fix(coding-agent): improve subagent parallel summaries closes #4710

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed the subagent extension's parallel mode to return useful per-task output and failed-task diagnostics to the parent model instead of 100-character previews ([#4710](https://github.com/earendil-works/pi/issues/4710)).
+
 ## [0.75.3] - 2026-05-18
 
 ### Fixed

packages/coding-agent/examples/extensions/subagent/README.md

@@ -113,6 +113,8 @@ Use a chain: first have scout find the read tool, then have planner suggest impr
 - Shows all tasks with live status (⏳ running, ✓ done, ✗ failed)
 - Updates as each task makes progress
 - Shows "2/3 done, 1 running" status
+- Returns each completed task's final output to the parent model, capped at 50 KB per task
+- Returns failure diagnostics from stderr/error messages when a child exits before producing output
 
 **Tool call formatting** (mimics built-in tools):
 - `$ command` for bash
@@ -168,5 +170,6 @@ Project agents override user agents with the same name when `agentScope: "both"`
 ## Limitations
 
 - Output truncated to last 10 items in collapsed view (expand to see all)
+- Parallel model-visible output is capped at 50 KB per task; full results remain in tool details
 - Agents discovered fresh on each invocation (allows editing mid-session)
 - Parallel mode limited to 8 tasks, 4 concurrent

packages/coding-agent/examples/extensions/subagent/index.ts

@@ -27,6 +27,7 @@ import { type AgentConfig, type AgentScope, discoverAgents } from "./agents.js";
 const MAX_PARALLEL_TASKS = 8;
 const MAX_CONCURRENCY = 4;
 const COLLAPSED_ITEM_COUNT = 10;
+const PER_TASK_OUTPUT_CAP = 50 * 1024;
 
 function formatTokens(count: number): string {
 	if (count < 1000) return count.toString();
@@ -172,6 +173,28 @@ function getFinalOutput(messages: Message[]): string {
 	return "";
 }
 
+function isFailedResult(result: SingleResult): boolean {
+	return result.exitCode !== 0 || result.stopReason === "error" || result.stopReason === "aborted";
+}
+
+function getResultOutput(result: SingleResult): string {
+	if (isFailedResult(result)) {
+		return result.errorMessage || result.stderr || getFinalOutput(result.messages) || "(no output)";
+	}
+	return getFinalOutput(result.messages) || "(no output)";
+}
+
+function truncateParallelOutput(output: string): string {
+	const byteLength = Buffer.byteLength(output, "utf8");
+	if (byteLength <= PER_TASK_OUTPUT_CAP) return output;
+
+	let truncated = output.slice(0, PER_TASK_OUTPUT_CAP);
+	while (Buffer.byteLength(truncated, "utf8") > PER_TASK_OUTPUT_CAP) {
+		truncated = truncated.slice(0, -1);
+	}
+	return `${truncated}\n\n[Output truncated: ${byteLength - Buffer.byteLength(truncated, "utf8")} bytes omitted. Full output preserved in tool details.]`;
+}
+
 type DisplayItem = { type: "text"; text: string } | { type: "toolCall"; name: string; args: Record<string, any> };
 
 function getDisplayItems(messages: Message[]): DisplayItem[] {
@@ -534,11 +557,9 @@ export default function (pi: ExtensionAPI) {
 					);
 					results.push(result);
 
-					const isError =
-						result.exitCode !== 0 || result.stopReason === "error" || result.stopReason === "aborted";
+					const isError = isFailedResult(result);
 					if (isError) {
-						const errorMsg =
-							result.errorMessage || result.stderr || getFinalOutput(result.messages) || "(no output)";
+						const errorMsg = getResultOutput(result);
 						return {
 							content: [{ type: "text", text: `Chain stopped at step ${i + 1} (${step.agent}): ${errorMsg}` }],
 							details: makeDetails("chain")(results),
@@ -617,17 +638,19 @@ export default function (pi: ExtensionAPI) {
 					return result;
 				});
 
