diff --git a/.changeset/env-availability.md b/.changeset/env-availability.md
new file mode 100644
index 000000000000..0ac9bfbdf485
--- /dev/null
+++ b/.changeset/env-availability.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': major
+---
+
+breaking: replace `EnvVarConfig.static` with `EnvVarConfig.availability`
diff --git a/documentation/docs/20-core-concepts/70-environment-variables.md b/documentation/docs/20-core-concepts/70-environment-variables.md
index d785468585c0..8e73d34e7606 100644
--- a/documentation/docs/20-core-concepts/70-environment-variables.md
+++ b/documentation/docs/20-core-concepts/70-environment-variables.md
@@ -121,27 +121,31 @@ export const variables = defineEnvVars({
 });
 ```
 
-If a value is invalid, the app will fail to start (or build). To opt out of one or the other, use [`building`]($app-env#building) from `$app/env` along with a validator that accepts an optional value:
+If a value is invalid, the app will fail to start (or build). If the variable is only available at run time (for example a secret that isn't set during the build), use `availability: 'runtime'` so that it is only validated when the app starts:
 
 ```ts
 /// file: src/env.ts
 import { defineEnvVars } from '@sveltejs/kit/hooks';
-+++import { building } from '$app/env'+++
 import * as v from 'valibot';
 
 export const variables = defineEnvVars({
 	SECRET: {
-		// optional when building but required when starting the app
-		+++schema: building ? v.optional(v.string()) : v.string()+++
+		// required when the app starts, but not validated during the build
+		+++availability: 'runtime',+++
+		schema: v.string()
 	}
 });
 ```
 
 You can use validators to make values optional, or transform them (such as turning a string into a boolean, or parsing JSON) — see your validation library's documentation to learn how.
 
-### Static variables
+### Availability
 
-By default, variables are dynamic. If a variable is configured with `static: true`, it will be inlined into your application code, enabling optimisations like dead-code elimination:
+The `availability` property controls when a variable's value is available, and therefore when it is validated. It has three possible values:
+
+- `'dynamic'` (default) — the value is validated and used during runtime and build time.
+- `'inline'` — the value is inlined into your application code at build time, enabling optimisations like dead-code elimination. It is validated at build time.
+- `'runtime'` — the value is validated and used during runtime only. The value is `undefined` during the build, so the variable is typed as `T | undefined`.
 
 ```ts
 /// file: src/env.ts
@@ -151,7 +155,7 @@ import * as v from 'valibot';
 export const variables = defineEnvVars({
 	SHOW_DEBUG_OVERLAY: {
 		public: true,
-		+++static: true,+++
+		+++availability: 'inline',+++
 
 		// coerce to true/false
 		schema: v.pipe(
@@ -162,7 +166,7 @@ export const variables = defineEnvVars({
 });
 ```
 
-Because this variable is `static`, the `<DebugOverlay>` component shown here will be excluded from the JavaScript bundle unless `SHOW_DEBUG_OVERLAY` is truthy:
+Because this variable is `inline`, the `<DebugOverlay>` component shown here will be excluded from the JavaScript bundle unless `SHOW_DEBUG_OVERLAY` is truthy:
 
 ```svelte
 <script>
diff --git a/documentation/docs/25-build-and-deploy/90-adapter-vercel.md b/documentation/docs/25-build-and-deploy/90-adapter-vercel.md
index 29ba141a2535..12867a69c5a8 100644
--- a/documentation/docs/25-build-and-deploy/90-adapter-vercel.md
+++ b/documentation/docs/25-build-and-deploy/90-adapter-vercel.md
@@ -181,7 +181,7 @@ export function load() {
 <p>This staging environment was deployed from {data.deploymentGitBranch}.</p>
 ```
 
-Since all of these variables are unchanged between build time and run time when building on Vercel, we recommend configuring the variable with `static: true` — which will statically replace the variables, enabling optimisations like dead code elimination.
