diff --git a/extensions/README.md b/extensions/README.md
index 42b08d72..1313c614 100644
--- a/extensions/README.md
+++ b/extensions/README.md
@@ -89,6 +89,8 @@ convert(({ body, frontmatter, stem }) => ({
 }));
 ```
 
+For targets with required fields, validate them in `convert.js` and exit non-zero with a clear message. For example, Codex TOML agents require `description`, so `extensions/codex-agents` rejects inputs without `description` frontmatter instead of generating a TOML file that Codex cannot load.
+
 Plain values become TOML strings:
 
 ```toml
diff --git a/extensions/codex-agents/convert.js b/extensions/codex-agents/convert.js
index 4253dfe0..5ee114c4 100644
--- a/extensions/codex-agents/convert.js
+++ b/extensions/codex-agents/convert.js
@@ -1,9 +1,17 @@
 #!/usr/bin/env node
 const { block, convert } = require("./md-toml");
 
-convert(({ body, frontmatter, stem }) => ({
-  name: frontmatter.name || stem,
-  description: frontmatter.description,
-  model: frontmatter.model,
-  developer_instructions: block(body),
-}));
+convert(({ body, frontmatter, stem }) => {
+  if (!frontmatter.description || !frontmatter.description.trim()) {
+    throw new Error(
+      "codex-agents: missing required frontmatter 'description' (Codex custom agents require description)"
+    );
+  }
+
+  return {
+    name: frontmatter.name || stem,
+    description: frontmatter.description,
+    model: frontmatter.model,
+    developer_instructions: block(body),
+  };
+});
diff --git a/extensions/codex-agents/md-toml.js b/extensions/codex-agents/md-toml.js
index d3e77b2b..5b27b143 100644
--- a/extensions/codex-agents/md-toml.js
+++ b/extensions/codex-agents/md-toml.js
@@ -20,6 +20,9 @@ function convert(mapFields) {
     }
 
     process.stdout.write(lines.join("\n") + (lines.length ? "\n" : ""));
+  }).catch((err) => {
+    process.stderr.write((err && err.message ? err.message : String(err)) + "\n");
+    process.exit(1);
   });
 }
 
diff --git a/internal/sync/extension_test.go b/internal/sync/extension_test.go
index 9802c4c9..d6fd5bc2 100644
--- a/internal/sync/extension_test.go
+++ b/internal/sync/extension_test.go
@@ -2,6 +2,7 @@ package sync
 
 import (
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -439,3 +440,90 @@ func TestSyncExtra_TransformRejectsNonCopyMode(t *testing.T) {
 		}
 	}
 }
+
+func TestBundledCodexAgentExtension_TransformsRequiredFields(t *testing.T) {
+	node := requireNode(t)
+	root := testRepoRoot(t)
+
+	cmd := exec.Command(node, filepath.Join(root, "extensions", "codex-agents", "convert.js"))
+	cmd.Env = append(os.Environ(), "SS_REL_PATH=reviewer.md")
+	cmd.Stdin = strings.NewReader("---\nname: reviewer\ndescription: Review PRs for correctness\nmodel: gpt-5.4\n---\nReview code like an owner.\n")
+
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("codex-agents convert failed: %v\n%s", err, out)
+	}
+
+	got := string(out)
+	for _, want := range []string{
+		`name = "reviewer"`,
+		`description = "Review PRs for correctness"`,
+		`model = "gpt-5.4"`,
+		`developer_instructions = """`,
+		`Review code like an owner.`,
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("converted output missing %q:\n%s", want, got)
+		}
+	}
+}
+
+func TestBundledCodexAgentExtension_RequiresDescription(t *testing.T) {
+	node := requireNode(t)
+	root := testRepoRoot(t)
+
+	for _, tc := range []struct {
+		name  string
+		input string
+	}{
+		{
+			name:  "missing",
+			input: "# Reviewer\nReview code like an owner.\n",
+		},
+		{
+			name:  "whitespace-only",
+			input: "---\ndescription: \"   \"\n---\nReview code like an owner.\n",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(node, filepath.Join(root, "extensions", "codex-agents", "convert.js"))
+			cmd.Env = append(os.Environ(), "SS_REL_PATH=reviewer.md")
+			cmd.Stdin = strings.NewReader(tc.input)
+
+			out, err := cmd.CombinedOutput()
+			if err == nil {
+				t.Fatalf("expected missing description to fail, got success:\n%s", out)
+			}
+			if !strings.Contains(string(out), "missing required frontmatter 'description'") {
+				t.Fatalf("expected missing description error, got:\n%s", out)
+			}
+		})
+	}
+}
+
+func requireNode(t *testing.T) string {
+	t.Helper()
+	node, err := exec.LookPath("node")
+	if err != nil {
+		t.Skip("node not available; skipping bundled JavaScript extension check")
+	}
+	return node
+}
+
+func testRepoRoot(t *testing.T) string {
+	t.Helper()
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("getwd: %v", err)
+	}
+	for {
+		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
+			return dir
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			t.Fatal("could not find repo root")
+		}
+		dir = parent
+	}
+}
diff --git a/website/docs/reference/commands/extras.md b/website/docs/reference/commands/extras.md
index 70b368a3..5320ef59 100644
--- a/website/docs/reference/commands/extras.md
+++ b/website/docs/reference/commands/extras.md
@@ -338,7 +338,7 @@ extras:
         extension: codex-agents
 ```
 
-`skillshare sync extras` converts each `<agent>.md` into `~/.codex/agents/<agent>.toml`, mapping frontmatter `name`, `description`, and `model` and folding the markdown body into `developer_instructions` (other frontmatter keys are dropped). No separate copy of the agents is needed.
+`skillshare sync extras` converts each `<agent>.md` into `~/.codex/agents/<agent>.toml`, mapping frontmatter `name`, `description`, and `model` and folding the markdown body into `developer_instructions` (other frontmatter keys are dropped). Codex requires `description`, so the reference transform reports a clear error when that frontmatter field is missing. No separate copy of the agents is needed.
 
 ---