diff --git a/packages/coding-agent/test/agent-session-message-pipeline.test.ts b/packages/coding-agent/test/agent-session-message-pipeline.test.ts
index 96ba0e79e..4c1d4db3a 100644
--- a/packages/coding-agent/test/agent-session-message-pipeline.test.ts
+++ b/packages/coding-agent/test/agent-session-message-pipeline.test.ts
@@ -14,12 +14,15 @@ import {
 } from "@oh-my-pi/pi-ai";
 import { AssistantMessageEventStream } from "@oh-my-pi/pi-ai/utils/event-stream";
 import { buildModel } from "@oh-my-pi/pi-catalog/build";
+import { ModelRegistry } from "@oh-my-pi/pi-coding-agent/config/model-registry";
 import { Settings } from "@oh-my-pi/pi-coding-agent/config/settings";
 import * as memoryBackend from "@oh-my-pi/pi-coding-agent/memory-backend";
 import type { MemoryBackend } from "@oh-my-pi/pi-coding-agent/memory-backend/types";
 import { type MnemopiSessionState, setMnemopiSessionState } from "@oh-my-pi/pi-coding-agent/mnemopi/state";
+import { createAgentSession, type ExtensionFactory } from "@oh-my-pi/pi-coding-agent/sdk";
 import { obfuscateProviderContext, SecretObfuscator } from "@oh-my-pi/pi-coding-agent/secrets";
 import { AgentSession, type AgentSessionEvent } from "@oh-my-pi/pi-coding-agent/session/agent-session";
+import { AuthStorage } from "@oh-my-pi/pi-coding-agent/session/auth-storage";
 import { convertToLlm, wrapSteeringForModel } from "@oh-my-pi/pi-coding-agent/session/messages";
 import { SessionManager } from "@oh-my-pi/pi-coding-agent/session/session-manager";
 import { TempDir } from "@oh-my-pi/pi-utils";
@@ -689,6 +692,94 @@ describe("AgentSession message pipeline", () => {
 		expect(contexts[1]!.systemPrompt).toEqual(firstSystemPrompt);
 	});
 
+	it("preserves append-only prefixes in subagent sessions when context handlers rewrite prior turns", async () => {
+		using tempDir = TempDir.createSync("@pi-subagent-append-only-");
+		const api = "test-subagent-append-only-cache";
+		const contexts: Context[] = [];
+		registerCustomApi(api, (_model, context) => {
+			contexts.push(context);
+			const stream = new AssistantMessageEventStream();
+			queueMicrotask(() => {
+				const message = createAssistantMessage(`ok-${contexts.length}`);
+				stream.push({ type: "text_delta", contentIndex: 0, delta: "ok", partial: message });
+				stream.push({ type: "done", reason: "stop", message });
+			});
+			return stream;
+		});
+		const model = buildModel({
+			id: "local-subagent-model",
+			name: "Local Subagent Model",
+			api,
+			provider: "llama.cpp",
+			baseUrl: "http://127.0.0.1:8080/v1",
+			reasoning: false,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 4096,
+			maxTokens: 1024,
+		} as ModelSpec<Api>) as Model<Api>;
+		const rewritePriorAssistant: ExtensionFactory = pi => {
+			pi.on("context", async event => {
+				const hasSecondTurn = event.messages.some(message => {
+					if (message.role !== "user") return false;
+					const content = message.content;
+					if (typeof content === "string") return content.includes("second");
+					return content.some(part => part.type === "text" && part.text.includes("second"));
+				});
+				if (!hasSecondTurn) return undefined;
+				return {
+					messages: event.messages.map(message =>
+						message.role === "assistant"
+							? { ...message, content: [{ type: "text" as const, text: "rewritten assistant" }] }
+							: message,
+					),
+				};
+			});
+		};
+		const authStorage = await AuthStorage.create(tempDir.join("auth.db"));
+		const modelRegistry = new ModelRegistry(authStorage, tempDir.join("models.yml"));
+		const { session } = await createAgentSession({
+			cwd: tempDir.path(),
+			agentDir: tempDir.path(),
+			sessionManager: SessionManager.inMemory(tempDir.path()),
+			authStorage,
+			modelRegistry,
+			settings: Settings.isolated({
+				"compaction.enabled": false,
+				"provider.appendOnlyContext": "auto",
+			}),
+			model,
+			disableExtensionDiscovery: true,
+			extensions: [rewritePriorAssistant],
+			skills: [],
+			contextFiles: [],
+			promptTemplates: [],
+			slashCommands: [],
+			enableMCP: false,
+			enableLsp: false,
+			skipPythonPreflight: true,
+			taskDepth: 1,
+			agentId: "SubAgent",
+		});
+		try {
+			expect(session.agent.appendOnlyContext).toBeDefined();
+
+			await session.sendUserMessage("first");
+			await session.sendUserMessage("second");
+
+			expect(contexts).toHaveLength(2);
+			expect(contexts[0]!.messages).toHaveLength(1);
+			expect(contexts[1]!.messages).toHaveLength(3);
+			expect(contexts[1]!.messages[0]).toBe(contexts[0]!.messages[0]);
+			expect((contexts[1]!.messages[1] as { content: unknown }).content).toEqual([
+				{ type: "text", text: "rewritten assistant" },
+			]);
+		} finally {
+			await session.dispose();
+			authStorage.close();
+		}
+	});
+
 	it("clears promoted memory from the base prompt when switching sessions", async () => {
 		using tempDir = TempDir.createSync("@pi-injected-memory-switch-");
 		const sessionManager = SessionManager.create(tempDir.path(), tempDir.join("sessions"));