feat: Expose BuildSystemPromptOptions on before-agent-start event. (#3473)

Resolves: #3463

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

@@ -36,6 +36,7 @@ export type {
 	BeforeAgentStartEventResult,
 	BeforeProviderRequestEvent,
 	BeforeProviderRequestEventResult,
+	BuildSystemPromptOptions,
 	// Context
 	CompactOptions,
 	// Events - Agent

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

@@ -10,6 +10,7 @@ import type { ResourceDiagnostic } from "../diagnostics.js";
 import type { KeybindingsConfig } from "../keybindings.js";
 import type { ModelRegistry } from "../model-registry.js";
 import type { SessionManager } from "../session-manager.js";
+import type { BuildSystemPromptOptions } from "../system-prompt.js";
 import type {
 	BeforeAgentStartEvent,
 	BeforeAgentStartEventResult,
@@ -789,6 +790,7 @@ export class ExtensionRunner {
 		prompt: string,
 		images: ImageContent[] | undefined,
 		systemPrompt: string,
+		systemPromptOptions: BuildSystemPromptOptions,
 	): Promise<BeforeAgentStartCombinedResult | undefined> {
 		const ctx = this.createContext();
 		const messages: NonNullable<BeforeAgentStartEventResult["message"]>[] = [];
@@ -806,6 +808,7 @@ export class ExtensionRunner {
 						prompt,
 						images,
 						systemPrompt: currentSystemPrompt,
+						systemPromptOptions,
 					};
 					const handlerResult = await handler(event, ctx);
 

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

@@ -57,6 +57,7 @@ import type {
 } from "../session-manager.js";
 import type { SlashCommandInfo } from "../slash-commands.js";
 import type { SourceInfo } from "../source-info.js";
+import type { BuildSystemPromptOptions } from "../system-prompt.js";
 import type { BashOperations } from "../tools/bash.js";
 import type { EditToolDetails } from "../tools/edit.js";
 import type {
@@ -75,6 +76,7 @@ import type {
 } from "../tools/index.js";
 
 export type { ExecOptions, ExecResult } from "../exec.js";
+export type { BuildSystemPromptOptions } from "../system-prompt.js";
 export type { AgentToolResult, AgentToolUpdateCallback, ToolExecutionMode };
 export type { AppKeybinding, KeybindingsManager } from "../keybindings.js";
 
@@ -582,9 +584,14 @@ export interface AfterProviderResponseEvent {
 /** Fired after user submits prompt but before agent loop. */
 export interface BeforeAgentStartEvent {
 	type: "before_agent_start";
+	/** The raw user prompt text (after expansion). */
 	prompt: string;
+	/** Images attached to the user prompt, if any. */
 	images?: ImageContent[];
+	/** The fully assembled system prompt string. */
 	systemPrompt: string;
+	/** Structured options used to build the system prompt. Extensions can inspect this to understand what Pi loaded without re-discovering resources. */
+	systemPromptOptions: BuildSystemPromptOptions;
 }
 
 /** Fired when an agent loop starts */