feat(mcp): generate an MCP server in TypeScript (--target mcp)

pnpm-workspace.yaml

@@ -1,6 +1,7 @@
 packages:
   - packages/*
   - src/bin/forge/ts/lib
+  - src/bin/forge/mcp/lib
 allowBuilds:
   esbuild: true
   workerd: true

rfcs/mcp.md

@@ -0,0 +1,204 @@
+# The MCP target
+
+`forge openapi.json --target mcp --out tools` generates the tools of an MCP server in TypeScript: one tool for each API method, with the code that turns a tool call into the method's HTTP request. This document says how forge maps the API onto MCP and what it generates. It describes the generator as it is.
+
+Forge generates the tools, not a server. Every server around them is different: how it authenticates, which tools, resources or prompts of its own it serves alongside, what it does with an API error. So the output is data a server lists and calls, plus the few functions every server needs to do that, typed for the official MCP TypeScript SDK, `@modelcontextprotocol/server`.
+
+```ts
+import { buildRequest, listTools, toolsByName, tools } from './tools/index.js';
+
+// tools/list
+const listing = listTools(tools);
+
+// tools/call
+const tool = toolsByName(tools).get(name);
+const request = buildRequest(tool, args, 'https://api.example.com');
+const response = await fetch(request.url, { ...request, headers: { ...request.headers, authorization } });
+```
+
+## Contents
+
+- [What it generates](#what-it-generates)
+- [Which methods get a tool](#which-methods-get-a-tool)
+- [Names](#names)
+- [Arguments](#arguments)
+- [The request](#the-request)
+- [Schemas](#schemas)
+- [Description and annotations](#description-and-annotations)
+- [Extensions](#extensions)
+- [Response schemas](#response-schemas)
+- [Output schemas](#output-schemas)
+- [The runtime](#the-runtime)
+- [What clients see](#what-clients-see)
+
+## What it generates
+
+```text
+tools/
+  index.ts          every tool, the API's base URL and version, and the runtime
+  README.md         the number of tools, and each method without one and why
+  _/tool.ts         the runtime: the Tool type, listing tools, building requests
+  <group>/tools.ts  the tools of a group's methods
+  <group>/schemas.ts  the schema of each model used in arguments, placed in its group
+  <group>/outputs.ts  the schema of each model used in responses
+```
+
+Each `tools.ts` exports `tools`, an array of the group's tools sorted by name, and `index.ts` concatenates them. A tool is plain data plus one function:
+
+```ts
+{
+  name: 'dns_records_create',
+  title: 'Create DNS Record',
+  description: '…',
+  inputSchema: { type: 'object', properties: { … }, required: [ … ], $defs: { … } },
+  annotations: { title, readOnlyHint, destructiveHint, idempotentHint, openWorldHint },
+  method: 'POST',
+  path: '/zones/{zone_id}/dns_records',
+  extensions: { 'x-api-token-group': [ … ] },
+  responses: { schemas: { '200': { … }, '4XX': { … } }, $defs: { … } },
+  outputSchema: { … , $defs: { … } },
+  request: (args) => ({ path: `/zones/${pathParam(args, 'zone_id')}/dns_records`, body: { value: args['body'], mediaType: 'application/json' } }),
+}
+```
+
+The runtime imports nothing at runtime, only the SDK's types, so the tools load anywhere: in the server, or in a separate process that only lists them and plans requests.
+
+## Which methods get a tool
+
+Every method forge reads gets a tool, except:
+
+| Method                                                                                                               | Why                                                                                                         |
+| -------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------- |
+| Deprecated (`deprecated: true`)                                                                                      | Left out silently.                                                                                          |
+| A required request body with no JSON media type (`multipart/form-data`, `application/octet-stream`, `text/plain`, …) | A tool's arguments are JSON. An optional non-JSON body is left off the tool, and the method still gets one. |
+| A tool name over 128 characters                                                                                      | MCP's limit.                                                                                                |
+| A tool name another method already has                                                                               | Names must be unique across the server; the first method keeps it.                                          |
+| Two arguments with the same name, even after [prefixing](#arguments)                                                 | The tool can't take both.                                                                                   |
+| A path parameter with no argument                                                                                    | The tool couldn't fill the path.                                                                            |
+
+Each of these except deprecation is listed in `README.md` with the method, path and reason, so a change to the spec that loses a tool shows in review. Operations forge itself doesn't read, such as those with `x-fern-ignore`, never reach the generator. Hidden methods (`x-forge-hidden`) get a tool.
+
+## Names
+
+A tool is named after its groups and its method, each in snake_case, joined by `_`:
+
+| Group              | Method   | Tool                    |
+| ------------------ | -------- | ----------------------- |
+| `dns.records`      | `create` | `dns_records_create`    |
+| `accounts.members` | `list`   | `accounts_members_list` |
+
+The name comes from `x-fern-sdk-group-name` and `x-fern-sdk-method-name`, as for every target, so it is stable while those are. Renaming a group renames its tools.
+
+## Arguments
+
+`inputSchema` is always an object. Its properties are:
+
+- **Path parameters**, named after the parameter in snake_case, always required.
+- **Query parameters**, likewise, required if the spec says so.
+- **Header parameters**, likewise. Read-only headers are left out.
+- **The JSON body**, as one argument named `body` that takes the body as the API does, with the body's description. If a parameter is already called `body`, it is `request_body`. It is required if the body is.
+
+A parameter whose snake_case name is empty takes its wire name. When two parameters in different places would share a name, the later one gets its place in front: `query_name`, `header_name`.
+
+## The request
+
+`request(args)` returns the request relative to the API's base URL:
+
+- `path`: the template with each parameter substituted. `pathParam` percent-encodes it and throws `InvalidArguments` if it is missing or empty.
+- `query`: each parameter's wire name, value and whether it is exploded.
+- `headers`: each header's wire name and value.
+- `body`: the value and its media type.
+
+`buildRequest(tool, args, baseUrl)` turns that into the HTTP request, as plain data: `{ method, url, headers, body? }`, with lower-case header names and the body as a string. It crosses RPC or `postMessage` as it is, and `fetch(request.url, request)` sends it. It checks the required arguments are present, then:
+
+- serializes query parameters in OpenAPI's default `form` style (`a=1&a=2` exploded, `a=1,2` not; objects as `k=v` pairs or `name=k,v`);
+- joins an array header with `,`;
+- leaves out every query parameter and header that is `undefined` or `null`;
+- sends `Accept: application/json`, and the body as JSON with its media type.
+
+It doesn't validate values against the schema. The API does that, and its error says more than ours would.
+
+## Schemas
+
+Schemas are JSON Schema 2020-12, built from forge's model of the spec rather than copied from the OpenAPI document, so every target agrees on what a type is.
