diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 860266c53..afda49031 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed mnemopi auto-retain storing cumulative full-session transcripts on every retention interval; subsequent retains now store only newly completed user-turn suffixes. ([#4396](https://github.com/can1357/oh-my-pi/issues/4396))
+
 ## [16.3.4] - 2026-07-03
 
 ### Fixed
diff --git a/packages/coding-agent/src/mnemopi/state.ts b/packages/coding-agent/src/mnemopi/state.ts
index da3c384da..8831661d6 100644
--- a/packages/coding-agent/src/mnemopi/state.ts
+++ b/packages/coding-agent/src/mnemopi/state.ts
@@ -116,6 +116,22 @@ interface MnemopiStoredMemoryRow {
 	session_id?: unknown;
 }
 
+type MnemopiRetentionMessage = { role: string; content: string };
+
+function sliceUnretainedMessages(
+	messages: MnemopiRetentionMessage[],
+	lastRetainedTurn: number,
+): MnemopiRetentionMessage[] {
+	if (lastRetainedTurn <= 0) return messages;
+	let userTurns = 0;
+	for (let index = 0; index < messages.length; index++) {
+		if (messages[index].role !== "user") continue;
+		userTurns++;
+		if (userTurns > lastRetainedTurn) return messages.slice(index);
+	}
+	return [];
+}
+
 export function getMnemopiSessionState(session: AgentSession | undefined): MnemopiSessionState | undefined {
 	return session ? (session as AgentSessionWithMnemopiState)[kMnemopiSessionState] : undefined;
 }
@@ -339,7 +355,10 @@ export class MnemopiSessionState {
 		const flat = extractMessages(this.session.sessionManager);
 		const userTurns = flat.filter(message => message.role === "user").length;
 		if (userTurns - this.lastRetainedTurn < this.config.retainEveryNTurns) return;
-		await this.retainMessages(flat, `${this.sessionId}-${Date.now()}`);
+		await this.retainMessages(
+			sliceUnretainedMessages(flat, this.lastRetainedTurn),
+			`${this.sessionId}-${Date.now()}`,
+		);
 		this.lastRetainedTurn = userTurns;
 	}
 
diff --git a/packages/coding-agent/test/memory-tools.test.ts b/packages/coding-agent/test/memory-tools.test.ts
index ffc850e67..8afe7e1b3 100644
--- a/packages/coding-agent/test/memory-tools.test.ts
+++ b/packages/coding-agent/test/memory-tools.test.ts
@@ -450,14 +450,15 @@ describe("Mnemopi backend lifecycle", () => {
 		tempDbPath = undefined;
 	});
 
-	it("auto-retain uses the cumulative transcript turn count", async () => {
+	it("auto-retain stores only the not-yet-retained suffix", async () => {
 		const entries = Array.from({ length: 4 }, (_, index) => ({
 			type: "message",
 			message: { role: "user", content: `turn ${index + 1}` },
 		}));
-		const state = registerMnemopiState(makeMnemopiConfig({ retainEveryNTurns: 4 }), {
+		const state = registerMnemopiState(makeMnemopiConfig({ retainEveryNTurns: 2 }), {
 			cwd: "/work/project-alpha",
 		});
+		state.lastRetainedTurn = 2;
 		(state.session.sessionManager as { getEntries: () => unknown[] }).getEntries = () => entries;
 		const retainSpy = vi.spyOn(state, "retainMessages").mockResolvedValue();
 
@@ -465,8 +466,6 @@ describe("Mnemopi backend lifecycle", () => {
 
 		expect(retainSpy).toHaveBeenCalledTimes(1);
 		expect(retainSpy.mock.calls[0][0]).toEqual([
-			{ role: "user", content: "turn 1" },
-			{ role: "user", content: "turn 2" },
 			{ role: "user", content: "turn 3" },
 			{ role: "user", content: "turn 4" },
 		]);