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/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