diff --git a/src/target-go.ts b/src/target-go.ts
index 7bb6989..87c3a7a 100644
--- a/src/target-go.ts
+++ b/src/target-go.ts
@@ -550,6 +550,21 @@ func parse(t []Tok) int32 {
 \tnodes = nodes[:0]; kids = kids[:0]; scratch = scratch[:0]
 \treturn parse${ir.entry}()
 }
+
+type Edit struct { Start, End int; Text string }
+type Doc struct { text string; root int32 }
+func NewDoc(src string) *Doc {
+\td := &Doc{text: src}
+\td.root = parse(tokenize(src))
+\treturn d
+}
+func (d *Doc) Text() string { return d.text }
+func (d *Doc) Root() int32 { return d.root }
+func (d *Doc) Edit(edits []Edit) int32 {
+\tfor _, e := range edits { d.text = d.text[:e.Start] + e.Text + d.text[e.End:] }
+\td.root = parse(tokenize(d.text))
+\treturn d.root
+}
 `;
   },
   emitRunner(): string {
@@ -559,6 +574,7 @@ func parse(t []Tok) int32 {
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"os"
@@ -571,7 +587,7 @@ func main() {
 \tdata, _ := io.ReadAll(os.Stdin)
 \tsrc := string(data)
 \t// Self-bench: a numeric arg N times the lex+parse loop and prints ms/iteration.
-\tif len(os.Args) > 1 {
+\tif len(os.Args) > 1 && os.Args[1] != "edit-session" {
 \t\tif iters, err := strconv.Atoi(os.Args[1]); err == nil && iters > 0 {
 \t\t\tfor i := 0; i < 3; i++ { parse(tokenize(src)) }
 \t\t\tt0 := time.Now()
@@ -580,6 +596,27 @@ func main() {
 \t\t\treturn
 \t\t}
 \t}
+\tif len(os.Args) > 1 && os.Args[1] == "edit-session" {
+\t\tvar sess struct {
+\t\t\tInit    string              \`json:"init"\`
+\t\t\tBatches [][][3]interface{}   \`json:"batches"\`
+\t\t}
+\t\tif json.Unmarshal(data, &sess) != nil { os.Exit(1) }
+\t\td := NewDoc(sess.Init)
+\t\tfor _, batch := range sess.Batches {
+\t\t\tedits := make([]Edit, len(batch))
+\t\t\tfor i, t := range batch {
+\t\t\t\tedits[i] = Edit{Start: int(t[0].(float64)), End: int(t[1].(float64)), Text: t[2].(string)}
+\t\t\t}
+\t\t\td.Edit(edits)
+\t\t}
+\t\troot := d.Root()
+\t\tif root < 0 || pos != len(toks) { fmt.Fprintf(os.Stderr, "parse error (pos %d/%d)\\n", pos, len(toks)); os.Exit(1) }
+\t\tvar b strings.Builder
+\t\twriteJSON(root, &b)
+\t\tos.Stdout.WriteString(b.String())
+\t\treturn
+\t}
 \troot := parse(tokenize(src))
 \tif root < 0 || pos != len(toks) {
 \t\tfmt.Fprintf(os.Stderr, "parse error (pos %d/%d)\\n", pos, len(toks))
diff --git a/src/target-rust.ts b/src/target-rust.ts
index c39fd6b..1961568 100644
--- a/src/target-rust.ts
+++ b/src/target-rust.ts
@@ -546,6 +546,17 @@ fn parse<'a>(tokens: Tokens<'a>) -> Option<(Parser<'a>, i32)> {
         _ => None,
     }
 }
+
+pub struct Edit { pub start: usize, pub end: usize, pub text: String }
+pub struct Doc { text: String }
+impl Doc {
+    pub fn new(text: String) -> Doc { Doc { text } }
+    pub fn text(&self) -> &str { &self.text }
+    pub fn edit(&mut self, edits: &[Edit]) {
+        for e in edits { let s = e.start; let en = e.end; self.text = format!("{}{}{}", &self.text[..s], e.text, &self.text[en..]); }
+    }
+    pub fn parse(&self) -> Option<(Parser<'_>, i32)> { parse(tokenize(&self.text)) }
+}
 `;
   },
   emitRunner(): string {
@@ -553,12 +564,115 @@ fn parse<'a>(tokens: Tokens<'a>) -> Option<(Parser<'a>, i32)> {
 // CLI runner (harness only): stdin -> CST JSON + a self-bench mode. Appended to the parser
 // library by the gate (same file/crate, so it calls \`parse\`/\`write_json\` directly); NOT part
 // of the parser.
