Wire context event to preprocessor for per-LLM-call execution

- Change from contextTransform (runs once at agent start) to preprocessor
- preprocessor runs before EACH LLM call inside the agent loop
- ContextEvent now uses Message[] (pi-ai format) instead of AppMessage[]
- Deep copy handled by pi-ai preprocessor, not Agent

This enables:
- Pruning rules applied on every turn (not just agent start)
- /prune during long agent loop takes effect immediately
- Compaction can use same transforms (future work)

packages/agent/src/agent.ts

@@ -55,8 +55,8 @@ export interface AgentOptions {
 	transport: AgentTransport;
 	// Transform app messages to LLM-compatible messages before sending to transport
 	messageTransformer?: (messages: AppMessage[]) => Message[] | Promise<Message[]>;
-	// Called before messageTransformer - can modify messages before they're sent to LLM (non-destructive)
+	// Called before each LLM call inside the agent loop - can modify messages (e.g., for pruning)
-	contextTransform?: (messages: AppMessage[]) => Promise<AppMessage[] | undefined>;
+	preprocessor?: (messages: Message[]) => Promise<Message[]>;
 	// Queue mode: "all" = send all queued messages at once, "one-at-a-time" = send one queued message per turn
 	queueMode?: "all" | "one-at-a-time";
 }
@@ -77,7 +77,7 @@ export class Agent {
 	private abortController?: AbortController;
 	private transport: AgentTransport;
 	private messageTransformer: (messages: AppMessage[]) => Message[] | Promise<Message[]>;
-	private contextTransform?: (messages: AppMessage[]) => Promise<AppMessage[] | undefined>;
+	private preprocessor?: (messages: Message[]) => Promise<Message[]>;
 	private messageQueue: Array<QueuedMessage<AppMessage>> = [];
 	private queueMode: "all" | "one-at-a-time";
 	private runningPrompt?: Promise<void>;
@@ -87,7 +87,7 @@ export class Agent {
 		this._state = { ...this._state, ...opts.initialState };
 		this.transport = opts.transport;
 		this.messageTransformer = opts.messageTransformer || defaultMessageTransformer;
-		this.contextTransform = opts.contextTransform;
+		this.preprocessor = opts.preprocessor;
 		this.queueMode = opts.queueMode || "one-at-a-time";
 	}
 
@@ -286,6 +286,7 @@ export class Agent {
 			tools: this._state.tools,
 			model,
 			reasoning,
+			preprocessor: this.preprocessor,
 			getQueuedMessages: async <T>() => {
 				if (this.queueMode === "one-at-a-time") {
 					if (this.messageQueue.length > 0) {
@@ -302,18 +303,7 @@ export class Agent {
 			},
 		};
 
-		// Apply context transform (hooks can modify messages non-destructively)
+		const llmMessages = await this.messageTransformer(this._state.messages);
-		// Deep copy so modifications don't affect the original state
-		let messagesToSend = this._state.messages;
-		if (this.contextTransform) {
-			const messagesCopy = JSON.parse(JSON.stringify(messagesToSend)) as AppMessage[];
-			const transformed = await this.contextTransform(messagesCopy);
-			if (transformed) {
-				messagesToSend = transformed;
-			}
-		}
-
-		const llmMessages = await this.messageTransformer(messagesToSend);
 
 		return { llmMessages, cfg, model };
 	}

packages/agent/src/transports/ProviderTransport.ts

@@ -60,6 +60,7 @@ export class ProviderTransport implements AgentTransport {
 			// Resolve API key per assistant response (important for expiring OAuth tokens)
 			getApiKey: this.options.getApiKey,
 			getQueuedMessages: cfg.getQueuedMessages,
+			preprocessor: cfg.preprocessor,
 		};
 	}
 

packages/agent/src/transports/types.ts

@@ -9,6 +9,8 @@ export interface AgentRunConfig {
 	model: Model<any>;
 	reasoning?: ReasoningEffort;
 	getQueuedMessages?: <T>() => Promise<QueuedMessage<T>[]>;
+	/** Called before each LLM call - can modify messages (e.g., for pruning) */
+	preprocessor?: (messages: Message[]) => Promise<Message[]>;
 }
 
 /**

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

@@ -2,7 +2,7 @@
  * Hook runner - executes hooks and manages their lifecycle.
  */
 
-import type { AppMessage } from "@mariozechner/pi-agent-core";
+import type { Message } from "@mariozechner/pi-ai";
 import type { ModelRegistry } from "../model-registry.js";
 import type { SessionManager } from "../session-manager.js";
 import type { AppendEntryHandler, LoadedHook, SendMessageHandler } from "./loader.js";
@@ -311,12 +311,13 @@ export class HookRunner {
 	/**
 	 * Emit a context event to all hooks.
 	 * Handlers are chained - each gets the previous handler's output (if any).
-	 * Returns the final modified messages, or undefined if no modifications.
+	 * Returns the final modified messages, or the original if no modifications.
+	 *
+	 * Note: Messages are already deep-copied by the caller (pi-ai preprocessor).
 	 */
-	async emitContext(messages: AppMessage[]): Promise<AppMessage[] | undefined> {
+	async emitContext(messages: Message[]): Promise<Message[]> {
 		const ctx = this.createContext();
 		let currentMessages = messages;
-		let modified = false;
 
 		for (const hook of this.hooks) {
 			const handlers = hook.handlers.get("context");
@@ -331,7 +332,6 @@ export class HookRunner {
 
 					if (handlerResult && (handlerResult as ContextEventResult).messages) {
 						currentMessages = (handlerResult as ContextEventResult).messages!;
-						modified = true;
 					}
 				} catch (err) {
 					const message = err instanceof Error ? err.message : String(err);
@@ -344,6 +344,6 @@ export class HookRunner {
 			}
 		}
 
-		return modified ? currentMessages : undefined;
+		return currentMessages;
 	}
 }

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

@@ -6,7 +6,7 @@
  */
 
 import type { AppMessage } from "@mariozechner/pi-agent-core";
-import type { ImageContent, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
+import type { ImageContent, Message, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
 import type { Component } from "@mariozechner/pi-tui";
 import type { Theme } from "../../modes/interactive/theme/theme.js";
 import type { CompactionPreparation, CompactionResult } from "../compaction.js";
@@ -148,13 +148,14 @@ export type SessionEvent =
 
 /**
  * Event data for context event.
- * Fired before messages are sent to the LLM, allowing hooks to modify context non-destructively.
+ * Fired before each LLM call, allowing hooks to modify context non-destructively.
  * Original session messages are NOT modified - only the messages sent to the LLM are affected.
+ * Messages are already in LLM format (Message[], not AppMessage[]).
  */
 export interface ContextEvent {
 	type: "context";
-	/** Messages about to be sent to the LLM */
+	/** Messages about to be sent to the LLM (deep copy, safe to modify) */
-	messages: AppMessage[];
+	messages: Message[];
 }
 
 /**
@@ -330,7 +331,7 @@ export type HookEvent =
  */
 export interface ContextEventResult {
 	/** Modified messages to send instead of the original */
-	messages?: AppMessage[];
+	messages?: Message[];
 }
 
 /**

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

@@ -589,7 +589,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: allToolsArray,
 		},
 		messageTransformer,
-		contextTransform: hookRunner
+		preprocessor: hookRunner
 			? async (messages) => {
 					return hookRunner.emitContext(messages);
 				}