+Since all of these variables are unchanged between build time and run time when building on Vercel, we recommend configuring the variable with `availability: 'inline'` — which will statically replace the variables, enabling optimisations like dead code elimination.
 
 ## Skew protection
 
diff --git a/packages/adapter-static/test/apps/prerendered/src/env.ts b/packages/adapter-static/test/apps/prerendered/src/env.ts
index 01d2ca41b971..9c0332ea4dcf 100644
--- a/packages/adapter-static/test/apps/prerendered/src/env.ts
+++ b/packages/adapter-static/test/apps/prerendered/src/env.ts
@@ -3,6 +3,6 @@ import { defineEnvVars } from '@sveltejs/kit/hooks';
 export const variables = defineEnvVars({
 	PUBLIC_ANSWER: {
 		public: true,
-		static: true
+		availability: 'inline'
 	}
 });
diff --git a/packages/adapter-static/test/apps/spa/src/env.ts b/packages/adapter-static/test/apps/spa/src/env.ts
index 70bc639e00bd..81a364cb9bc2 100644
--- a/packages/adapter-static/test/apps/spa/src/env.ts
+++ b/packages/adapter-static/test/apps/spa/src/env.ts
@@ -3,6 +3,6 @@ import { defineEnvVars } from '@sveltejs/kit/hooks';
 export const variables = defineEnvVars({
 	PUBLIC_VALUE: {
 		public: true,
-		static: true
+		availability: 'inline'
 	}
 });
diff --git a/packages/kit/src/core/adapt/builder.js b/packages/kit/src/core/adapt/builder.js
index 57d10b1e15c2..6ae07d10b31d 100644
--- a/packages/kit/src/core/adapt/builder.js
+++ b/packages/kit/src/core/adapt/builder.js
@@ -167,7 +167,10 @@ export function create_builder({
 			const issues = {};
 
 			for (const [name, config] of Object.entries(variables)) {
-				if (config.static || !config.public) continue;
+				const availability = config.availability ?? 'dynamic';
+				// only `dynamic` public vars are included in the prerendered env.js.
+				// `inline` is inlined, and `runtime` isn't available at build time
+				if (!config.public || availability !== 'dynamic') continue;
 				values[name] = validate(variables, env[name], name, issues);
 			}
 
diff --git a/packages/kit/src/core/env.js b/packages/kit/src/core/env.js
index 63c493e61875..04830db4dd3f 100644
--- a/packages/kit/src/core/env.js
+++ b/packages/kit/src/core/env.js
@@ -99,33 +99,46 @@ export async function load_explicit_env(kit, file, root, mode) {
 export function create_sveltekit_env(variables, env, entry, is_dev) {
 	const imports = entry
 		? [
+				`import { building } from '$app/env/internal';`,
 				`import { variables } from ${JSON.stringify(entry)};`,
 				`import { validate, handle_issues } from '@sveltejs/kit/internal/env';`
 			]
 		: [`const variables = {};`, `const handle_issues = () => {};`];
 
+	/** @type {string[]} */
 	const declarations = [];
+	/** @type {string[]} */
 	const setters = [];
+	/** @type {string[]} */
+	const runtime_setters = [];
 
 	/** @type {Record<string, StandardSchemaV1.Issue[]>} */
 	const issues = {};
 
 	for (const [name, config] of Object.entries(variables ?? {})) {
-		if (config?.static) {
-			if (config.public) {
+		if (/** @type {any} */ (config)?.static) {
+			throw new Error('`static: true` has been removed. Use `availability: "inline"` instead.');
+		}
+
+		const availability = config?.availability ?? 'dynamic';
+
+		if (availability === 'inline') {
+			if (config?.public) {
 				const value = validate(variables ?? {}, env[name], name, issues);
 				declarations.push(`explicit_public_env.${name} = ${devalue.uneval(value)};`);
 			}
 		} else {
-			setters.push(
+			const target = availability === 'runtime' ? runtime_setters : setters;
