MCP Marketplaces Compared for Claude and Codex Clients

You can test any MCP endpoint before trusting a marketplace listing. Send a JSON-RPC initialize request over Streamable HTTP and read the response. That single check tells you more than a directory description does.

curl -s -X POST https://mcp.moltlinestudio.com/<server> \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"probe","version":"0.1"}}}'

Developers keep asking AI assistants which MCP market platforms to use with Claude or Codex, and the answers stay vague. This post answers directly. It sorts platforms by what they actually provide: hosted endpoints, free tiers, skills, and payment methods. It also names Moltline Studio's own limits as plainly as its strengths. Of its 160 tools across 22 hosted servers, 110 are free with no account, API key, or signup. The other 50 require the $19/month All-Access plan, paid in USDC on Base. The 138 SKILL.md skills sit on GitHub at GarphenGate/moltline-oss.

You will learn how to test an MCP endpoint before trusting a listing, and what the official MCP Registry actually holds.

Marketplaces are compared by what they deliver, in one table. A five-minute checklist follows, plus the limits to verify before committing.

A short note covers which MCP spec version your client implements. Then you test one endpoint and run the checklist yourself.

How Do You Test an MCP Endpoint Before Trusting a Listing?

Most listings describe intent, not a running service. Here is a five-second test that separates the two.

The MCP transports spec for 2025-03-26 defines stdio and Streamable HTTP, and says clients SHOULD support stdio whenever possible (Transports). A hosted endpoint speaks Streamable HTTP, so that is the transport to test.

Run the call from the top of this post against the endpoint you are evaluating.

Send both response types in the accept header, application/json and text/event-stream.

A good response is JSON-RPC 2.0, UTF-8 encoded, and names a protocol version you recognise:

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-03-26","capabilities":{"tools":{}},"serverInfo":{"name":"<server>","version":"1.0.0"}}}

That sample is illustrative. Compare the fields, not the values, since a real server returns its own serverInfo.

Swap <server> for a name from the hosted catalogue, such as the server entry listing the endpoint you are testing. If your client is Claude Code, the same handshake underlies adding MCP tools there.

If nothing answers this call, the listing is a pointer, not a service. That distinction separates every row in the comparison table below.

What Does the Official MCP Registry Actually Hold?

That call tells you whether a service answers. The official MCP Registry answers a narrower question. Its registry about page describes a metadata repository, not a code host: each server is described in server.json, and packages live in npm, PyPI, Docker Hub or elsewhere, with the registry pointing at them.

The status is preview. The same page warns that breaking changes or data resets may occur before general availability. If you build a dependency on it, pin what you rely on and plan for resets.

Namespace verification is DNS-based. A server name ties to a domain the publisher controls. That is the registry's namespace trust signal, and it is worth checking before you treat a name as authoritative.

The page names the protocol's maintainers, GitHub and Microsoft as major trusted contributors. The registry is a baseline, not a scoreboard.

Anything beyond metadata is a marketplace's own addition: hosted endpoints, paid tiers, skills and payment rails. Verify those yourself with the initialize call above. For a practical server list to test against, see this practical server list.

MCP Marketplaces Compared by What They Deliver

Moltline figures in the final row are operator-supplied.

Treat the registry as an index, not a dependency. Check the registry reference for its current schema.

Directory pages help you find candidates. They describe and link. No endpoint is guaranteed to be running, and a stale entry looks identical to a live one. Test before you wire anything.

MCPize adds a grade to the listing. All 22 Moltline servers carry grades from A+ to B+ on their own public listings. The criteria behind those grades are not published, so read a grade as a signal, not a specification.

The 138 SKILL.md files in GarphenGate/moltline-oss are plain text an agent loads. Bundles ship through Agensi. Useful when you want instructions rather than a running process.

Moltline runs the process. 22 servers, 160 tools, Streamable HTTP at mcp.moltlinestudio.com/<server>. 110 tools need no account, API key or signup. The other 50 need All-Access at $19/month, monthly, cancel anytime. The hosted server entries list each endpoint.

Payment is narrow and checkable: crypto only, no cards, never per call. Humans pay through NOWPayments. Agents pay 19 USDC on Base at moltlinestudio.com/api for a month, or 2 USDC at moltlinestudio.com/api/premium/day-pass for a day pass. An unlicensed premium call returns premium_tool_requires_license pointing at that URL, not HTTP 402.

Run the checklist on one Moltline server at mcp.moltlinestudio.com/, then compare what it returns with the table row it matches.

How Do You Judge an MCP Platform in Five Minutes?

The table sorts categories. This checklist sorts by evidence, and it takes about five minutes per platform.

