From 115f79a82ee6dc50fae66257f78b6664fa391ecc Mon Sep 17 00:00:00 2001
From: PierrunoYT <pierrebruno@hotmail.ch>
Date: Mon, 28 Sep 2026 20:16:15 +0200
Subject: [PATCH] fix(condense): summarize effective history, excluding
 truncated messages

summarizeConversation selected its input with getMessagesSinceLastSummary,
which ignores truncationParent. Messages already hidden by sliding-window
truncation were sent to the condensing request, which could exceed the
model context window even though normal requests still fit.

Use getEffectiveApiHistory, the same selection normal requests use. The
stored history is unchanged; all messages are still tagged with
condenseParent.

Fixes #1823

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 src/core/condense/__tests__/condense.spec.ts | 41 ++++++++++++++++++++
 src/core/condense/index.ts                   |  6 ++-
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/src/core/condense/__tests__/condense.spec.ts b/src/core/condense/__tests__/condense.spec.ts
index c209fa9724..45a725d29f 100644
--- a/src/core/condense/__tests__/condense.spec.ts
+++ b/src/core/condense/__tests__/condense.spec.ts
@@ -186,6 +186,47 @@ Line 2
 			}
 		})
 
+		it("should not send messages hidden by sliding-window truncation to the summarizer", async () => {
+			const truncationId = "trunc-1"
+			const messages: ApiMessage[] = [
+				{ role: "user", content: "Hidden first", truncationParent: truncationId },
+				{ role: "assistant", content: "Hidden second", truncationParent: truncationId },
+				{ role: "assistant", content: "Truncation marker", isTruncationMarker: true, truncationId },
+				{ role: "user", content: "Visible third" },
+				{ role: "assistant", content: "Visible fourth" },
+				{ role: "user", content: "Visible fifth" },
+			]
+			const original = structuredClone(messages)
+			const sentMessages: unknown[] = []
+			const createMessageSpy = vi
+				.spyOn(mockApiHandler, "createMessage")
+				.mockImplementation((...args: unknown[]) => {
+					sentMessages.push(args[1])
+					return (async function* () {
+						yield { type: "text", text: "Mock summary" }
+					})()
+				})
+
+			const result = await summarizeConversation({
+				messages,
+				apiHandler: mockApiHandler,
+				systemPrompt: "System prompt",
+				taskId,
+				isAutomaticTrigger: false,
+			})
+
+			const sentText = JSON.stringify(sentMessages[0])
+			expect(sentText).not.toContain("Hidden first")
+			expect(sentText).not.toContain("Hidden second")
+			expect(sentText).toContain("Visible third")
+
+			// Complete stored history is preserved (non-destructive) and the input is not mutated
+			expect(result.messages.filter((msg) => !msg.isSummary)).toHaveLength(messages.length)
+			expect(messages).toEqual(original)
+
+			createMessageSpy.mockRestore()
+		})
+
 		it("should preserve <command> blocks in the summary", async () => {
 			const messages: ApiMessage[] = [
 				{
diff --git a/src/core/condense/index.ts b/src/core/condense/index.ts
index 0438bf6bcb..920f047d44 100644
--- a/src/core/condense/index.ts
+++ b/src/core/condense/index.ts
@@ -275,8 +275,10 @@ export async function summarizeConversation(options: SummarizeConversationOption
 
 	const response: SummarizeResponse = { messages, cost: 0, summary: "" }
 
-	// Get messages to summarize (all messages since the last summary, if any)
-	const messagesToSummarize = getMessagesSinceLastSummary(messages)
+	// Summarize the effective history (what normal requests send): excludes messages already hidden
+	// by sliding-window truncation or a previous condense, and starts at the last summary if any.
+	// The stored history in `messages` is left intact and tagged below.
+	const messagesToSummarize = getEffectiveApiHistory(messages)
 
 	if (messagesToSummarize.length <= 1) {
 		const error =