diff --git a/fep/c551/fep-c551.md b/fep/c551/fep-c551.md
new file mode 100644
index 0000000..21bd878
--- /dev/null
+++ b/fep/c551/fep-c551.md
@@ -0,0 +1,134 @@
+---
+slug: "c551"
+authors: bengo <ben@bengo.co>
+status: DRAFT
+dateReceived: 2024-07-11
+trackingIssue:
+discussionsTo:
+---
+
+# FEP-c551: Use ECMAScript Modules to Create Conformance Tests for Fediverse Enhancement Proposals
+
+## Summary
+
+This is a proposal to enhance the fediverse by creating test cases for FEPs as ECMAScript Modules.
+
+<!-- TOC -->
+
+## Contents
+
+* [Context](#context)
+* [Test Specifications](#test-specifications)
+* [Test Modules](#test-modules)
+* [Test Objects](#test-objects)
+* [Test Functions](#test-functions)
+* [Test Inputs](#test-inputs)
+* [Test Results](#test-results)
+
+<!-- section break -->
+
+## Context
+
+[FEP-d9ad][] proposes to Create Conformance Tests for Fediverse Enhancement Proposals, and specifies components that all Conformance Tests may use and describe in their Test Specifications. It *does not* specify a format for implementing FEP-d9ad Conformance Tests in any programming language.
+
+This FEP-c551 proposes to supplement human-readable FEP-d9ad Conformance Tests with implementations of the test as [Test Objects](#test-objects) exported from [Test Modules](#test-modules). Each [Test Object][] has a `run` function parameterized by a [Test Input] and returning a Promise of a [Test Result][].
+
+## Overview
+
+When a tester comes up with a new test for a FEP, they create a human-readable [Test Specification](#test-specifications) describing how to test whether some subject conforms to the FEP.
+
+ECMAScript developers implement Test Specifications as automatable code by using ECMAScript to create [Test Functions][] that execute the test logic and [Test Objects][] that group the Test Function with more info like the test's name, required input, and possible outcomes. Test Objects are distributed in ECMAScript Modules published on the web, e.g. in `.js` or `.mjs` files.
+
+Testers invoke the Test Function once for each Test Input, await any returned Promises, and receive a [Test Result][] describing the `outcome` of running the test.
+
+## Test Specifications
+
+Test Specifications are human-readable documents that specify the behavior of a test.
+
+Test Specifications SHOULD include Conformance Test Component specifications from [FEP-d9ad][].
+
+An example of a test specification is [fep-521a-test-case.md](https://codeberg.org/fediverse/fep/src/branch/main/fep/521a/fep-521a-test-case.md).
+
+## Test Modules
+
+FEP testers MAY publish implementations of their test specifications as an [ECMAScript Module][] following the recommendations in this proposal. Such modules may be referred to as Test Modules.
+
+Test Modules SHOULD have no imports. This is to ensure portability of the test modules.
+
+Test Modules MAY export a default export object that is a Test Object
+
+Test Modules SHOULD be resilient to being parsed and evaluated in various ECMAScript runtimes (e.g. node.js or a web browser like Firefox).
+
+An example of a test module can be found [in activitypub-testing-fep-521a](https://codeberg.org/socialweb.coop/activitypub-testing-fep-521a/src/branch/main/fep/521a/actor-objects-must-express-signing-key-as-assertionMethod-multikey.js).
+
+### Example Test Module
+
+```javascript
+export default {
+  name: 'invalid script module name',
+  run: (input) => ({ outcome: 'passed' }),
+  type: ['https://w3id.org/fep/c551#Test'],
+  '@context': ["https://www.w3.org/ns/activitystreams"],
+}
+```
+
+## Test Objects
+
+Test Objects are ECMAScript Objects that represent a named, runnable test, e.g. a test specified by a [FEP-d9ad Conformance Test][].
+
+Test Objects MUST have a property named `type` whose value is either the string `https://w3id.org/fep/c551#Test` or  an Array containing that string.
+
+Test Objects MUST have a property named `name` whose value is a string.
