diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index d1fbaf0b..2d2e61df 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -27,9 +27,13 @@ permissions:
 
 jobs:
   # ── Job 1: Agent 크로스컴파일 ────────────────────────────────────────────
-  # ubuntu-latest를 쓰는 이유: CGO 미사용 Go 바이너리 + Node runtime download +
-  # LSP bundle은 OS 독립적이라 어느 runner에서도 OK. macOS runner 분당 과금이
-  # ubuntu 대비 10x이므로 ubuntu에서 먼저 처리 후 artifact로 공유한다.
+  # ubuntu-latest를 쓰는 이유: 에이전트는 CGO_ENABLED=0(정적 링크)로 빌드돼
+  # 빌드 호스트의 glibc/OS 와 무관 + Node runtime download + LSP bundle 도 OS
+  # 독립적이라 어느 runner 에서도 OK. (과거 linux/amd64 는 네이티브 빌드라 CGO 가
+  # 켜져 빌드 호스트 glibc 에 동적 링크됐고, 구버전 배포판에서 'GLIBC_x.xx not
+  # found' 로 실패했다 — build-agent.ts 에서 CGO_ENABLED=0 을 강제해 해결.)
+  # macOS runner 분당 과금이 ubuntu 대비 10x 이므로 ubuntu 에서 먼저 처리 후
+  # artifact 로 공유한다.
   build-agent:
     name: Build agent (cross-compile)
     runs-on: ubuntu-latest
diff --git a/package.json b/package.json
index 6409d1ae..320f2702 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "nexus-code",
   "productName": "NexusCode",
-  "version": "0.5.0",
+  "version": "0.5.1",
   "description": "Multi-workspace VSCode-style editor for macOS. Monaco editor + terminal in one window.",
   "license": "MIT",
   "private": true,
diff --git a/scripts/build-agent.ts b/scripts/build-agent.ts
index 804c2fb5..7edb9536 100644
--- a/scripts/build-agent.ts
+++ b/scripts/build-agent.ts
@@ -214,10 +214,25 @@ async function buildAgentBinaries(args: {
     const name = `agent-${args.version}-${target.os}-${target.arch}`;
     await args.run(
       "go",
-      ["build", "-ldflags=-s -w", "-o", path.join(args.outDir, name), "./cmd/agent"],
+      [
+        "build",
+        "-tags=netgo,osusergo",
+        "-ldflags=-s -w",
+        "-o",
+        path.join(args.outDir, name),
+        "./cmd/agent",
+      ],
       {
         cwd: args.rootDir,
-        env: { ...process.env, GOOS: target.os, GOARCH: target.arch },
+        // CGO_ENABLED=0 forces a fully static binary. Without it the linux/amd64
+        // target is a *native* build on the CI runner, where Go defaults
+        // CGO_ENABLED to 1; because the agent imports "net" (and os/user), the
+        // binary then dynamically links the runner's glibc and requires that
+        // exact version at runtime — failing on older distros such as Ubuntu
+        // 20.04 (glibc 2.31) with "version `GLIBC_x.xx' not found". The
+        // netgo/osusergo tags keep the net and os/user resolvers in pure Go so
+        // disabling cgo does not silently change name resolution behavior.
+        env: { ...process.env, GOOS: target.os, GOARCH: target.arch, CGO_ENABLED: "0" },
       },
     );
     artifacts.push({ ...target, path: name });
diff --git a/src/main/infra/agent/channel/reconnecting-process-channel.ts b/src/main/infra/agent/channel/reconnecting-process-channel.ts
index d58b2550..139129a0 100644
--- a/src/main/infra/agent/channel/reconnecting-process-channel.ts
+++ b/src/main/infra/agent/channel/reconnecting-process-channel.ts
@@ -22,10 +22,22 @@ export interface AgentReconnectOptions {
   readonly maxDelayMs?: number;
 }
 
+/**
+ * Diagnostic context captured at the moment a child process closes. Passed to
+ * `closeError` so the resulting terminal error can record the exit code,
+ * signal, and the tail of the process's stderr (e.g. a loader error) — making
+ * the file log self-sufficient instead of an empty-cause `ssh.unknown`.
+ */
+export interface ChannelCloseContext {
+  readonly code?: number | null;
+  readonly signal?: NodeJS.Signals | null;
+  readonly stderrTail?: string;
+}
+
 export interface ReconnectingProcessChannelOptions {
   readonly spawn: () => ChildProcessWithoutNullStreams;
   readonly classifyStderr: StderrClassifier;
-  readonly closeError: (wasReady: boolean) => Error;
+  readonly closeError: (wasReady: boolean, context?: ChannelCloseContext) => Error;
   readonly requestTimeoutMs?: number;
   readonly expectedProtocolMajor?: string;
   readonly reconnect?: AgentReconnectOptions;
@@ -218,12 +230,13 @@ export function createReconnectingProcessChannel(
     if (active !== attempt) return;
     attempt.closed = true;
     clearForceKillTimer(attempt);
-    const { wasReady } = attempt.pipe.notifyClose();
+    const { wasReady, stderrTail } = attempt.pipe.notifyClose();
+    const closeContext: ChannelCloseContext = { code, signal, stderrTail };
 
     if (state === "disposed" || terminalError) return;
     if (code === 0 && wasReady) {
       state = "terminal";
-      terminalError = options.closeError(true);
+      terminalError = options.closeError(true, closeContext);
       emitLifecycle({ type: "exit", code, signal });
       return;
     }
@@ -244,7 +257,7 @@ export function createReconnectingProcessChannel(
       return;
     }
 
