diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3f51456fa1..94dcb0758a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ This is the log of notable changes to EAS CLI and related packages.
 - [eas-cli] Validate local composite functions referenced in workflow files during `eas workflow:validate`, checking that the YAML files exist and are well-formed. ([#3931](https://github.com/expo/eas-cli/pull/3931) by [@sswrk](https://github.com/sswrk))
 - [build-tools] Load local composite function catalogs for hook steps, in both steps-based and native builds. ([#4063](https://github.com/expo/eas-cli/pull/4063) by [@sswrk](https://github.com/sswrk))
 - [eas-cli] Validate local composite functions referenced from workflow job hooks during `eas workflow:validate`. ([#4064](https://github.com/expo/eas-cli/pull/4064) by [@sswrk](https://github.com/sswrk))
+- [build-tools] Load local functions declaring `command` or `path`, so custom build functions moved from `.eas/build` configs into `.eas/functions` can be called from workflows. ([#4097](https://github.com/expo/eas-cli/pull/4097) by [@sswrk](https://github.com/sswrk))
 
 ### 🐛 Bug fixes
 
diff --git a/packages/build-tools/src/steps/__tests__/compositeFunctions.test.ts b/packages/build-tools/src/steps/__tests__/compositeFunctions.test.ts
index bb9d61e1d3..bfb45ac198 100644
--- a/packages/build-tools/src/steps/__tests__/compositeFunctions.test.ts
+++ b/packages/build-tools/src/steps/__tests__/compositeFunctions.test.ts
@@ -191,6 +191,121 @@ describe(buildCompositeFunctionCatalogAsync, () => {
     expect(catalog).toEqual({});
   });
 
+  it('loads a referenced single-step command function', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'say-hi',
+      [
+        'name: Say hi',
+        'inputs:',
+        '  - name',
+        'outputs:',
+        '  - greeting',
+        'command: set-output greeting "Hi, ${ inputs.name }!"',
+        'shell: sh',
+        'supported_platforms:',
+        '  - linux',
+      ].join('\n')
+    );
+
+    const catalog = await buildCompositeFunctionCatalogAsync(projectRoot, {
+      steps: [{ uses: './.eas/functions/say-hi', id: 'greet' }],
+    });
+
+    const localFunction = catalog['./.eas/functions/say-hi'];
+    assert(isLegacyFunctionConfig(localFunction));
+    expect(localFunction.name).toBe('Say hi');
+    expect(localFunction.command).toBe('set-output greeting "Hi, ${ inputs.name }!"');
+    expect(localFunction.shell).toBe('sh');
+    expect(localFunction.supported_platforms).toEqual(['linux']);
+    expect(localFunction.inputs).toEqual(['name']);
+    expect(localFunction.outputs).toEqual(['greeting']);
+  });
+
+  it('resolves the module path of a single-step path function against the function directory', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'say-hi',
+      ['path: ./my-function'].join('\n')
+    );
+    const moduleDir = path.join(projectRoot, '.eas', 'functions', 'say-hi', 'my-function');
+    await fs.mkdir(moduleDir, { recursive: true });
+    await fs.writeFile(path.join(moduleDir, 'package.json'), '{}', 'utf-8');
+
+    const catalog = await buildCompositeFunctionCatalogAsync(projectRoot, {
+      steps: [{ uses: './.eas/functions/say-hi', id: 'greet' }],
+    });
+
+    const localFunction = catalog['./.eas/functions/say-hi'];
+    assert(isLegacyFunctionConfig(localFunction));
+    expect(localFunction.path).toBe(moduleDir);
+  });
+
+  it('throws when the module directory of a path function does not exist', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'say-hi',
+      'path: ./my-function'
+    );
+
+    await expect(
+      buildCompositeFunctionCatalogAsync(projectRoot, {
+        steps: [{ uses: './.eas/functions/say-hi', id: 'greet' }],
+      })
+    ).rejects.toThrow(
+      /Local function "\.\/\.eas\/functions\/say-hi" declares "path: \.\/my-function", but there is no such directory at .*my-function\./
+    );
+  });
+
+  it('throws when the module directory of a path function has no package.json', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'say-hi',
+      'path: ./my-function'
+    );
+    await fs.mkdir(path.join(projectRoot, '.eas', 'functions', 'say-hi', 'my-function'), {
+      recursive: true,
+    });
+
+    await expect(
+      buildCompositeFunctionCatalogAsync(projectRoot, {
+        steps: [{ uses: './.eas/functions/say-hi', id: 'greet' }],
+      })
+    ).rejects.toThrow(/does not contain a package.json file/);
+  });
+
+  it('loads a catalog mixing composite and single-step functions', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'setup',
+      ['runs:', '  steps:', '    - uses: ./.eas/functions/say-hi'].join('\n')
+    );
+    const sayHiDir = path.join(projectRoot, '.eas', 'functions', 'say-hi');
+    await fs.mkdir(sayHiDir, { recursive: true });
+    await fs.writeFile(path.join(sayHiDir, 'function.yml'), 'command: echo hi', 'utf-8');
+
+    const catalog = await buildCompositeFunctionCatalogAsync(projectRoot, {
+      steps: [{ uses: './.eas/functions/setup', id: 'setup' }],
+    });
+
+    expect(Object.keys(catalog).sort()).toEqual([
+      './.eas/functions/say-hi',
+      './.eas/functions/setup',
+    ]);
+    expect(isLegacyFunctionConfig(catalog['./.eas/functions/say-hi'])).toBe(true);
+    expect(isLegacyFunctionConfig(catalog['./.eas/functions/setup'])).toBe(false);
+  });
+
+  it('throws a clear error for a function config mixing both shapes', async () => {
+    const projectRoot = await makeProjectWithCompositeFunctionAsync(
+      'broken',
+      ['command: echo hi', 'runs:', '  steps:', '    - run: echo hi'].join('\n')
+    );
+
+    await expect(
+      buildCompositeFunctionCatalogAsync(projectRoot, {
+        steps: [{ uses: './.eas/functions/broken', id: 'broken' }],
+      })
+    ).rejects.toThrow(
+      'Invalid local function "./.eas/functions/broken": ✖ A local function must declare either "runs.steps" (a composite function) or "command"/"path" (a single-step function), not both.'
+    );
+  });
+
   it('throws a clear error for a malformed referenced composite function config', async () => {
     const projectRoot = await makeProjectWithCompositeFunctionAsync(
       'broken',
diff --git a/packages/build-tools/src/steps/compositeFunctions.ts b/packages/build-tools/src/steps/compositeFunctions.ts
index 9c68902c5d..28df3b0821 100644
--- a/packages/build-tools/src/steps/compositeFunctions.ts
+++ b/packages/build-tools/src/steps/compositeFunctions.ts
@@ -5,14 +5,16 @@
 import { bunyan } from '@expo/logger';
 import {
   CompositeFunctionCatalog,
-  CompositeFunctionConfig,
-  CompositeFunctionConfigZ,
   Hooks,
+  LocalFunctionConfig,
+  LocalFunctionConfigZ,
   Step,
+  isLegacyFunctionConfig,
   parseHookKey,
 } from '@expo/eas-build-job';
 import {
   buildCompositeFunctionCatalogFromStepsAsync,
+  resolveLegacyFunctionModulePath,
   resolveLocalCompositeFunctionPath,
 } from '@expo/steps';
 import fs from 'fs/promises';
@@ -24,7 +26,7 @@ async function loadLocalCompositeFunctionConfigAsync(
   projectRoot: string,
   compositeFunctionPath: string,
   { logger }: { logger?: bunyan } = {}
-): Promise<CompositeFunctionConfig> {
+): Promise<LocalFunctionConfig> {
   const resolvedPath = resolveLocalCompositeFunctionPath(projectRoot, compositeFunctionPath);
 
