What's happening to my colour outputs from my shader?

Hello, I have the following simplified Shader, which outputs grey rgb(0.5,0.5,0.5) - I have tried it in both linear and gamma modes of rendering and when I screenshot I get RGBs of 174.174.174 (linear mode) or 109.109.109 (gamma mode), when it should really be 128.128.128. For my application I need to render pixels exactly without any correction, from a shader, but I can't work out how to do this - is there a setting I am missing somewhere? Any help much appreciated!

 Shader "Custom/ColourDemo"
 {
     Properties {}
     SubShader
     {
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
 
             struct v2f
             {
                 float4 grabPos : TEXCOORD0;
                 float4 pos : SV_POSITION;
             };
 
             v2f vert(appdata_base v) {
                 v2f o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.grabPos = ComputeGrabScreenPos(o.pos);
                 return o;
             }
 
             float4 frag(v2f i) : SV_Target
             {
                 float4 output = float4(0.5,0.5,0.5,1);
                 return output;
             }
             ENDCG
         }
     }
 }