+fn skip_ws(s: &[u8], mut i: usize) -> usize { while i < s.len() && (s[i] as char).is_whitespace() { i += 1; } i }
+fn parse_str(s: &[u8], mut i: usize) -> Option<(String, usize)> {
+    if s.get(i)? != &b'"' { return None; }
+    i += 1;
+    let mut out = String::new();
+    while i < s.len() {
+        match s[i] {
+            b'"' => return Some((out, i + 1)),
+            b'\\\\' => { i += 1; if i >= s.len() { return None; }
+                out.push(match s[i] { b'n' => '\\n', b'r' => '\\r', b't' => '\\t', b'"' => '"', b'\\\\' => '\\\\', b'/' => '/', c => c as char });
+                i += 1; }
+            c => { out.push(c as char); i += 1; }
+        }
+    }
+    None
+}
+fn parse_num(s: &[u8], mut i: usize) -> Option<(usize, usize)> {
+    let start = i;
+    while i < s.len() && s[i].is_ascii_digit() { i += 1; }
+    if i == start { return None; }
+    s[start..i].iter().fold(Some(0usize), |a, &d| a.and_then(|n| n.checked_mul(10).and_then(|m| m.checked_add((d - b'0') as usize))))
+        .map(|n| (n, i))
+}
+fn parse_triple(s: &[u8], mut i: usize) -> Option<((usize, usize, String), usize)> {
+    if s.get(i)? != &b'[' { return None; }
+    i = skip_ws(s, i + 1);
+    let (a, mut i) = parse_num(s, i)?;
+    i = skip_ws(s, i); if s.get(i)? != &b',' { return None; }
+    i = skip_ws(s, i + 1);
+    let (b, mut i) = parse_num(s, i)?;
+    i = skip_ws(s, i); if s.get(i)? != &b',' { return None; }
+    i = skip_ws(s, i + 1);
+    let (t, mut i) = parse_str(s, i)?;
+    i = skip_ws(s, i); if s.get(i)? != &b']' { return None; }
+    Some(((a, b, t), i + 1))
+}
+fn parse_batch(s: &[u8], mut i: usize) -> Option<(Vec<(usize, usize, String)>, usize)> {
+    if s.get(i)? != &b'[' { return None; }
+    i = skip_ws(s, i + 1);
+    let mut batch = Vec::new();
+    if s.get(i)? == &b']' { return Some((batch, i + 1)); }
+    loop {
+        let (t, ni) = parse_triple(s, i)?;
+        batch.push(t);
+        i = skip_ws(s, ni);
+        if s.get(i)? == &b']' { return Some((batch, i + 1)); }
+        if s.get(i)? != &b',' { return None; }
+        i = skip_ws(s, i + 1);
+    }
+}
+fn parse_edit_session(s: &str) -> Option<(String, Vec<Vec<(usize, usize, String)>>)> {
+    let b = s.as_bytes();
+    let mut i = skip_ws(b, 0);
+    if b.get(i)? != &b'{' { return None; }
+    i = skip_ws(b, i + 1);
+    let mut init = None;
+    let mut batches = None;
+    loop {
+        let (key, ni) = parse_str(b, i)?;
+        i = skip_ws(b, ni);
+        if b.get(i)? != &b':' { return None; }
+        i = skip_ws(b, i + 1);
+        if key == "init" {
+            let (v, ni) = parse_str(b, i)?;
+            init = Some(v);
+            i = skip_ws(b, ni);
+        } else if key == "batches" {
+            if b.get(i)? != &b'[' { return None; }
+            i = skip_ws(b, i + 1);
+            let mut bs = Vec::new();
+            if b.get(i)? == &b']' { batches = Some(bs); i += 1; }
+            else {
+                loop {
+                    let (batch, ni) = parse_batch(b, i)?;
+                    bs.push(batch);
+                    i = skip_ws(b, ni);
+                    if b.get(i)? == &b']' { batches = Some(bs); i += 1; break; }
+                    if b.get(i)? != &b',' { return None; }
+                    i = skip_ws(b, i + 1);
+                }
+            }
+        } else { return None; }
+        if b.get(i)? == &b'}' { break; }
+        if b.get(i)? != &b',' { return None; }
+        i = skip_ws(b, i + 1);
+    }
+    Some((init?, batches?))
