feat(coding-agent): expose post-compaction token estimates

closes #5877

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

@@ -45,6 +45,7 @@ import {
 	collectEntriesForBranchSummary,
 	compact,
 	estimateContextTokens,
+	estimateTokens,
 	generateBranchSummary,
 	prepareCompaction,
 	shouldCompact,
@@ -242,6 +243,14 @@ interface ToolDefinitionEntry {
 	sourceInfo: SourceInfo;
 }
 
+function estimateMessagesTokens(messages: AgentMessage[]): number {
+	let tokens = 0;
+	for (const message of messages) {
+		tokens += estimateTokens(message);
+	}
+	return tokens;
+}
+
 // ============================================================================
 // Constants
 // ============================================================================
@@ -1726,6 +1735,7 @@ export class AgentSession {
 			const newEntries = this.sessionManager.getEntries();
 			const sessionContext = this.sessionManager.buildSessionContext();
 			this.agent.state.messages = sessionContext.messages;
+			const estimatedTokensAfter = estimateMessagesTokens(sessionContext.messages);
 
 			// Get the saved compaction entry for the extension event
 			const savedCompactionEntry = newEntries.find((e) => e.type === "compaction" && e.summary === summary) as
@@ -1740,10 +1750,11 @@ export class AgentSession {
 				});
 			}
 
-			const compactionResult = {
+			const compactionResult: CompactionResult = {
 				summary,
 				firstKeptEntryId,
 				tokensBefore,
+				estimatedTokensAfter,
 				details,
 			};
 			this._emit({
@@ -1999,6 +2010,7 @@ export class AgentSession {
 			const newEntries = this.sessionManager.getEntries();
 			const sessionContext = this.sessionManager.buildSessionContext();
 			this.agent.state.messages = sessionContext.messages;
+			const estimatedTokensAfter = estimateMessagesTokens(sessionContext.messages);
 
 			// Get the saved compaction entry for the extension event
 			const savedCompactionEntry = newEntries.find((e) => e.type === "compaction" && e.summary === summary) as
@@ -2017,6 +2029,7 @@ export class AgentSession {
 				summary,
 				firstKeptEntryId,
 				tokensBefore,
+				estimatedTokensAfter,
 				details,
 			};
 			this._emit({ type: "compaction_end", reason, result, aborted: false, willRetry });

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

@@ -104,6 +104,7 @@ export interface CompactionResult<T = unknown> {
 	summary: string;
 	firstKeptEntryId: string;
 	tokensBefore: number;
+	estimatedTokensAfter?: number;
 	/** Extension-specific data (e.g., ArtifactIndex, version markers for structured compaction) */
 	details?: T;
 }