diff --git a/__tests__/celery-dispatch-synthesizer.test.ts b/__tests__/celery-dispatch-synthesizer.test.ts
index c94f9d6a5..e70cffca4 100644
--- a/__tests__/celery-dispatch-synthesizer.test.ts
+++ b/__tests__/celery-dispatch-synthesizer.test.ts
@@ -103,6 +103,34 @@ def handle_request(req):
     cg.close?.();
   });
 
+  it('numbers each dispatch by its own file, when two files of one length are scanned in a row', async () => {
+    // The pass reuses a line index while the text is the same; two files of the
+    // same length but different line breaks must still each get their own lines.
+    fs.writeFileSync(path.join(dir, 'tasks.py'), 'from celery import shared_task\n\n\n@shared_task\ndef send_email(to):\n    return to\n');
+    const a = 'from tasks import send_email\n\n\n\ndef first(req):\n    send_email.delay(req.addr)\n';
+    const b = 'from tasks import send_email\ndef second(req):\n    send_email.delay(req.addr)\n';
+    const padTo = (s: string, n: number): string => `${s}#${'x'.repeat(n - s.length - 2)}\n`;
+    const len = Math.max(a.length, b.length) + 8;
+    fs.writeFileSync(path.join(dir, 'a.py'), padTo(a, len));
+    fs.writeFileSync(path.join(dir, 'b.py'), padTo(b, len));
+    expect(fs.statSync(path.join(dir, 'a.py')).size).toBe(fs.statSync(path.join(dir, 'b.py')).size);
+
+    const cg = await CodeGraph.init(dir, { silent: true });
+    await cg.indexAll();
+    const rows = (cg as any).db.db
+      .prepare(
+        `SELECT s.name source, e.line line, json_extract(e.metadata,'$.registeredAt') at
+         FROM edges e JOIN nodes s ON s.id = e.source
+         WHERE json_extract(e.metadata,'$.synthesizedBy') = 'celery-dispatch' ORDER BY s.name`
+      )
+      .all();
+    expect(rows).toEqual([
+      { source: 'first', line: 6, at: 'a.py:6' },
+      { source: 'second', line: 3, at: 'b.py:3' },
+    ]);
+    cg.close?.();
+  });
+
   it('produces no edges in a Celery-free project (clean control)', async () => {
     fs.writeFileSync(
       path.join(dir, 'app.py'),
diff --git a/src/resolution/callback-synthesizer.ts b/src/resolution/callback-synthesizer.ts
index d747898ab..db842d158 100644
--- a/src/resolution/callback-synthesizer.ts
+++ b/src/resolution/callback-synthesizer.ts
@@ -132,9 +132,38 @@ function nuxtComponentName(filePath: string): string | null {
   return out.join('');
 }
 
+// Passes call sliceLines once per node, so splitting the whole file each
+// time is quadratic in a file's node count. Keep the last file's lines.
+let sliceLinesSource: string | null = null;
+let sliceLinesSplit: string[] = [];
 function sliceLines(content: string, startLine?: number, endLine?: number): string | null {
   if (!startLine || !endLine) return null;
-  return content.split('\n').slice(startLine - 1, endLine).join('\n');
+  if (content !== sliceLinesSource) {
+    sliceLinesSource = content;
+    sliceLinesSplit = content.split('\n');
+  }
+  return sliceLinesSplit.slice(startLine - 1, endLine).join('\n');
+}
+
+// `src.slice(0, idx).split('\n').length` per match is quadratic in a file's
+// match count. The newline index is built once per text, and the last text's
+// index is kept.
+let lineOfSource: string | null = null;
+let lineOfIndex: (idx: number) => number = () => 1;
+function lineOf(src: string, idx: number): number {
+  if (src !== lineOfSource) {
+    lineOfSource = src;
+    lineOfIndex = makeLineAt(src, 1);
+  }
+  return lineOfIndex(idx);
+}
+
+/** Forget the last file sliceLines and lineOf keep, once a synthesis run is over. */
+function releaseLineCaches(): void {
+  sliceLinesSource = null;
+  sliceLinesSplit = [];
+  lineOfSource = null;
+  lineOfIndex = () => 1;
 }
 