+}
+
 fn main() {
     use std::io::Read;
     let mut src = String::new();
     std::io::stdin().read_to_string(&mut src).unwrap();
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() > 1 && args[1] == "edit-session" {
+        let (init, batches) = parse_edit_session(&src).unwrap();
+        let mut doc = Doc::new(init);
+        for batch in &batches {
+            let edits: Vec<Edit> = batch.iter().map(|&(s, e, ref t)| Edit { start: s, end: e, text: t.clone() }).collect();
+            doc.edit(&edits);
+        }
+        match doc.parse() {
+            Some((p, root)) => { let mut out = String::new(); write_json(&p, root, &mut out); print!("{}", out); }
+            None => { eprintln!("parse error"); std::process::exit(1); }
+        }
+        return;
+    }
     // Self-bench: a numeric arg N times the lex+parse loop and prints ms/iteration.
-    if let Some(iters) = std::env::args().nth(1).and_then(|a| a.parse::<u64>().ok()) {
+    if let Some(iters) = args.get(1).and_then(|a| a.parse::<u64>().ok()) {
         for _ in 0..3 { let s = std::hint::black_box(&src); if let Some((p, r)) = parse(tokenize(s)) { std::hint::black_box((&p.nodes[r as usize], p.pos)); } }
         let t = std::time::Instant::now();
         for _ in 0..iters { let s = std::hint::black_box(&src); if let Some((p, r)) = parse(tokenize(s)) { std::hint::black_box((&p.nodes[r as usize], p.pos)); } }
diff --git a/src/target-ts.ts b/src/target-ts.ts
index ec8f5d6..5cb84a1 100644
--- a/src/target-ts.ts
+++ b/src/target-ts.ts
@@ -471,15 +471,40 @@ export function parse(tokens: Tok[]): Cst | null {
   const root = parse${ir.entry}();
   return root !== null && pos === toks.length ? root : null;
 }
+
+export type Edit = { start: number; end: number; text: string };
+export function createDoc(src: string): { text(): string; root(): Node | null; edit(edits: Edit[]): Node | null } {
+  let text = src;
+  let root: Node | null = parse(tokenize(src));
+  return {
+    text(): string { return text; },
+    root(): Node | null { return root; },
+    edit(edits: Edit[]): Node | null {
+      for (const e of edits) text = text.slice(0, e.start) + e.text + text.slice(e.end);
+      root = parse(tokenize(text));
+      return root;
+    },
+  };
+}
 `;
   },
   emitRunner(): string {
     return `// CLI runner (harness only): stdin → CST JSON. Appended to the parser library by the gate;
 // NOT part of the emitted parser. The import is hoisted, so it may follow the library code.
