;
+				currentImages = inputResult.images ?? currentImages;
+			}
+		}
+
 		// Expand file-based prompt templates if requested
-		const expandedText = expandPromptTemplates ? expandPromptTemplate(text, [...this._promptTemplates]) : text;
+		const expandedText = expandPromptTemplates
+			? expandPromptTemplate(currentText, [...this._promptTemplates])
+			: currentText;
 
 		// If streaming, queue via steer() or followUp() based on option
 		if (this.isStreaming) {
@@ -616,8 +639,8 @@ export class AgentSession {
 
 		// Add user message
 		const userContent: (TextContent | ImageContent)[] = [{ type: "text", text: expandedText }];
-		if (options?.images) {
-			userContent.push(...options.images);
+		if (currentImages) {
+			userContent.push(...currentImages);
 		}
 		messages.push({
 			role: "user",
@@ -635,7 +658,7 @@ export class AgentSession {
 		if (this._extensionRunner) {
 			const result = await this._extensionRunner.emitBeforeAgentStart(
 				expandedText,
-				options?.images,
+				currentImages,
 				this._baseSystemPrompt,
 			);
 			// Add all custom messages from extensions
@@ -853,6 +876,7 @@ export class AgentSession {
 			expandPromptTemplates: false,
 			streamingBehavior: options?.deliverAs,
 			images,
+			source: "extension",
 		});
 	}
 

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

@@ -61,6 +61,10 @@ export type {
 	GetAllToolsHandler,
 	GetThinkingLevelHandler,
 	GrepToolResultEvent,
+	// Events - Input
+	InputEvent,
+	InputEventResult,
+	InputSource,
 	KeybindingsManager,
 	LoadExtensionsResult,
 	LsToolResultEvent,

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

@@ -25,6 +25,9 @@ import type {
 	ExtensionRuntime,
 	ExtensionShortcut,
 	ExtensionUIContext,
+	InputEvent,
+	InputEventResult,
+	InputSource,
 	MessageRenderer,
 	RegisteredCommand,
 	RegisteredTool,
@@ -538,4 +541,35 @@ export class ExtensionRunner {
 
 		return undefined;
 	}
+
+	/** Emit input event. Transforms chain, "handled" short-circuits. */
+	async emitInput(text: string, images: ImageContent[] | undefined, source: InputSource): Promise<InputEventResult> {
+		const ctx = this.createContext();
+		let currentText = text;
+		let currentImages = images;
+
+		for (const ext of this.extensions) {
+			for (const handler of ext.handlers.get("input") ?? []) {
+				try {
+					const event: InputEvent = { type: "input", text: currentText, images: currentImages, source };
+					const result = (await handler(event, ctx)) as InputEventResult | undefined;
+					if (result?.action === "handled") return result;
+					if (result?.action === "transform") {
+						currentText = result.text;
+						currentImages = result.images ?? currentImages;
+					}
+				} catch (err) {
+					this.emitError({
+						extensionPath: ext.path,
+						event: "input",
+						error: err instanceof Error ? err.message : String(err),
+						stack: err instanceof Error ? err.stack : undefined,
+					});
+				}
+			}
+		}
+		return currentText !== text || currentImages !== images
+			? { action: "transform", text: currentText, images: currentImages }
+			: { action: "continue" };
+	}
 }

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

@@ -446,6 +446,30 @@ export interface UserBashEvent {
 	cwd: string;
 }
 
+// ============================================================================
+// Input Events
+// ============================================================================
+
+/** Source of user input */
+export type InputSource = "interactive" | "rpc" | "extension";
+
+/** Fired when user input is received, before agent processing */
+export interface InputEvent {
+	type: "input";
+	/** The input text */
+	text: string;
+	/** Attached images, if any */
+	images?: ImageContent[];
+	/** Where the input came from */
+	source: InputSource;
+}
+
+/** Result from input event handler */
+export type InputEventResult =
+	| { action: "continue" }
+	| { action: "transform"; text: string; images?: ImageContent[] }
+	| { action: "handled" };
+
 // ============================================================================
 // Tool Events
 // ============================================================================
@@ -551,6 +575,7 @@ export type ExtensionEvent =
 	| TurnEndEvent
 	| ModelSelectEvent
 	| UserBashEvent
+	| InputEvent
 	| ToolCallEvent
 	| ToolResultEvent;
 
@@ -675,6 +700,7 @@ export interface ExtensionAPI {
 	on(event: "tool_call", handler: ExtensionHandler<ToolCallEvent, ToolCallEventResult>): void;
 	on(event: "tool_result", handler: ExtensionHandler<ToolResultEvent, ToolResultEventResult>): void;
 	on(event: "user_bash", handler: ExtensionHandler<UserBashEvent, UserBashEventResult>): void;
+	on(event: "input", handler: ExtensionHandler<InputEvent, InputEventResult>): void;
 
 	// =========================================================================
 	// Tool Registration

packages/coding-agent/src/index.ts

@@ -64,6 +64,9 @@ export type {
 	ExtensionShortcut,
 	ExtensionUIContext,
 	ExtensionUIDialogOptions,
+	InputEvent,
+	InputEventResult,
+	InputSource,
 	KeybindingsManager,
 	LoadExtensionsResult,
 	MessageRenderer,

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

@@ -99,7 +99,7 @@ export async function runPrintMode(session: AgentSession, options: PrintModeOpti
 					return { cancelled: result.cancelled };
 				},
 			},
-			// No UI context
+			// No UI context - hasUI will be false
 		);
 		extensionRunner.onError((err) => {
 			console.error(`Extension error (${err.extensionPath}): ${err.error}`);

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

@@ -348,6 +348,7 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 					.prompt(command.message, {
 						images: command.images,
 						streamingBehavior: command.streamingBehavior,
+						source: "rpc",
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");