+
+			target.push(
 				`const ${name} = validate(variables, env.${name}, ${JSON.stringify(name)}, issues);`
 			);
 
 			if (config?.public) {
-				setters.push(`explicit_public_env.${name} = ${name};`);
-				setters.push(`rendered_env.${name} = ${name};`);
+				target.push(`explicit_public_env.${name} = ${name};`);
+				target.push(`rendered_env.${name} = ${name};`);
 			} else {
-				setters.push(`dynamic_private_env.${name} = ${name};`);
+				target.push(`dynamic_private_env.${name} = ${name};`);
 			}
 		}
 	}
@@ -146,6 +159,11 @@ export function create_sveltekit_env(variables, env, entry, is_dev) {
 			export function set_env(env) {
 				const issues = {};
 				${setters.join('\n')}
+				${
+					runtime_setters.length > 0
+						? `if (!building) {\n\t\t\t\t\t${runtime_setters.join('\n')}\n\t\t\t\t}`
+						: ''
+				}
 				handle_issues(issues);
 			}`
 	];
@@ -187,9 +205,10 @@ export function create_sveltekit_env_private(variables, env) {
 	for (const [name, config] of Object.entries(variables)) {
 		if (config.public) continue;
 
-		const value = config.static
-			? devalue.uneval(validate(variables, env[name], name, issues))
-			: `env.${name}`;
+		const value =
+			config.availability === 'inline'
+				? devalue.uneval(validate(variables, env[name], name, issues))
+				: `env.${name}`;
 
 		exports.push(`export const ${name} = ${value};\n`);
 	}
@@ -219,9 +238,10 @@ export function create_sveltekit_env_public(variables, env, prelude) {
 	for (const [name, config] of Object.entries(variables)) {
 		if (!config.public) continue;
 
-		const value = config.static
-			? devalue.uneval(validate(variables, env[name], name, issues))
-			: `env.${name}`;
+		const value =
+			config.availability === 'inline'
+				? devalue.uneval(validate(variables, env[name], name, issues))
+				: `env.${name}`;
 
 		exports.push(`export const ${name} = ${value};\n`);
 	}
@@ -243,7 +263,7 @@ export function create_sveltekit_env_public(variables, env, prelude) {
  */
 export function create_sveltekit_env_service_worker(variables, env, global, base, app_dir) {
 	const has_dynamic_public_env = Object.values(variables ?? {}).some(
-		(config) => config.public && !config.static
+		(config) => config.public && config.availability !== 'inline'
 	);
 
 	if (!has_dynamic_public_env) {
@@ -310,9 +330,12 @@ export function create_explicit_env_types(variables, relative, type) {
 		.filter(([_, config]) => !!config.public === (type === 'public'))
 		.map(([name, config]) => {
 			const comment = config.description ? `${create_jsdoc(config.description)}\n` : '';
+			const maybe_undefined = config.availability === 'runtime';
 			const type = config.schema
-				? `import('@sveltejs/kit/internal/types').StandardSchemaV1.InferOutput<typeof import('${relative}').variables.${name}.schema>`
-				: 'string';
+				? `import('@sveltejs/kit/internal/types').StandardSchemaV1.InferOutput<typeof import('${relative}').variables.${name}.schema>${maybe_undefined ? ' | undefined' : ''}`
+				: maybe_undefined
+					? 'string | undefined'
+					: 'string';
 			return `${comment}export const ${name}: ${type};`;
 		});
 
diff --git a/packages/kit/src/exports/public.d.ts b/packages/kit/src/exports/public.d.ts
index 450da13f7674..c8f78eb1dbe9 100644
--- a/packages/kit/src/exports/public.d.ts
+++ b/packages/kit/src/exports/public.d.ts
@@ -2452,12 +2452,13 @@ export interface EnvVarConfig<T> {
 	 */
 	public?: boolean;
 	/**
-	 * Whether the value is determined at build time or when the app runs.
