diff --git a/.github/workflows/lgb-diff.yml b/.github/workflows/lgb-diff.yml
new file mode 100644
index 000000000..f126cf3e7
--- /dev/null
+++ b/.github/workflows/lgb-diff.yml
@@ -0,0 +1,95 @@
+name: lgb-diff
+
+# Opt-in bundle diff: label a PR `lgb-diff` and this posts (and keeps updated)
+# a sticky comment with the semantic bytecode diff of pkg/rt/core_compiled.lgb
+# against the merge-base, rendered by scripts/lgbdump.lg. Same-repo PRs only:
+# fork PRs get a read-only token, so the comment step would fail there.
+
+on:
+  pull_request:
+    types: [labeled, synchronize]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+concurrency:
+  group: lgb-diff-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  diff:
+    if: contains(github.event.pull_request.labels.*.name, 'lgb-diff')
+    runs-on: ubuntu-latest
+    steps:
+      # Head SHA, not the merge ref: the lg built here must share an opcode
+      # enum with the bundle it dumps, and the head tree is that producer.
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+
+      - name: Locate bundle change
+        id: bundle
+        run: |
+          MB=$(git merge-base HEAD "origin/${{ github.event.pull_request.base.ref }}")
+          echo "mb=$MB" >> "$GITHUB_OUTPUT"
+          if git diff --quiet "$MB" HEAD -- pkg/rt/core_compiled.lgb; then
+            echo "changed=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "changed=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      - uses: actions/setup-go@v5
+        if: steps.bundle.outputs.changed == 'true'
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Build lg, dump both bundles, diff
+        if: steps.bundle.outputs.changed == 'true'
+        run: |
+          make lg
+          MB=${{ steps.bundle.outputs.mb }}
+          git show "$MB:pkg/rt/core_compiled.lgb" > /tmp/base.lgb
+          ./lg scripts/lgbdump.lg /tmp/base.lgb > /tmp/base.txt
+          ./lg scripts/lgbdump.lg pkg/rt/core_compiled.lgb > /tmp/head.txt
+          # diff(1) semantics: exit 1 just means the bundles differ.
+          diff -u --label "base ($(git rev-parse --short "$MB"))" --label head \
+            /tmp/base.txt /tmp/head.txt > /tmp/lgb.diff || true
+
+      - name: Post sticky comment
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          PR: ${{ github.event.pull_request.number }}
+        run: |
+          MARKER='<!-- lgb-diff -->'
+          MB_SHORT=$(git rev-parse --short "${{ steps.bundle.outputs.mb }}")
+          {
+            echo "$MARKER"
+            echo "### lgb-diff: \`pkg/rt/core_compiled.lgb\` vs merge-base \`$MB_SHORT\`"
+            echo ""
+            if [ "${{ steps.bundle.outputs.changed }}" = "false" ]; then
+              echo "Bundle unchanged."
+            else
+              LINES=$(wc -l < /tmp/lgb.diff)
+              echo "<details><summary>unified diff ($LINES lines)</summary>"
+              echo ""
+              echo '```diff'
+              head -400 /tmp/lgb.diff
+              echo '```'
+              echo "</details>"
+              if [ "$LINES" -gt 400 ]; then
+                echo ""
+                echo "_Truncated at 400 lines; dump both bundles with \`scripts/lgbdump.lg\` locally for the rest._"
+              fi
+            fi
+          } > /tmp/comment.md
+          CID=$(gh api "repos/$REPO/issues/$PR/comments" --paginate \
+                --jq ".[] | select(.body | startswith(\"$MARKER\")) | .id" | head -1)
+          if [ -n "$CID" ]; then
+            gh api -X PATCH "repos/$REPO/issues/comments/$CID" -F body=@/tmp/comment.md
+          else
+            gh pr comment "$PR" --repo "$REPO" --body-file /tmp/comment.md
+          fi
diff --git a/pkg/compiler/compiler.go b/pkg/compiler/compiler.go
index d692cfd1f..e806b463f 100644
--- a/pkg/compiler/compiler.go
+++ b/pkg/compiler/compiler.go
@@ -195,6 +195,7 @@ func (c *Context) CompileMultiple(reader io.Reader) (compiled *vm.CodeChunk, res
 		compiledForms++
 		return nil
 	}
+	sawVoid := false
 	for {
 		o, err := r.Read()
 		if err != nil {
@@ -203,10 +204,23 @@ func (c *Context) CompileMultiple(reader io.Reader) (compiled *vm.CodeChunk, res
 			}
 			return nil, result, err
 		}
+		// Comments, #_ discards, and empty reader conditionals read as the
+		// VOID sentinel. Compiling them would emit a dead LOAD_CONST/POP pair
+		// per occurrence (and let a trailing comment clobber the last value),
+		// so skip them here like every other read loop does.
