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Merged
tasercake merged 1 commit into from
Jul 27, 2026
Merged

fix(pi-subagents): prevent oversized stdout string failures #46

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1 change: 1 addition & 0 deletions extensions/pi-subagents/CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@
### Added

### Fixed
- Run minimal child subagents in print mode instead of persisting cumulative JSON streaming events, and bound legacy output recovery so oversized logs cannot exceed V8's string limit.
- Prevent the parent-death lifeline watcher from keeping successfully settled subagents alive indefinitely.

## [0.24.2] - 2026-05-10
Expand Down
213 changes: 105 additions & 108 deletions extensions/pi-subagents/src/extension/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -258,6 +258,9 @@ function extractFinalOutput(stdout: string): string {
if (event.message?.role === "assistant") {
const text = extractTextFromMessageContent(event.message.content);
if (text.trim()) lastAssistant = text.trim();
} else if (!event.type) {
// Print-mode answers may themselves be valid JSON.
rawLines.push(line);
}
} catch {
rawLines.push(line);
Expand All @@ -266,6 +269,94 @@ function extractFinalOutput(stdout: string): string {
return lastAssistant || rawLines.join("\n").trim();
}

const MAX_RECOVERY_FILE_BYTES = 16 * 1024 * 1024;

function hasNonEmptyFile(filePath: string | undefined): boolean {
if (!filePath) return false;
try {
return fs.statSync(filePath).size > 0;
} catch {
return false;
}
}

function readRecoveryFile(
filePath: string | undefined,
label: string,
): { text: string; issue?: string } {
if (!filePath) return { text: "" };
try {
const size = fs.statSync(filePath).size;
if (size > MAX_RECOVERY_FILE_BYTES) {
return {
text: "",
issue: `${label} exceeds the ${MAX_RECOVERY_FILE_BYTES}-byte recovery limit: ${filePath}`,
};
}
return { text: fs.readFileSync(filePath, "utf-8") };
} catch (error) {
const code =
error && typeof error === "object" && "code" in error
? String(error.code)
: undefined;
if (code === "ENOENT") return { text: "" };
return {
text: "",
issue: `Could not read ${label} ${filePath}: ${error instanceof Error ? error.message : String(error)}`,
};
}
}

function recoverFinalOutput(record: PersistedSubagentRecord): {
output: string;
issue?: string;
} {
try {
recordDiscoveredSessionFile(record);
} catch {
// Discovery is best-effort; stdout remains the compatibility fallback.
}
let issue: string | undefined;
for (const [filePath, label] of [
[record.sessionFile, "child session"],
[record.stdoutFile, "child stdout"],
] as const) {
const recovered = readRecoveryFile(filePath, label);
issue ??= recovered.issue;
const output = extractFinalOutput(recovered.text);
if (output) return { output };
}
return { output: "", issue };
}

function ensureResultFile(
record: PersistedSubagentRecord,
stderr: { text: string; issue?: string },
): void {
if (!record.outputFile || hasNonEmptyFile(record.outputFile)) return;
try {
const recovered = recoverFinalOutput(record);
if (record.error) {
fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
} else if (recovered.output) {
fs.writeFileSync(record.outputFile, `${recovered.output}\n`, {
mode: 0o600,
});
} else if (stderr.text.trim()) {
record.error = stderr.text.trim();
fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
} else if (recovered.issue || stderr.issue) {
record.error = recovered.issue ?? stderr.issue;
fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
} else {
fs.writeFileSync(record.outputFile, "(no output)\n", { mode: 0o600 });
}
} catch (error) {
record.error ??=
`Could not create subagent result: ${error instanceof Error ? error.message : String(error)}`;
}
}

function refreshRecord(record: PersistedSubagentRecord): {
record: PersistedSubagentRecord;
changed: boolean;
Expand All @@ -274,40 +365,12 @@ function refreshRecord(record: PersistedSubagentRecord): {
return { record, changed: false };
}
const refreshed: PersistedSubagentRecord = { ...record };
const stdout = fs.existsSync(refreshed.stdoutFile)
? fs.readFileSync(refreshed.stdoutFile, "utf-8")
: "";
const stderr = fs.existsSync(refreshed.stderrFile)
? fs.readFileSync(refreshed.stderrFile, "utf-8")
: "";
refreshed.running = false;
refreshed.updatedAt = Date.now();
refreshed.completedAt ??= Date.now();

// Check if subagent already wrote to the result file.
const hasExistingResult =
refreshed.outputFile &&
fs.existsSync(refreshed.outputFile) &&
fs.readFileSync(refreshed.outputFile, "utf-8").trim().length > 0;

if (!hasExistingResult) {
const finalOutput = extractFinalOutput(stdout);
if (finalOutput && refreshed.outputFile) {
fs.writeFileSync(refreshed.outputFile, `${finalOutput}\n`, {
mode: 0o600,
});
} else if (stderr.trim()) {
// Subagent produced only stderr, no stdout output.
if (!refreshed.error) refreshed.error = stderr.trim();
if (refreshed.outputFile) {
fs.writeFileSync(refreshed.outputFile, "(error)\n", { mode: 0o600 });
}
} else if (refreshed.outputFile) {
// Edge case: neither stdout nor stderr produced content.
// Write a placeholder so the parent gets a valid result file.
fs.writeFileSync(refreshed.outputFile, "(no output)\n", { mode: 0o600 });
}
}
const stderr = readRecoveryFile(refreshed.stderrFile, "child stderr");
ensureResultFile(refreshed, stderr);
refreshed.result = refreshed.outputFile;
return { record: refreshed, changed: true };
}
Expand Down Expand Up @@ -907,37 +970,7 @@ function startChild(
}
lifelines.set(record.id, lifeline);