-	 * - if `true`, the build time value is inlined into the bundle. This enables optimisations like dead-code elimination
-	 * - if `false`, the value is read from the environment when the app starts
-	 * @default false
+	 * When the variable's value is available, and therefore when it is validated.
+	 * - `'dynamic'` — the value is validated and used during runtime and build time.
+	 * - `'inline'` — the value is inlined into your application code at build time, enabling optimisations like dead-code elimination. It is validated at build time.
+	 * - `'runtime'` — the value is validated and used during runtime only. The value is `undefined` during the build, so the variable is typed as `T | undefined`.
+	 * @default 'dynamic'
 	 */
-	static?: boolean;
+	availability?: 'inline' | 'dynamic' | 'runtime';
 	/**
 	 * A [Standard Schema](https://standardschema.dev/) validator that is applied to the value when the app starts.
 	 * The validator can output any value — not necessarily a string — but public, non-static values must be
diff --git a/packages/kit/src/exports/vite/index.js b/packages/kit/src/exports/vite/index.js
index 0e000666ae71..8e4b7934bfa0 100644
--- a/packages/kit/src/exports/vite/index.js
+++ b/packages/kit/src/exports/vite/index.js
@@ -1655,11 +1655,11 @@ function kit({ svelte_config }) {
 					find_deps(vite_manifest, posixify(path.relative(root, entry)), add_dynamic_css, root);
 
 				const has_explicit_dynamic_public_env = Object.values(explicit_env_config ?? {}).some(
-					(variable) => variable.public && !variable.static
+					(variable) => variable.public && variable.availability !== 'inline'
 				);
 
 				// the app only depends on runtime public env if it imports `$app/env/public`
-				// *and* at least one public env var is actually dynamic (non-static)
+				// *and* at least one public env var is actually dynamic (not inlined at build time)
 				const uses_env_dynamic_public =
 					has_explicit_dynamic_public_env &&
 					client_chunks.some(
diff --git a/packages/kit/test/apps/async/src/env.ts b/packages/kit/test/apps/async/src/env.ts
index 3ebd2aecc14e..e05b0bb80d96 100644
--- a/packages/kit/test/apps/async/src/env.ts
+++ b/packages/kit/test/apps/async/src/env.ts
@@ -3,18 +3,18 @@ import { defineEnvVars } from '@sveltejs/kit/hooks';
 export const variables = defineEnvVars({
 	PRIVATE_STATIC: {
 		public: false,
-		static: true
+		availability: 'inline'
 	},
 	PRIVATE_DYNAMIC: {
 		public: false,
-		static: false
+		availability: 'dynamic'
 	},
 	PUBLIC_STATIC: {
 		public: true,
-		static: true
+		availability: 'inline'
 	},
 	PUBLIC_DYNAMIC: {
 		public: true,
-		static: false
+		availability: 'dynamic'
 	}
 });
diff --git a/packages/kit/test/apps/basics/src/env.ts b/packages/kit/test/apps/basics/src/env.ts
index cdf5d6a8d72b..d17ef59bad69 100644
--- a/packages/kit/test/apps/basics/src/env.ts
+++ b/packages/kit/test/apps/basics/src/env.ts
@@ -3,30 +3,30 @@ import { defineEnvVars } from '@sveltejs/kit/hooks';
 export const variables = defineEnvVars({
 	PRIVATE_STATIC: {
 		public: false,
-		static: true
+		availability: 'inline'
 	},
 	PRIVATE_DYNAMIC: {
 		public: false,
-		static: false
+		availability: 'dynamic'
 	},
 	PUBLIC_STATIC: {
 		public: true,
-		static: true
+		availability: 'inline'
 	},
 	PUBLIC_DYNAMIC: {
 		public: true,
-		static: false
+		availability: 'dynamic'
 	},
 	PUBLIC_THEME: {
 		public: true,
-		static: false
+		availability: 'dynamic'
 	},
 	PUBLIC_PRERENDERING: {