+
+Test Objects SHOULD have a property named `@context` whose value is an Array containing `https://www.w3.org/ns/activitystreams`.
+
+An example of a test object is [exported in activitypub-testing-fep-521a](https://codeberg.org/socialweb.coop/activitypub-testing-fep-521a/src/commit/b6e49fd5f490b05f04a958f5f3c5c584e66f592b/fep/521a/actor-objects-must-express-signing-key-as-assertionMethod-multikey.js#L38).
+
+Test Objects SHOULD have a property named `run` whose value is a [Test Function][]].
+
+## Test Functions
+
+Test Object `run` calls SHOULD return a `Promise` that resolves to a [Test Result][].
+
+Test Object `run` functions SHOULD be resilient to being run in various ECMAScript runtimes (e.g. node.js or a web browser like Firefox).
+
+## Test Inputs
+
+A Test Input is the first parameter to a test's `run` function.
+
+Test Input MUST be an object. A test with several logically distinct inputs should give each input a name, and add each named input as a property within a top-level input object.
+
+Test Input values SHOULD conform to the specification of the called test's [Input](https://bengo.is/fep/d9ad/#input) spec.
+
+## Test Results
+
+[Test Results][] MUST have a property named `outcome` whose value is a string.
+
+Test Results SHOULD have a property named `info` whose value is a string.
+
+Test Results MAY have a property named `pointer` that contextualizes the `outcome`, e.g. an object with a property for each value that led to the `outcome`. For example, if a test outcome is `failed` because some number was too low, you can set the result `info` to "number too low" and `pointer` to `{ number: 100 }`.
+
+<!-- section break -->
+
+<section id="conformance">
+Conformance requirements are indicated by sentences containing MUST a la <a href="https://datatracker.ietf.org/doc/html/rfc2119">RFC2119</a>.
+</section>
+
+## Copyright
+
+CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
+
+To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.
+
+[ECMAScript Module]: https://tc39.es/ecma262/#sec-modules
+[Test Module]: #test-modules
+[Test Modules]: #test-modules
+[Test Inputs]: #test-inputs
+[Test Result]: #test-results
+[Test Results]: #test-results
+[Test Object]: #test-objects
+[Test Objects]: #test-objects
+[Test Function]: #test-functions
+[Test Functions]: #test-functions
+[FEP-d9ad]: https://bengo.is/fep/d9ad/
+[FEP-d9ad Conformance Test]: https://bengo.is/fep/d9ad/
diff --git a/fep/c551/tests/.gitignore b/fep/c551/tests/.gitignore
new file mode 100644
index 0000000..c925c21
--- /dev/null
+++ b/fep/c551/tests/.gitignore
@@ -0,0 +1,2 @@
+/dist
+/node_modules
diff --git a/fep/c551/tests/README.md b/fep/c551/tests/README.md
new file mode 100644
index 0000000..b5a4827
--- /dev/null
+++ b/fep/c551/tests/README.md
@@ -0,0 +1,44 @@
+# fep-c551-tests
+
+A test suite that tests for conformance to [FEP-c551: Use ECMAScript Modules to Create Conformance Tests for Fediverse Enhancement Proposals][FEP-c551].
+
+## Tests
+
+### fep-c551 module must export test object
+
+* slug: `fep-c551-module-must-export-test-object`
+* [Test Module](./fep-c551-module-must-export-test-object.js)
+
+## Usage
+
+### Running local test files via Data URL
+
+```shell
+activitypub-testing run test \
+--url="$(data-url ./fep-c551-module-must-export-test-object.js)" \
+--input.module="$(cat <<EOF
+export default {
+  name: 'sample test',
+  run: () => {
+    return { outcome: 'inapplicable' }
+  }
+}
+EOF
+)"
+```
+
+The `data-url` command is provided by the following shell function:
+
+```shell
+data-url() {
+  if [ -z "$1" ]; then
+    echo "usage: data-url file" >&2
+    exit 1
+  fi
+  mimetype=$(file -bN --mime-type "$1")
+  content=$(base64 < "$1")
+  echo "data:$mimetype;base64,$content"
+}
+```
+
+[FEP-c551]: https://codeberg.org/fediverse/fep/src/branch/main/fep/c551/fep-c551.md
diff --git a/fep/c551/tests/fep-c551-module-must-export-test-object.js b/fep/c551/tests/fep-c551-module-must-export-test-object.js
new file mode 100644
index 0000000..bfb2b7c
--- /dev/null
+++ b/fep/c551/tests/fep-c551-module-must-export-test-object.js
@@ -0,0 +1,220 @@
+const name = 'fep-c551 module must export test object'
+const slug = 'fep-c551-module-must-export-test-object'
+const uuid = '14bab0ae-e682-4f4c-9474-ef65ca47d527'
+const attributedTo = [
+  'https://bengo.is',
+]
+
+/** ID URL for type of FEP-C551 Test */
+const fepC551TestType = 'https://w3id.org/fep/c551#Test'
+
+/**
+ * Expected input to the test rule.