+
+| Forge type                             | Schema                                                                                     |
+| -------------------------------------- | ------------------------------------------------------------------------------------------ |
+| unknown                                | `{}`                                                                                       |
+| null, boolean, string, integer, number | the `type`                                                                                 |
+| uuid, date-time, date                  | `string` with that `format`                                                                |
+| bytes                                  | `string` with `contentEncoding: base64`                                                    |
+| binary                                 | `string` with `format: binary`                                                             |
+| enum                                   | `enum` of the values, with `type` when they share one                                      |
+| array                                  | `items`, with `minItems`/`maxItems`                                                        |
+| object                                 | `properties`, `required`, and `additionalProperties` when the extra properties have a type |
+| exclusive union / union                | `oneOf` / `anyOf`                                                                          |
+| intersection                           | `allOf`                                                                                    |
+| model                                  | `{ "$ref": "#/$defs/<name>" }`                                                             |
+
+A nullable type in the spec is already a union with `null` in forge, so it becomes `anyOf` with `{ "type": "null" }`.
+
+Models are `$ref`s into the schema's own `$defs`, so recursive models terminate, and each tool's `$defs` holds only the models its schema reaches, so every tool's schema stands alone. A model's schema is built once, as a constant in its group's `schemas.ts` or `outputs.ts`, and every tool that uses it refers to the constant. The `$defs` key is the model's schema name with characters that a JSON Pointer or URI fragment would need escaped replaced by `_`, and a number added if two models would share one.
+
+Arguments and responses are built separately, because a model differs between them:
+
+|                                      | Arguments (`schemas.ts`)               | Responses (`outputs.ts`)                              |
+| ------------------------------------ | -------------------------------------- | ----------------------------------------------------- |
+| Read-only fields                     | left out: a client doesn't send them   | kept: the API returns them                            |
+| Descriptions, defaults, `deprecated` | kept, for the model choosing arguments | left out: these schemas check data, nobody reads them |
+
+## Description and annotations
+
+`title` is the method's summary. `description` is the method's description, or its summary, or `METHOD /path`, followed by:
+
+- `This method is alpha.` (or `beta`, `in preview`, `deprecated`) when `x-fern-availability` says so;
+- `Confirm with the user before calling this tool.` and the sentence from `x-forge-require-confirmation`, when the method has one.
+
+The annotations come from the HTTP method:
+
+|                                                | `readOnlyHint` | `destructiveHint` | `idempotentHint` |
+| ---------------------------------------------- | -------------- | ----------------- | ---------------- |
+| `GET`, `HEAD`, `OPTIONS`                       | true           | false             | true             |
+| `POST`, `PATCH`                                | false          | false             | false            |
+| `PUT`, `DELETE`                                | false          | true              | true             |
+| any method with `x-forge-require-confirmation` | false          | true              | as above         |
+
+`openWorldHint` is always true: every tool calls an API. The hints describe the method; they authorize nothing.
+
+## Extensions
+
+`extensions` holds the operation's `x-` extensions that forge doesn't read itself, as forge's `Method::extensions` returns them, such as `x-api-token-group`. They are for the server, for example to explain a refused call or to filter tools by permission, and are never listed to clients.
+
+## Response schemas
+
+`responses.schemas` has the schema of every response's JSON body, success or error, by status as the API declares it: a code (`200`), an uppercase range (`4XX`), or `default`. A response without a JSON body is `null`. `responses.$defs` holds the models they reach.
+
+They are for the server to check what the API returns against what the spec says: pick the status's schema, then its range, then `default`, and validate. A server can use the mismatches to find where the spec is wrong before it promises clients an output schema. They are never listed to clients.
+
+## Output schemas
+
+`outputSchema` is the JSON body schema of the method's successful (`2XX`) responses: the one schema when they agree, or `anyOf` the distinct ones, with `$defs` holding only the models those reach. A method with a successful response that has no JSON body gets none, since some calls would have no structured result.
+
+It is often not an object: a `$ref`, an `allOf`, an array or a scalar. MCP `2026-07-28` allows that; `2025` clients expect an object.
+
+`listTools` leaves output schemas out unless called with `{ outputSchema: true }`. A server that wants them lists them for `2026-07-28` requests and returns the parsed body as `structuredContent` itself. Until the spec's response schemas match what the API returns, an output schema can make a strict client reject a correct result, so serving them is the server's choice.
+
+## The runtime
+
+`_/tool.ts` is copied into every output. It is small enough to read:
+
+- `listTools(tools, { outputSchema })` returns the tools as `tools/list` sends them, without `request`, `method`, `path`, `extensions` and `responses`, and without `outputSchema` unless asked. The listing is built once per array of tools, so a server can answer `tools/list` with one handler rather than registering thousands of tools on every request.
+- `toolsByName(tools)` indexes them for `tools/call`, once per array.
+- `buildRequest(tool, args, baseUrl)` is described in [The request](#the-request). It throws `InvalidArguments` for arguments the tool can't be called with, which a server returns as a tool error.
+
+Everything else is the server's: sending the request with its credential, turning the response into a tool result, and anything it changes about a tool, such as making a parameter optional and filling it from the session. It can change a tool by mapping over `tools` before listing them, since a tool is plain data and its `request` reads only the arguments.
+
+## What clients see
+
+| Field                                                        | `tools/list` | Server only |
+| ------------------------------------------------------------ | ------------ | ----------- |
+| `name`, `title`, `description`, `inputSchema`, `annotations` | ✓            |             |
+| `outputSchema`                                               | when asked   |             |
+| `method`, `path`, `request`                                  |              | ✓           |
+| `extensions`                                                 |              | ✓           |
+| `responses`                                                  |              | ✓           |

