diff --git a/ts/project.test.ts b/ts/project.test.ts
index d9cd67d9..48712a65 100644
--- a/ts/project.test.ts
+++ b/ts/project.test.ts
@@ -1,13 +1,15 @@
 import { Check } from "./check.ts";
 import { Executable } from "./executable.ts";
-import { Project } from "./project.ts";
+import { Plugin, Project } from "./project.ts";
 import { describe, it } from "https://deno.land/std@0.206.0/testing/bdd.ts";
 import * as garn from "./mod.ts";
 import * as nix from "./nix.ts";
 import { assertStdout, assertSuccess, runExecutable } from "./testUtils.ts";
+import { Package } from "./mod.ts";
 
 const assertTypeIsCheck = (_c: Check) => {};
 const assertTypeIsExecutable = (_e: Executable) => {};
+const assertTypeIsPackage = (_p: Package) => {};
 
 const _testTypeCheckingOfAddCheck = (project: Project) => {
   const p = project
@@ -133,4 +135,78 @@ describe("Project.add", () => {
     assertSuccess(output);
     assertStdout(output, "bar\n");
   });
+
+  it("provides a nice type synonym for plugins that add a field", () => {
+    const plugin: Plugin<object, { addedField: garn.Package }> = (p) => ({
+      ...p,
+      addedField: garn.build``,
+    });
+    const project = garn
+      .mkProject(
+        { description: "", defaultEnvironment: garn.emptyEnvironment },
+        {},
+      )
+      .addExecutable("foo", "")
+      .add(plugin);
+    assertTypeIsExecutable(project.foo);
+    assertTypeIsPackage(project.addedField);
+  });
+
+  it("provides a nice type synonym for plugins that add multiple fields", () => {
+    const plugin: Plugin<object, { one: garn.Package; two: garn.Check }> = (
+      p,
+    ) => ({
+      ...p,
+      one: garn.build``,
+      two: garn.check(""),
+    });
+    const project = garn
+      .mkProject(
+        { description: "", defaultEnvironment: garn.emptyEnvironment },
+        {},
+      )
+      .addExecutable("foo", "")
+      .add(plugin);
+    assertTypeIsExecutable(project.foo);
+    assertTypeIsPackage(project.one);
+    assertTypeIsCheck(project.two);
+  });
+
+  it("provides a nice interface for plugins that depend on a non-standard field", () => {
+    const plugin: Plugin<{ dep: Package }, { addedField: Executable }> = (
+      p,
+    ) => ({
+      ...p,
+      addedField: garn.shell`${p.dep}/bin/whatever`,
+    });
+    const project = garn
+      .mkProject(
+        { description: "", defaultEnvironment: garn.emptyEnvironment },
+        { dep: garn.build`` },
+      )
+      .addExecutable("foo", "")
+      .add(plugin);
+    assertTypeIsExecutable(project.foo);
+    assertTypeIsExecutable(project.addedField);
+    // @ts-expect-error - `dep` is missing
+    () => garn.mkProject({ description: "" }, {}).add(plugin);
+  });
+
+  it("allows to overwrite fields", () => {
+    const plugin: Plugin<object, { field: Package }> = (p) => ({
+      ...p,
+      field: garn.build``,
+    });
+    const project = garn
+      .mkProject(
+        { description: "", defaultEnvironment: garn.emptyEnvironment },
+        { field: garn.shell("") },
+      )
+      .addExecutable("foo", "")
+      .add(plugin);
+    assertTypeIsExecutable(project.foo);
+    // @ts-expect-error - should not be an `Executable` anymore
+    assertTypeIsExecutable(project.field);
+    assertTypeIsPackage(project.field);
+  });
 });
diff --git a/ts/project.ts b/ts/project.ts
index d24b20ca..301053d0 100644
--- a/ts/project.ts
+++ b/ts/project.ts
@@ -1,11 +1,11 @@
 import "./internal/registerInternalLib.ts";
 
 import { Check, mkCheck } from "./check.ts";
-import { Environment, emptyEnvironment } from "./environment.ts";
+import { Environment } from "./environment.ts";
 import { Executable, mkShellExecutable } from "./executable.ts";
 import { hasTag } from "./internal/utils.ts";
 import { NixStrLitInterpolatable } from "./nix.ts";
-import { Package, mkShellPackage } from "./package.ts";
+import { mkShellPackage, Package } from "./package.ts";
 import { markAsMayNotExport } from "./internal/may_not_export.ts";
 
 /**
@@ -15,13 +15,15 @@ import { markAsMayNotExport } from "./internal/may_not_export.ts";
  */
 export type Project = ProjectHelpers & ProjectData;
 
-type ProjectData = {
+export type ProjectData = {
   tag: "project";
   description: string;
   defaultEnvironment?: Environment;
   defaultExecutable?: Executable;
 };
 
+export type Plugin<Input, Output> = (project: Input) => Output;
+
 type ProjectHelpers = {
   /**
    * Returns a new Project with the provided devtools added to the default
@@ -78,10 +80,10 @@ type ProjectHelpers = {
    *   .add(self => self.addExecutable("codegen")`${self.mainPackage}/bin/codegen`)
    * ```
    */
-  add<T extends ProjectData, U extends ProjectData>(
-    this: T,
-    fn: (p: T) => U,
-  ): U;
+  add<Input extends ProjectData, Output>(
+    this: Input,
+    fn: Plugin<Input, Output>,
+  ): Omit<Input, keyof Output> & Output;
 
   /**
    * Adds an `Executable` with the given name to the Project
@@ -215,11 +217,11 @@ const proxyEnvironmentHelpers = (): ProjectHelpers => ({
     return mkShellPackage(defaultEnvironment, s, ...args);
   },
 
-  add<T extends ProjectData, U extends ProjectData>(
-    this: T,
-    fn: (p: T) => U,
-  ): U {
-    return fn(this);
+  add<Input extends ProjectData, Output>(
+    this: Input,
+    fn: Plugin<Input, Output>,
+  ): Omit<Input, keyof Output> & Output {
+    return { ...this, ...fn(this) };
   },
 
   addExecutable<T extends ProjectData, Name extends string>(