// Monitor stdout for terminal provider errors (e.g. HTTP 402).
// When stopReason==="error" is emitted, the child Pi process may
// stay alive indefinitely. Kill it so the close handler finalizes
// the record promptly.
let terminalErrorText: string | undefined;
let errorTimer: NodeJS.Timeout | undefined;
child.stdout.on("data", (chunk: Buffer) => {
// Write to file (replaces pipe).
stdoutStream.write(chunk);
if (terminalErrorText) return;
const text = chunk.toString("utf-8");
if (/"stopReason"\s*:\s*"error"/.test(text)) {
const errMatch = text.match(/"errorMessage"\s*:\s*"([^"]+)"/);
terminalErrorText = errMatch?.[1] ?? "Provider error";
if (!record.error) {
record.error = terminalErrorText;
}
errorTimer = setTimeout(() => {
errorTimer = undefined;
if (child.exitCode === null && child.signalCode === null) {
child.kill("SIGTERM");
setTimeout(() => {
if (child.exitCode === null && child.signalCode === null) {
child.kill("SIGKILL");
}
}, 1000).unref();
}
}, 1000);
errorTimer.unref();
}
});
child.stdout.pipe(stdoutStream);
child.stderr.pipe(stderrStream);

const done = new Promise<PersistedSubagentRecord>((resolve) => {
Expand All @@ -950,12 +983,6 @@ function startChild(
if (finalized) return;
finalized = true;

// Clear error-detection timer if still pending.
if (errorTimer) {
clearTimeout(errorTimer);
errorTimer = undefined;
}

await Promise.all([
new Promise<void>((r) => stdoutStream.end(r)),
new Promise<void>((r) => stderrStream.end(r)),
Expand All @@ -966,55 +993,25 @@ function startChild(
closeLifeline(record.id);
cleanupTempDir(built.tempDir);

const stdout = fs.existsSync(record.stdoutFile)
? fs.readFileSync(record.stdoutFile, "utf-8")
: "";
const stderr = fs.existsSync(record.stderrFile)
? fs.readFileSync(record.stderrFile, "utf-8")
: "";

record.running = false;
record.completedAt = Date.now();
record.updatedAt = Date.now();

// Only assign a code-based error for actual non-zero exit
// codes (not null signals) and when we don't already have
// a provider error from stdout monitoring.
if (code !== null && code !== 0 && !record.error)
const stderr = readRecoveryFile(record.stderrFile, "child stderr");
if (code !== null && code !== 0 && !record.error) {
record.error =
stderr.trim() ||
stderr.text.trim() ||
stderr.issue ||
`Subagent exited with code ${code}${signal ? ` (${signal})` : ""}`;

// Check if subagent already wrote to result file
const hasExistingResult =
record.outputFile &&
fs.existsSync(record.outputFile) &&
fs.readFileSync(record.outputFile, "utf-8").trim().length > 0;

if (!hasExistingResult) {
// Subagent did not write to result file — auto-save final output
const finalOutput = extractFinalOutput(stdout);
// When we have a provider error captured from stdout
// monitoring, use the (error) fallback.
if (record.error && record.outputFile) {
fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
} else if (finalOutput && record.outputFile) {
fs.writeFileSync(record.outputFile, `${finalOutput}\n`, {
mode: 0o600,
});
} else if (stderr.trim()) {
// Subagent produced only stderr, no stdout output.
if (!record.error) record.error = stderr.trim();
if (record.outputFile) {
fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
}
} else if (record.outputFile) {
// Edge case: neither stdout nor stderr produced content.
fs.writeFileSync(record.outputFile, "(no output)\n", { mode: 0o600 });
}
}

ensureResultFile(record, stderr);
record.result = record.outputFile;
recordDiscoveredSessionFile(record);
try {
recordDiscoveredSessionFile(record);
} catch {
// Session-file discovery is best-effort after terminal persistence.
}

upsertRecord(record);

Expand Down
6 changes: 3 additions & 3 deletions extensions/pi-subagents/src/runs/shared/pi-args.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -65,9 +65,9 @@ export function buildPiArgs(input: BuildPiArgsInput): BuildPiArgsResult {

if (input.model) args.push("--model", input.model);

// Use JSON mode so child output is machine-parseable for error
// detection and result extraction.
args.push("--mode", "json");
// Child stdout is only a fallback for the final answer. Full JSON event
// streams repeat cumulative message state and can grow to hundreds of MB.
args.push("--print");

// Add only the minimal runtime extension. Do not disable normal extensions,
// skills, tools, MCP direct tools, or project context.
Expand Down
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