src/bin/forge/main.rs

@@ -2,6 +2,7 @@
 //!
 //! ```text
 //! forge openapi.json --target ts --out sdk
+//! forge openapi.json --target mcp --out tools
 //! forge openapi.json --target wasm://plugin.wasm --out sdk
 //! ```
 
@@ -13,6 +14,7 @@ use std::process::ExitCode;
 use clap::Parser;
 use forge::{Err, Forge};
 
+mod mcp;
 mod ts;
 
 /// Generates an SDK from an OpenAPI document.
@@ -21,8 +23,9 @@ mod ts;
 struct Args {
     /// The OpenAPI JSON document, or - to read it from standard input.
     spec: PathBuf,
-    /// The language of the SDK: `ts` for TypeScript, or `wasm://PATH` for
-    /// the plugin compiled to a WebAssembly component at PATH.
+    /// The language of the SDK: `ts` for TypeScript, `mcp` for the tools of
+    /// an MCP server in TypeScript, or `wasm://PATH` for the plugin compiled
+    /// to a WebAssembly component at PATH.
     #[arg(long, value_parser = Target::parse)]
     target: Target,
     /// The directory to write the SDK to.
@@ -35,18 +38,22 @@ struct Args {
 enum Target {
     /// The TypeScript generator.
     Ts,
+    /// The MCP generator, which generates the tools of an MCP server in TypeScript.
+    Mcp,
     /// A plugin compiled to a WebAssembly component, at the path.
     #[cfg(feature = "wasm")]
     Wasm(PathBuf),
 }
 
 impl Target {
     fn parse(s: &str) -> Result<Target, String> {
-        if s == "ts" {
-            return Ok(Target::Ts);
+        match s {
+            "ts" => return Ok(Target::Ts),
+            "mcp" => return Ok(Target::Mcp),
+            _ => {}
         }
         let Some(path) = s.strip_prefix("wasm://") else {
-            return Err("possible values: ts, wasm://PATH".to_owned());
+            return Err("possible values: ts, mcp, wasm://PATH".to_owned());
         };
         if path.is_empty() {
             return Err("wasm:// needs the path of a plugin, as in wasm://plugin.wasm".to_owned());
@@ -75,6 +82,10 @@ fn run(args: Args) -> Result<(), Err> {
             let forge = read(&args.spec)?;
             forge::write(&args.out, forge::run(&forge, &ts::Generator)?)
         }
+        Target::Mcp => {
+            let forge = read(&args.spec)?;
+            forge::write(&args.out, forge::run(&forge, &mcp::Generator)?)
+        }
         #[cfg(feature = "wasm")]
         Target::Wasm(path) => {
             // Compiling the plugin and reading the document are both slow,