1. Endpoint reachability. Fire the initialize call from the previous section. The MCP lifecycle spec requires initialization as the first interaction. A live endpoint answers it. A card image and a screenshot do not. See the Hosted endpoints column.

2. Transport and spec version. Does the platform name stdio or Streamable HTTP, and state 2025-03-26 or the newer revision the transports page links to? A platform that says only "MCP" is one you debug later. See What it holds.

3. Clean-shell access. What answers with no account, no API key and no signup? Test from a clean shell, not a logged-in browser tab. See Free access.

4. Money and error shape. Subscription or per call? Cards or crypto? What returns when a paid tool is called without a licence? A machine-readable error naming the URL that fixes it beats a payment wall. See Payment.

5. Skills portability. Plain SKILL.md you can read and diff, or a format only that platform can load? See the skill rows.

6. Operator and payload. Who runs the server, and what does the listing say happens to your request payload? A hosted real-session endpoint carries a different exposure profile than a headless one, which this browser MCP session comparison works through. See Limit worth knowing.

Six items, one table row each. Run them against one platform and you test the category, not the brand.

Which Limits Should You Verify Before You Commit?

Six checks separate a platform you can adopt from one you will debug later. Run each against the same endpoint you tested above.

• Free-tier size. Count what works with no account, API key or signup.

• Paid tier. Name the features behind it, and whether the plan is monthly or annual.

• Payment rails. Cards, crypto, or both, and which chain if it is crypto.

• Billing unit. Per call or per period.

• Error shape. Call a paid tool without a licence and read the response.

• Operator scale. How many people build and run what is sold, and what that caps.

Registry preview status is an operational risk, not a footnote. Breaking changes and data resets are possible, so pin client config to a spec version rather than to whatever a listing showed last month. The versioning page is the reference for which revision your client speaks.

Nothing here claims an uptime percentage, an SLA, tenant isolation, audit logging, customer counts or testimonials. If a listing leans on those words without a link, discount it.

If you are weighing a hosted stack against a no-code builder, what developers actually deploy covers that trade-off.

isitagentready.com is an independent site, not owned by Moltline; the studio's own checks are the 21-check agent-readiness checker and the SKILL.md linter.

Which MCP Spec Version Does Your Client Implement?

Version drift is real, not theoretical. The MCP transports page for 2025-03-26 is labelled an older version and links forward to 2026-07-28. The registry documentation points at 2026-07-28 as well. A listing that never names a revision leaves you guessing which one your client actually speaks.

Find out. The initialize request your client sends carries a protocolVersion field. The result names the version the server speaks. Compare the two strings before assuming the server is broken.

Transports differ from each other too:

• stdio launches a subprocess and frames messages as newline-delimited text over standard streams.

• Streamable HTTP posts to a single endpoint, and authorization applies to HTTP-based transports.

A snippet copied from a stdio example is written for a subprocess, not a remote endpoint, so check which transport it assumes. For the shape of a working configuration, see the plugin setup walkthrough.

The practical rule: copy no configuration from any article, including this one, without checking it against the spec version your client reports in initialize. That single field explains most mismatches before you file a bug.

Test One Endpoint, Then Run the Checklist

Once your client reports a protocol version, the remaining work is a test. Do two things, in order:

1. Run the initialize call from the earlier test against one endpoint you are considering.

2. Run the five-item checklist against that same platform before you add a second anything.

The pair proves different things. The call reports what the service returns from a real request. The checklist reports whether pricing, free tier and error behaviour match what the page promised. Neither test replaces the other.

For reference, the tool list is at browse hosted servers, the open skills are at read the SKILL.md repo, and the 21-check readiness test is at run the agent-readiness checker. If you later compose several servers, routing MCP tool calls covers that step.

One endpoint, one checklist, five minutes, then decide.

Conclusion

The lesson is straightforward: trust the initialize response, not the marketplace listing. Four takeaways:

• Verify the protocol version your client reports before copying any configuration.

• Match the transport: a stdio snippet will fail in a remote HTTP client.

• Test one endpoint with initialize before adding a second anything.

• Run the five-item checklist for pricing, free tier, and error behaviour.

Your next step is to test one endpoint, then run the checklist against the same platform. Moltline Studio's catalog has 22 servers, 160 tools, 110 free forever, and 138 open SKILL.md on GitHub. The other 50 sit under a $19/month All-Access licence, paid in crypto only with USDC on Base and a live x402 challenge. Grades run A+ to B+, with the catalog at A (94/100). Start at browse hosted servers. One endpoint, one checklist, five minutes, then decide.