   for (const ext of ['yml', 'yaml'] as const) {
@@ -54,17 +56,25 @@ async function loadLocalCompositeFunctionConfigAsync(
         }
       );
     }
-    let config: CompositeFunctionConfig;
+    let config: LocalFunctionConfig;
     try {
-      config = CompositeFunctionConfigZ.parse(parsed);
+      config = LocalFunctionConfigZ.parse(parsed);
     } catch (err) {
       if (err instanceof ZodError) {
         throw new Error(
-          `Invalid composite function "${compositeFunctionPath}": ${z.prettifyError(err)}`
+          `Invalid local function "${compositeFunctionPath}": ${z.prettifyError(err)}`
         );
       }
       throw err;
     }
+    // The catalog is consumed by the steps parser, which expects an absolute module path.
+    if (isLegacyFunctionConfig(config) && config.path !== undefined) {
+      config.path = await resolveFunctionModulePathAsync({
+        projectRoot,
+        functionPath: compositeFunctionPath,
+        modulePath: config.path,
+      });
+    }
     logger?.debug(
       `Loaded local composite function "${compositeFunctionPath}" from ${path.relative(projectRoot, absolutePath)}`
     );
@@ -76,6 +86,47 @@ async function loadLocalCompositeFunctionConfigAsync(
   );
 }
 
+/**
+ * Single-step functions never join `workflow.buildFunctions`, so
+ * `BuildWorkflowValidator.validateCustomFunctionModulesAsync` will not check their modules. This
+ * is where a missing module has to be caught, before the job is parsed.
+ */
+async function resolveFunctionModulePathAsync({
+  projectRoot,
+  functionPath,
+  modulePath,
+}: {
+  projectRoot: string;
+  functionPath: string;
+  modulePath: string;
+}): Promise<string> {
+  const resolvedModulePath = resolveLegacyFunctionModulePath({
+    projectRoot,
+    functionPath,
+    modulePath,
+  });
+  if (!(await pathExistsAsync(resolvedModulePath))) {
+    throw new Error(
+      `Local function "${functionPath}" declares "path: ${modulePath}", but there is no such directory at ${resolvedModulePath}.`
+    );
+  }
+  if (!(await pathExistsAsync(path.join(resolvedModulePath, 'package.json')))) {
+    throw new Error(
+      `Local function "${functionPath}" declares "path: ${modulePath}", but the module directory ${resolvedModulePath} does not contain a package.json file.`
+    );
+  }
+  return resolvedModulePath;
+}
+
+async function pathExistsAsync(target: string): Promise<boolean> {
+  try {
+    await fs.access(target);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 export async function buildCompositeFunctionCatalogAsync(
   projectRoot: string,
   { steps, hooks, logger }: { steps: readonly Step[]; hooks?: Hooks; logger?: bunyan }