-    const error = options.closeError(wasReady);
+    const error = options.closeError(wasReady, closeContext);
     attempt.pipe.fail(error);
     state = "terminal";
     terminalError = error;
diff --git a/src/main/infra/agent/pipe.ts b/src/main/infra/agent/pipe.ts
index d25d2368..8695b297 100644
--- a/src/main/infra/agent/pipe.ts
+++ b/src/main/infra/agent/pipe.ts
@@ -184,8 +184,13 @@ export interface NdjsonPipe {
   dispose(): void;
   /** Marks the pipe terminally failed from the orchestrator's side. */
   fail(error: Error): void;
-  /** Flushes buffered lines and reports whether ready had settled before close. */
-  notifyClose(): { wasReady: boolean };
+  /**
+   * Flushes buffered lines and reports whether ready had settled before close.
+   * `stderrTail` carries the last few raw stderr lines (loader errors, panics,
+   * unclassified warnings) so the orchestrator can attach them to the terminal
+   * error's cause — the file log is then self-sufficient for diagnosis.
+   */
+  notifyClose(): { wasReady: boolean; stderrTail: string };
 }
 
 /**
@@ -207,6 +212,13 @@ export function createNdjsonPipe(deps: NdjsonPipeDependencies): NdjsonPipe {
   });
   const stderrLines = createLineSplitter(handleStderrLine);
 
+  // Ring buffer of the most recent raw stderr lines. Retained regardless of
+  // classification so an unrecognized fatal line (e.g. a glibc loader error)
+  // is still available as diagnostic context when the process closes.
+  const STDERR_TAIL_MAX_LINES = 20;
+  const STDERR_TAIL_MAX_LINE_CHARS = 200;
+  const recentStderr: string[] = [];
+
   let nextRequestId = 1;
   let disposed = false;
   let terminalError: Error | null = null;
@@ -388,9 +400,23 @@ export function createNdjsonPipe(deps: NdjsonPipeDependencies): NdjsonPipe {
       }
     }
 
+    // Retain the raw line before classification so it survives even when no
+    // pattern matches — the close handler reads this tail for the cause.
+    recordStderr(line);
+
     const code = deps.classifyStderr(line);
     if (code) {
-      selfFail(createSshError(code));
+      // Attach the offending line as cause so the file log shows *why* the
+      // agent failed to start, not just the classified code.
+      selfFail(createSshError(code, line));
+    }
+  }
+
+  /** Appends one stderr line to the bounded diagnostic tail buffer. */
+  function recordStderr(line: string): void {
+    recentStderr.push(line.slice(0, STDERR_TAIL_MAX_LINE_CHARS));
+    if (recentStderr.length > STDERR_TAIL_MAX_LINES) {
+      recentStderr.shift();
     }
   }
 
@@ -529,10 +555,10 @@ export function createNdjsonPipe(deps: NdjsonPipeDependencies): NdjsonPipe {
       rejectReady(error);
       rejectPendingRequests(error);
     },
-    notifyClose(): { wasReady: boolean } {
+    notifyClose(): { wasReady: boolean; stderrTail: string } {
       stdoutLines.flush();
       stderrLines.flush();
-      return { wasReady: readySettled };
+      return { wasReady: readySettled, stderrTail: recentStderr.join(" | ") };
     },
   };
 }
@@ -553,7 +579,9 @@ export function createSshError(code: SshErrorCode, cause?: unknown): SshError {
   try {
     const causeMsg =
       cause instanceof Error ? cause.message : cause === undefined ? "" : String(cause);
-    const causeSnippet = causeMsg.slice(0, 300);
+    // 600 chars (up from 300) so a close diagnostic carrying an exit code plus
+    // a multi-line stderr tail is not truncated before the actual error text.
+    const causeSnippet = causeMsg.slice(0, 600);
     const stack = (error.stack ?? "").split("\n").slice(1, 6).join(" | ");
     getMalformedStdoutLogger().warn(
       `SshError throw: code=${code} cause=${causeSnippet} stack=${stack}`,
diff --git a/src/main/infra/agent/ssh/channel.ts b/src/main/infra/agent/ssh/channel.ts
index 05936474..81f5f508 100644
--- a/src/main/infra/agent/ssh/channel.ts
+++ b/src/main/infra/agent/ssh/channel.ts
@@ -8,6 +8,7 @@ import { ChannelEventRegistry } from "../channel/event-registry";
 import { createDisposedError, createSshError } from "../pipe";
 import {
   type AgentReconnectOptions,
+  type ChannelCloseContext,
   createReconnectingProcessChannel,
 } from "../channel/reconnecting-process-channel";
 import { classifyAuthLine } from "./auth";
@@ -40,6 +41,29 @@ export interface SshChannelDependencies {
   readonly reconnect?: AgentReconnectOptions;
 }
 
