diff --git a/platform-api/internal/service/mcp_test.go b/platform-api/internal/service/mcp_test.go
index 5a1d4e722..ebbd30b9d 100644
--- a/platform-api/internal/service/mcp_test.go
+++ b/platform-api/internal/service/mcp_test.go
@@ -251,3 +251,26 @@ func TestFetchServerInfoSuppliedURLWithStoredCredential(t *testing.T) {
 		assert.Equal(t, "stored-secret", h, "the proxy's stored credential must still be sent")
 	}
 }
+
+// TestFetchServerInfoUpstreamUnauthorizedReturnsValidationFailed verifies that when the upstream
+// MCP server responds with 401 Unauthorized to the initialize request, FetchServerInfo returns an
+// apperror.ValidationFailed (HTTP 400 Bad Request) rather than apperror.Unauthorized (HTTP 401).
+// This prevents the frontend API client from treating the upstream credential failure as an
+// expired workspace session and logging out the user.
+func TestFetchServerInfoUpstreamUnauthorizedReturnsValidationFailed(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		_, _ = w.Write([]byte(`{"error":"Unauthorized"}`))
+	}))
+	defer srv.Close()
+
+	service := NewMCPProxyService(&mockMCPProxyRepository{}, nil, nil, nil, nil, slog.Default(), &noopAuditRepo{}, &config.Server{}, newTestIdentityService())
+
+	targetURL := srv.URL + "/mcp"
+	_, err := service.FetchServerInfo("org-1", &api.MCPServerInfoFetchRequest{
+		Url: &targetURL,
+	})
+	require.Error(t, err)
+	assert.True(t, apperror.ValidationFailed.Is(err), "expected apperror.ValidationFailed when upstream returns 401, got: %v", err)
+	assert.False(t, apperror.Unauthorized.Is(err), "should NOT be apperror.Unauthorized (which would log out workspace session)")
+}
diff --git a/platform-api/internal/utils/mcp.go b/platform-api/internal/utils/mcp.go
index d46f6f851..9beeae1ff 100644
--- a/platform-api/internal/utils/mcp.go
+++ b/platform-api/internal/utils/mcp.go
@@ -294,11 +294,16 @@ func initializeMCPServer(url string, headerName string, headerValue string) (str
 
 	// Check HTTP status code
 	if resp.StatusCode == http.StatusUnauthorized {
-		return "", nil, apperror.Unauthorized.New().
+		return "", nil, apperror.ValidationFailed.New("The MCP server requires authentication credentials or the provided credentials are invalid.").
 			WithLogMessage("MCP server returned 401 Unauthorized to the initialize request")
 	}
+	if resp.StatusCode == http.StatusForbidden {
+		return "", nil, apperror.ValidationFailed.New("Access to the MCP server was forbidden (403).").
+			WithLogMessage("MCP server returned 403 Forbidden to the initialize request")
+	}
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return "", nil, fmt.Errorf("initialize request failed with status %d: %s", resp.StatusCode, string(body))
+		return "", nil, apperror.ValidationFailed.New(fmt.Sprintf("MCP server initialize request failed with status %d.", resp.StatusCode)).
+			WithLogMessage(fmt.Sprintf("initialize request failed with status %d", resp.StatusCode))
 	}
 
 	// Check if response is event stream and parse it
@@ -315,10 +320,10 @@ func initializeMCPServer(url string, headerName string, headerValue string) (str
 	if err := json.Unmarshal(body, &initResult); err != nil {
 		// Only ignore unmarshal error if this was a valid event stream (parsed above)
 		if !isEventStreamResp {
-			return "", nil, fmt.Errorf("failed to parse initialize response: %w, body: %s", err, string(body))
+			return "", nil, fmt.Errorf("failed to parse initialize response: %w", err)
 		}
 		// For event stream, if unmarshal fails after successful parsing, that's still an error
-		return "", nil, fmt.Errorf("failed to parse initialize response from event stream: %w, body: %s", err, string(body))
+		return "", nil, fmt.Errorf("failed to parse initialize response from event stream: %w", err)
 	}
 
 	if initResult.Error != nil {
@@ -374,7 +379,7 @@ func postJSONRPCWithSession(url string, req any, sessionID string, headerName st
 
 	// Check HTTP status code
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return nil, fmt.Errorf("request failed with status %d: %s", resp.StatusCode, string(body))
+		return nil, fmt.Errorf("request failed with status %d", resp.StatusCode)
 	}
 
 	// Check if response is event stream
@@ -382,7 +387,7 @@ func postJSONRPCWithSession(url string, req any, sessionID string, headerName st
 		// Extract JSON data from event stream
 		data, err := parseEventStream(body)
 		if err != nil {
-			return nil, fmt.Errorf("failed to parse event stream: %w, body: %s", err, string(body))
+			return nil, fmt.Errorf("failed to parse event stream: %w", err)
 		}
 		return data, nil
 	}