+ * This will be checked for test applicability.
+ * If the test is applicable, the rule will derive a test target from the Input,
+ * then check expectations against the Target
+ * returning a result with Outcomes
+ * @typedef Input
+ * @property {unknown} module
+ */
+
+/**
+ * The test will check expectations against test Target derived from Input
+ * @typedef Target
+ * @property {string} module
+ */
+
+/** 
+ * Outcome - every test Target has an outcome
+ * @typedef {"inapplicable"|"passed"|"failed"} Outcome
+ */
+
+/**
+ * @typedef {any} TestResult
+ */
+
+export default {
+  attributedTo,
+  testCases: [
+
+    {
+      name: 'valid actor - type value is array',
+      input: {
+        module: `
+        export default {
+          type: ['https://w3id.org/fep/c551#Test'],
+          name: 'invalid script module name',
+          run: () => ({ outcome: 'passed' }),
+          '@context': ["https://www.w3.org/ns/activitystreams"],
+        };
+        `
+      },
+      result: {
+        outcome: 'passed',
+      }
+    },
+
+    {
+      name: 'valid actor - type value is string',
+      input: {
+        module: `
+        export default {
+          name: 'invalid script module name',
+          run: () => ({ outcome: 'passed' }),
+          type: 'https://w3id.org/fep/c551#Test',
+          '@context': ["https://www.w3.org/ns/activitystreams"],
+        };
+        `
+      },
+      result: {
+        outcome: 'passed',
+      }
+    },
+
+    {
+      name: 'without run property',
+      input: {
+        module: `
+        export default {
+          name: 'invalid script module name',
+          type: 'https://w3id.org/fep/c551#Test',
+          '@context': ["https://www.w3.org/ns/activitystreams"]
+        };
+        `
+      },
+      result: {
+        outcome: 'failed',
+      }
+    },
+
+    {
+      name: 'with empty type array',
+      input: {
+        module: `
+        export default {
+          name: 'invalid script module name',
+          run: () => ({ outcome: 'passed' }),
+          type: [],
+          '@context': ["https://www.w3.org/ns/activitystreams"],
+        };
+        `
+      },
+      result: {
+        outcome: 'failed',
+      }
+    },
+  ],
+  input: {
+    module: {
+      help: 'ECMAScript Module that exports a test object',
+      required: true,
+    }
+  },
+  name,
+  run,
+  slug,
+  uuid,
+}
+
+/**
+ * given test rule inputs, check for applicability.
+ * If the input does not pass test rule applicability requirements,
+ *   return a result with outcome "inapplicable".
+ * (does some checks from 'Applicability' section of test rule)
+ * @param {Input} input
+ * @returns {{ outcome: "inapplicable", info: string }
+ *          |{ module: string }}
+ */
+function checkApplicability(input) {
+  if (typeof input.module !== 'string') return {
+    outcome: "inapplicable",
+    info: 'applicability requires input.module MUST be a string'
+  }
+  return {
+    module: input.module,
+  }
+}
+
+/**
+ * given test rule inputs, return test targets.
