From 7bd11aa5d3d29ab06551fa19fb96958691c9b695 Mon Sep 17 00:00:00 2001
From: Kyle Mistele <kyle@humanlayer.dev>
Date: Tue, 11 Aug 2026 14:46:50 -0700
Subject: [PATCH] fix(agentlayer): allow short manual compaction

HumanLayer-Session: https://cloud.dev.codelayer.gg/sessions/019feeba-ab18-7035-b0f3-a47b76384add
---
 packages/agentlayer-core/src/agent.ts         | 19 +++++---
 .../agentlayer-core/src/compaction/engine.ts  | 15 +++++++
 .../agentlayer-core/test/compaction.test.ts   | 44 +++++++++++++++++++
 3 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/packages/agentlayer-core/src/agent.ts b/packages/agentlayer-core/src/agent.ts
index 48d4500..d436681 100644
--- a/packages/agentlayer-core/src/agent.ts
+++ b/packages/agentlayer-core/src/agent.ts
@@ -8,6 +8,7 @@ import {
 	COMPACTION_SYSTEM_PROMPT,
 	type CompactionTrigger,
 	compactionSummaryMessage,
+	fitsCompactionTail,
 	isContextOverflowError,
 	parseCompactCommand,
 	planCompaction,
@@ -709,11 +710,19 @@ export class Agent<TTools extends Record<string, Tool<any, any>> = Record<string
 					const messagesWithoutCommand = allMessages.filter(
 						(_, index) => index !== manualCommand.messageIndex,
 					)
-					await applyCompaction(
-						'manual',
-						{ ...buildState(), messages: messagesWithoutCommand },
-						manualCommand.additionalInstructions,
-					)
+					const stateWithoutCommand = { ...buildState(), messages: messagesWithoutCommand }
+					if (
+						fitsCompactionTail(messagesWithoutCommand, {
+							keepRecentTokens: compactionPolicy.keepRecentTokens,
+							requiredToolCallIds: new Set(
+								(stateWithoutCommand.pendingToolCalls ?? []).map((pending) => pending.toolCallId),
+							),
+						})
+					) {
+						allMessages.splice(0, allMessages.length, ...messagesWithoutCommand)
+					} else {
+						await applyCompaction('manual', stateWithoutCommand, manualCommand.additionalInstructions)
+					}
 				} else if (
 					shouldCompactForThreshold({
 						contextWindowTokens: contextWindowTokens > 0 ? contextWindowTokens : undefined,
diff --git a/packages/agentlayer-core/src/compaction/engine.ts b/packages/agentlayer-core/src/compaction/engine.ts
index 337374f..96f6502 100644
--- a/packages/agentlayer-core/src/compaction/engine.ts
+++ b/packages/agentlayer-core/src/compaction/engine.ts
@@ -270,6 +270,21 @@ export function planCompaction(
 	}
 }
 
+/**
+ * Whether a complete, valid conversation already fits in its configured native tail.
+ *
+ * This is distinct from a missing compaction plan caused by malformed tool traffic or
+ * insufficient conversation structure. Manual compaction commands can safely become a
+ * no-op only in this case.
+ */
+export function fitsCompactionTail(messages: ReadonlyArray<ModelMessage>, options: FindCompactionCutOptions): boolean {
+	if (messages.length < 2 || findCompactionCut(messages, options) !== 0) return false
+	if (!hasValidToolCallResultPairs(messages)) return false
+	if (!containsRequiredToolCalls(messages, options.requiredToolCallIds ?? new Set<string>())) return false
+	const totalTokens = messages.reduce((total, message) => total + estimateMessageTokens(message), 0)
+	return totalTokens <= Math.max(1, options.keepRecentTokens)
+}
+
 export const CONTEXT_OVERFLOW_PATTERNS: ReadonlyArray<RegExp> = [
 	/context[_ ]length[_ ]exceeded/i,
 	/model_context_window_exceeded/i,
diff --git a/packages/agentlayer-core/test/compaction.test.ts b/packages/agentlayer-core/test/compaction.test.ts
index fe2042b..7aa475b 100644
--- a/packages/agentlayer-core/test/compaction.test.ts
+++ b/packages/agentlayer-core/test/compaction.test.ts
@@ -532,6 +532,50 @@ describe('automatic loop compaction', () => {
 		}
 	})
 
+	test('consumes bare and instructed compact commands as no-ops when the full conversation fits the retained tail', async () => {
+		for (const command of ['/compact', '/compact Preserve verification commands.']) {
+			const calls: LanguageModelV3CallOptions[] = []
+			const events: AgentEvent[] = []
+			const priorMessages = [
+				userMessage('Short completed request.'),
+				{ role: 'assistant' as const, content: 'Short completed answer.' },
+			]
+			const run = new Agent({
+				model: scriptedModel([{ text: 'normal answer' }], calls),
+				tools: {},
+			}).run({
+				state: startState([...priorMessages, userMessage(command)]),
+				stream: true,
+			})
+			for await (const event of run) events.push(event)
+			const result = await run.result
+
+			expect(result.finishReason).toBe('complete')
+			expect(calls).toHaveLength(1)
+			expect(callContains(calls[0]!, '/compact')).toBe(false)
+			expect(callContains(calls[0]!, 'Short completed request.')).toBe(true)
+			expect(events.filter((event) => event.type === 'compaction')).toHaveLength(0)
+			expect(result.state.compaction).toBeUndefined()
+			expect(result.state.messages.slice(0, -1)).toEqual(priorMessages)
+			expect(result.state.messages.at(-1)).toMatchObject({
+				role: 'assistant',
+				content: [{ type: 'text', text: 'normal answer' }],
+			})
+		}
+	})
+
+	test('keeps the coherent-prefix error for a compact command without prior conversation', async () => {
+		const calls: LanguageModelV3CallOptions[] = []
+		const result = await new Agent({
+			model: scriptedModel([], calls),
+			tools: {},
+		}).run({ state: startState([userMessage('/compact')]) }).result
+
+		expect(result.finishReason).toBe('error')
+		expect(result.error?.message).toBe('Compaction requires a coherent message prefix to summarize.')
+		expect(calls).toHaveLength(0)
+	})
+
 	test('compacts and retries exactly once after context overflow', async () => {
 		const calls: LanguageModelV3CallOptions[] = []
 		const events: AgentEvent[] = []