+/**
+ * Renders the close context into a single-line cause string for the terminal
+ * `ssh.unknown` error. `createSshError` logs this to the file transport, so an
+ * otherwise-opaque transport close ("disconnected by user", empty cause) now
+ * records the exit code, signal, and the tail of the agent's stderr. Returns
+ * undefined when nothing useful was captured, preserving the prior behavior.
+ */
+function formatCloseDiagnostic(context?: ChannelCloseContext): string | undefined {
+  if (!context) return undefined;
+  const parts: string[] = [];
+  if (context.code !== undefined && context.code !== null) {
+    parts.push(`exit code=${context.code}`);
+  }
+  if (context.signal) {
+    parts.push(`signal=${context.signal}`);
+  }
+  const stderrTail = context.stderrTail?.trim();
+  if (stderrTail) {
+    parts.push(`stderr=${stderrTail}`);
+  }
+  return parts.length > 0 ? `agent process closed (${parts.join(" ")})` : undefined;
+}
+
 /**
  * Opens an SSH-backed NDJSON request channel to the remote agent. The
  * orchestrator spawns the SSH client (via ssh-master) and composes an NDJSON
@@ -60,7 +84,8 @@ export function createSshChannel(
         spawn: dependencies.spawn,
       }),
     classifyStderr: classifyAuthLine,
-    closeError: () => createSshError("ssh.unknown"),
+    closeError: (_wasReady, context) =>
+      createSshError("ssh.unknown", formatCloseDiagnostic(context)),
     requestTimeoutMs: dependencies.requestTimeoutMs,
     expectedProtocolMajor: REMOTE_AGENT_PROTOCOL_MAJOR,
     reconnect: dependencies.reconnect,
diff --git a/src/main/infra/agent/ssh/stderr-patterns.ts b/src/main/infra/agent/ssh/stderr-patterns.ts
index 3336130a..4dde2ba2 100644
--- a/src/main/infra/agent/ssh/stderr-patterns.ts
+++ b/src/main/infra/agent/ssh/stderr-patterns.ts
@@ -31,6 +31,13 @@ const SERVER_SPAWN_FAILED_PATTERNS = [
   /\bexec: .*: not found\b/i,
   /\bexec: .*: no such file or directory\b/i,
   /cannot execute: required file not found/i,
+  // Dynamic-loader failures when the agent binary cannot run on the remote:
+  // a glibc too old for the binary's required symbol version (the exact case
+  // that surfaced as an empty-cause ssh.unknown before this classifier
+  // existed), a missing shared library, or an architecture mismatch.
+  /version `?GLIBC_[\d.]+'? not found/i,
+  /error while loading shared libraries/i,
+  /cannot execute binary file/i,
 ];
 
 /**
diff --git a/tests/unit/main/agent/stderr-patterns.test.ts b/tests/unit/main/agent/stderr-patterns.test.ts
new file mode 100644
index 00000000..aed5f9be
--- /dev/null
+++ b/tests/unit/main/agent/stderr-patterns.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test";
+import { classifyStderrLine } from "../../../../src/main/infra/agent/ssh/stderr-patterns";
+
+// Regression coverage for the dynamic-loader patterns. These were added after a
+// glibc-too-old failure surfaced as an empty-cause `ssh.unknown` (the agent
+// binary was dynamically linked against a newer glibc than the remote had).
+// Classifying them as `server.spawn-failed` gives the user "Remote agent failed
+// to start" instead of the generic transport error, and the offending line is
+// attached as the SshError cause in the file log.
+describe("classifyStderrLine — agent loader failures", () => {
+  test("glibc version mismatch", () => {
+    const line =
+      "agent: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by agent)";
+    expect(classifyStderrLine(line)).toBe("server.spawn-failed");
+  });
+
+  test("missing shared library", () => {
+    expect(
+      classifyStderrLine("agent: error while loading shared libraries: libfoo.so.1: cannot open"),
+    ).toBe("server.spawn-failed");
+  });
+
+  test("architecture mismatch", () => {
+    expect(classifyStderrLine("bash: /x/agent: cannot execute binary file: Exec format error")).toBe(
+      "server.spawn-failed",
+    );
+  });
+
+  test("benign login banner is not misclassified", () => {
+    expect(classifyStderrLine("Welcome to monolith! Have a great day.")).toBeNull();
+  });
+
+  test("existing auth/connect classification is unchanged", () => {
+    expect(classifyStderrLine("Permission denied (publickey).")).toBe("ssh.auth-failed");
+    expect(classifyStderrLine("ssh: connect to host x port 22: Connection refused")).toBe(
+      "ssh.connect-failed",
+    );
+  });
+});