diff --git a/packages/codemode/README.md b/packages/codemode/README.md
index 83e2408bd64..4c2074c0966 100644
--- a/packages/codemode/README.md
+++ b/packages/codemode/README.md
@@ -114,8 +114,9 @@ that returns a Promise or an Effect, so a denied call rejects its promise like a
 Extension.make({ name: "web", globals: { fetch: Extension.async((url: string) => globalThis.fetch(url)) } })
 ```
 
-A global may return a value, a Promise, or an Effect. An Effect's requirements join the runtime's environment the same
-way a tool's do, so the host can provide per-execution services such as the calling session:
+A global may return a value, a Promise, or an Effect. A synchronous global's Effect runs to completion before the call
+returns, so the host can offer blocking calls such as `fs.readFileSync`. An Effect's requirements join the runtime's
+environment the same way a tool's do, so the host can provide per-execution services such as the calling session:
 
 ```ts
 const web = Extension.make({
diff --git a/packages/codemode/src/extension.ts b/packages/codemode/src/extension.ts
index e7a6103e7a2..ea969601146 100644
--- a/packages/codemode/src/extension.ts
+++ b/packages/codemode/src/extension.ts
@@ -8,7 +8,9 @@ import type { Effect } from "effect"
  * function inside a result is callable the same way. Extension calls are not tool calls. A global may return a value,
  * a Promise, or an Effect, and an Effect's requirements join the runtime's environment.
  *
- * A global is synchronous unless it is an `async` function or is marked with `async`. The `extension.before` hook
+ * A global is synchronous unless it is an `async` function or is marked with `async`. A synchronous global that
+ * returns an Effect blocks the program until the Effect completes and returns its value, as `fs.readFileSync` does;
+ * an asynchronous one hands the program a promise. The `extension.before` hook
  * runs before the call, so a denied call never reaches the host; it fails the way the global would, by rejecting the
  * promise of an asynchronous global and by throwing from a synchronous one. Functions inside a global object are
  * called the same way, named by their path (`fs.readFile`).
@@ -20,13 +22,13 @@ export type Extension<Globals extends Readonly<Record<string, unknown>> = Readon
   readonly globals: Globals
 }
 
-/** The Effect environment required by an extension's Effect-returning functions, at the top level or one level in. */
+/** The Effect environment required by an extension's Effect-returning functions, at any depth (`fs.promises.readFile`). */
 export type Services<E> = E extends { readonly globals: infer Globals } ? GlobalServices<Globals[keyof Globals]> : never
 
 type GlobalServices<Global> = Global extends (...args: never) => unknown
   ? Requirements<Global>
   : Global extends object
-    ? Requirements<Global[keyof Global]>
+    ? GlobalServices<Global[keyof Global]>
     : never
 
 type Requirements<F> = F extends (...args: never) => infer Result ? Effect.Services<Result> : never
diff --git a/packages/codemode/src/interpreter/extensions.ts b/packages/codemode/src/interpreter/extensions.ts
index 87a03d07bec..9274d8c68fd 100644
--- a/packages/codemode/src/interpreter/extensions.ts
+++ b/packages/codemode/src/interpreter/extensions.ts
@@ -185,7 +185,8 @@ export const extensionGlobals = <R>(
           )
         if (before) yield ctx.runner.blocking(before)
         const host = callHost<R>(value, args)
-        if (host.async) return pending(settle(host.effect), site)
+        // A synchronous global's Effect runs to completion before the call returns, like `readFileSync`.
+        if (host.result === "promise") return pending(settle(host.effect), site)
         return (yield ctx.runner.blocking(settle(host.effect))) as Value
       },
     )
@@ -216,20 +217,17 @@ export const extensionGlobals = <R>(
   )
 }
 
-/**
- * Calls a host function and describes what the program waits on. A returned Effect or Promise settles later; a
- * returned value or a throw settles now.
- */
+/** Calls a host function and describes what it returned: an Effect, a Promise, or a value (or a throw) as an Effect. */
 const callHost = <R>(value: Function, args: ReadonlyArray<unknown>) => {
   try {
     const result: unknown = value.apply(undefined, args)
     // Extension.Services adds an Effect global's requirements to R.
-    if (Effect.isEffect(result)) return { async: true, effect: result as Effect.Effect<unknown, unknown, R> }
+    if (Effect.isEffect(result)) return { result: "effect", effect: result as Effect.Effect<unknown, unknown, R> }
     if (result instanceof Promise)
-      return { async: true, effect: Effect.tryPromise({ try: () => result, catch: (reason) => reason }) }
-    return { async: false, effect: Effect.succeed(result) }
+      return { result: "promise", effect: Effect.tryPromise({ try: () => result, catch: (reason) => reason }) }
+    return { result: "value", effect: Effect.succeed(result) }
   } catch (reason) {
-    return { async: false, effect: Effect.fail(reason) }
+    return { result: "value", effect: Effect.fail(reason) }
   }
 }
 
diff --git a/packages/codemode/test/extensions.test.ts b/packages/codemode/test/extensions.test.ts
index 5e66727559e..236a149fcac 100644
--- a/packages/codemode/test/extensions.test.ts
+++ b/packages/codemode/test/extensions.test.ts
@@ -417,13 +417,18 @@ describe("Effect globals", () => {
   const web = Extension.make({
     name: "web",
     globals: {
-      fetch: (url: string) =>
+      fetch: Extension.async((url: string) =>
         Effect.gen(function* () {
           const session = yield* Session
           sent.push(url)
           return { url, session: session.id }
         }),
-      fail: () => Effect.fail(new RangeError("denied")),
+      ),
+      fail: Extension.async(() => Effect.fail(new RangeError("denied"))),
+      fs: {
+        readSync: (path: string) => Session.use((session) => Effect.succeed(`${session.id}:${path}`)),
+        failSync: () => Effect.fail(new RangeError("missing")),
+      },
     },
   })
 
@@ -458,6 +463,18 @@ describe("Effect globals", () => {
     expect(result).toMatchObject({ ok: true, value: [true, "denied"] })
   })
 
+  test("a synchronous Effect global returns its value, or throws, before the next statement", async () => {
+    const result = await Effect.runPromise(
+      CodeMode.make({ extensions: [web] })
+        .execute(
+          `const text = fs.readSync("a")
+           try { fs.failSync() } catch (e) { return [text, e instanceof RangeError, e.message] }`,
+        )
+        .pipe(Effect.provideService(Session, { id: "ses_1" })),
+    )
+    expect(result).toMatchObject({ ok: true, value: ["ses_1:a", true, "missing"] })
+  })
+
   test("a denying before hook stops the Effect before it runs", async () => {
     sent.length = 0
     const runtime = CodeMode.make({