diff --git a/package.json b/package.json
index 1e190d830..c4523ec9c 100644
--- a/package.json
+++ b/package.json
@@ -227,6 +227,7 @@
     "test:php-patch-approval-filter": "php scripts/php-patch-approval-filter-smoke.php",
     "test:schema-parity": "tsx tests/schema-parity.test.ts",
     "test:recipe-validation-descriptors": "tsx tests/recipe-validation-descriptors.test.ts",
+    "test:runtime-workers": "tsx tests/runtime-workers.test.ts",
     "test:recipe-runtime-backend-normalization": "tsx tests/recipe-runtime-backend-normalization.test.ts",
     "test:recipe-runtime-setup-staged-materialization": "tsx tests/recipe-runtime-setup-staged-materialization.test.ts",
     "test:recipe-run-provenance": "tsx tests/recipe-run-provenance.test.ts",
diff --git a/packages/cli/src/commands/recipe-build.ts b/packages/cli/src/commands/recipe-build.ts
index d23c21118..b903480b7 100644
--- a/packages/cli/src/commands/recipe-build.ts
+++ b/packages/cli/src/commands/recipe-build.ts
@@ -1,5 +1,5 @@
 import { readFile, writeFile } from "node:fs/promises"
-import { buildGenericAbilityRuntimeRunRecipe, buildRuntimePackageRunRecipe, buildWordPressBenchRecipe, buildWordPressPhpunitRecipe, compileRecipeTemplate, type GenericAbilityRuntimeRunOptions, type RecipeTemplateInput, type RuntimePackageRunRecipeOptions, type RuntimeWordPressInstallMode, type WorkspaceRecipe, type WorkspaceRecipeExtraPlugin, type WorkspaceRecipeMount, type WorkspaceRecipePHPWasmExtensionManifest, type WorkspaceRecipeRuntimeBackendPackage, type WorkspaceRecipeRuntimeService, type WorkspaceRecipeStep } from "@automattic/wp-codebox-core"
+import { buildGenericAbilityRuntimeRunRecipe, buildRuntimePackageRunRecipe, buildWordPressBenchRecipe, buildWordPressPhpunitRecipe, compileRecipeTemplate, type GenericAbilityRuntimeRunOptions, type RecipeTemplateInput, type RuntimePackageRunRecipeOptions, type RuntimeWorkerCount, type RuntimeWordPressInstallMode, type WorkspaceRecipe, type WorkspaceRecipeExtraPlugin, type WorkspaceRecipeMount, type WorkspaceRecipePHPWasmExtensionManifest, type WorkspaceRecipeRuntimeBackendPackage, type WorkspaceRecipeRuntimeService, type WorkspaceRecipeStep } from "@automattic/wp-codebox-core"
 
 interface RecipeBuildOptions {
   recipeType: "phpunit" | "bench" | "template" | "generic-ability-runtime-run" | "runtime-package-run"
@@ -11,6 +11,7 @@ interface WordPressPhpunitBuilderOptions {
   blueprint?: unknown
   wordpressVersion?: string
   phpVersion?: string
+  workers?: RuntimeWorkerCount
   databaseType?: "sqlite" | "mysql"
   wordpressInstallMode?: RuntimeWordPressInstallMode
   extensions?: WorkspaceRecipePHPWasmExtensionManifest[]
@@ -82,6 +83,7 @@ function buildRecipe(recipeType: RecipeBuildOptions["recipeType"], options: Word
         blueprint: phpunitOptions.blueprint,
         wordpressVersion: stringOrUndefined(phpunitOptions.wordpressVersion),
         phpVersion: stringOrUndefined(phpunitOptions.phpVersion),
+        workers: workerCountOrUndefined(phpunitOptions.workers),
         databaseType: phpunitOptions.databaseType,
         wordpressInstallMode: phpunitOptions.wordpressInstallMode,
         extensions: Array.isArray(phpunitOptions.extensions) ? phpunitOptions.extensions : [],
@@ -182,6 +184,12 @@ function requiredString(value: unknown, name: string): string {
   return value
 }
 
+function workerCountOrUndefined(value: unknown): RuntimeWorkerCount | undefined {
+  if (value === undefined || value === "auto") return value
+  if (typeof value === "number" && Number.isSafeInteger(value) && value >= 1 && value <= 64) return value
+  throw new Error("workers must be an integer from 1 to 64 or auto.")