+		if o.Type() == vm.VoidType {
+			sawVoid = true
+			continue
+		}
 		if err := evalTopForm(o); err != nil {
 			return nil, result, err
 		}
 	}
+	// Input that held only no-value forms still evaluates to VOID (not nil),
+	// so the REPL keeps echoing nothing for a comment-only line.
+	if compiledForms == 0 && sawVoid {
+		result = vm.VOID
+	}
 
 	c.chunk = chunk
 
diff --git a/pkg/compiler/void_toplevel_test.go b/pkg/compiler/void_toplevel_test.go
new file mode 100644
index 000000000..7899b713f
--- /dev/null
+++ b/pkg/compiler/void_toplevel_test.go
@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2026 let-go contributors
+ * SPDX-License-Identifier: MIT
+ */
+
+package compiler
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/nooga/let-go/pkg/rt"
+	"github.com/nooga/let-go/pkg/vm"
+)
+
+// A trailing comment (or #_ discard) must not clobber the last value: the
+// reader surfaces those as the VOID sentinel and CompileMultiple used to
+// compile-and-run it like a real form, so `42 ;; done` evaluated to VOID.
+func TestTrailingCommentKeepsLastValue(t *testing.T) {
+	for _, src := range []string{
+		"42 ;; done",
+		"42\n;; done\n",
+		"42 #_(discarded)",
+		"42 #?(:cljr 1)",
+	} {
+		v, err := Eval(src)
+		if err != nil {
+			t.Fatalf("Eval(%q): %v", src, err)
+		}
+		if n, ok := v.(vm.Int); !ok || int(n) != 42 {
+			t.Fatalf("Eval(%q) = %v (%s), want 42", src, v, v.Type().Name())
+		}
+	}
+}
+
+// Input holding only no-value forms still evaluates to VOID, not nil — the
+// REPL relies on this to echo nothing for a comment-only line.
+func TestCommentOnlyInputEvaluatesToVoid(t *testing.T) {
+	for _, src := range []string{
+		";; just a comment",
+		"#_(all discarded)",
+	} {
+		v, err := Eval(src)
+		if err != nil {
+			t.Fatalf("Eval(%q): %v", src, err)
+		}
+		if v != vm.VOID {
+			t.Fatalf("Eval(%q) = %v (%s), want VOID", src, v, v.Type().Name())
+		}
+	}
+}
+
+// Skipped forms must not leave dead LOAD_CONST/POP pairs in the chunk: a
+// comment between two forms compiles identically to no comment at all.
+func TestVoidFormsEmitNoCode(t *testing.T) {
+	compile := func(src string) []int32 {
+		c := NewTransientCompiler(consts, rt.NS(rt.NameCoreNS))
+		chunk, _, err := c.CompileMultiple(strings.NewReader(src))
+		if err != nil {
+			t.Fatalf("CompileMultiple(%q): %v", src, err)
+		}
+		return chunk.Code()
+	}
+	plain := compile("1 2")
+	commented := compile("1 ;; between\n#_(dead) 2")
+	if len(plain) != len(commented) {
+		t.Fatalf("comments changed emitted code size: %d words vs %d", len(plain), len(commented))
+	}
+}
diff --git a/pkg/rt/core_compiled.lgb b/pkg/rt/core_compiled.lgb
index 22e4f1525..8a893fb48 100644
Binary files a/pkg/rt/core_compiled.lgb and b/pkg/rt/core_compiled.lgb differ
diff --git a/scripts/lgbdump.lg b/scripts/lgbdump.lg
new file mode 100644
index 000000000..959c1086a
--- /dev/null
+++ b/scripts/lgbdump.lg
@@ -0,0 +1,75 @@
+;; scripts/lgbdump.lg — canonical text dump of a .lgb bundle, for diffing.
