Add context event for non-destructive message modification before LLM calls

- Add contextTransform option to Agent (runs before messageTransformer)
- Deep copy messages before passing to contextTransform (modifications are ephemeral)
- Add ContextEvent and ContextEventResult types
- Add emitContext() to HookRunner (chains multiple handlers)
- Wire up in sdk.ts when creating Agent with hooks

Enables dynamic context pruning: hooks can modify messages sent to LLM
without changing session data. See discussion #330.

packages/coding-agent/docs/session-tree-plan.md

@@ -227,7 +227,7 @@ interface HookUIContext {
 
 See also: `CustomEntry<T>` for storing hook state that does NOT participate in context.
 
-**New: `context` event (TODO)**
+**New: `context` event ✅**
 
 Fires before messages are sent to the LLM, allowing hooks to modify context non-destructively.
 

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

@@ -13,6 +13,8 @@ export type {
 	AgentEndEvent,
 	AgentStartEvent,
 	BashToolResultEvent,
+	ContextEvent,
+	ContextEventResult,
 	CustomToolResultEvent,
 	EditToolResultEvent,
 	ExecOptions,

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

@@ -2,10 +2,13 @@
  * Hook runner - executes hooks and manages their lifecycle.
  */
 
+import type { AppMessage } from "@mariozechner/pi-agent-core";
 import type { ModelRegistry } from "../model-registry.js";
 import type { SessionManager } from "../session-manager.js";
 import type { AppendEntryHandler, LoadedHook, SendMessageHandler } from "./loader.js";
 import type {
+	ContextEvent,
+	ContextEventResult,
 	HookError,
 	HookEvent,
 	HookEventContext,
@@ -304,4 +307,43 @@ export class HookRunner {
 
 		return result;
 	}
+
+	/**
+	 * 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.
+	 */
+	async emitContext(messages: AppMessage[]): Promise<AppMessage[] | undefined> {
+		const ctx = this.createContext();
+		let currentMessages = messages;
+		let modified = false;
+
+		for (const hook of this.hooks) {
+			const handlers = hook.handlers.get("context");
+			if (!handlers || handlers.length === 0) continue;
+
+			for (const handler of handlers) {
+				try {
+					const event: ContextEvent = { type: "context", messages: currentMessages };
+					const timeout = createTimeout(this.timeout);
+					const handlerResult = await Promise.race([handler(event, ctx), timeout.promise]);
+					timeout.clear();
+
+					if (handlerResult && (handlerResult as ContextEventResult).messages) {
+						currentMessages = (handlerResult as ContextEventResult).messages!;
+						modified = true;
+					}
+				} catch (err) {
+					const message = err instanceof Error ? err.message : String(err);
+					this.emitError({
+						hookPath: hook.path,
+						event: "context",
+						error: message,
+					});
+				}
+			}
+		}
+
+		return modified ? currentMessages : undefined;
+	}
 }

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

@@ -146,6 +146,17 @@ export type SessionEvent =
 			fromHook: boolean;
 	  });
 
+/**
+ * Event data for context event.
+ * Fired before messages are sent to the LLM, allowing hooks to modify context non-destructively.
+ * Original session messages are NOT modified - only the messages sent to the LLM are affected.
+ */
+export interface ContextEvent {
+	type: "context";
+	/** Messages about to be sent to the LLM */
+	messages: AppMessage[];
+}
+
 /**
  * Event data for agent_start event.
  * Fired when an agent loop starts (once per user prompt).
@@ -301,6 +312,7 @@ export function isLsToolResult(e: ToolResultEvent): e is LsToolResultEvent {
  */
 export type HookEvent =
 	| SessionEvent
+	| ContextEvent
 	| AgentStartEvent
 	| AgentEndEvent
 	| TurnStartEvent
@@ -312,6 +324,15 @@ export type HookEvent =
 // Event Results
 // ============================================================================
 
+/**
+ * Return type for context event handlers.
+ * Allows hooks to modify messages before they're sent to the LLM.
+ */
+export interface ContextEventResult {
+	/** Modified messages to send instead of the original */
+	messages?: AppMessage[];
+}
+
 /**
  * Return type for tool_call event handlers.
  * Allows hooks to block tool execution.
@@ -417,6 +438,8 @@ export interface RegisteredCommand {
 export interface HookAPI {
 	// biome-ignore lint/suspicious/noConfusingVoidType: void allows handlers to not return anything
 	on(event: "session", handler: HookHandler<SessionEvent, SessionEventResult | void>): void;
+	// biome-ignore lint/suspicious/noConfusingVoidType: void allows handlers to not return anything
+	on(event: "context", handler: HookHandler<ContextEvent, ContextEventResult | void>): void;
 	on(event: "agent_start", handler: HookHandler<AgentStartEvent>): void;
 	on(event: "agent_end", handler: HookHandler<AgentEndEvent>): void;
 	on(event: "turn_start", handler: HookHandler<TurnStartEvent>): void;

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

@@ -589,6 +589,11 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: allToolsArray,
 		},
 		messageTransformer,
+		contextTransform: hookRunner
+			? async (messages) => {
+					return hookRunner.emitContext(messages);
+				}
+			: undefined,
 		queueMode: settingsManager.getQueueMode(),
 		transport: new ProviderTransport({
 			getApiKey: async () => {