diff --git a/.github/workflows/ci-matrix.yml b/.github/workflows/ci-matrix.yml
index 8516d99..8de7d9f 100644
--- a/.github/workflows/ci-matrix.yml
+++ b/.github/workflows/ci-matrix.yml
@@ -30,12 +30,12 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: build
-          args: --no-default-features --features ${{ matrix.features }}
+          args: --no-default-features --features strict,${{ matrix.features }}
 
       - uses: actions-rs/cargo@v1
         with:
           command: test
-          args: --no-default-features --features ${{ matrix.features }}
+          args: --no-default-features --features strict,${{ matrix.features }}
 
   clippy:
     runs-on: ubuntu-latest
@@ -59,13 +59,13 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: build
-          args: --features surf-client,reqwest-client,reqwest-client-rustls --all-targets
+          args: --features strict,surf-client,reqwest-client,reqwest-client-rustls --all-targets
 
       - uses: actions-rs/cargo@v1
-        # We test with all-features to ensure that that does build
+        # We test with approximately all-features to ensure that that does build (excludes nightly only backtrace)
         with:
           command: test
-          args: --features surf-client,reqwest-client,reqwest-client-rustls --all-targets
+          args: --features strict,surf-client,reqwest-client,reqwest-client-rustls --all-targets
 
       - uses: actions-rs/cargo@v1
         with:
@@ -75,4 +75,4 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: clippy
-          args: --features surf-client,reqwest-client,reqwest-client-rustls --all-targets -- -D warnings
+          args: --features strict,surf-client,reqwest-client,reqwest-client-rustls --all-targets -- -D warnings
diff --git a/Cargo.toml b/Cargo.toml
index 605cf90..853b22a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,6 +24,8 @@ functional = []
 reqwest-client = ["reqwest", "reqwest/default-tls"]
 # For users that don't want to depend on OpenSSL.
 reqwest-client-rustls = ["reqwest", "reqwest/rustls-tls"]
+# To error if an unsupported API feature is present
+strict = []
 # For use with --no-default-features
 surf-client = ["surf", "async-std"]
 
diff --git a/client-specification b/client-specification
index 3d91a8f..d069632 160000
--- a/client-specification
+++ b/client-specification
@@ -1 +1 @@
-Subproject commit 3d91a8f18e1679994d308e5dde47954ae2def56d
+Subproject commit d0696320bb8999acaaaa4b29291ee85e6172d736
diff --git a/src/api.rs b/src/api.rs
index 79fbc0c..261a4fa 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -7,7 +7,7 @@ use chrono::Utc;
 use serde::{Deserialize, Serialize};
 
 #[derive(Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Features {
     pub version: u8,
     pub features: Vec<Feature>,
@@ -20,7 +20,7 @@ impl Features {
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Feature {
     pub name: String,
     #[serde(default)]
@@ -33,7 +33,7 @@ pub struct Feature {
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Strategy {
     pub constraints: Option<Vec<Constraint>>,
     pub name: String,
@@ -41,7 +41,7 @@ pub struct Strategy {
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Constraint {
     #[serde(rename = "contextName")]
     pub context_name: String,
@@ -51,7 +51,7 @@ pub struct Constraint {
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
 #[serde(tag = "operator", content = "values")]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub enum ConstraintExpression {
     #[serde(rename = "IN")]
     In(Vec<String>),
@@ -60,7 +60,7 @@ pub enum ConstraintExpression {
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Variant {
     pub name: String,
     pub weight: u8,
@@ -69,7 +69,7 @@ pub struct Variant {
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
-// #[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct VariantOverride {
     #[serde(rename = "contextName")]
     pub context_name: String,
diff --git a/src/context.rs b/src/context.rs
index eb8ff54..89ad438 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -26,7 +26,7 @@ impl<'de> de::Deserialize<'de> for IPAddress {
 }
 
 #[derive(Debug, Default, Deserialize)]
-#[serde(deny_unknown_fields)]
+#[cfg_attr(feature = "strict", serde(deny_unknown_fields))]
 pub struct Context {
     #[serde(rename = "userId")]
     pub user_id: Option<String>,
diff --git a/src/lib.rs b/src/lib.rs
index 6335911..cf99657 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -106,6 +106,8 @@ fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
   Enables reqwest with OpenSSL TLS support
 * **reqwest-client-rustls** -
   Enables reqwest with RusTLS support
+* **strict** -
+  Turn unexpected fields in API responses into errors
 * **surf-client** -
   Enables Surf as the HTTP client to retrieve flags
 */