Improve before_compact hook: add messagesToKeep, replace apiKey with resolveApiKey

packages/coding-agent/CHANGELOG.md

@@ -4,7 +4,10 @@
 
 ### Added
 
-- **Compaction hook `resolveApiKey`**: The `before_compact` session event now includes `resolveApiKey` function to resolve API keys for any model (checks settings, OAuth, env vars). Useful for hooks that need to call different models during custom compaction.
+- **Compaction hook improvements**: The `before_compact` session event now includes:
+  - `messagesToKeep`: Messages that will be kept after the summary (recent turns), in addition to `messagesToSummarize`
+  - `resolveApiKey`: Function to resolve API keys for any model (checks settings, OAuth, env vars)
+  - Removed `apiKey` string in favor of `resolveApiKey` for more flexibility
 
 ## [0.27.5] - 2025-12-24
 

packages/coding-agent/docs/hooks.md

@@ -180,13 +180,15 @@ For `before_branch` and `branch` events, `event.targetTurnIndex` contains the en
 
 For `before_compact` events, additional fields are available:
 - `event.cutPoint`: Where the context will be cut (`firstKeptEntryIndex`, `isSplitTurn`)
-- `event.messagesToSummarize`: Messages that will be summarized
+- `event.messagesToSummarize`: Messages that will be summarized and discarded
+- `event.messagesToKeep`: Messages that will be kept after the summary (recent turns)
 - `event.tokensBefore`: Current context token count
 - `event.model`: Model to use for summarization
-- `event.apiKey`: API key for the model
 - `event.resolveApiKey`: Function to resolve API key for any model (checks settings, OAuth, env vars)
 - `event.customInstructions`: Optional custom focus for summary (from `/compact` command)
 
+To get all messages since the last compaction: `[...event.messagesToSummarize, ...event.messagesToKeep]`
+
 Return `{ compactionEntry }` to provide a custom summary instead of the default. The `compactionEntry` must have: `type: "compaction"`, `timestamp`, `summary`, `firstKeptEntryIndex` (from `cutPoint`), `tokensBefore`.
 
 For `compact` events (after compaction):

packages/coding-agent/examples/hooks/full-compaction.ts

@@ -3,8 +3,8 @@
  *
  * Replaces the default compaction behavior with a full summary of the entire context.
  * Instead of keeping the last 20k tokens of conversation turns, this hook:
- * 1. Summarizes ALL messages being compacted into a single comprehensive summary
+ * 1. Summarizes ALL messages (both messagesToSummarize and messagesToKeep)
- * 2. Discards all old turns completely
+ * 2. Discards all old turns completely, keeping only the summary
  *
  * This is useful when you want maximum context window space for new work
  * at the cost of losing exact conversation history.
@@ -21,9 +21,12 @@ export default function (pi: HookAPI) {
 	pi.on("session", async (event, ctx) => {
 		if (event.reason !== "before_compact") return;
 
-		const { messagesToSummarize, tokensBefore, model, resolveApiKey, cutPoint } = event;
+		const { messagesToSummarize, messagesToKeep, tokensBefore, model, resolveApiKey, entries } = event;
 
-		ctx.ui.notify(`Compacting ${tokensBefore.toLocaleString()} tokens with full summary...`, "info");
+		// Combine all messages for full summary
+		const allMessages = [...messagesToSummarize, ...messagesToKeep];
+
+		ctx.ui.notify(`Full compaction: summarizing ${allMessages.length} messages (${tokensBefore.toLocaleString()} tokens)...`, "info");
 
 		// Resolve API key for the model
 		const apiKey = await resolveApiKey(model);
@@ -33,7 +36,7 @@ export default function (pi: HookAPI) {
 		}
 
 		// Transform app messages to LLM-compatible format
-		const transformedMessages = messageTransformer(messagesToSummarize);
+		const transformedMessages = messageTransformer(allMessages);
 
 		// Build messages that ask for a comprehensive summary
 		const summaryMessages = [
@@ -43,7 +46,7 @@ export default function (pi: HookAPI) {
 				content: [
 					{
 						type: "text" as const,
-						text: `You are a conversation summarizer. Create a comprehensive summary of this conversation that captures:
+						text: `You are a conversation summarizer. Create a comprehensive summary of this entire conversation that captures:
 
 1. The main goals and objectives discussed
 2. Key decisions made and their rationale
@@ -52,7 +55,7 @@ export default function (pi: HookAPI) {
 5. Any blockers, issues, or open questions
 6. Next steps that were planned or suggested
 
-Be thorough but concise. The summary will replace the entire conversation history, so include all information needed to continue the work effectively.
+Be thorough but concise. The summary will replace the ENTIRE conversation history, so include all information needed to continue the work effectively.
 
 Format the summary as structured markdown with clear sections.`,
 					},
@@ -75,14 +78,14 @@ Format the summary as structured markdown with clear sections.`,
 				return; // Fall back to default compaction
 			}
 
-			// Return a compaction entry that discards ALL old messages
+			// Return a compaction entry that discards ALL messages
-			// firstKeptEntryIndex points to after all summarized content
+			// firstKeptEntryIndex points past all current entries
 			return {
 				compactionEntry: {
 					type: "compaction" as const,
 					timestamp: new Date().toISOString(),
 					summary,
-					firstKeptEntryIndex: cutPoint.firstKeptEntryIndex,
+					firstKeptEntryIndex: entries.length,
 					tokensBefore,
 				},
 			};

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

@@ -767,11 +767,11 @@ export class AgentSession {
 					previousSessionFile: null,
 					reason: "before_compact",
 					cutPoint: preparation.cutPoint,
-					messagesToSummarize: preparation.messagesToSummarize,
+					messagesToSummarize: [...preparation.messagesToSummarize],
+					messagesToKeep: [...preparation.messagesToKeep],
 					tokensBefore: preparation.tokensBefore,
 					customInstructions,
 					model: this.model,
-					apiKey,
 					resolveApiKey: this._resolveApiKey,
 				})) as SessionEventResult | undefined;
 