+}
+
 function stringOrUndefined(value: unknown): string | undefined {
   return typeof value === "string" && value !== "" ? value : undefined
 }
diff --git a/packages/cli/src/commands/recipe-run.ts b/packages/cli/src/commands/recipe-run.ts
index e61183344..5b15338da 100644
--- a/packages/cli/src/commands/recipe-run.ts
+++ b/packages/cli/src/commands/recipe-run.ts
@@ -217,6 +217,7 @@ export async function runRecipe(options: RecipeRunOptions, interruption?: Recipe
       name: plan.runtime.name,
       version: plan.runtime.wp,
       phpVersion: plan.runtime.phpVersion,
+      workers: plan.runtime.workers,
       wordpressInstallMode: plan.runtime.wordpressInstallMode,
       databaseSetup: recipe.inputs?.services?.some(({ kind }) => kind === "mysql") ? "external" as const : undefined,
       blueprint: plan.runtime.blueprint,
diff --git a/packages/cli/src/recipe-dry-run.ts b/packages/cli/src/recipe-dry-run.ts
index f0729a86a..77ad187a0 100644
--- a/packages/cli/src/recipe-dry-run.ts
+++ b/packages/cli/src/recipe-dry-run.ts
@@ -54,6 +54,7 @@ export interface RecipePlan {
     name: string
     wp: string
     phpVersion?: string
+    workers?: number | "auto"
     wordpressInstallMode?: RuntimeWordPressInstallMode
     assets?: RuntimeAssetSpec
     blueprint: unknown
@@ -442,6 +443,7 @@ export async function planWorkspaceRecipe(recipe: WorkspaceRecipe, recipeDirecto
       name: recipe.runtime?.name ?? "wp-codebox-recipe",
       wp: recipe.runtime?.wp ?? context.defaultWordPressVersion,
       ...(recipe.runtime?.phpVersion ? { phpVersion: recipe.runtime.phpVersion } : {}),
+      ...(recipe.runtime?.workers !== undefined ? { workers: recipe.runtime.workers } : {}),
       ...(recipe.runtime?.wordpressInstallMode ? { wordpressInstallMode: recipe.runtime.wordpressInstallMode } : {}),
       ...(recipe.runtime?.assets ? { assets: recipe.runtime.assets } : {}),
       ...(recipe.runtime?.extensions ? { extensions: recipe.runtime.extensions } : {}),
diff --git a/packages/runtime-core/src/recipe-builders.ts b/packages/runtime-core/src/recipe-builders.ts
index 159db6b0c..713d1f362 100644
--- a/packages/runtime-core/src/recipe-builders.ts
+++ b/packages/runtime-core/src/recipe-builders.ts
@@ -1,4 +1,4 @@
-import type { RuntimePreviewSpec, RuntimeWordPressInstallMode, WorkspaceRecipe, WorkspaceRecipeExtraPlugin, WorkspaceRecipeMount, WorkspaceRecipePHPWasmExtensionManifest, WorkspaceRecipeRuntimeBackendPackage, WorkspaceRecipeRuntimeService, WorkspaceRecipeStep } from "./runtime-contracts.js"
+import type { RuntimePreviewSpec, RuntimeWorkerCount, RuntimeWordPressInstallMode, WorkspaceRecipe, WorkspaceRecipeExtraPlugin, WorkspaceRecipeMount, WorkspaceRecipePHPWasmExtensionManifest, WorkspaceRecipeRuntimeBackendPackage, WorkspaceRecipeRuntimeService, WorkspaceRecipeStep } from "./runtime-contracts.js"
 import { commandArg, commandJsonArg, commandStringListArg } from "./command-codecs.js"
 export { buildRuntimePackageRunRecipe, CODEBOX_RUN_RUNTIME_PACKAGE_ABILITY, RUNTIME_PACKAGE_ARTIFACT_DECLARATION_SCHEMA, RUNTIME_PACKAGE_EXECUTION_INPUT_SCHEMA, RUNTIME_PACKAGE_EXECUTION_RESULT_SCHEMA, RUNTIME_PACKAGE_OUTPUT_PROJECTION_SCHEMA, runtimePackageExecutionInput, type RuntimePackageArtifactDeclaration, type RuntimePackageExecutionInput, type RuntimePackageOutputProjection, type RuntimePackageRunRecipeOptions } from "./runtime-package-execution.js"
