diff --git a/test/runtime/terminal/session-manager-auto-restart.test.ts b/test/runtime/terminal/session-manager-auto-restart.test.ts
index 61282b19a7..91da8a7531 100644
--- a/test/runtime/terminal/session-manager-auto-restart.test.ts
+++ b/test/runtime/terminal/session-manager-auto-restart.test.ts
@@ -195,4 +195,50 @@ describe("TerminalSessionManager auto-restart", () => {
 		expect(session.write).toHaveBeenCalledWith(deferredStartupInput);
 		expect(session.write).toHaveBeenCalledTimes(1);
 	});
+
+	it("auto-restarts up to MAX_AUTO_RESTARTS_PER_WINDOW times, then refuses the next exit in the same window", async () => {
+		const spawnedSessions: Array<ReturnType<typeof createMockPtySession>> = [];
+		ptySessionSpawnMock.mockImplementation((request: MockSpawnRequest) => {
+			const session = createMockPtySession(100 + spawnedSessions.length, request);
+			spawnedSessions.push(session);
+			return session;
+		});
+
+		const manager = new TerminalSessionManager();
+		manager.attach("task-1", {
+			onState: vi.fn(),
+			onOutput: vi.fn(),
+			onExit: vi.fn(),
+		});
+
+		await manager.startTaskSession({
+			taskId: "task-1",
+			agentId: "codex",
+			binary: "codex",
+			args: [],
+			cwd: "/tmp/task-1",
+			prompt: "Fix the bug",
+		});
+
+		expect(ptySessionSpawnMock).toHaveBeenCalledTimes(1);
+
+		// First 3 qualifying exits (within the trailing AUTO_RESTART_WINDOW_MS) each auto-restart.
+		for (let restart = 1; restart <= 3; restart += 1) {
+			spawnedSessions[spawnedSessions.length - 1]?.triggerExit(1);
+			await vi.waitFor(() => {
+				expect(ptySessionSpawnMock).toHaveBeenCalledTimes(restart + 1);
+			});
+			expect(manager.getSummary("task-1")?.state).toBe("running");
+		}
+
+		// The 4th qualifying exit within the same window hits MAX_AUTO_RESTARTS_PER_WINDOW and is refused.
+		spawnedSessions[spawnedSessions.length - 1]?.triggerExit(1);
+		await Promise.resolve();
+		await Promise.resolve();
+
+		expect(ptySessionSpawnMock).toHaveBeenCalledTimes(4);
+		expect(manager.getSummary("task-1")?.state).toBe("awaiting_review");
+		expect(manager.getSummary("task-1")?.reviewReason).toBe("error");
+		expect(manager.getSummary("task-1")?.pid).toBeNull();
+	});
 });
diff --git a/test/runtime/terminal/session-state-machine.test.ts b/test/runtime/terminal/session-state-machine.test.ts
new file mode 100644
index 0000000000..e3c65aa13e
--- /dev/null
+++ b/test/runtime/terminal/session-state-machine.test.ts
@@ -0,0 +1,150 @@
+import { describe, expect, it } from "vitest";
+
+import type { RuntimeTaskSessionSummary } from "../../../src/core/api-contract";
+import { reduceSessionTransition, type SessionTransitionEvent } from "../../../src/terminal/session-state-machine";
+
+function createSummary(overrides: Partial<RuntimeTaskSessionSummary> = {}): RuntimeTaskSessionSummary {
+	return {
+		taskId: "task-1",
+		state: "running",
+		agentId: "claude",
+		workspacePath: "/tmp/worktree",
+		pid: 1234,
+		startedAt: Date.now(),
+		updatedAt: Date.now(),
+		lastOutputAt: Date.now(),
+		reviewReason: null,
+		exitCode: null,
+		lastHookAt: null,
+		latestHookActivity: null,
+		...overrides,
+	};
+}
+
+describe("reduceSessionTransition", () => {
+	it("hook.to_review moves a running session to awaiting_review with reason hook", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "hook.to_review" });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "awaiting_review", reviewReason: "hook" });
+		expect(result.clearAttentionBuffer).toBe(true);
+	});
+
+	it("hook.to_review is a no-op when the session is not running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "attention" });
+		const result = reduceSessionTransition(summary, { type: "hook.to_review" });
+
+		expect(result).toEqual({ changed: false, patch: {}, clearAttentionBuffer: false });
+	});
+
+	it("hook.to_in_progress returns an awaiting_review session with a resumable reason to running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "attention" });
+		const result = reduceSessionTransition(summary, { type: "hook.to_in_progress" });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "running", reviewReason: null });
+		expect(result.clearAttentionBuffer).toBe(true);
+	});
+
