diff --git a/packages/shared-ai/src/planner/loop.test.ts b/packages/shared-ai/src/planner/loop.test.ts
index 82263a0e9..1acd349f0 100644
--- a/packages/shared-ai/src/planner/loop.test.ts
+++ b/packages/shared-ai/src/planner/loop.test.ts
@@ -328,6 +328,190 @@ describe('runPlannerLoop — parallel reads', () => {
 	});
 });
 
+describe('runPlannerLoop — compactor', () => {
+	it('does not compact below the threshold', async () => {
+		const llm = new MockLlmClient();
+		(llm as unknown as { queue: unknown[] }).queue.push({
+			content: null,
+			toolCalls: [{ id: 'c1', name: 'list_things', arguments: {} }],
+			finishReason: 'tool_calls',
+			usage: { promptTokens: 500, completionTokens: 0, totalTokens: 500 }, // 50%
+		});
+		llm.enqueueStop('done');
+
+		const compactSpy = vi.fn();
+		await runPlannerLoop({
+			llm,
+			input: {
+				systemPrompt: 's',
+				userPrompt: 'u',
+				tools,
+				model: 'm',
+				compactor: {
+					maxContextTokens: 1000,
+					compact: async (m) => {
+						compactSpy();
+						return { messages: m, compactedTurns: 0 };
+					},
+				},
+			},
+			onToolCall: async () => ({ success: true, message: 'ok' }),
+		});
+
+		expect(compactSpy).not.toHaveBeenCalled();
+	});
+
+	it('fires when usage crosses the threshold and replaces messages', async () => {
+		const llm = new MockLlmClient();
+		// Round 1: tool call that reports 92% of the 1000-token budget
+		(llm as unknown as { queue: unknown[] }).queue.push({
+			content: null,
+			toolCalls: [{ id: 'c1', name: 'list_things', arguments: {} }],
+			finishReason: 'tool_calls',
+			usage: { promptTokens: 920, completionTokens: 0, totalTokens: 920 },
+		});
+		// Round 2: after compaction fires, the LLM stops
+		llm.enqueueStop('done');
+
+		let compactorInput: readonly { role: string; content?: string | null }[] = [];
+		await runPlannerLoop({
+			llm,
+			input: {
+				systemPrompt: 's-prompt',
+				userPrompt: 'u-prompt',
+				tools,
+				model: 'm',
+				compactor: {
+					maxContextTokens: 1000,
+					compact: async (m) => {
+						compactorInput = m;
+						return {
+							messages: [
+								{ role: 'system', content: 's-prompt' },
+								{ role: 'user', content: 'u-prompt' },
+								{ role: 'assistant', content: '<compact-summary>FOLDED</compact-summary>' },
+							],
+							compactedTurns: 2,
+						};
+					},
+				},
+			},
+			onToolCall: async () => ({ success: true, message: 'ok' }),
+		});
+
+		// The compactor received the full post-round-1 history
+		expect(compactorInput.length).toBeGreaterThan(2);
+		// The round-2 LLM request saw the compacted history, not the raw one
+		const round2Seen = llm.calls[1].messages;
+		expect(round2Seen).toHaveLength(3);
+		expect(round2Seen[2].content).toContain('FOLDED');
+	});
+
+	it('fires at most once per run', async () => {
+		const llm = new MockLlmClient();
+		for (let i = 0; i < 4; i++) {
+			(llm as unknown as { queue: unknown[] }).queue.push({
+				content: null,
+				toolCalls: [{ id: `c${i}`, name: 'list_things', arguments: {} }],
+				finishReason: 'tool_calls',
+				usage: { promptTokens: 950, completionTokens: 0, totalTokens: 950 }, // always over threshold
+			});
+		}
+		llm.enqueueStop('done');
+
+		let compactCallCount = 0;
+		await runPlannerLoop({
+			llm,
+			input: {
+				systemPrompt: 's',
+				userPrompt: 'u',
+				tools,
+				model: 'm',
+				maxRounds: 10,
+				compactor: {
+					maxContextTokens: 1000,
+					compact: async () => {
+						compactCallCount++;
+						return {
+							messages: [
+								{ role: 'system', content: 's' },
+								{ role: 'user', content: 'u' },
+								{ role: 'assistant', content: '<compact>' },
+							],
+							compactedTurns: 2,
+						};
+					},
+				},
+			},
+			onToolCall: async () => ({ success: true, message: 'ok' }),
+		});
+
+		expect(compactCallCount).toBe(1);
+	});
+
+	it('bails out silently when maxContextTokens is 0', async () => {
+		const llm = new MockLlmClient();
+		(llm as unknown as { queue: unknown[] }).queue.push({
+			content: 'done',
+			toolCalls: [],
+			finishReason: 'stop',
+			usage: { promptTokens: 9_999, completionTokens: 0, totalTokens: 9_999 },
+		});
+
+		const compactSpy = vi.fn();
+		await runPlannerLoop({
+			llm,
+			input: {
+				systemPrompt: 's',
+				userPrompt: 'u',
+				tools,
+				model: 'm',
+				compactor: {
+					maxContextTokens: 0, // disabled
+					compact: async (m) => {
+						compactSpy();
+						return { messages: m, compactedTurns: 0 };
+					},
+				},
+			},
+			onToolCall: async () => ({ success: true, message: 'ok' }),
+		});
+
+		expect(compactSpy).not.toHaveBeenCalled();
+	});
+
+	it('skips when the compactor returns 0 compacted turns', async () => {
+		const llm = new MockLlmClient();
+		(llm as unknown as { queue: unknown[] }).queue.push({
+			content: null,
+			toolCalls: [{ id: 'c1', name: 'list_things', arguments: {} }],
+			finishReason: 'tool_calls',
+			usage: { promptTokens: 950, completionTokens: 0, totalTokens: 950 },
+		});
+		llm.enqueueStop('done');
+
+		await runPlannerLoop({
+			llm,
+			input: {
+				systemPrompt: 's',
+				userPrompt: 'u',
+				tools,
+				model: 'm',
+				compactor: {
+					maxContextTokens: 1000,
+					compact: async (m) => ({ messages: m, compactedTurns: 0 }),
+				},
+			},
+			onToolCall: async () => ({ success: true, message: 'ok' }),
+		});
+
+		// Round 2 should have seen the ORIGINAL history (untouched by the
+		// no-op compactor) — just system + user + assistant + tool
+		const round2Seen = llm.calls[1].messages;
+		expect(round2Seen).toHaveLength(4);
+	});
+});
+
 describe('runPlannerLoop — reminderChannel', () => {
 	it('injects reminders as transient system messages on the LLM call', async () => {
 		const llm = new MockLlmClient().enqueueStop('done');
diff --git a/packages/shared-ai/src/planner/loop.ts b/packages/shared-ai/src/planner/loop.ts
index 747cb247f..706a16689 100644
--- a/packages/shared-ai/src/planner/loop.ts
+++ b/packages/shared-ai/src/planner/loop.ts
@@ -144,6 +144,35 @@ export interface PlannerLoopInput {
 	 * constant-time lookups are expected (registry hit, name-prefix check).
 	 */
 	readonly isParallelSafe?: (toolName: string) => boolean;
+	/**
+	 * Context-window compactor wiring (Claude-Code `wU2` pattern).
+	 *
+	 * When set AND usage crosses the threshold, the loop replaces the
+	 * middle of the message history with a compact summary before the
+	 * next LLM call. The compact summary is persisted in the returned
+	 * `messages` — unlike reminders, this IS part of the canonical
+	 * history because raw turns got dropped.
+	 *
+	 * Contract:
+	 *   - `maxContextTokens`: provider ceiling; compactor skips when unset
+	 *     (matches `shouldCompact()`'s safe-bail behaviour).
+	 *   - `compact`: async callback that performs the compaction. Pass
+	 *     `compactHistory` from this package or an adapter that uses a
+	 *     cheaper model (e.g. Haiku) for the compactor's LLM call.
+	 *   - `threshold`: optional override, default 0.92.
+	 *
+	 * Compaction fires at MOST once per loop run — once a round has been
+	 * compacted, we don't re-trigger until the next run, even if the
+	 * fresh history hits the threshold again (defence-in-depth against
+	 * a runaway tool that keeps bloating turns).
+	 */
+	readonly compactor?: {
+		readonly maxContextTokens: number;
+		readonly threshold?: number;
+		readonly compact: (
+			messages: readonly ChatMessage[]
+		) => Promise<{ readonly messages: readonly ChatMessage[]; readonly compactedTurns: number }>;
+	};
 }
 