 export { RUNTIME_PACKAGE_DIAGNOSTIC_SCHEMA, RUNTIME_PACKAGE_RESULT_SCHEMA, RUNTIME_PACKAGE_TASK_SCHEMA, normalizeRuntimePackageResult, normalizeRuntimePackageTask, validateRuntimePackageTask, type RuntimePackageDiagnostic, type RuntimePackageResult, type RuntimePackageTask } from "./runtime-package-contracts.js"
@@ -14,6 +14,7 @@ export interface NormalizeRecipeMountsOptions {
 export interface WordPressPhpunitRecipeOptions {
   wordpressVersion?: string
   phpVersion?: string
+  workers?: RuntimeWorkerCount
   databaseType?: "sqlite" | "mysql"
   wordpressInstallMode?: RuntimeWordPressInstallMode
   blueprint?: unknown
@@ -81,6 +82,7 @@ export function buildWordPressPhpunitRecipe(options: WordPressPhpunitRecipeOptio
     runtime: {
       wp: options.wordpressVersion ?? DEFAULT_WORDPRESS_VERSION,
       ...(options.phpVersion ? { phpVersion: options.phpVersion } : {}),
+      ...(options.workers !== undefined ? { workers: options.workers } : {}),
       ...(options.wordpressInstallMode ? { wordpressInstallMode: options.wordpressInstallMode } : {}),
       blueprint: blueprintWithMultisite(options.blueprint ?? { steps: [] }, (options.multisite ?? false) && options.databaseType !== "mysql"),
       ...(options.preview || options.multisite ? { preview: multisitePreview(options.preview, options.multisite ?? false) } : {}),
diff --git a/packages/runtime-core/src/recipe-schema.ts b/packages/runtime-core/src/recipe-schema.ts
index 83e96c2e4..de3291072 100644
--- a/packages/runtime-core/src/recipe-schema.ts
+++ b/packages/runtime-core/src/recipe-schema.ts
@@ -119,6 +119,13 @@ export function createWorkspaceRecipeJsonSchema(options: WorkspaceRecipeJsonSche
             pattern: "^[0-9]+\\.[0-9]+$",
             description: "PHP runtime version passed to WordPress Playground, for example 8.3 or 8.4.",
           },
+          workers: {
+            anyOf: [
+              { type: "integer", minimum: 1, maximum: 64 },
+              { const: "auto" },
+            ],
+            description: "Runtime request worker count. Omit to use the backend default.",
+          },
           wordpressInstallMode: {
             enum: enumValues(options.runtimeWordPressInstallModes, defaultRuntimeWordPressInstallModes),
             description: "Controls how Playground prepares a mounted WordPress directory. Use do-not-attempt-installing for custom distributions that own their own boot/readiness probes.",
diff --git a/packages/runtime-core/src/runtime-contracts.ts b/packages/runtime-core/src/runtime-contracts.ts
index a93e9b822..43fe1fb1e 100644
--- a/packages/runtime-core/src/runtime-contracts.ts
+++ b/packages/runtime-core/src/runtime-contracts.ts
@@ -3,7 +3,7 @@ import type { RuntimePolicy } from "./runtime-policy.js"
 import { SANDBOX_WORKSPACE_ROOT } from "./runtime-action-adapter.js"
 import type { ArtifactFileDigest, ArtifactManifestFile, ArtifactSpec, ArtifactViewerMetadata } from "./artifact-manifest.js"
 import type { HostToolDefinition, HostToolRegistry } from "./host-tool-registry.js"
-import type { BackendNeutralRuntimeProvenance, RuntimePHPWasmExtensionManifest, RuntimeWordPressAssetSpec, RuntimeWordPressDatabaseSetupContract, RuntimeWordPressEnvironmentSpec, RuntimeWordPressInstallModeContract, RuntimeWordPressProvenance } from "./runtime-neutral-contracts.js"