@@ -918,11 +918,11 @@ export class AgentSession {
 					previousSessionFile: null,
 					reason: "before_compact",
 					cutPoint: preparation.cutPoint,
-					messagesToSummarize: preparation.messagesToSummarize,
+					messagesToSummarize: [...preparation.messagesToSummarize],
+					messagesToKeep: [...preparation.messagesToKeep],
 					tokensBefore: preparation.tokensBefore,
 					customInstructions: undefined,
 					model: this.model,
-					apiKey,
 					resolveApiKey: this._resolveApiKey,
 				})) as SessionEventResult | undefined;
 

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

@@ -327,7 +327,10 @@ export async function generateSummary(
 
 export interface CompactionPreparation {
 	cutPoint: CutPointResult;
+	/** Messages that will be summarized and discarded */
 	messagesToSummarize: AppMessage[];
+	/** Messages that will be kept after the summary (recent turns) */
+	messagesToKeep: AppMessage[];
 	tokensBefore: number;
 	boundaryStart: number;
 }
@@ -353,6 +356,8 @@ export function prepareCompaction(entries: SessionEntry[], settings: CompactionS
 	const cutPoint = findCutPoint(entries, boundaryStart, boundaryEnd, settings.keepRecentTokens);
 
 	const historyEnd = cutPoint.isSplitTurn ? cutPoint.turnStartIndex : cutPoint.firstKeptEntryIndex;
+
+	// Messages to summarize (will be discarded after summary)
 	const messagesToSummarize: AppMessage[] = [];
 	for (let i = boundaryStart; i < historyEnd; i++) {
 		const entry = entries[i];
@@ -361,7 +366,16 @@ export function prepareCompaction(entries: SessionEntry[], settings: CompactionS
 		}
 	}
 
-	return { cutPoint, messagesToSummarize, tokensBefore, boundaryStart };
+	// Messages to keep (recent turns, kept after summary)
+	const messagesToKeep: AppMessage[] = [];
+	for (let i = cutPoint.firstKeptEntryIndex; i < boundaryEnd; i++) {
+		const entry = entries[i];
+		if (entry.type === "message") {
+			messagesToKeep.push(entry.message);
+		}
+	}
+
+	return { cutPoint, messagesToSummarize, messagesToKeep, tokensBefore, boundaryStart };
 }
 
 // ============================================================================

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

@@ -130,11 +130,13 @@ export type SessionEvent =
 	| (SessionEventBase & {
 			reason: "before_compact";
 			cutPoint: CutPointResult;
+			/** Messages that will be summarized and discarded */
 			messagesToSummarize: AppMessage[];
+			/** Messages that will be kept after the summary (recent turns) */
+			messagesToKeep: AppMessage[];
 			tokensBefore: number;
 			customInstructions?: string;
 			model: Model<any>;
-			apiKey: string;
 			/** Resolve API key for any model (checks settings, OAuth, env vars) */
 			resolveApiKey: (model: Model<any>) => Promise<string | undefined>;
 	  })

packages/coding-agent/test/compaction-hooks-example.test.ts

@@ -15,17 +15,19 @@ describe("Documentation example", () => {
 
 				// After narrowing, these should all be accessible
 				const messages = event.messagesToSummarize;
+				const messagesToKeep = event.messagesToKeep;
 				const cutPoint = event.cutPoint;
 				const tokensBefore = event.tokensBefore;
 				const model = event.model;
-				const apiKey = event.apiKey;
+				const resolveApiKey = event.resolveApiKey;
 
 				// Verify types
 				expect(Array.isArray(messages)).toBe(true);
+				expect(Array.isArray(messagesToKeep)).toBe(true);
 				expect(typeof cutPoint.firstKeptEntryIndex).toBe("number");
 				expect(typeof tokensBefore).toBe("number");
 				expect(model).toBeDefined();
-				expect(typeof apiKey).toBe("string");
+				expect(typeof resolveApiKey).toBe("function");
 
 				const summary = messages
 					.filter((m) => m.role === "user")

packages/coding-agent/test/compaction-hooks.test.ts

@@ -129,9 +129,10 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			expect(beforeEvent.cutPoint).toBeDefined();
 			expect(beforeEvent.cutPoint.firstKeptEntryIndex).toBeGreaterThanOrEqual(0);
 			expect(beforeEvent.messagesToSummarize).toBeDefined();
+			expect(beforeEvent.messagesToKeep).toBeDefined();
 			expect(beforeEvent.tokensBefore).toBeGreaterThanOrEqual(0);
 			expect(beforeEvent.model).toBeDefined();
-			expect(beforeEvent.apiKey).toBeDefined();
+			expect(beforeEvent.resolveApiKey).toBeDefined();
 		}
 
 		const afterEvent = compactEvents[0];
@@ -336,14 +337,14 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 		expect(event.cutPoint).toHaveProperty("turnStartIndex");
 
 		expect(Array.isArray(event.messagesToSummarize)).toBe(true);
+		expect(Array.isArray(event.messagesToKeep)).toBe(true);
 
 		expect(typeof event.tokensBefore).toBe("number");
 
 		expect(event.model).toHaveProperty("provider");
 		expect(event.model).toHaveProperty("id");
 
-		expect(typeof event.apiKey).toBe("string");
+		expect(typeof event.resolveApiKey).toBe("function");
-		expect(event.apiKey.length).toBeGreaterThan(0);
 
 		expect(Array.isArray(event.entries)).toBe(true);
 		expect(event.entries.length).toBeGreaterThan(0);