+ * (does some checks from 'Applicability' section of test rule)
+ * @param {Input & {console?:globalThis.console}} input
+ */
+function getTarget({ module, console = globalThis.console }) {
+  if (typeof module !== 'string') {
+    return {
+      result: {
+        outcome: 'inapplicable',
+        info: 'input.module MUST be a string',
+      }
+    }
+  }
+
+  return {
+    targets: [{ module }]
+  }
+}
+
+/**
+ * run expectations against target
+ * @param {Target} target
+ */
+async function expect({ module }) {
+  if (typeof module !== 'string') return { result: { outcome: 'failed', info: 'input.module MUST be a string' } }
+  const moduleUri = `data:text/javascript;charset=utf-8;base64,${btoa(module)}`
+  const test = await import(moduleUri).then(m => m.default)
+  if (typeof test?.run !== 'function') return { result: { outcome: 'failed', info: 'exported test.run MUST be a function', pointer: { run: test.run } } }
+
+  // The default export MUST have a property named `@type` whose value is either the string `https://w3id.org/fep/c551#Test` or an Array containing that string.
+  const testTypeValues = Array.isArray(test.type) ? test.type : test.type || []
+  if ( ! testTypeValues.includes(fepC551TestType)) {
+    return {
+      result: {
+        outcome: "failed",
+        info: "test must have type https://w3id.org/fep/c551#Test",
+        pointer: {
+          type: test.type
+        }
+      }
+    }
+  }
+
+  return { result: { outcome: "passed" } }
+}
+
+/**
+ * @param {Input} input
+ */
+async function run(input) {
+  // check input for whether this test applies
+  const applicability = await checkApplicability(input)
+  if ('outcome' in applicability && applicability.outcome === "inapplicable") {
+    return applicability
+  }
+
+  // get test targets
+  const targeting = getTarget(input)
+  if ('result' in targeting) return targeting.result
+  /** @type {Array<{ target: Target, result: TestResult}>} */
+  const results = []
+  for (const target of targeting.targets) {
+    if (!target) throw new Error(`got undefined target. this should not happen`)
+    // check expectations against targets
+    const expectations = await expect(target)
+    if (expectations && 'result' in expectations) results.push({
+      target,
+      result: expectations.result,
+    })
+  }
+  if (results.length === 1) {
+    return results[0].result
+  } else if (results.length) {
+    return {
+      outcome: results.every(r => r.result.outcome === "passed") ? "passed" : "failed",
+      pointer: {
+        results
+      }
+    }
+  }
+  throw Object.assign(new Error('unexpected input'), { input })
+}
diff --git a/fep/c551/tests/fep-c551-module-must-export-test-object.test.js b/fep/c551/tests/fep-c551-module-must-export-test-object.test.js
new file mode 100644
index 0000000..d94a4c7
--- /dev/null
+++ b/fep/c551/tests/fep-c551-module-must-export-test-object.test.js
@@ -0,0 +1,54 @@
+import { describe, it } from 'node:test';
+import assert from "node:assert";
+
+import testCase from "./fep-c551-module-must-export-test-object.js"
+
+await describe(`activitypub-testing test ${testCase.slug}`, async () => {
+  await describe('default export', async () => {
+    await it('has a uuid', () => {
+      assert.equal(typeof testCase.uuid, 'string')
+    })
+  })
+
+  await it('has testCases', async () => {
+    await testHasTestCases(testCase, { minimum: 1 })
+  })
+
+  await it('when inputs are {}, outcome is inapplicable', async () => {
+    // @ts-expect-error - testing even though typechecker should prevent
+    const result = await testCase.run({})
+    assert.equal(result.outcome, 'inapplicable')
+  })
+});
+
+/**
+ * @template Inputs
+ * @template Outcome
+ * @param {object} test
+ * @param {Array<any>} [test.testCases]
+ * @param {(input: Inputs) => Promise<any>} test.run
