diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9eb5e8de..1190f72a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+
+## [0.21.0] - 2024-04-30
+
 ### Added
 
 - `libcnb`:
@@ -289,7 +292,8 @@ version number. See the changelog below for other changes.
 
 - Remove support for legacy BOM. Remove `Launch::bom`, `Build::bom`, `bom::Bom`, `bom::Entry`. ([#489](https://github.com/heroku/libcnb.rs/pull/489))
 
-[unreleased]: https://github.com/heroku/libcnb.rs/compare/v0.20.0...HEAD
+[unreleased]: https://github.com/heroku/libcnb.rs/compare/v0.21.0...HEAD
+[0.21.0]: https://github.com/heroku/libcnb.rs/compare/v0.20.0...v0.21.0
 [0.20.0]: https://github.com/heroku/libcnb.rs/compare/v0.19.0...v0.20.0
 [0.19.0]: https://github.com/heroku/libcnb.rs/compare/v0.18.0...v0.19.0
 [0.18.0]: https://github.com/heroku/libcnb.rs/compare/v0.17.0...v0.18.0
diff --git a/Cargo.toml b/Cargo.toml
index f5db376d..7466cf33 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,7 +18,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.20.0"
+version = "0.21.0"
 rust-version = "1.76"
 edition = "2021"
 license = "BSD-3-Clause"
@@ -38,10 +38,10 @@ missing_errors_doc = "allow"
 module_name_repetitions = "allow"
 
 [workspace.dependencies]
-libcnb = { version = "=0.20.0", path = "libcnb" }
-libcnb-common = { version = "=0.20.0", path = "libcnb-common" }
-libcnb-data = { version = "=0.20.0", path = "libcnb-data" }
-libcnb-package = { version = "=0.20.0", path = "libcnb-package" }
-libcnb-proc-macros = { version = "=0.20.0", path = "libcnb-proc-macros" }
-libcnb-test = { version = "=0.20.0", path = "libcnb-test" }
+libcnb = { version = "=0.21.0", path = "libcnb" }
+libcnb-common = { version = "=0.21.0", path = "libcnb-common" }
+libcnb-data = { version = "=0.21.0", path = "libcnb-data" }
+libcnb-package = { version = "=0.21.0", path = "libcnb-package" }
+libcnb-proc-macros = { version = "=0.21.0", path = "libcnb-proc-macros" }
+libcnb-test = { version = "=0.21.0", path = "libcnb-test" }
 toml = { version = "0.8.12" }
diff --git a/examples/execd/Cargo.toml b/examples/execd/Cargo.toml
index bcac5001..ef891a69 100644
--- a/examples/execd/Cargo.toml
+++ b/examples/execd/Cargo.toml
@@ -7,7 +7,7 @@ rust-version.workspace = true
 workspace = true
 
 [dependencies]
-fastrand = "2.0.2"
+fastrand = "2.1.0"
 libcnb.workspace = true
 
 [dev-dependencies]
diff --git a/libcnb-cargo/Cargo.toml b/libcnb-cargo/Cargo.toml
index dc2df6d3..f80dd45e 100644
--- a/libcnb-cargo/Cargo.toml
+++ b/libcnb-cargo/Cargo.toml
@@ -29,7 +29,7 @@ clap = { version = "4.5.4", default-features = false, features = [
 libcnb-data.workspace = true
 libcnb-package.workspace = true
 pathdiff = "0.2.1"
-thiserror = "1.0.58"
+thiserror = "1.0.59"
 
 [dev-dependencies]
 libcnb-common.workspace = true
diff --git a/libcnb-common/Cargo.toml b/libcnb-common/Cargo.toml
index bbfae22f..2b1d04c2 100644
--- a/libcnb-common/Cargo.toml
+++ b/libcnb-common/Cargo.toml
@@ -14,6 +14,6 @@ include = ["src/**/*", "LICENSE", "README.md"]
 workspace = true
 
 [dependencies]
-serde = { version = "1.0.197", features = ["derive"] }
-thiserror = "1.0.58"
+serde = { version = "1.0.199", features = ["derive"] }
+thiserror = "1.0.59"
 toml.workspace = true
diff --git a/libcnb-data/Cargo.toml b/libcnb-data/Cargo.toml
index 503026d9..5e654a37 100644
--- a/libcnb-data/Cargo.toml
+++ b/libcnb-data/Cargo.toml
@@ -17,8 +17,8 @@ workspace = true
 [dependencies]
 fancy-regex = { version = "0.13.0", default-features = false, features = ["std"] }
 libcnb-proc-macros.workspace = true
-serde = { version = "1.0.197", features = ["derive"] }
-thiserror = "1.0.58"
+serde = { version = "1.0.199", features = ["derive"] }
+thiserror = "1.0.59"
 toml.workspace = true
 uriparse = "0.6.4"
 
