diff --git a/.github/workflows/sdk-spec.yml b/.github/workflows/sdk-spec.yml
new file mode 100644
index 0000000..cfb2ea1
--- /dev/null
+++ b/.github/workflows/sdk-spec.yml
@@ -0,0 +1,17 @@
+name: SDK spec
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  sdk-spec:
+    # Keep the ref in `uses:` and `spec-ref` in sync.
+    uses: Flagsmith/sdk-spec/.github/workflows/run-spec.yml@v0.3.0
+    with:
+      spec-ref: v0.3.0
+      language: nodejs
diff --git a/.npmignore b/.npmignore
index c4c7620..f13707b 100644
--- a/.npmignore
+++ b/.npmignore
@@ -1 +1,2 @@
 ./.idea
+sdk-spec/
diff --git a/renovate.json b/renovate.json
index b7a5f2e..b1830e2 100644
--- a/renovate.json
+++ b/renovate.json
@@ -4,6 +4,15 @@
     "ignorePresets": [":semanticPrefixFixDepsChoreOthers"],
     "semanticCommitType": "deps",
     "git-submodules": { "enabled": true },
+    "customManagers": [
+        {
+            "customType": "regex",
+            "managerFilePatterns": ["/^\\.github/workflows/sdk-spec\\.yml$/"],
+            "matchStrings": ["spec-ref: (?<currentValue>v[\\d.]+)"],
+            "depNameTemplate": "Flagsmith/sdk-spec",
+            "datasourceTemplate": "github-tags"
+        }
+    ],
     "packageRules": [
         {
             "matchUpdateTypes": [
@@ -23,6 +32,11 @@
             "matchManagers": ["git-submodules"],
             "versioning": "semver-coerced",
             "enabled": true
+        },
+        {
+            "matchPackageNames": ["Flagsmith/sdk-spec"],
+            "groupName": "SDK spec",
+            "enabled": true
         }
     ]
 }
diff --git a/sdk-spec/Dockerfile b/sdk-spec/Dockerfile
new file mode 100644
index 0000000..faea9da
--- /dev/null
+++ b/sdk-spec/Dockerfile
@@ -0,0 +1,23 @@
+# Runs this SDK, built from source, under the Flagsmith SDK behavioural spec.
+# docker build -f sdk-spec/Dockerfile --build-context harness=<sdk-spec>/harness/nodejs .
+FROM docker.io/library/node:24-slim AS build
+
+WORKDIR /sdk
+COPY package.json package-lock.json tsconfig*.json .npmignore ./
+# Skip the `prepare` script, which installs Git hooks.
+RUN npm ci --ignore-scripts
+COPY index.ts ./
+COPY sdk ./sdk
+COPY flagsmith-engine ./flagsmith-engine
+RUN npm run build && npm pack --ignore-scripts --pack-destination /tmp
+
+FROM docker.io/library/node:24-slim
+
+WORKDIR /sdk
+COPY --from=build /tmp/flagsmith-nodejs-*.tgz /tmp/
+RUN npm install --no-save --no-audit --no-fund /tmp/flagsmith-nodejs-*.tgz && rm /tmp/*.tgz
+
+COPY --from=harness harness.mts /harness/harness.mts
+COPY sdk-spec/steps.mts ./sdk-spec/steps.mts
+
+ENTRYPOINT ["node", "/harness/harness.mts", "sdk-spec/steps.mts"]
diff --git a/sdk-spec/steps.mts b/sdk-spec/steps.mts
new file mode 100644
index 0000000..9d4b3c8
--- /dev/null
+++ b/sdk-spec/steps.mts
@@ -0,0 +1,34 @@
+/**
+ * Steps for the Flagsmith SDK behavioural spec: https://github.com/Flagsmith/sdk-spec
+ *
+ * Each function implements the Gherkin step in the comment above it.
+ */
+
+import { Flagsmith, type Flags, type FlagsmithValue } from '@flagsmith/nodejs';
+
+function asSpecFlags(flags: Flags) {
+    return flags.allFlags().map(flag => ({
+        feature: flag.featureName,
+        enabled: flag.enabled,
+        value: flag.value
+    }));
+}
+
+// Create the SDK client
+export function createClient(environmentKey: string, apiUrl: string) {
+    return new Flagsmith({ environmentKey, apiUrl });
+}
+
+// When the application gets the environment flags
+export async function getEnvironmentFlags(flagsmith: Flagsmith) {
+    return asSpecFlags(await flagsmith.getEnvironmentFlags());
+}
+
+// When the application gets the flags for identity "{identifier}" with traits:
+export async function getIdentityFlags(
+    flagsmith: Flagsmith,
+    identifier: string,
+    traits: Record<string, FlagsmithValue>
+) {
+    return asSpecFlags(await flagsmith.getIdentityFlags(identifier, traits));
+}