-				const successCount = results.filter((r) => r.exitCode === 0).length;
+				const successCount = results.filter((r) => !isFailedResult(r)).length;
 				const summaries = results.map((r) => {
-					const output = getFinalOutput(r.messages);
-					const preview = output.slice(0, 100) + (output.length > 100 ? "..." : "");
-					return `[${r.agent}] ${r.exitCode === 0 ? "completed" : "failed"}: ${preview || "(no output)"}`;
+					const output = truncateParallelOutput(getResultOutput(r));
+					const status = isFailedResult(r)
+						? `failed${r.stopReason && r.stopReason !== "end" ? ` (${r.stopReason})` : ""}`
+						: "completed";
+					return `### [${r.agent}] ${status}\n\n${output}`;
 				});
 				return {
 					content: [
 						{
 							type: "text",
-							text: `Parallel: ${successCount}/${results.length} succeeded\n\n${summaries.join("\n\n")}`,
+							text: `Parallel: ${successCount}/${results.length} succeeded\n\n${summaries.join("\n\n---\n\n")}`,
 						},
 					],
 					details: makeDetails("parallel")(results),
@@ -646,10 +669,9 @@ export default function (pi: ExtensionAPI) {
 					onUpdate,
 					makeDetails("single"),
 				);
-				const isError = result.exitCode !== 0 || result.stopReason === "error" || result.stopReason === "aborted";
+				const isError = isFailedResult(result);
 				if (isError) {
-					const errorMsg =
-						result.errorMessage || result.stderr || getFinalOutput(result.messages) || "(no output)";
+					const errorMsg = getResultOutput(result);
 					return {
 						content: [{ type: "text", text: `Agent ${result.stopReason || "failed"}: ${errorMsg}` }],
 						details: makeDetails("single")([result]),
@@ -740,7 +762,7 @@ export default function (pi: ExtensionAPI) {
 
 			if (details.mode === "single" && details.results.length === 1) {
 				const r = details.results[0];
-				const isError = r.exitCode !== 0 || r.stopReason === "error" || r.stopReason === "aborted";
+				const isError = isFailedResult(r);
 				const icon = isError ? theme.fg("error", "✗") : theme.fg("success", "✓");
 				const displayItems = getDisplayItems(r.messages);
 				const finalOutput = getFinalOutput(r.messages);
@@ -893,8 +915,8 @@ export default function (pi: ExtensionAPI) {
 
 			if (details.mode === "parallel") {
 				const running = details.results.filter((r) => r.exitCode === -1).length;
-				const successCount = details.results.filter((r) => r.exitCode === 0).length;
-				const failCount = details.results.filter((r) => r.exitCode > 0).length;
+				const successCount = details.results.filter((r) => r.exitCode !== -1 && !isFailedResult(r)).length;
+				const failCount = details.results.filter((r) => r.exitCode !== -1 && isFailedResult(r)).length;
 				const isRunning = running > 0;
 				const icon = isRunning
 					? theme.fg("warning", "⏳")
@@ -916,7 +938,7 @@ export default function (pi: ExtensionAPI) {
 					);
 
 					for (const r of details.results) {
-						const rIcon = r.exitCode === 0 ? theme.fg("success", "✓") : theme.fg("error", "✗");
+						const rIcon = isFailedResult(r) ? theme.fg("error", "✗") : theme.fg("success", "✓");
 						const displayItems = getDisplayItems(r.messages);
 						const finalOutput = getFinalOutput(r.messages);
 
@@ -963,9 +985,9 @@ export default function (pi: ExtensionAPI) {
 					const rIcon =
 						r.exitCode === -1
 							? theme.fg("warning", "⏳")
-							: r.exitCode === 0
-								? theme.fg("success", "✓")
-								: theme.fg("error", "✗");
+							: isFailedResult(r)
+								? theme.fg("error", "✗")
+								: theme.fg("success", "✓");
 					const displayItems = getDisplayItems(r.messages);
 					text += `\n\n${theme.fg("muted", "─── ")}${theme.fg("accent", r.agent)} ${rIcon}`;
 					if (displayItems.length === 0)