diff --git a/libcnb-package/Cargo.toml b/libcnb-package/Cargo.toml
index 250af6e5..e2efdc3d 100644
--- a/libcnb-package/Cargo.toml
+++ b/libcnb-package/Cargo.toml
@@ -21,6 +21,6 @@ indoc = "2.0.5"
 libcnb-common.workspace = true
 libcnb-data.workspace = true
 petgraph = { version = "0.6.4", default-features = false }
-thiserror = "1.0.58"
+thiserror = "1.0.59"
 uriparse = "0.6.4"
 which = "6.0.1"
diff --git a/libcnb-proc-macros/Cargo.toml b/libcnb-proc-macros/Cargo.toml
index 9c12c8c1..4aa0247c 100644
--- a/libcnb-proc-macros/Cargo.toml
+++ b/libcnb-proc-macros/Cargo.toml
@@ -20,4 +20,4 @@ workspace = true
 cargo_metadata = "0.18.1"
 fancy-regex = { version = "0.13.0", default-features = false, features = ["std"] }
 quote = "1.0.36"
-syn = { version = "2.0.58", features = ["full"] }
+syn = { version = "2.0.60", features = ["full"] }
diff --git a/libcnb-test/Cargo.toml b/libcnb-test/Cargo.toml
index 3b786003..8985d4d5 100644
--- a/libcnb-test/Cargo.toml
+++ b/libcnb-test/Cargo.toml
@@ -15,15 +15,15 @@ include = ["src/**/*", "LICENSE", "README.md"]
 workspace = true
 
 [dependencies]
-fastrand = "2.0.2"
+fastrand = "2.1.0"
 fs_extra = "1.3.0"
 libcnb-common.workspace = true
 libcnb-data.workspace = true
 libcnb-package.workspace = true
 tempfile = "3.10.1"
-thiserror = "1.0.58"
+thiserror = "1.0.59"
 
 [dev-dependencies]
 indoc = "2.0.5"
 libcnb.workspace = true
-ureq = { version = "2.9.6", default-features = false }
+ureq = { version = "2.9.7", default-features = false }
diff --git a/libcnb/Cargo.toml b/libcnb/Cargo.toml
index 9eb6ee4a..5aae7c78 100644
--- a/libcnb/Cargo.toml
+++ b/libcnb/Cargo.toml
@@ -26,10 +26,10 @@ libcnb-proc-macros.workspace = true
 opentelemetry = { version = "0.21.0", optional = true }
 opentelemetry_sdk = { version = "0.21.2", optional = true }
 opentelemetry-stdout = { version = "0.2.0", optional = true, features = ["trace"] }
-serde = { version = "1.0.197", features = ["derive"] }
-thiserror = "1.0.58"
+serde = { version = "1.0.199", features = ["derive"] }
+thiserror = "1.0.59"
 toml.workspace = true
 
 [dev-dependencies]
-serde_json = "1.0.115"
+serde_json = "1.0.116"
 tempfile = "3.10.1"
diff --git a/libcnb/src/runtime.rs b/libcnb/src/runtime.rs
index be9f5a9f..2d1e50d5 100644
--- a/libcnb/src/runtime.rs
+++ b/libcnb/src/runtime.rs
@@ -364,6 +364,9 @@ where
     let os = env::var("CNB_TARGET_OS").map_err(Error::CannotDetermineTargetOs)?;
     let arch = env::var("CNB_TARGET_ARCH").map_err(Error::CannotDetermineTargetArch)?;
     let arch_variant = env::var("CNB_TARGET_ARCH_VARIANT").ok();
+    // Whilst the Buildpack API spec says these env vars are optional they will always be set in
+    // practice, so we treat them as mandatory to improve buildpack author UX. See:
+    // https://github.com/heroku/libcnb.rs/issues/820
     let distro_name =
         env::var("CNB_TARGET_DISTRO_NAME").map_err(Error::CannotDetermineTargetDistroName)?;
     let distro_version =
diff --git a/libherokubuildpack/Cargo.toml b/libherokubuildpack/Cargo.toml
index b8343c75..3aeb832c 100644
--- a/libherokubuildpack/Cargo.toml
+++ b/libherokubuildpack/Cargo.toml
@@ -37,15 +37,15 @@ crossbeam-utils = { version = "0.8.19", optional = true }
 # Ideally we'd use the fastest `zlib-ng` backend, however it fails to cross-compile:
 # https://github.com/rust-lang/libz-sys/issues/93
 # As such we have to use the next best alternate backend, which is `zlib`.
-flate2 = { version = "1.0.28", default-features = false, features = ["zlib"], optional = true }
+flate2 = { version = "1.0.30", default-features = false, features = ["zlib"], optional = true }
 libcnb = { workspace = true, optional = true }
 pathdiff = { version = "0.2.1", optional = true }
 sha2 = { version = "0.10.8", optional = true }
 tar = { version = "0.4.40", default-features = false, optional = true }
 termcolor = { version = "1.4.1", optional = true }
-thiserror = { version = "1.0.58", optional = true }
+thiserror = { version = "1.0.59", optional = true }
 toml = { workspace = true, optional = true }
-ureq = { version = "2.9.6", default-features = false, features = ["tls"], optional = true }
+ureq = { version = "2.9.7", default-features = false, features = ["tls"], optional = true }
 
 [dev-dependencies]
 indoc = "2.0.5"