diff --git a/CHANGELOG.md b/CHANGELOG.md
index cbc0b623..325edfe9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1185,6 +1185,12 @@ npm release are grouped under the in-development version that introduced them.
   variable. See ADR 0005 Decision 1's addendum for why this was deferred and what implementing
   it would take; relaxing the guard later is non-breaking.
 
+- **The bundle-size gate's externals now match what the build actually emits.**
+  ([#709](https://github.com/rejifald/StitchAPI/issues/709)) `check:size` externalised `node:*`, but
+  `tsup` strips the prefix — `lib/` emits bare `from"fs"` — so the pattern matched nothing, and
+  measuring a builtin-using entry failed outright with `Could not resolve "fs"`. No measured or
+  advertised number moves; the fix is repo tooling only.
+
 ## [1.0.0-rc.7] — 2026-08-01
 
 ### Added
diff --git a/packages/core/scripts/bundle-size.mjs b/packages/core/scripts/bundle-size.mjs
index 3ae98034..3eaea4f6 100644
--- a/packages/core/scripts/bundle-size.mjs
+++ b/packages/core/scripts/bundle-size.mjs
@@ -335,7 +335,23 @@ function measure(code) {
         format: 'esm',
         treeShaking: true,
         platform: 'neutral',
-        external: ['node:*'], // zero deps; the root entry is browser-safe
+        // Node builtins, in BOTH spellings — the package has zero deps, so builtins are
+        // the only thing that is ever external and the root entry stays browser-safe.
+        //
+        // The bare forms are not belt-and-braces: they are the ones that actually match.
+        // The source writes the prefix (`src/registry.ts` imports `node:fs`/`node:path`/
+        // `node:url`) but tsup STRIPS it, so `lib/` emits `from"fs"`, `require("path")`.
+        // Across the shipped files the only surviving `node:` string is inside
+        // `process?.getBuiltinModule?.("node:fs")` — a runtime lookup no bundler resolves.
+        // `node:*` alone therefore matched NOTHING here: measuring a builtin-using entry
+        // (`stitchapi/registry`) failed outright with `Could not resolve "fs"`, and a bare
+        // `fs` left non-external can be shadowed by a stray `node_modules/fs` and silently
+        // inlined into the measurement. The prefixed forms are kept so this keeps working
+        // if the build is ever changed to preserve `node:` (the better fix for shadowing).
+        //
+        // A builtin the list misses fails loudly rather than measuring something wrong —
+        // extend it when the artifacts start reaching for a new one.
+        external: ['node:*', 'fs', 'fs/promises', 'path', 'url', 'http'],
         write: false,
         logLevel: 'silent',
     });