M2M Token only authentication in Clerk Middleware

[x-delfino]

Preliminary Checks

• I have reviewed the documentation: https://clerk.com/docs

• I have searched for existing issues: https://github.com/clerk/javascript/issues

• I have not already reached out to Clerk support via email or Discord (if you have, no need to open an issue here)

• This issue is not a question, general help request, or anything other than a bug report directly related to Clerk. Please ask questions in our Discord community: https://clerk.com/discord.

Reproduction

https://github.com/x-delfino/clerk-fastify-issue-repro

Publishable key

pk_test_dG91Y2hlZC1mb3dsLTk5MDYuY2xlcmsuYWNjb3VudHMuZGV2JA

Description

I'm trying to use M2M tokens to authenticate one machine to another, without any subsequent backend API access. I'm testing this with the clerk fastify plugin, but I don't think the issue is necessarily specific to that package

Steps to reproduce:

1. Create M2M Machines in Clerk:

• MachineA: no scopes
• MachineB: scope for MachineA

2. Setup workspace (using MachineA secret key):

git clone https://github.com/x-delfino/clerk-fastify-issue-repro
cd clerk-fastify-issue-repro
pnpm install
export CLERK_MACHINE_SECRET_KEY="ak_XXXXXXXXX"
pnpm run serve

3. Generate token for MachineB
4. make HTTP request:

TOKEN="MACHINE_B_TOKEN"
curl -H "Authorization: Bearer $TOKEN" localhost:8080/protected

Expected behavior:

To receive response:

{"message":"Machine authenticated successfully","subject":"mch_XXXXXXXXX","scopes":["mch_XXXXXXXXX"]}

Actual behavior:

Response received:

{"statusCode":500,"error":"Internal Server Error","message":"Publishable key is missing.\n\nTo create a new Clerk app, run:\nnpx clerk@latest init\n\nTo use an existing Clerk app, run:\nnpx clerk@latest link\nnpx clerk@latest env pull\n\nFor production keys, run:\nnpx clerk@latest env pull --instance prod\n\nOr copy keys from https://dashboard.clerk.com/~/api-keys into your .env file."}

Providing CLERK_PUBLISHABLE_KEY changes the response to:

{"statusCode":500,"error":"Internal Server Error","message":"Missing Clerk Secret Key. Go to https://dashboard.clerk.com and get your key for your instance."}

Detail:

The clerk middleware for fastify hardcodes the acceptsToken to 'any':

javascript/packages/fastify/src/withClerkMiddleware.ts

Lines 110 to 116 in dfb6204

	const requestState = await clerkClient.authenticateRequest(req, {
	...clerkOptions,
	secretKey,
	publishableKey,
	proxyUrl: resolvedProxyUrl,
	acceptsToken: 'any',
	});

When the token is then processed, as it's not explicitly M2MToken or ApiKey - it tries to load the publishable key:

javascript/packages/backend/src/tokens/authenticateContext.ts

Lines 86 to 98 in 26feba7

	if (options.acceptsToken === TokenType.M2MToken || options.acceptsToken === TokenType.ApiKey) {
	// For non-session tokens, we only want to set the header values.
	this.initHeaderValues();
	} else {
	// Even though the options are assigned to this later in this function
	// we set the publishableKey here because it is being used in cookies/headers/handshake-values
	// as part of getMultipleAppsCookie.
	this.initPublishableKeyValues(options);
	this.initHeaderValues();
	// initCookieValues should be used before initHandshakeValues because it depends on suffixedCookies
	this.initCookieValues();
	this.initHandshakeValues();
	}

I patched the fastify clerk middleware to allow the acceptsToken key to be provided:

    const requestState = await clerkClient.authenticateRequest(req, {
      ...clerkOptions,
      secretKey,
      publishableKey,
      proxyUrl: resolvedProxyUrl,
      // acceptsToken: 'any',
    });

Allowing me to update my sample code to:

fastify.register(clerkPlugin, { acceptsToken: "m2m_token" })

Allowing me to receive and validate M2M tokens using M2M tokens only

However, this isn't type correct. ClerkFastifyOptions, through a chain of intersections, doesn't pull in acceptsToken from AuthenticateRequestOptions

javascript/packages/backend/src/tokens/factory.ts

Lines 8 to 22 in 2ee976b

	type BuildTimeOptions = Partial<
	Pick<
	AuthenticateRequestOptions,
	| 'apiUrl'
	| 'apiVersion'
	| 'audience'
	| 'domain'
	| 'isSatellite'
	| 'jwtKey'
	| 'proxyUrl'
	| 'publishableKey'
	| 'secretKey'
	| 'machineSecretKey'
	>
	>;

I'm not sure the best way to proceed from here as changes may affect more than fastify

Environment

System:
    OS: macOS 26.5
    CPU: (10) arm64 Apple M4
    Memory: 134.00 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 26.10.0 - /nix/store/xs1l2nzllqp7imnhpd2n6cm0m70yzp10-nodejs-26.10.0/bin/node
    npm: 11.19.1 - /nix/store/xs1l2nzllqp7imnhpd2n6cm0m70yzp10-nodejs-26.10.0/bin/npm
    pnpm: 11.27.0 - /nix/store/gjkd8wjh079v7i5j2rb8y0b5r19addpa-pnpm-11.27.0/bin/pnpm
  Browsers:
    Safari: 26.5
  npmPackages:
    @clerk/fastify: ^3.1.82 => 3.1.82 
    @types/node: ^26.6.3 => 26.6.3 
    fastify: ^5.12.5 => 5.12.5 
    typescript: ^7.0.2 => 7.0.2