fix(coding-agent): fell back for kernel field when os.version returns unknown

Bun on macOS 15+ (Darwin 24/25) returns the literal string "unknown" from
os.version() when uv_os_uname() cannot populate the version field. That
value was piped straight into the <workstation> block as `Kernel:
unknown`, and the model then guessed the OS glyph from the ambiguous
signal — displaying a Windows logo on Macs.

Fall back to `${os.type()} ${os.release()}` (uname -s + uname -r, e.g.
`Darwin 25.5.0`) whenever os.version() is empty or the literal
"unknown". Linux keeps its build string when Bun returns one.

Fixes #4141

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed the `<workstation>` block rendering `Kernel: unknown` on macOS 15+, which caused the model to display the wrong OS glyph, by falling back to `<type> <release>` (e.g. `Darwin 25.5.0`) when `os.version()` returns `"unknown"` or an empty string ([#4141](https://github.com/can1357/oh-my-pi/issues/4141)).
+
 ## [16.2.12] - 2026-07-01
 
 ### Breaking Changes

packages/coding-agent/src/system-prompt.ts

@@ -249,6 +249,20 @@ async function getCachedGpu(): Promise<string | undefined> {
 	await logger.time("getCachedGpu:saveGpuCache", saveGpuCache, { gpu });
 	return gpu ?? undefined;
 }
+/**
+ * Kernel identity for the workstation block. Prefers the uname build string
+ * from `os.version()`, but Bun on macOS 15+ (Darwin 24/25) returns the literal
+ * `"unknown"` when `uv_os_uname()`'s `version` field is empty — which surfaces
+ * `Kernel: unknown` in the system prompt and makes the model misidentify the
+ * host as Windows (#4141). Fall back to `<type> <release>` (uname -s + -r) so
+ * macOS is always tagged as `Darwin <release>` and Linux keeps its build info.
+ */
+function getKernelIdentity(): string {
+	const version = os.version()?.trim();
+	if (version && version.toLowerCase() !== "unknown") return version;
+	return `${os.type()} ${os.release()}`.trim();
+}
+
 function getEnvironmentInfo(gpu: string | undefined): Array<{ label: string; value: string }> {
 	let cpuModel: string | undefined;
 	try {
@@ -259,7 +273,7 @@ function getEnvironmentInfo(gpu: string | undefined): Array<{ label: string; val
 	const entries: Array<{ label: string; value: string | undefined }> = [
 		{ label: "OS", value: `${os.platform()} ${os.release()}` },
 		{ label: "Distro", value: os.type() },
-		{ label: "Kernel", value: os.version() },
+		{ label: "Kernel", value: getKernelIdentity() },
 		{ label: "Arch", value: os.arch() },
 		{ label: "CPU", value: cpuModel },
 		{ label: "GPU", value: gpu },

packages/coding-agent/test/system-prompt-kernel.test.ts

@@ -0,0 +1,86 @@
+import { afterEach, beforeEach, describe, expect, it, spyOn } from "bun:test";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { buildSystemPrompt } from "@oh-my-pi/pi-coding-agent/system-prompt";
+import { cleanupTempHome } from "./helpers/temp-home-cleanup";
+
+const EMPTY_TREE = {
+	rootPath: "",
+	rendered: "",
+	truncated: false,
+	totalLines: 0,
+	agentsMdFiles: [],
+};
+
+// Regression: Bun on macOS 15+ (Darwin 24/25) makes `os.version()` return the
+// literal "unknown", which used to leak into the <workstation> block as
+// `Kernel: unknown` and caused the model to display the wrong OS glyph
+// (issue #4141). The Kernel field must always carry a real identity.
+describe("system prompt Kernel field", () => {
+	let tempDir = "";
+	let tempHomeDir = "";
+	let originalHome: string | undefined;
+
+	beforeEach(() => {
+		tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-prompt-kernel-"));
+		tempHomeDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-prompt-kernel-home-"));
+		originalHome = process.env.HOME;
+		process.env.HOME = tempHomeDir;
+	});
+
+	afterEach(cleanupTempHome(() => ({ tempDir, tempHomeDir, originalHome })));
+
+	it(`falls back to "<type> <release>" when os.version() returns "unknown" (Bun on macOS 15+)`, async () => {
+		spyOn(os, "version").mockReturnValue("unknown");
+		spyOn(os, "type").mockReturnValue("Darwin");
+		spyOn(os, "release").mockReturnValue("25.5.0");
+
+		const { systemPrompt } = await buildSystemPrompt({
+			cwd: tempDir,
+			contextFiles: [],
+			skills: [],
+			rules: [],
+			toolNames: [],
+			workspaceTree: { ...EMPTY_TREE, rootPath: tempDir },
+		});
+
+		const rendered = systemPrompt.join("\n\n");
+		expect(rendered).not.toContain("Kernel: unknown");
+		expect(rendered).toContain("Kernel: Darwin 25.5.0");
+	});
+
+	it("also falls back when os.version() is empty or whitespace", async () => {
+		spyOn(os, "version").mockReturnValue("   ");
+		spyOn(os, "type").mockReturnValue("Darwin");
+		spyOn(os, "release").mockReturnValue("25.5.0");
+
+		const { systemPrompt } = await buildSystemPrompt({
+			cwd: tempDir,
+			contextFiles: [],
+			skills: [],
+			rules: [],
+			toolNames: [],
+			workspaceTree: { ...EMPTY_TREE, rootPath: tempDir },
+		});
+
+		expect(systemPrompt.join("\n\n")).toContain("Kernel: Darwin 25.5.0");
+	});
+
+	it("keeps the real uname build string when os.version() is populated", async () => {
+		spyOn(os, "version").mockReturnValue(
+			"Darwin Kernel Version 25.5.0: Tue Nov  7 21:48:04 PST 2026; root:xnu-11215.1.12~1/RELEASE_ARM64_T6031",
+		);
+
+		const { systemPrompt } = await buildSystemPrompt({
+			cwd: tempDir,
+			contextFiles: [],
+			skills: [],
+			rules: [],
+			toolNames: [],
+			workspaceTree: { ...EMPTY_TREE, rootPath: tempDir },
+		});
+
+		expect(systemPrompt.join("\n\n")).toContain("Kernel: Darwin Kernel Version 25.5.0:");
+	});
+});