fix(pi-subagents): model inheritance and fatal-error lifecycle

extensions/pi-subagents/.gitignore

@@ -7,3 +7,4 @@ node_modules/
 package-lock.json
 
 .spec/
+bun.lock

extensions/pi-subagents/skills/pi-subagents/SKILL.md

@@ -16,6 +16,7 @@ Use this tool to launch unrestricted child Pi sessions. The caller must include
   - Timeout is only a notification threshold: the parent is informed that the child is still running; the child is not killed.
   - The returned subagent id is also the child Pi session id.
   - Child output is written to `result.log` under the child subagent directory.
+  - When `model` is omitted, the child inherits the parent session's active model (e.g., `openai-codex/gpt-5.6-sol`). Pass an explicit `model` to override (e.g., `"anthropic/claude-sonnet-4-5"`).
 
 ## Usage
 

extensions/pi-subagents/src/extension/index.ts

@@ -1188,7 +1188,38 @@ function startChild(
 		throw new Error("Subagent process lifeline pipe was not created.");
 	}
 	lifelines.set(record.id, lifeline);
-	child.stdout.pipe(stdoutStream);
+
+	// 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.stderr.pipe(stderrStream);
 
 	const done = new Promise<PersistedSubagentRecord>((resolve) => {
@@ -1201,6 +1232,12 @@ 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)),
@@ -1227,7 +1264,10 @@ function startChild(
 			record.completedAt = Date.now();
 			record.updatedAt = Date.now();
 
-			if (code !== 0 && !record.error)
+			// 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)
 				record.error =
 					stderr.trim() ||
 					`Subagent exited with code ${code}${signal ? ` (${signal})` : ""}`;
@@ -1241,7 +1281,11 @@ function startChild(
 			if (!hasExistingResult) {
 				// Subagent did not write to result file — auto-save final output
 				const finalOutput = extractFinalOutput(stdout);
-				if (finalOutput && record.outputFile) {
+				// When we have a provider error captured from stdout
+			// monitoring, use the (error) fallback.
+			if (record.error && record.outputFile) {
+				fs.writeFileSync(record.outputFile, "(error)\n", { mode: 0o600 });
+			} else if (finalOutput && record.outputFile) {
 					fs.writeFileSync(record.outputFile, `${finalOutput}\n`, {
 						mode: 0o600,
 					});

extensions/pi-subagents/src/runs/shared/pi-args.ts

@@ -65,6 +65,10 @@ 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");
+
 	// Add only the minimal runtime extension. Do not disable normal extensions,
 	// skills, tools, MCP direct tools, or project context.
 	args.push("--extension", PROMPT_RUNTIME_EXTENSION_PATH);

extensions/pi-subagents/test/support/mock-pi-script.mjs

@@ -39,7 +39,7 @@ function claimNextResponse(dir) {
 	return JSON.parse(fs.readFileSync(defaultPath, "utf-8"));
 }
 
-function defaultAssistantMessage(output) {
+function defaultAssistantMessage(output, overrides = {}) {
 	return {
 		type: "message_end",
 		message: {
@@ -54,6 +54,7 @@ function defaultAssistantMessage(output) {
 				cacheWrite: 0,
 				cost: { total: 0.001 },
 			},
+			...overrides,
 		},
 	};
 }
@@ -191,6 +192,14 @@ async function main() {
 		await new Promise((resolve) => setTimeout(resolve, response.delay));
 	}
 
+	const assistantOverrides = {};
+	if (typeof response.stopReason === "string") {
+		assistantOverrides.stopReason = response.stopReason;
+	}
+	if (typeof response.errorMessage === "string") {
+		assistantOverrides.errorMessage = response.errorMessage;
+	}
+
 	if (Array.isArray(response.steps) && response.steps.length > 0) {
 		for (const step of response.steps) {
 			if (typeof step?.delay === "number" && step.delay > 0) {
@@ -210,10 +219,10 @@ async function main() {
 			response.echoEnv.map((key) => [key, process.env[key] ?? null]),
 		);
 		if (jsonMode)
-			writeJsonlLine(defaultAssistantMessage(JSON.stringify(envSnapshot)));
+			writeJsonlLine(defaultAssistantMessage(JSON.stringify(envSnapshot), assistantOverrides));
 		else process.stdout.write(`${JSON.stringify(envSnapshot)}\n`);
 	} else if (typeof response.output === "string") {
-		if (jsonMode) writeJsonlLine(defaultAssistantMessage(response.output));
+		if (jsonMode) writeJsonlLine(defaultAssistantMessage(response.output, assistantOverrides));
 		else process.stdout.write(`${response.output}\n`);
 	}