+import type { BackendNeutralRuntimeProvenance, RuntimePHPWasmExtensionManifest, RuntimeWorkerCount, RuntimeWordPressAssetSpec, RuntimeWordPressDatabaseSetupContract, RuntimeWordPressEnvironmentSpec, RuntimeWordPressInstallModeContract, RuntimeWordPressProvenance } from "./runtime-neutral-contracts.js"
 import type {
   RUNTIME_EPISODE_ACTION_SCHEMA,
   RUNTIME_EPISODE_OBSERVATION_SCHEMA,
@@ -20,6 +20,7 @@ export interface EnvironmentSpec extends RuntimeWordPressEnvironmentSpec {}
 
 export type RuntimeWordPressInstallMode = RuntimeWordPressInstallModeContract
 export type RuntimeWordPressDatabaseSetup = RuntimeWordPressDatabaseSetupContract
+export type { RuntimeWorkerCount }
 
 export interface RuntimeAssetSpec extends RuntimeWordPressAssetSpec {}
 
@@ -647,6 +648,7 @@ export interface WorkspaceRecipe {
     name?: string
     wp?: string
     phpVersion?: string
+    workers?: RuntimeWorkerCount
     wordpressInstallMode?: RuntimeWordPressInstallMode
     blueprint?: unknown
     preview?: RuntimePreviewSpec
diff --git a/packages/runtime-core/src/runtime-neutral-contracts.ts b/packages/runtime-core/src/runtime-neutral-contracts.ts
index a0cccbcce..407ae23db 100644
--- a/packages/runtime-core/src/runtime-neutral-contracts.ts
+++ b/packages/runtime-core/src/runtime-neutral-contracts.ts
@@ -47,6 +47,7 @@ export interface BackendNeutralEnvironmentSpec {
 
 export type RuntimeWordPressInstallModeContract = "install-from-existing-files" | "install-from-existing-files-if-needed" | "do-not-attempt-installing"
 export type RuntimeWordPressDatabaseSetupContract = "runtime-managed" | "external"
+export type RuntimeWorkerCount = number | "auto"
 
 export interface RuntimeWordPressAssetSpec extends BackendNeutralRuntimeAssetSpec {
   wordpressDirectory?: string
@@ -61,6 +62,7 @@ export interface RuntimePHPWasmExtensionManifest {
 export interface RuntimeWordPressEnvironmentSpec extends BackendNeutralEnvironmentSpec {
   blueprint?: unknown
   phpVersion?: string
+  workers?: RuntimeWorkerCount
   assets?: RuntimeWordPressAssetSpec
   wordpressInstallMode?: RuntimeWordPressInstallModeContract
   databaseSetup?: RuntimeWordPressDatabaseSetupContract
@@ -115,6 +117,7 @@ export function normalizeRuntimeWordPressEnvironmentSpec(input: unknown): Runtim
     ...neutral,
     blueprint: value.blueprint,
     phpVersion: optionalString(value.phpVersion, "environment.phpVersion"),
+    workers: normalizeRuntimeWorkerCount(value.workers),
     assets: normalizeRuntimeWordPressAssetSpec(value.assets),
     wordpressInstallMode: optionalString(value.wordpressInstallMode, "environment.wordpressInstallMode") as RuntimeWordPressInstallModeContract | undefined,
     databaseSetup: optionalString(value.databaseSetup, "environment.databaseSetup") as RuntimeWordPressDatabaseSetupContract | undefined,
@@ -122,6 +125,15 @@ export function normalizeRuntimeWordPressEnvironmentSpec(input: unknown): Runtim
   })
 }
 
+function normalizeRuntimeWorkerCount(value: unknown): RuntimeWorkerCount | undefined {
+  if (value === undefined) return undefined
+  if (value === "auto") return value
+  if (typeof value !== "number" || !Number.isSafeInteger(value) || value < 1 || value > 64) {
+    throw new Error("environment.workers must be an integer from 1 to 64 or auto.")
