Add previousSummary to before_compact hook event

packages/coding-agent/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Added
 
 - **Compaction hook improvements**: The `before_compact` session event now includes:
+  - `previousSummary`: Summary from the last compaction (if any), so hooks can preserve accumulated context
   - `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

packages/coding-agent/docs/hooks.md

@@ -194,6 +194,7 @@ The `before_compact` event lets you implement custom compaction strategies. Unde
 |-------|-------------|
 | `entries` | All session entries (header, messages, model changes, previous compactions). Use this for custom schemes that need full session history. |
 | `cutPoint` | Where default compaction would cut. `firstKeptEntryIndex` is the entry index where kept messages start. `isSplitTurn` indicates if cutting mid-turn. |
+| `previousSummary` | Summary from the last compaction, if any. Include this in your summary to preserve accumulated context. |
 | `messagesToSummarize` | Messages that will be summarized and discarded (from after last compaction to cut point). |
 | `messagesToKeep` | Messages that will be kept verbatim after the summary (from cut point to end). |
 | `tokensBefore` | Current context token count (why compaction triggered). |

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

@@ -21,7 +21,7 @@ export default function (pi: HookAPI) {
 	pi.on("session", async (event, ctx) => {
 		if (event.reason !== "before_compact") return;
 
-		const { messagesToSummarize, messagesToKeep, tokensBefore, model, resolveApiKey, entries } = event;
+		const { messagesToSummarize, messagesToKeep, previousSummary, tokensBefore, model, resolveApiKey, entries } = event;
 
 		// Combine all messages for full summary
 		const allMessages = [...messagesToSummarize, ...messagesToKeep];
@@ -38,6 +38,9 @@ export default function (pi: HookAPI) {
 		// Transform app messages to LLM-compatible format
 		const transformedMessages = messageTransformer(allMessages);
 
+		// Include previous summary context if available
+		const previousContext = previousSummary ? `\n\nPrevious session summary for context:\n${previousSummary}` : "";
+
 		// Build messages that ask for a comprehensive summary
 		const summaryMessages = [
 			...transformedMessages,
@@ -46,7 +49,7 @@ export default function (pi: HookAPI) {
 				content: [
 					{
 						type: "text" as const,
-						text: `You are a conversation summarizer. Create a comprehensive summary of this entire conversation that captures:
+						text: `You are a conversation summarizer. Create a comprehensive summary of this entire conversation that captures:${previousContext}
 
 1. The main goals and objectives discussed
 2. Key decisions made and their rationale

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

@@ -756,6 +756,15 @@ export class AgentSession {
 				throw new Error("Already compacted");
 			}
 
+			// Find previous compaction summary if any
+			let previousSummary: string | undefined;
+			for (let i = entries.length - 1; i >= 0; i--) {
+				if (entries[i].type === "compaction") {
+					previousSummary = (entries[i] as CompactionEntry).summary;
+					break;
+				}
+			}
+
 			let compactionEntry: CompactionEntry | undefined;
 			let fromHook = false;
 
@@ -767,6 +776,7 @@ export class AgentSession {
 					previousSessionFile: null,
 					reason: "before_compact",
 					cutPoint: preparation.cutPoint,
+					previousSummary,
 					messagesToSummarize: [...preparation.messagesToSummarize],
 					messagesToKeep: [...preparation.messagesToKeep],
 					tokensBefore: preparation.tokensBefore,
@@ -907,6 +917,15 @@ export class AgentSession {
 				return;
 			}
 
+			// Find previous compaction summary if any
+			let previousSummary: string | undefined;
+			for (let i = entries.length - 1; i >= 0; i--) {
+				if (entries[i].type === "compaction") {
+					previousSummary = (entries[i] as CompactionEntry).summary;
+					break;
+				}
+			}
+
 			let compactionEntry: CompactionEntry | undefined;
 			let fromHook = false;
 
@@ -918,6 +937,7 @@ export class AgentSession {
 					previousSessionFile: null,
 					reason: "before_compact",
 					cutPoint: preparation.cutPoint,
+					previousSummary,
 					messagesToSummarize: [...preparation.messagesToSummarize],
 					messagesToKeep: [...preparation.messagesToKeep],
 					tokensBefore: preparation.tokensBefore,

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

@@ -130,6 +130,8 @@ export type SessionEvent =
 	| (SessionEventBase & {
 			reason: "before_compact";
 			cutPoint: CutPointResult;
+			/** Summary from previous compaction, if any. Include this in your summary to preserve context. */
+			previousSummary?: string;
 			/** Messages that will be summarized and discarded */
 			messagesToSummarize: AppMessage[];
 			/** Messages that will be kept after the summary (recent turns) */