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

closes #5877

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added estimated post-compaction token counts to compact results and compaction events ([#5877](https://github.com/earendil-works/pi/issues/5877)).
+
 ### Fixed
 
 - Fixed compaction to refuse sessions with no eligible messages instead of producing empty summaries ([#4811](https://github.com/earendil-works/pi/issues/4811)).

packages/coding-agent/docs/rpc.md

@@ -374,11 +374,14 @@ Response:
     "summary": "Summary of conversation...",
     "firstKeptEntryId": "abc123",
     "tokensBefore": 150000,
+    "estimatedTokensAfter": 32000,
     "details": {}
   }
 }
 ```
 
+`estimatedTokensAfter` is a heuristic estimate over the rebuilt message context immediately after compaction, not a provider-exact token count.
+
 #### set_auto_compaction
 
 Enable or disable automatic compaction when context is nearly full.
@@ -924,6 +927,7 @@ The `reason` field is `"manual"`, `"threshold"`, or `"overflow"`.
     "summary": "Summary of conversation...",
     "firstKeptEntryId": "abc123",
     "tokensBefore": 150000,
+    "estimatedTokensAfter": 32000,
     "details": {}
   },
   "aborted": false,

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;
 }

packages/coding-agent/test/suite/agent-session-compaction.test.ts

@@ -5,6 +5,7 @@ import {
 	type Model,
 } from "@earendil-works/pi-ai";
 import { afterEach, describe, expect, it, vi } from "vitest";
+import { estimateTokens } from "../../src/core/compaction/index.ts";
 import { createHarness, type Harness } from "./harness.ts";
 
 type SessionWithCompactionInternals = {
@@ -118,8 +119,10 @@ describe("AgentSession compaction characterization", () => {
 
 		const result = await harness.session.compact();
 		const compactionEntries = harness.sessionManager.getEntries().filter((entry) => entry.type === "compaction");
+		const estimatedTokensAfter = harness.session.messages.reduce((sum, message) => sum + estimateTokens(message), 0);
 
 		expect(result.summary).toBe("summary from extension");
+		expect(result.estimatedTokensAfter).toBe(estimatedTokensAfter);
 		expect(compactionEntries).toHaveLength(1);
 		expect(harness.session.messages[0]?.role).toBe("compactionSummary");
 	});
@@ -161,7 +164,9 @@ describe("AgentSession compaction characterization", () => {
 		await sessionInternals._runAutoCompaction("threshold", false);
 
 		const compactionEntries = harness.sessionManager.getEntries().filter((entry) => entry.type === "compaction");
+		const compactionEnd = harness.eventsOfType("compaction_end").at(-1);
 		expect(compactionEntries).toHaveLength(1);
+		expect(compactionEnd?.result?.estimatedTokensAfter).toBeGreaterThan(0);
 		expect(getStreamCallCount()).toBe(1);
 	});