+  }
+  return value
+}
+
 function normalizeRuntimePHPWasmExtensionManifests(input: unknown): RuntimePHPWasmExtensionManifest[] | undefined {
   if (input === undefined) return undefined
   if (!Array.isArray(input)) throw new Error("environment.extensions must be an array.")
diff --git a/packages/runtime-playground/src/playground-cli-runner.ts b/packages/runtime-playground/src/playground-cli-runner.ts
index 77591566c..6c643f20d 100644
--- a/packages/runtime-playground/src/playground-cli-runner.ts
+++ b/packages/runtime-playground/src/playground-cli-runner.ts
@@ -150,7 +150,7 @@ export async function startPlaygroundCliServer(spec: RuntimeCreateSpec, mounts:
           quiet: true,
           verbosity: "quiet",
           skipBrowser: true,
-          workers: 6,
+          workers: spec.environment.workers ?? 6,
           mount: [
             ...postinstallMounts.map((mount) => ({
               hostPath: mount.source,
diff --git a/tests/playground-cli-runner-bootstrap-ini.test.ts b/tests/playground-cli-runner-bootstrap-ini.test.ts
index 1a438d43f..625b063f2 100644
--- a/tests/playground-cli-runner-bootstrap-ini.test.ts
+++ b/tests/playground-cli-runner-bootstrap-ini.test.ts
@@ -33,6 +33,7 @@ try {
     environment: {
       version: "mounted-wordpress-source",
       phpVersion: "8.4",
+      workers: 1,
       wordpressInstallMode: "do-not-attempt-installing",
       databaseSetup: "external",
       assets: { wordpressDirectory: wordpressDevelopDirectory },
@@ -84,7 +85,7 @@ try {
   assert.equal(calls[0]["mount-before-install"]?.[4]?.vfsPath, "/wordpress/wp-config.php")
   assert.deepEqual(calls[0]["mount-before-install"]?.[5], { hostPath: wordpressDevelopDirectory, vfsPath: "/wordpress" })
   assert.deepEqual(calls[0].mount, [])
-  assert.equal(calls[0].workers, 6)
+  assert.equal(calls[0].workers, 1)
   assert.equal(calls[0].wordpressInstallMode, "do-not-attempt-installing")
   assert.equal(calls[0].skipSqliteSetup, true)
   assert.equal(calls[0].phpEnv?.DB_PASSWORD, "secret")
@@ -135,6 +136,7 @@ try {
   await defaultRuntimeIniServer[Symbol.asyncDispose]()
 
   assert.equal(calls.length, 1)
+  assert.equal(calls[0].workers, 1)
   assert.deepEqual(calls[0].phpIniEntries, { memory_limit: "512M" })
   assert.equal(calls[0].skipSqliteSetup, false)
   assert.equal(shouldUseProgrammaticPlaygroundRunner(defaultRuntimeIniSpec), true)
@@ -146,6 +148,7 @@ try {
       ...defaultRuntimeIniSpec.environment,
       version: "latest",
       wordpressInstallMode: undefined,
+      workers: undefined,
       assets: undefined,
     },
   }
@@ -158,6 +161,7 @@ try {
   assert.equal(calls[0]["mount-before-install"]?.[0]?.vfsPath, "/internal/wp-codebox")
   assert.match(calls[0]["mount-before-install"]?.[1]?.vfsPath ?? "", /^\/wordpress\/wp-codebox-execute-[a-f0-9]{24}\.php$/)
   assert.equal(calls[0].wordpressInstallMode, undefined)
+  assert.equal(calls[0].workers, 6)
   assert.equal(shouldUseProgrammaticPlaygroundRunner(downloadedWordPressSpec), false)
 
   calls.length = 0