+;;
+;;     ./lg scripts/lgbdump.lg <bundle.lgb>
+;;
+;; Diff two bundles by diffing two dumps:
+;;
+;;     diff -u <(./lg scripts/lgbdump.lg old.lgb) <(./lg scripts/lgbdump.lg new.lgb)
+;;
+;; Builds on disasm/decode-bundle (decodes without executing) and
+;; disassemble-resolved (LOAD_CONST/LOAD_VAR rows carry the referenced
+;; identifier, so const-pool index shifts don't drown a diff). Output is
+;; deterministic: namespaces sorted by name, no addresses or pool order.
+;;
+;; Named functions from the shared const pool are dumped under their name;
+;; anonymous ones under a bare "fn anon" label (a pool index would shift on
+;; any insertion, making identical fns diff as different).
+
+(ns lgbdump
+  (:require [disasm]))
+
+;; A resolved LOAD_CONST/LOAD_VAR row is [op pool-idx identifier]. The pool
+;; index shifts whenever any earlier const is added or removed, which turns a
+;; one-fn change into a whole-bundle diff — so for diffing, keep only the
+;; identifier. Unresolved rows keep their index (it's all we have).
+(defn scrub-row [row]
+  (if (and (>= (count row) 3)
+           (or (= :LOAD_CONST (first row)) (= :LOAD_VAR (first row))))
+    (vector (first row) (nth row 2))
+    row))
+
+;; Top-level comments and #_ discards compile to a LOAD_CONST-VOID/POP pair
+;; (dead code; being removed by #600). Rendering each pair would make "added
+;; a comment" look like bytecode churn, so suppress them and print one count
+;; line per chunk instead — the count still diffs when it changes.
+(defn void-load? [row]
+  (and (= :LOAD_CONST (first row))
+       (let [x (nth row (dec (count row)))]
+         (and (vector? x) (= :opaque (first x)) (= "VOID" (second x))))))
+
+(defn dump-chunk [label chunk]
+  (println (str "== " label " =="))
+  (loop [rows (seq (disasm/disassemble-resolved chunk)) skipped 0]
+    (if (nil? rows)
+      (when (> skipped 0)
+        (println (str ";; " skipped " no-value pairs suppressed")))
+      (let [row (first rows)
+            nxt (next rows)]
+        (if (and (void-load? row) nxt (= :POP (first (first nxt))))
+          (recur (next nxt) (inc skipped))
+          (do (println (pr-str (scrub-row row)))
+              (recur nxt skipped)))))))
+
+(defn dump-bundle [path]
+  (let [b (disasm/decode-bundle path)
+        nss (sort-by (fn [m] (get m :ns)) (get b :namespaces))]
+    (println (str ";; lgbdump: " (count nss) " namespaces, "
+                  (count (get b :consts)) " consts"))
+    (dump-chunk "main" (get b :main))
+    (doseq [m nss]
+      (dump-chunk (str "ns " (get m :ns)) (get m :chunk)))
+    ;; Nested fns live in the bundle-wide shared const pool: raw values give
+    ;; the chunks, the projected view gives stable [:fn name] labels.
+    (let [raw (disasm/constants (get b :main))
+          proj (get b :consts)]
+      (doseq [i (range (count raw))]
+        (let [p (nth proj i)]
+          (when (and (vector? p) (= :fn (first p)))
+            (dump-chunk (str "fn " (if (> (count p) 1) (second p) "anon"))
+                        (nth raw i))))))))
+
+(let [path (first *command-line-args*)]
+  (if (nil? path)
+    (do (println "usage: lg scripts/lgbdump.lg <bundle.lgb>")
+        (os/exit 2))
+    (dump-bundle path)))