+ * @param {object} options
+ * @param {number} [options.minimum=0] - minimum required testCases
+ */
+export async function testHasTestCases({ testCases = [], run }, { minimum = 0 } = {}) {
+  let remainingForMinimum = minimum
+  for (const testCase of testCases) {
+    await it(`test case: "${testCase.name}"`, async () => {
+      const result = await run(testCase.input)
+      if (result.outcome !== testCase.result.outcome) {
+        throw Object.assign(
+          new Error(`expected result.outcome to be "${testCase.result.outcome}" but got '${result.outcome}'`),
+          {
+            name: 'UnexpectedOutcome',
+            testCase,
+            result
+          }
+        )
+      }
+      remainingForMinimum--
+    })
+  }
+  if (remainingForMinimum > 0) {
+    throw new Error(`test had ${minimum - remainingForMinimum} testCases but failed to meet required minimum of ${minimum}`)
+  }
+}
diff --git a/fep/c551/tests/index.js b/fep/c551/tests/index.js
new file mode 100644
index 0000000..e440b69
--- /dev/null
+++ b/fep/c551/tests/index.js
@@ -0,0 +1,5 @@
+import testfepC551ModuleMustExportTestObject from "./fep-c551-module-must-export-test-object";
+
+export default [
+  testfepC551ModuleMustExportTestObject,
+]
diff --git a/fep/c551/tests/package-lock.json b/fep/c551/tests/package-lock.json
new file mode 100644
index 0000000..25bc86d
--- /dev/null
+++ b/fep/c551/tests/package-lock.json
@@ -0,0 +1,44 @@
+{
+  "name": "activitypub-testing-fep-c551",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "activitypub-testing-fep-c551",
+      "version": "1.0.0",
+      "devDependencies": {
+        "@types/node": "^20.14.10",
+        "typescript": "^5.5.3"
+      }
+    },
+    "node_modules/@types/node": {
+      "version": "20.14.10",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-20.14.10.tgz",
+      "integrity": "sha512-MdiXf+nDuMvY0gJKxyfZ7/6UFsETO7mGKF54MVD/ekJS6HdFtpZFBgrh6Pseu64XTb2MLyFPlbW6hj8HYRQNOQ==",
+      "dev": true,
+      "dependencies": {
+        "undici-types": "~5.26.4"
+      }
+    },
+    "node_modules/typescript": {
+      "version": "5.5.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.5.3.tgz",
+      "integrity": "sha512-/hreyEujaB0w76zKo6717l3L0o/qEUtRgdvUBvlkhoWeOVMjMuHNHk0BRBzikzuGDqNmPQbg5ifMEqsHLiIUcQ==",
+      "dev": true,
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
+    "node_modules/undici-types": {
+      "version": "5.26.5",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-5.26.5.tgz",
+      "integrity": "sha512-JlCMO+ehdEIKqlFxk6IfVoAUVmgz7cU7zD/h9XZ0qzeosSHmUJVOzSQvvYSYWXkFXC+IfLKSIffhv0sVZup6pA==",
+      "dev": true
+    }
+  }
+}
diff --git a/fep/c551/tests/package.json b/fep/c551/tests/package.json
new file mode 100644
index 0000000..7037b78
--- /dev/null
+++ b/fep/c551/tests/package.json
@@ -0,0 +1,14 @@
+{
+  "name": "fep-c551-tests",
+  "type": "module",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "test": "node --test",
+    "typecheck": "tsc --build"
+  },
+  "devDependencies": {
+    "@types/node": "^20.14.10",
+    "typescript": "^5.5.3"
+  }
+}
diff --git a/fep/c551/tests/tsconfig.json b/fep/c551/tests/tsconfig.json
new file mode 100644
index 0000000..2135fa2
--- /dev/null
+++ b/fep/c551/tests/tsconfig.json
@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "composite": true,
+    "target": "ESNext",
+    "module": "ESNext",
+    "moduleResolution": "Bundler",
+    "esModuleInterop": true,
+    "allowJs": true,
+    "checkJs": true,
+    "strict": true,
+    "skipLibCheck": true,
+    "emitDeclarationOnly": true,
+    "outDir": "dist",
+    "lib": [
+      "esnext",
+      "DOM"
+    ],
+    "jsx": "react-jsx",
+    "jsxImportSource": "hono/jsx"
+  },
+  "include": ["*.js", "src", "test"],
+  "references": [
+  ]
+}