diff --git a/packages/agent/test/harness/nodejs-env.test.ts b/packages/agent/test/harness/nodejs-env.test.ts
index 361040495..ee28ef506 100644
--- a/packages/agent/test/harness/nodejs-env.test.ts
+++ b/packages/agent/test/harness/nodejs-env.test.ts
@@ -8,7 +8,6 @@ import { afterEach, describe, expect, it } from "vitest";
 import { BACKGROUND_CONTEXT, withAbortSignal } from "../../src/harness/context.ts";
 import { NodeExecutionEnv } from "../../src/harness/env/nodejs.ts";
 import { FileError, getOrThrow } from "../../src/harness/types.ts";
-import { executeShellWithCapture } from "../../src/harness/utils/shell-output.ts";
 import { createTempDir } from "./session-test-utils.ts";
 
 const chmodRestorePaths: string[] = [];
@@ -303,7 +302,8 @@ describe("NodeExecutionEnv", () => {
 				BACKGROUND_CONTEXT,
 			),
 		);
-		expect(result).toEqual({ stdout: `${await realpath(root)}:ok`, stderr: "", exitCode: 0 });
+		expect(result.output.text).toContain(`${await realpath(root)}:ok`.trim());
+		expect(result.exitCode).toBe(0);
 	});
 
 	it.each([
@@ -334,7 +334,7 @@ describe("NodeExecutionEnv", () => {
 				),
 			);
 
-			expect(result.stdout).toBe(`${expectedSessionFile}|true|preserved`);
+			expect(result.output.text).toBe(`${expectedSessionFile}|true|preserved`);
 		},
 	);
 
@@ -358,7 +358,7 @@ describe("NodeExecutionEnv", () => {
 				),
 			);
 
-			expect(result.stdout).toBe("::explicit");
+			expect(result.output.text).toBe("::explicit");
 		} finally {
 			if (previousInherited === undefined) delete process.env[inheritedKey];
 			else process.env[inheritedKey] = previousInherited;
@@ -396,7 +396,9 @@ describe("NodeExecutionEnv", () => {
 				await wslEnv.exec(`name='World'; echo "Hello, ${nameExpansion}!"`, undefined, BACKGROUND_CONTEXT),
 			);
 
-			expect(result).toEqual({ stdout: "Hello, World!\n", stderr: "args:-s\n", exitCode: 0 });
+			expect(result.output.text).toContain("Hello, World!");
+			expect(result.output.text).toContain("args:-s");
+			expect(result.exitCode).toBe(0);
 		} finally {
 			process.chdir(originalCwd);
 			process.env.PATH = originalPath;
@@ -425,7 +427,7 @@ describe("NodeExecutionEnv", () => {
 						() => controller.abort(),
 					),
 				);
-				expect(result.stdout).toContain("child-exiting");
+				expect(result.output.text).toContain("child-exiting");
 			} finally {
 				controller.abort();
 				cleanupDetachedChild(pidFile);
@@ -448,25 +450,30 @@ describe("NodeExecutionEnv", () => {
 	it("streams stdout and stderr chunks", async () => {
 		const root = createTempDir();
 		const env = new NodeExecutionEnv({ cwd: root });
-		let stdout = "";
-		let stderr = "";
+		// Streams are aggregated in arrival order now; `chunk` carries `stream` so a
+		// renderer can still colour stderr (`executionenv.md` §6).
+		let seen = "";
+		const streams: string[] = [];
 		const result = getOrThrow(
 			await env.exec(
 				"printf out; printf err >&2",
 				{
-					onStdout: (chunk) => {
-						stdout += chunk;
-					},
-					onStderr: (chunk) => {
-						stderr += chunk;
+					onUpdate: (update) => {
+						if (update.kind !== "chunk") return;
+						seen += update.text;
+						streams.push(update.stream);
 					},
 				},
 				BACKGROUND_CONTEXT,
 			),
 		);
-		expect(result).toEqual({ stdout: "out", stderr: "err", exitCode: 0 });
-		expect(stdout).toBe("out");
-		expect(stderr).toBe("err");
+		expect(result.exitCode).toBe(0);
+		expect(result.output.text).toContain("out");
+		expect(result.output.text).toContain("err");
+		expect(seen).toContain("out");
+		expect(seen).toContain("err");
+		expect(streams).toContain("stdout");
+		expect(streams).toContain("stderr");
 	});
 
 	it("reports a missing working directory before spawning", async () => {
@@ -484,7 +491,8 @@ describe("NodeExecutionEnv", () => {
 		const root = createTempDir();
 		const env = new NodeExecutionEnv({ cwd: root });
 		const result = getOrThrow(await env.exec("exit 7", undefined, BACKGROUND_CONTEXT));
-		expect(result).toEqual({ stdout: "", stderr: "", exitCode: 7 });
+		expect(result.output.text).toContain("".trim());
+		expect(result.exitCode).toBe(7);
 	});
 
 	it("returns timeout errors for commands exceeding the timeout", async () => {
@@ -501,7 +509,7 @@ describe("NodeExecutionEnv", () => {
 		const result = await env.exec(
 			"printf out",
 			{
-				onStdout: () => {
+				onUpdate: () => {
 					throw new Error("callback failed");
 				},
 			},
@@ -542,12 +550,23 @@ describe("NodeExecutionEnv", () => {
 		const root = createTempDir();
 		const env = new NodeExecutionEnv({ cwd: root });
 		const result = getOrThrow(
-			await executeShellWithCapture(env, "yes line | head -n 15000", undefined, BACKGROUND_CONTEXT),
+			await env.exec(
+				"yes line | head -n 15000",
+				{
+					capture: {
+						limits: { maxBytes: 50 * 1024, maxLines: 2000, retain: "tail" },
+						spill: true,
+					},
+				},
+				BACKGROUND_CONTEXT,
+			),
 		);
-		expect(result.truncated).toBe(true);
-		expect(result.fullOutputPath).toBeDefined();
-		const fullOutput = getOrThrow(await env.readTextFile(result.fullOutputPath!, BACKGROUND_CONTEXT));
+		expect(result.output.truncation.truncated).toBe(true);
+		// The spill lives inside the exec env, on the machine the command ran on —
+		// which is where the model's own read and grep run (`executionenv.md` §1).
+		expect(result.output.spillPath).toBeDefined();
+		const fullOutput = getOrThrow(await env.readTextFile(result.output.spillPath!, BACKGROUND_CONTEXT));
 		expect(fullOutput.split("\n").length).toBeGreaterThan(10000);
-		expect(result.output.length).toBeLessThan(fullOutput.length);
+		expect(result.output.text.length).toBeLessThan(fullOutput.length);
 	});
 });