Flipt 1.5.0 evaluate API invalid value for string type null on null context values

TL;DR

For "proto: (line 1:102): invalid value for string type: null": this was a breaking change in v1.5.0 that was not in the changelog. Fixed in Flipt v1.5.1, upgrade and null context values are accepted again.

proto: (line 1:102): invalid value for string type: null

Steps

1. POSTing to /api/v1/evaluate with a null context value, e.g. {"flagkey":"profile","entityid":"...","context":{"cohort":null}}, returns HTTP 400 with {"code":3,"message":"proto: (line 1:102): invalid value for string type: null"}. This was a breaking change in v1.5.0 that was not in the changelog.
2. Fixed in Flipt v1.5.1, upgrade and null context values are accepted again. The stricter null handling came from the grpc-gateway layer; the v2 API plans to address nulls properly, but for v1 the 1.5.1 patch restored the previous behavior.

When to use

You are seeing this: POSTing to /api/v1/evaluate with a null context value, e.g. {"flagkey":"profile","entityid":"...","context":{"cohort":null}}, returns HTTP 400 with {"code":3,"message":"proto: (line 1:102): invalid value for string type: null"}. Use this skill when you run into "Flipt 1.5.0 evaluate API invalid value for string type null on null context values".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: v1.5.0, Flipt v1.5.1, 1.5.1. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

A user asked: POSTing to /api/v1/evaluate with a null context value, e.g. {"flagkey":"profile","entityid":"...","context":{"cohort":null}}, returns HTTP 400 with {"code":3,"message":"proto: (line 1:102): invalid value for string type: null"}. This was a breaking change in v1.5.0 that was not in the changelog. The verified answer: Fixed in Flipt v1.5.1, upgrade and null context values are accepted again. The stricter null handling came from the grpc-gateway layer; the v2 API plans to address nulls properly, but for v1 the 1.5.1 patch restored the previous behavior. Thread: https://vectle.com/threads/thr_mlQ37RJShSI8hWrSmfisGA