diff --git a/packages/agent/src/harness/env/nodejs.ts b/packages/agent/src/harness/env/nodejs.ts
index 732dc84db..db9eb9a57 100644
--- a/packages/agent/src/harness/env/nodejs.ts
+++ b/packages/agent/src/harness/env/nodejs.ts
@@ -19,6 +19,7 @@ import { basename, isAbsolute, join, resolve } from "node:path";
 import { createInterface } from "node:readline";
 import { fileURLToPath } from "node:url";
 import type { Context } from "../context.ts";
+import { OutputCapture } from "../utils/output-capture.ts";
 import {
 	type ExecutionEnv,
 	ExecutionError,
@@ -28,7 +29,10 @@ import {
 	type FileKind,
 	ok,
 	type Result,
+	type ShellCapabilities,
 	type ShellExecOptions,
+	type ShellExecResult,
+	type ShellStream,
 	toError,
 } from "../types.ts";
 
@@ -365,11 +369,15 @@ export class NodeExecutionEnv implements ExecutionEnv {
 		return ok(join(...parts));
 	}
 
+	capabilities(): ShellCapabilities {
+		return { capture: true, spill: true, liveUpdates: true };
+	}
+
 	async exec(
 		command: string,
 		options: ShellExecOptions | undefined,
 		context: Context,
-	): Promise<Result<{ stdout: string; stderr: string; exitCode: number }, ExecutionError>> {
+	): Promise<Result<ShellExecResult, ExecutionError>> {
 		const signal = context.abortSignal;
 		if (signal?.aborted) return err(new ExecutionError("aborted", "aborted"));
 		const timeoutMsResult = resolveTimeoutMs(options?.timeout);
@@ -392,9 +400,28 @@ export class NodeExecutionEnv implements ExecutionEnv {
 			);
 		}
 
+		// Spill lives in the exec env, on the machine where the model's own `read`
+		// and `grep` run — the whole reason capping happens here rather than on the
+		// agent side (`executionenv.md` §1).
+		let spillHandle: string | undefined;
+		let spillChain: Promise<void> = Promise.resolve();
+		const wantSpill = options?.capture?.spill === true;
+
 		return await new Promise((resolvePromise) => {
-			let stdout = "";
-			let stderr = "";
+			const capture = new OutputCapture(options?.capture, options?.onUpdate);
+			const spillChunk = (chunk: string): Promise<void> => {
+				if (!wantSpill) return Promise.resolve();
+				spillChain = spillChain.then(async () => {
+					if (spillHandle === undefined) {
+						const created = await this.createTempFile({ prefix: "pi-output-", suffix: ".log" }, context);
+						if (!created.ok) return;
+						spillHandle = created.value;
+						capture.setSpillPath(spillHandle);
+					}
+					await this.appendFile(spillHandle, chunk, context);
+				});
+				return spillChain;
+			};
 			let settled = false;
 			let timedOut = false;
 			let callbackError: ExecutionError | undefined;
@@ -407,7 +434,7 @@ export class NodeExecutionEnv implements ExecutionEnv {
 				}
 			};
 
-			const settle = (result: Result<{ stdout: string; stderr: string; exitCode: number }, ExecutionError>) => {
+			const settle = (result: Result<ShellExecResult, ExecutionError>) => {
 				if (timeoutId) clearTimeout(timeoutId);
 				if (signal) signal.removeEventListener("abort", onAbort);
 				if (child?.pid) this.activeChildPids.delete(child.pid);
@@ -458,31 +485,32 @@ export class NodeExecutionEnv implements ExecutionEnv {
 				}
 			}
 
+			// setEncoding("utf8") is what keeps a chunk boundary from splitting a
+			// codepoint; without it a multi-byte sequence straddling a read bakes in a
+			// replacement character (`executionenv.md` §8).
 			child.stdout?.setEncoding("utf8");
 			child.stderr?.setEncoding("utf8");
-			child.stdout?.on("data", (chunk: string) => {
-				stdout += chunk;
-				try {
-					options?.onStdout?.(chunk, context);
-				} catch (error) {
-					const cause = toError(error);
-					callbackError = new ExecutionError("callback_error", cause.message, cause);
-					onAbort();
-				}
-			});
-			child.stderr?.on("data", (chunk: string) => {
-				stderr += chunk;
+			const feed = (stream: ShellStream, chunk: string) => {
 				try {
-					options?.onStderr?.(chunk, context);
+					capture.push(stream, chunk, context);
+					void spillChunk(chunk);
 				} catch (error) {
 					const cause = toError(error);
 					callbackError = new ExecutionError("callback_error", cause.message, cause);
 					onAbort();
 				}
-			});
+			};
+			child.stdout?.on("data", (chunk: string) => feed("stdout", chunk));
+			child.stderr?.on("data", (chunk: string) => feed("stderr", chunk));
 
 			void waitForChildProcess(child).then(
-				(code) => {
+				async (code) => {
+					// The spill chain is async and lazy: the file is created on the first
+					// chunk. Await it before snapshotting, or the returned CapturedOutput
+					// reports no spillPath for output that was in fact archived.
+					await spillChain;
+					capture.flush(context);
+					capture.dispose();
 					if (callbackError) {
 						settle(err(callbackError));
 						return;
@@ -495,7 +523,7 @@ export class NodeExecutionEnv implements ExecutionEnv {
 						settle(err(new ExecutionError("aborted", "aborted")));
 						return;
 					}
-					settle(ok({ stdout, stderr, exitCode: code ?? 0 }));
+					settle(ok({ output: capture.snapshot(), exitCode: code ?? 0 }));
 				},
 				(error: Error) => settle(err(new ExecutionError("spawn_error", error.message, error))),
 			);