 /** Max concurrent tool executions per round. Mirrors Claude Code's gW5
@@ -206,10 +235,30 @@ export async function runPlannerLoop(opts: {
 	let rounds = 0;
 	let promptTokens = 0;
 	let completionTokens = 0;
+	let compactionsDone = 0;
 
 	while (rounds < maxRounds) {
 		rounds++;
 
+		// Context-window compactor (Claude-Code `wU2`): check BEFORE the
+		// next LLM call whether the previous round's usage crossed the
+		// threshold; if so, replace the middle of `messages` with a
+		// compact summary. Fire at most once per loop run so a runaway
+		// tool can't keep re-triggering.
+		if (input.compactor && compactionsDone === 0) {
+			const total = promptTokens + completionTokens;
+			const cap = input.compactor.maxContextTokens;
+			const threshold = input.compactor.threshold ?? 0.92;
+			if (cap > 0 && total > 0 && total / cap >= threshold) {
+				const compactResult = await input.compactor.compact(messages);
+				if (compactResult.compactedTurns > 0) {
+					messages.length = 0;
+					for (const m of compactResult.messages) messages.push(m);
+					compactionsDone++;
+				}
+			}
+		}
+
 		// Per-round reminder injection: ask the channel for transient
 		// hints, wrap each in <reminder> tags, and prepend them as system
 		// messages to THIS request only. Nothing gets pushed to `messages`
@@ -277,10 +326,11 @@ export async function runPlannerLoop(opts: {
 		// In both modes we append to `messages` in the LLM's original
 		// call order, not completion order, so the debug-log stays linear.
 		const calls = response.toolCalls;
+		const parallelSafePredicate = input.isParallelSafe;
 		const allParallelSafe =
-			!!input.isParallelSafe &&
+			!!parallelSafePredicate &&
 			calls.length > 1 &&
-			calls.every((c) => input.isParallelSafe!(c.name));
+			calls.every((c) => parallelSafePredicate(c.name));
 
 		if (allParallelSafe) {
 			for (let i = 0; i < calls.length; i += PARALLEL_TOOL_BATCH_SIZE) {