diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,3 +30,6 @@
 
 0.5.5.1, released 2025-12-02
     Revise README (reference to client)
+
+0.5.5.2, released 2026-03-22
+    Add flake.nix for development environment
diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -4,6 +4,9 @@
 help: ## print make targets
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'
 
+develop: ## Enter development environment
+	nix develop
+
 package: package.yaml
 	hpack
 
diff --git a/haskell-opentimestamps.cabal b/haskell-opentimestamps.cabal
--- a/haskell-opentimestamps.cabal
+++ b/haskell-opentimestamps.cabal
@@ -1,11 +1,11 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.38.2.
+-- This file has been generated from package.yaml by hpack version 0.38.3.
 --
 -- see: https://github.com/sol/hpack
 
 name:           haskell-opentimestamps
-version:        0.5.5.1
+version:        0.5.5.2
 synopsis:       Haskell implementation of OpenTimestamps
 description:    Please see the README on Codeberg at <https://codeberg.org/photonsphere/haskell-opentimestamps#readme>
 category:       Bitcoin, Finance, Network, Cryptography
diff --git a/package.yaml b/package.yaml
--- a/package.yaml
+++ b/package.yaml
@@ -1,5 +1,5 @@
 name:                haskell-opentimestamps
-version:             0.5.5.1
+version:             0.5.5.2
 license:             BSD3
 #x-license:           BSD-3-Clause OR Apache-2.0
 git:                 "https://codeberg.org/photonsphere/haskell-opentimestamps"
diff --git a/src/OpenTimestamps/Config.hs b/src/OpenTimestamps/Config.hs
--- a/src/OpenTimestamps/Config.hs
+++ b/src/OpenTimestamps/Config.hs
@@ -34,7 +34,7 @@
 
 -- | User-Agent string for HTTP requests.
 userAgent :: ByteString
-userAgent = "Haskell-OpenTimestamps-Client/0.5.5.1"
+userAgent = "Haskell-OpenTimestamps-Client/0.5.5.2"
 
 -- | URL string for digest HTTP requests.
 apiUrlDigest :: String