 import { readFileSync } from 'node:fs';
-const _root = parse(tokenize(readFileSync(0, 'utf8')));
-if (_root === null) { process.stderr.write('parse error\\n'); process.exit(1); }
-process.stdout.write(JSON.stringify(_root));
+const _raw = readFileSync(0, 'utf8');
+if (process.argv.includes('edit-session')) {
+  const { init, batches } = JSON.parse(_raw) as { init: string; batches: [number, number, string][][] };
+  const doc = createDoc(init);
+  for (const batch of batches) doc.edit(batch.map(([start, end, text]) => ({ start, end, text })));
+  const root = doc.root();
+  if (root === null) { process.stderr.write('parse error\\n'); process.exit(1); }
+  process.stdout.write(JSON.stringify(root));
+} else {
+  const _root = parse(tokenize(_raw));
+  if (_root === null) { process.stderr.write('parse error\\n'); process.exit(1); }
+  process.stdout.write(JSON.stringify(_root));
+}
 `;
   },
 };
diff --git a/test/portable-targets.ts b/test/portable-targets.ts
index 5d55d73..666ab05 100644
--- a/test/portable-targets.ts
+++ b/test/portable-targets.ts
@@ -271,6 +271,28 @@ function runProc(cmd: string, args: string[], src: string): Outcome {
   catch { return { ok: false }; }
 }
 
+type EditBatch = [number, number, string][];
+type EditScenario = { init: string; batches: EditBatch[] };
+const EDIT_SCENARIOS: Record<string, EditScenario[]> = {
+  calc: [
+    { init: '1+2*3', batches: [[[3, 3, '4']]] },
+    { init: '1+2*3', batches: [[[1, 3, '']]] },
+    { init: '1+2*3', batches: [[[2, 5, '(7-8)']]] },
+    { init: '1+2*3', batches: [[[0, 0, '9-']], [[7, 8, '']]] },
+  ],
+  javascript: [
+    { init: 'let a = 1;\nf(a);', batches: [[[8, 9, '42']]] },
+    { init: 'let a = 1;\nf(a);', batches: [[[11, 16, '']]] },
+    { init: 'let a = 1;\nf(a);', batches: [[[4, 5, 'b'], [12, 13, 'b']]] },
+    { init: 'let a = 1;\nf(a);', batches: [[[16, 16, '\ng(b);']], [[0, 4, 'var']]] },
+  ],
+};
+const applyEdits = (init: string, batches: EditBatch[]): string => {
+  let text = init;
+  for (const batch of batches) for (const [start, end, repl] of batch) text = text.slice(0, start) + repl + text.slice(end);
+  return text;
+};
+
 let failures = 0;
 for (const c of CASES) {
   const grammar: CstGrammar = (await import(c.path)).default;
@@ -333,6 +355,27 @@ for (const c of CASES) {
       console.log(`  ${c.grammar}/${r.label}: REJECT mismatch on ${JSON.stringify(src)} (oracle ok=${want.ok}, ${r.label} ok=${got.ok})`);
     }
     console.log(`  ${c.grammar}/${r.label}: ${acc}/${c.accept.length} accept ≡ oracle · ${rej}/${c.reject.length} reject ≡ oracle${snap ? ` · ${snap} shape drift` : ''}`);
+
+    const editScenarios = EDIT_SCENARIOS[c.grammar];
+    if (editScenarios) {
+      let editOk = 0;
+      const runEdit = (json: string) => runProc(
+        r.label === 'typescript' ? 'node' : r.label === 'go' ? `${dir}/go/p` : `${dir}/pr`,
+        r.label === 'typescript' ? [`${dir}/p.ts`, 'edit-session'] : ['edit-session'],
+        json,
+      );
+      for (const sc of editScenarios) {
+        const final = applyEdits(sc.init, sc.batches);
+        const a = runEdit(JSON.stringify({ init: sc.init, batches: sc.batches }));
+        const b = r.run(final);
+        if (a.ok === b.ok && (!a.ok || a.cst === b.cst) && a.ok === oracleOut(final).ok) editOk++;
+        else {
+          failures++;
+          console.log(`  ${c.grammar}/${r.label}: edit-session mismatch (final=${JSON.stringify(final)}) A ok=${a.ok} B ok=${b.ok}`);
+        }
+      }
+      console.log(`  ${c.grammar}/${r.label}: ${editOk}/${editScenarios.length} edit-sessions ≡ fresh`);
+    }
   }
 }