 		public: true,
-		static: false
+		availability: 'dynamic'
 	},
 	SOME_JSON: {
 		public: false,
-		static: true
+		availability: 'inline'
 	}
 });
diff --git a/packages/kit/test/apps/options-2/src/env.ts b/packages/kit/test/apps/options-2/src/env.ts
index d61d03fddfec..916e4c0fe0ff 100644
--- a/packages/kit/test/apps/options-2/src/env.ts
+++ b/packages/kit/test/apps/options-2/src/env.ts
@@ -1,21 +1,21 @@
 import process from 'node:process';
-import { building } from '$app/env';
 import * as v from 'valibot';
 
 export const variables = {
 	MESSAGE: {
 		public: true,
 		description: 'Public env var loaded from the shared test env directory',
-		static: !process.env.DYNAMIC_PUBLIC_ENV
+		availability: process.env.DYNAMIC_PUBLIC_ENV ? 'dynamic' : 'inline'
 	},
 	PRIVATE_EXPLICIT_ENV: {},
 	PRIVATE_STATIC_EXPLICIT_ENV: {
-		static: true
+		availability: 'inline'
 	},
 	PRIVATE_VALIDATED_DEFAULT_ENV: {
 		schema: v.optional(v.picklist(['foo', 'bar']), 'foo')
 	},
 	RUNTIME_ONLY: {
-		schema: building ? v.optional(v.string()) : v.string()
+		availability: 'runtime',
+		schema: v.string()
 	}
 };
diff --git a/packages/kit/test/apps/options/source/env.ts b/packages/kit/test/apps/options/source/env.ts
index 0b49d8f3845b..c3a35d69181a 100644
--- a/packages/kit/test/apps/options/source/env.ts
+++ b/packages/kit/test/apps/options/source/env.ts
@@ -5,6 +5,6 @@ export const variables = defineEnvVars({
 		public: true
 	},
 	TOP_SECRET_SHH_PLS: {
-		static: true
+		availability: 'inline'
 	}
 });
diff --git a/packages/kit/test/prerendering/basics/src/env.ts b/packages/kit/test/prerendering/basics/src/env.ts
index e5b56561f084..4bcce90f9dce 100644
--- a/packages/kit/test/prerendering/basics/src/env.ts
+++ b/packages/kit/test/prerendering/basics/src/env.ts
@@ -3,10 +3,10 @@ import { defineEnvVars } from '@sveltejs/kit/hooks';
 export const variables = defineEnvVars({
 	PUBLIC_STATIC: {
 		public: true,
-		static: true
+		availability: 'inline'
 	},
 	PRIVATE_STATIC: {
 		public: false,
-		static: true
+		availability: 'inline'
 	}
 });
diff --git a/packages/kit/types/index.d.ts b/packages/kit/types/index.d.ts
index 6892b0aab145..b356f02e3536 100644
--- a/packages/kit/types/index.d.ts
+++ b/packages/kit/types/index.d.ts
@@ -2422,12 +2422,13 @@ declare module '@sveltejs/kit' {
 		 */
 		public?: boolean;
 		/**
-		 * Whether the value is determined at build time or when the app runs.
-		 * - if `true`, the build time value is inlined into the bundle. This enables optimisations like dead-code elimination
-		 * - if `false`, the value is read from the environment when the app starts
-		 * @default false
+		 * When the variable's value is available, and therefore when it is validated.
+		 * - `'dynamic'` — the value is validated and used during runtime and build time.
+		 * - `'inline'` — the value is inlined into your application code at build time, enabling optimisations like dead-code elimination. It is validated at build time.
+		 * - `'runtime'` — the value is validated and used during runtime only. The value is `undefined` during the build, so the variable is typed as `T | undefined`.
+		 * @default 'dynamic'
 		 */
-		static?: boolean;
+		availability?: 'inline' | 'dynamic' | 'runtime';
 		/**
 		 * A [Standard Schema](https://standardschema.dev/) validator that is applied to the value when the app starts.
 		 * The validator can output any value — not necessarily a string — but public, non-static values must be