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 =