diff --git a/.github/workflows/hugo.yml b/.github/workflows/hugo.yml
index 8cf4b7f..195cd15 100644
--- a/.github/workflows/hugo.yml
+++ b/.github/workflows/hugo.yml
@@ -85,7 +85,7 @@ jobs:
           GOWORK: "off"
           GOTOOLCHAIN: local
         run: |
-          GOOS=js GOARCH=wasm go build -trimpath -ldflags "-s -w" \
+          GOOS=js GOARCH=wasm go build -tags rbgo_core -trimpath -ldflags "-s -w" \
             -o ../../static/playground/rbgo.wasm github.com/go-embedded-ruby/ruby/cmd/wasm
           raw=$(stat -c %s ../../static/playground/rbgo.wasm); gz=$(gzip -9 -c ../../static/playground/rbgo.wasm | wc -c)
           echo "rbgo.wasm ($(go list -m -f '{{.Version}}' github.com/go-embedded-ruby/ruby)): $raw bytes, $gz gzipped" | tee -a "$GITHUB_STEP_SUMMARY"
diff --git a/.github/workflows/playground.yml b/.github/workflows/playground.yml
index 47171b1..f2c3bf9 100644
--- a/.github/workflows/playground.yml
+++ b/.github/workflows/playground.yml
@@ -127,7 +127,7 @@ jobs:
       - name: build go-embedded-ruby (js/wasm)
         working-directory: playground/rubywasm
         run: |
-          GOOS=js GOARCH=wasm go build -trimpath -ldflags "-s -w" -o "$RUNNER_TEMP/rbgo.wasm" github.com/go-embedded-ruby/ruby/cmd/wasm
+          GOOS=js GOARCH=wasm go build -tags rbgo_core -trimpath -ldflags "-s -w" -o "$RUNNER_TEMP/rbgo.wasm" github.com/go-embedded-ruby/ruby/cmd/wasm
           raw=$(stat -c %s "$RUNNER_TEMP/rbgo.wasm"); gz=$(gzip -9 -c "$RUNNER_TEMP/rbgo.wasm" | wc -c)
           echo "rbgo.wasm ($(go list -m -f '{{.Version}}' github.com/go-embedded-ruby/ruby)): $raw bytes, $gz gzipped" | tee -a "$GITHUB_STEP_SUMMARY"
       - name: run the proof
@@ -137,7 +137,7 @@ jobs:
           PLAYGROUND_CAPTURE_DIR: ${{ runner.temp }}/captures
           PLAYGROUND_RUBY_WASM: ${{ runner.temp }}/rbgo.wasm
         # TestRubyAgreesWithGo runs every operation's Ruby program on the
-        # interpreter. The proof downloads its ~100 MB twice in Chrome: give the
+        # interpreter. The proof downloads its ~23 MB twice in Chrome: give the
         # run more than go test's 10 minutes.
         run: go test -count=1 -timeout 30m -v -run 'TestBrowserProof|TestRubyAgreesWithGo' . ./engine
       - name: keep the screenshots
diff --git a/playground/README.md b/playground/README.md
index 60c534e..ccfb361 100644
--- a/playground/README.md
+++ b/playground/README.md
@@ -87,15 +87,18 @@ calls only the methods of that API.