+	it("hook.to_in_progress treats reviewReason error as resumable and returns the session to running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "error" });
+		const result = reduceSessionTransition(summary, { type: "hook.to_in_progress" });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "running", reviewReason: null });
+		expect(result.clearAttentionBuffer).toBe(true);
+	});
+
+	it("hook.to_in_progress is a no-op when the review reason cannot return to running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "exit" });
+		const result = reduceSessionTransition(summary, { type: "hook.to_in_progress" });
+
+		expect(result).toEqual({ changed: false, patch: {}, clearAttentionBuffer: false });
+	});
+
+	it("hook.to_in_progress is a no-op when the session is not awaiting review", () => {
+		const summary = createSummary({ state: "running", reviewReason: null });
+		const result = reduceSessionTransition(summary, { type: "hook.to_in_progress" });
+
+		expect(result).toEqual({ changed: false, patch: {}, clearAttentionBuffer: false });
+	});
+
+	it("agent.prompt-ready returns an awaiting_review session with a resumable reason to running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "hook" });
+		const result = reduceSessionTransition(summary, { type: "agent.prompt-ready" });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "running", reviewReason: null });
+		expect(result.clearAttentionBuffer).toBe(true);
+	});
+
+	it("agent.prompt-ready is a no-op when the review reason cannot return to running", () => {
+		const summary = createSummary({ state: "awaiting_review", reviewReason: "interrupted" });
+		const result = reduceSessionTransition(summary, { type: "agent.prompt-ready" });
+
+		expect(result).toEqual({ changed: false, patch: {}, clearAttentionBuffer: false });
+	});
+
+	it("process.exit with exit code 0 and no interruption sets reviewReason exit", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: 0, interrupted: false });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "awaiting_review", reviewReason: "exit", exitCode: 0, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("process.exit with exit code 1 and no interruption sets reviewReason error", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: 1, interrupted: false });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "awaiting_review", reviewReason: "error", exitCode: 1, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("process.exit with exit code 137 and no interruption sets reviewReason error", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: 137, interrupted: false });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "awaiting_review", reviewReason: "error", exitCode: 137, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("process.exit with a null exit code and no interruption sets reviewReason error", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: null, interrupted: false });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "awaiting_review", reviewReason: "error", exitCode: null, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("process.exit with interrupted true overrides a zero exit code to reviewReason interrupted", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: 0, interrupted: true });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "interrupted", reviewReason: "interrupted", exitCode: 0, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("process.exit with interrupted true overrides a nonzero exit code to reviewReason interrupted", () => {
+		const summary = createSummary({ state: "running" });
+		const result = reduceSessionTransition(summary, { type: "process.exit", exitCode: 1, interrupted: true });
+
+		expect(result.changed).toBe(true);
+		expect(result.patch).toEqual({ state: "interrupted", reviewReason: "interrupted", exitCode: 1, pid: null });
+		expect(result.clearAttentionBuffer).toBe(false);
+	});
+
+	it("is a no-op for an event type outside the known union", () => {
+		const summary = createSummary({ state: "running" });
+		const unknownEvent = { type: "unknown.event" } as unknown as SessionTransitionEvent;
+		const result = reduceSessionTransition(summary, unknownEvent);
+
+		expect(result).toEqual({ changed: false, patch: {}, clearAttentionBuffer: false });
+	});
+});