diff --git a/tests/runtime-workers.test.ts b/tests/runtime-workers.test.ts
new file mode 100644
index 000000000..b90095ba3
--- /dev/null
+++ b/tests/runtime-workers.test.ts
@@ -0,0 +1,46 @@
+import assert from "node:assert/strict"
+import { readFile, writeFile } from "node:fs/promises"
+import { join } from "node:path"
+
+import { runRecipeBuildCommand } from "../packages/cli/src/commands/recipe-build.js"
+import { planWorkspaceRecipe } from "../packages/cli/src/recipe-dry-run.js"
+import { buildWordPressPhpunitRecipe } from "../packages/runtime-core/src/recipe-builders.js"
+import { normalizeRuntimeWordPressEnvironmentSpec, validateWorkspaceRecipeJsonSchema, type WorkspaceRecipe } from "../packages/runtime-core/src/index.js"
+import { withTempDir } from "../scripts/test-kit.js"
+
+const recipe: WorkspaceRecipe = {
+  schema: "wp-codebox/workspace-recipe/v1",
+  runtime: { workers: 1 },
+  workflow: { steps: [{ command: "wordpress.run-php" }] },
+}
+
+assert.equal(validateWorkspaceRecipeJsonSchema(recipe).valid, true)
+assert.equal(validateWorkspaceRecipeJsonSchema({ ...recipe, runtime: { workers: "auto" } }).valid, true)
+assert.equal(validateWorkspaceRecipeJsonSchema({ ...recipe, runtime: { workers: 0 } }).valid, false)
+assert.equal(validateWorkspaceRecipeJsonSchema({ ...recipe, runtime: { workers: 65 } }).valid, false)
+assert.equal(validateWorkspaceRecipeJsonSchema({ ...recipe, runtime: { workers: 1.5 } }).valid, false)
+assert.deepEqual(normalizeRuntimeWordPressEnvironmentSpec({ kind: "wordpress", workers: 1 }), { kind: "wordpress", workers: 1 })
+assert.throws(() => normalizeRuntimeWordPressEnvironmentSpec({ kind: "wordpress", workers: 0 }), /workers must be an integer/)
+
+assert.equal(buildWordPressPhpunitRecipe({ pluginSlug: "example", workers: 1 }).runtime?.workers, 1)
+assert.equal(buildWordPressPhpunitRecipe({ pluginSlug: "example", workers: "auto" }).runtime?.workers, "auto")
+assert.equal(buildWordPressPhpunitRecipe({ pluginSlug: "example" }).runtime?.workers, undefined)
+
+const plan = await planWorkspaceRecipe(recipe, process.cwd(), { recipePath: "recipe.json" }, {
+  defaultWordPressVersion: "latest",
+  resolveExecutionSpec: async (step) => ({ command: step.command, args: step.args ?? [] }),
+})
+assert.equal(plan.runtime.workers, 1)
+
+await withTempDir("wp-codebox-runtime-workers-", async (directory) => {
+  const optionsPath = join(directory, "options.json")
+  const recipePath = join(directory, "recipe.json")
+  await writeFile(optionsPath, JSON.stringify({ pluginSlug: "example", workers: 1 }))
+  assert.equal(await runRecipeBuildCommand(["phpunit", "--options", optionsPath, "--output", recipePath]), 0)
+  assert.equal((JSON.parse(await readFile(recipePath, "utf8")) as WorkspaceRecipe).runtime?.workers, 1)
+
+  await writeFile(optionsPath, JSON.stringify({ pluginSlug: "example", workers: 0 }))
+  await assert.rejects(runRecipeBuildCommand(["phpunit", "--options", optionsPath, "--output", recipePath]), /workers must be an integer/)
+})
+
+console.log("runtime workers ok")