 
 The interpreter is a second Go program compiled to WebAssembly,
 `github.com/go-embedded-ruby/ruby/cmd/wasm`, at the version `rubywasm/go.mod`
-pins:
+pins, built with the `rbgo_core` tag: the Ruby language and its numeric stack
+(NDArray, FFT, Complex, Rational, Set, BigDecimal, Matrix, …) and none of the
+gem bindings, which the playground never calls:
 
 ```sh
 cd rubywasm
-GOOS=js GOARCH=wasm go build -trimpath -ldflags="-s -w" \
+GOOS=js GOARCH=wasm go build -tags rbgo_core -trimpath -ldflags="-s -w" \
   -o ../../static/playground/rbgo.wasm github.com/go-embedded-ruby/ruby/cmd/wasm
 ```
 
-It is far larger than the playground (about 100 MB, 21 MB gzipped), so the page
+The full build was about 100 MB (22 MB gzipped); the core one is about 23 MB
+(5.7 MB gzipped). It is still larger than the playground, so the page
 never loads it up front: the first time the visitor opens the Ruby tab, the
 ViewModel asks the host, the page's `ndarrayRubyLoad` fetches it with a
 progress line and bar counting the bytes, and starts it in its own Go runtime
diff --git a/playground/browser_test.go b/playground/browser_test.go
index 31f5175..5eba224 100644
--- a/playground/browser_test.go
+++ b/playground/browser_test.go
@@ -94,7 +94,7 @@ func TestBrowserProof(t *testing.T) {
 			t.Fatal(err)
 		}
 	} else {
-		run("rubywasm", []string{"GOOS=js", "GOARCH=wasm"}, "go", "build", "-trimpath", "-ldflags=-s -w", "-o",
+		run("rubywasm", []string{"GOOS=js", "GOARCH=wasm"}, "go", "build", "-tags", "rbgo_core", "-trimpath", "-ldflags=-s -w", "-o",
 			rbgo, "github.com/go-embedded-ruby/ruby/cmd/wasm")
 	}
 	execJS, err := os.ReadFile(filepath.Join(runtime.GOROOT(), "lib", "wasm", "wasm_exec.js"))
diff --git a/playground/rubywasm/go.mod b/playground/rubywasm/go.mod
index 6cd7daf..9fc7afd 100644
--- a/playground/rubywasm/go.mod
+++ b/playground/rubywasm/go.mod
@@ -80,7 +80,7 @@ require (
 	github.com/go-crdt/collab v0.74.0 // indirect
 	github.com/go-crdt/crdt v0.55.0 // indirect
 	github.com/go-datetime/dates v0.1.0 // indirect
-	github.com/go-embedded-ruby/ruby v0.5.0 // indirect
+	github.com/go-embedded-ruby/ruby v0.6.0 // indirect
 	github.com/go-encryptions/unixcrypt v0.1.0 // indirect
 	github.com/go-facter/facter v0.0.0-20260830120958-454b72e642ab // indirect
 	github.com/go-fft/fft v0.23.0 // indirect
diff --git a/playground/rubywasm/go.sum b/playground/rubywasm/go.sum
index 9aa3311..c0a776c 100644
--- a/playground/rubywasm/go.sum
+++ b/playground/rubywasm/go.sum
@@ -202,8 +202,8 @@ github.com/go-crdt/crdt v0.55.0 h1:D/LHQ4OgSJRomMvEeYzRwwnP0hnXGngWevXglmpXIxI=
 github.com/go-crdt/crdt v0.55.0/go.mod h1:vbfvns9B9FUaMi2sPKzMYttKmbxfgO7EK3Yu82SrlYw=
 github.com/go-datetime/dates v0.1.0 h1:Uj8TaTZ09DXdq75EGQ/teFdrXpfVAC3ZDHAPQvPTnaA=
 github.com/go-datetime/dates v0.1.0/go.mod h1:KeZ1wUeMScJD6HOvj3hINwbsB+GCSFjJLWcJhrBlwIo=
-github.com/go-embedded-ruby/ruby v0.5.0 h1:DcJxu4H6CJC9qr6utIa399Q41eq36Ti/zbqnR8fRaYA=
-github.com/go-embedded-ruby/ruby v0.5.0/go.mod h1:QS5klA3N6Wg6nWe90pesNQvRQPw9AXMxBNOi9I6ukCs=
+github.com/go-embedded-ruby/ruby v0.6.0 h1:uB+qHsTf+75YtXj2u76IHiNI/2/YYlZ8Gj/caM3ZEZg=
+github.com/go-embedded-ruby/ruby v0.6.0/go.mod h1:QS5klA3N6Wg6nWe90pesNQvRQPw9AXMxBNOi9I6ukCs=
 github.com/go-encryptions/unixcrypt v0.1.0 h1:XWk4h9uCDd+T2hovOzgO0B2vOdJvAwi9nbOBVb3o62c=
 github.com/go-encryptions/unixcrypt v0.1.0/go.mod h1:B1dJSjz3kdxvP7cpi5xSzyxezgBijmtNCUeCKqqDd3o=
 github.com/go-facter/facter v0.0.0-20260830120958-454b72e642ab h1:bXPxQDhADpiF/S/8jlRFhVisFPRaxPtPgjibLnOzvsE=