MCP Inspector vs Postman in 2026: which one I actually use

I use two of the three: MCP Inspector for live calls, and a small client-side validator for checking definitions before I ever start a server. Postman's MCP support works, but it was too much setup for the quick checks I do most. Same broken tool, run through all three, below.

Full disclosure: the MCP Tool Tester I link to below is one I built. I'd tried four other validators and every one either needed a running server, an npm install, or an account before it'd tell me my inputSchema had a typo. Mine doesn't. It's free, runs entirely in your browser, no signup, nothing uploaded. Paste it in, get an answer. If you've got a better one, tell me.

The task: a broken currency tool

Three weeks ago I was wiring up an MCP server for a currency tool, and the agent kept refusing to call it. No error message. It just ignored the tool. After 47 minutes of squinting I found it: my handler read a field called from_currency, but the schema I advertised defined currency_from. The model saw a contract it couldn't satisfy and quietly walked away.

Here's the thing about MCP tool definitions: the schema you advertise and the handler you write live in two different places, and nothing forces them to agree. JSON Schema will happily describe a field your code never reads. Most agents won't tell you why they skipped a tool, they just skip it. The expected behavior here was simple: send 100 USD with EUR as the target, get a converted amount back. What I actually got was nothing, no call attempted, which is the worst kind of bug because there's no stack trace to follow.

So I rebuilt that broken tool on purpose and ran it through three things people reach for when testing MCP: the official Inspector, Postman, and the validator I made. Here's the server, mismatch and all.

// server.js  (run: npm i @modelcontextprotocol/sdk zod && node server.js)

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";

import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

import { z } from "zod";



const server = new McpServer({ name: "fx", version: "1.0.0" });



server.registerTool(

  "convert_currency",

  {

    description: "Convert an amount between two ISO 4217 currency codes",

    inputSchema: {

      amount: z.number().describe("amount to convert"),

      currency_from: z.string().length(3),

      currency_to: z.string().length(3),

    },

  },

  // Bug: the schema defines currency_from, the handler reads from_currency.

  // The names never line up, so the agent sees a field it can't supply.

  async ({ amount, from_currency, currency_to }) => ({

    content: [{ type: "text", text: `${amount} ${from_currency} -> ${currency_to}` }],

  })

);



await server.connect(new StdioServerTransport());

MCP Inspector: what happened

MCP Inspector is the official debugger. You run npx @modelcontextprotocol/inspector node server.js and it opens a local UI where you can list a server's tools and call them by hand. I ran it last Tuesday against the broken server above.

It connected in about two seconds and the tool showed up. Inspector reads your inputSchema and renders a form, so the field labels it expected (including currency_from) were right there on screen. That's where the bug got visible to me, because I knew my handler wanted from_currency. When I filled the form and hit call, my own handler threw on an undefined field. Honest, but late: I had to boot a full server to learn something a static check could have told me in seconds.

One more thing worth flagging. Inspector caches the tool list per session, so when I edited the schema and restarted the server, I had to reconnect to see the change. Minor, but I lost a couple of minutes the first time wondering why my fix wasn't showing. Once you learn to reconnect after every restart, it's fine. The history panel is also handy for replaying a call you already got working.

Inspector's strength is that it talks to a real, running server over the actual transport. Its limit is the same thing. It can't say a word about a definition until there's a live process to connect to.

Postman: what happened

Postman shipped MCP support in 2025, and for HTTP-based servers it's solid. I pointed it at the same tool after switching the transport to streamable HTTP, because Postman won't drive a stdio process. It discovered the tool, showed the schema, and let me send a call.

The request builder is nicer than Inspector's plain form, I'll give it that. Two things bugged me, though. I had to change my transport just to test, so I was poking at a slightly different server than the one I ship. And the validation is shallow: it happily sent garbage and reported the failure as a generic error response instead of pointing at the field that was wrong. Setup ate about 15 minutes. If you already live in Postman, that cost is mostly paid. For a fast definition check, it's heavy.

To be fair to Postman, the collection sharing is real value if you're on a team. I could save the MCP connection and hand it to a coworker, and they'd get the same setup without me writing a README. That's something neither Inspector nor my validator does. It just doesn't help the specific thing I was testing, which was whether a definition is correct before anyone runs it.

MCP Tool Tester: what happened

This one I built, so weigh that accordingly. The workflow is intentionally dumb: paste the tool definition (the JSON a server advertises, or a WebMCP tools array) and it checks the shape against the MCP schema plus a handful of lint rules I kept tripping over in real projects.

On the broken currency tool it flagged the mismatch in under a second: a required name with no matching property. It also caught two issues the other two never looked at, a description longer than the cutoff where some clients truncate (I still don't know the exact limit for every client, but I've watched it break around 1,024 characters) and an enum with a duplicated value. No server to boot, no install. It runs in the browser, so the definition never leaves your machine, which I care about because my tool descriptions leak internal endpoint names.

The lint rules came straight from bugs that cost me time. A required field with no property to back it. A description left empty, which makes some clients drop the tool entirely. I keep adding rules as I get burned, so the list grows in an embarrassingly autobiographical way. A few days ago a teammate's PR defined a tool that required user_id but only declared userId. Same family of bug as mine. The validator flagged it before review started, which saved a confused back-and-forth in comments.

What it won't do is execute your tool. It checks definitions, not behavior. Think of it as the step you run before Inspector.

The scorecard, and which one I reach for

Here's the same task scored across the things I actually care about:

The pattern is clear enough: Postman never wins a row outright for my use case. It's competent everywhere and best nowhere, which is a perfectly respectable place to be for a general tool that happens to speak MCP.

So which do I actually use? Two of them, at different moments. While I'm authoring a definition or reviewing a teammate's pull request, I paste it into the validator first, because catching a name mismatch in three seconds beats catching it after a 90-second server boot and a confused agent. Once the definition is clean, I bring up Inspector and call the thing for real over the transport I'll ship. Postman stays in the box unless a project already runs on it.

If I had to give up two of the three and keep one, I'd actually struggle, because they solve different halves of the problem. Definition correctness and runtime behavior aren't the same question. The validator answers the first in seconds; Inspector answers the second properly. That split is why I run both rather than picking a single winner.

If you want to throw your own definitions at the validator, it's here: .