diff --git a/workspace-server/internal/handlers/handlers_extended_test.go b/workspace-server/internal/handlers/handlers_extended_test.go
index 97f45fabe..51d141063 100644
--- a/workspace-server/internal/handlers/handlers_extended_test.go
+++ b/workspace-server/internal/handlers/handlers_extended_test.go
@@ -293,6 +293,26 @@ func TestExtended_SecretsSet(t *testing.T) {
 	}
 }
 
+func TestExtended_SecretsSetRejectsHermesCustomProviderInPlatformManagedMode(t *testing.T) {
+	t.Setenv("MOLECULE_LLM_BILLING_MODE", "platform_managed")
+	_ = setupTestDB(t)
+	handler := NewSecretsHandler(nil)
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "22222222-2222-2222-2222-222222222222"}}
+
+	body := `{"key":"HERMES_CUSTOM_BASE_URL","value":"https://api.moonshot.ai/v1"}`
+	c.Request = httptest.NewRequest("POST", "/workspaces/22222222-2222-2222-2222-222222222222/secrets", bytes.NewBufferString(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	handler.Set(c)
+
+	if w.Code != http.StatusBadRequest {
+		t.Fatalf("expected status 400, got %d: %s", w.Code, w.Body.String())
+	}
+}
+
 // ---------- TestSecretsDelete (Extended) ----------
 
 func TestExtended_SecretsDelete(t *testing.T) {
diff --git a/workspace-server/internal/handlers/secrets.go b/workspace-server/internal/handlers/secrets.go
index 62368c513..4d085c3bf 100644
--- a/workspace-server/internal/handlers/secrets.go
+++ b/workspace-server/internal/handlers/secrets.go
@@ -5,7 +5,9 @@ import (
 	"database/sql"
 	"log"
 	"net/http"
+	"os"
 	"regexp"
+	"strings"
 
 	"git.moleculesai.app/molecule-ai/molecule-core/workspace-server/internal/audit"
 	"git.moleculesai.app/molecule-ai/molecule-core/workspace-server/internal/crypto"
@@ -16,6 +18,31 @@ import (
 
 var uuidRegex = regexp.MustCompile(`^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$`)
 
+var platformManagedDirectLLMBypassKeys = map[string]struct{}{
+	"HERMES_CUSTOM_API_KEY":  {},
+	"HERMES_CUSTOM_BASE_URL": {},
+}
+
+func isPlatformManagedDirectLLMBypassKey(key string) bool {
+	_, ok := platformManagedDirectLLMBypassKeys[strings.ToUpper(strings.TrimSpace(key))]
+	return ok
+}
+
+func platformManagedLLMMode() bool {
+	return strings.EqualFold(strings.TrimSpace(os.Getenv("MOLECULE_LLM_BILLING_MODE")), "platform_managed")
+}
+
+func rejectPlatformManagedDirectLLMBypass(c *gin.Context, key string) bool {
+	if !platformManagedLLMMode() || !isPlatformManagedDirectLLMBypassKey(key) {
+		return false
+	}
+	c.JSON(http.StatusBadRequest, gin.H{
+		"error": "direct Hermes custom provider secrets are blocked for platform-managed LLM workspaces; use MODEL/LLM_PROVIDER or the platform LLM proxy env instead",
+		"key":   key,
+	})
+	return true
+}
+
 type SecretsHandler struct {
 	restartFunc func(workspaceID string) // Optional: auto-restart after secret change
 }
@@ -238,6 +265,9 @@ func (h *SecretsHandler) Set(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request body"})
 		return
 	}
+	if rejectPlatformManagedDirectLLMBypass(c, body.Key) {
+		return
+	}
 
 	// Encrypt the value (AES-256-GCM if SECRETS_ENCRYPTION_KEY is set, plaintext otherwise)
 	encrypted, err := crypto.Encrypt([]byte(body.Value))
@@ -380,6 +410,9 @@ func (h *SecretsHandler) SetGlobal(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request body"})
 		return
 	}
+	if rejectPlatformManagedDirectLLMBypass(c, body.Key) {
+		return
+	}
 
 	encrypted, err := crypto.Encrypt([]byte(body.Value))
 	if err != nil {
diff --git a/workspace-server/internal/handlers/workspace.go b/workspace-server/internal/handlers/workspace.go
index 592b40540..88df77475 100644
--- a/workspace-server/internal/handlers/workspace.go
+++ b/workspace-server/internal/handlers/workspace.go
@@ -568,6 +568,10 @@ func (h *WorkspaceHandler) Create(c *gin.Context) {
 	// nil/empty map is a no-op.  Any failure rolls back the workspace insert
 	// so we never have a workspace row without its intended secrets.
 	for k, v := range payload.Secrets {
+		if rejectPlatformManagedDirectLLMBypass(c, k) {
+			tx.Rollback() //nolint:errcheck
+			return
+		}
 		encrypted, encErr := crypto.Encrypt([]byte(v))
 		if encErr != nil {
 			tx.Rollback() //nolint:errcheck