 function registrarField(src: string): string | null {
@@ -611,7 +640,7 @@ async function arkuiEmitterEdges(ctx: ResolutionContext, onYield: MaybeYield): P
     while ((m = ARKUI_EMITTER_CALL_RE.exec(safe))) {
       const verb = m[1]!;
       const arg = m[2]!.trim();
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const encl = nodes
         .filter((n) => n.startLine <= line && n.endLine >= line)
         .sort((a, b) => (a.endLine - a.startLine) - (b.endLine - b.startLine))[0];
@@ -715,7 +744,7 @@ async function arkuiRouterEdges(ctx: ResolutionContext, onYield: MaybeYield): Pr
     let m: RegExpExecArray | null;
     while ((m = ARKUI_ROUTER_RE.exec(safe))) {
       const url = m[1]!;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const encl = nodes
         .filter((n) => n.startLine <= line && n.endLine >= line)
         .sort((a, b) => (a.endLine - a.startLine) - (b.endLine - b.startLine))[0];
@@ -3462,7 +3491,7 @@ async function ginMiddlewareChainEdges(queries: QueryBuilder, ctx: ResolutionCon
       const parenIdx = m.index + m[0].length - 1;
       const argStr = goBalancedArgs(safe, parenIdx);
       if (!argStr) continue;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       for (const arg of goSplitArgs(argStr)) {
         const name = goHandlerIdent(arg);
         if (name && !registered.has(name)) registered.set(name, `${file}:${line}`);
@@ -3639,7 +3668,7 @@ async function reduxThunkEdges(queries: QueryBuilder, ctx: ResolutionContext, on
       const key = `${node.id}>${target.id}`;
       if (seen.has(key)) continue;
       seen.add(key);
-      const line = node.startLine + safe.slice(0, m.index).split('\n').length - 1;
+      const line = node.startLine + lineOf(safe, m.index) - 1;
       edges.push({
         source: node.id,
         target: target.id,
@@ -3957,7 +3986,7 @@ async function piniaStoreEdges(ctx: ResolutionContext, onYield: MaybeYield): Pro
       const storeFile = varStore.get(cm[1]!);
       if (!storeFile) continue;
       const method = cm[2]!;
-      const line = safe.slice(0, cm.index).split('\n').length;
+      const line = lineOf(safe, cm.index);
       const disp = enclosingFn(nodesInFile, line) ?? fallbackDispatcher;
       if (!disp) continue;
       const target = ctx
@@ -4049,7 +4078,7 @@ async function vuexDispatchEdges(ctx: ResolutionContext, onYield: MaybeYield): P
     let added = 0;
     while ((m = VUEX_DISPATCH_RE.exec(safe)) && added < VUEX_FANOUT_CAP) {
       const key = m[1]!;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line) ?? fallback;
       if (!disp) continue;
       const target = resolve(key, file);
@@ -4145,7 +4174,7 @@ async function celeryDispatchEdges(ctx: ResolutionContext, onYield: MaybeYield):
     let added = 0;
     while ((m = CELERY_DISPATCH_RE.exec(safe)) && added < CELERY_FANOUT_CAP) {
       const name = m[1]!;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue; // module-level dispatch — no source symbol to attribute
       const target = resolve(name, file);
@@ -4267,7 +4296,7 @@ async function springEventEdges(ctx: ResolutionContext, onYield: MaybeYield): Pr
     while ((m = SPRING_PUBLISH_RE.exec(safe)) && added < SPRING_FANOUT_CAP) {
       const targets = listeners.get(m[1]!);
       if (!targets || !targets.length) continue;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue;
       for (const target of targets) {
@@ -4380,7 +4409,7 @@ async function mediatrDispatchEdges(ctx: ResolutionContext, onYield: MaybeYield)
     let added = 0;
     while ((m = MEDIATR_DISPATCH_RE.exec(safe)) && added < MEDIATR_FANOUT_CAP) {
       if (!MEDIATR_RECEIVER_RE.test(m[1]!)) continue; // not a mediator (MessagingCenter, HttpClient, …)
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue;
       const type = resolveMediatrArgType(m[2]!, safeLines, disp.startLine, line);
@@ -4476,7 +4505,7 @@ async function sidekiqDispatchEdges(ctx: ResolutionContext, onYield: MaybeYield)
     let m: RegExpExecArray | null;
     let added = 0;
     while ((m = SIDEKIQ_DISPATCH_RE.exec(safe)) && added < SIDEKIQ_FANOUT_CAP) {
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue;
       const target = resolve(m[1]!);
@@ -4855,7 +4884,7 @@ async function erlangBehaviourDispatchEdges(queries: QueryBuilder, ctx: Resoluti
       const behaviour = behaviours[0]!;
       const targets = targetsOf(behaviour, fn, arity);
       if (targets.length === 0 || targets.length > ERLANG_BEHAVIOUR_FANOUT_CAP) continue;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue;
       for (const target of targets) {
@@ -5002,7 +5031,7 @@ async function laravelEventEdges(ctx: ResolutionContext, onYield: MaybeYield): P
     while ((m = LARAVEL_DISPATCH_RE.exec(safe)) && added < LARAVEL_FANOUT_CAP) {
       const targets = listeners.get(phpSimpleName(m[1]!));
       if (!targets) continue;
-      const line = safe.slice(0, m.index).split('\n').length;
+      const line = lineOf(safe, m.index);
       const disp = enclosingFn(nodesInFile, line);
       if (!disp) continue;
       for (const target of targets.values()) {
@@ -5439,6 +5468,10 @@ export async function synthesizeCallbackEdges(
       await runPassOnMain(i);
     }
   }
+  // The passes are done. In a long-lived process (the MCP server, `watch`),
+  // the two line caches would otherwise keep the last file read until the next
+  // synthesis; a pool worker's go with the worker.
+  releaseLineCaches();
 
   // A Jest or Vitest manual mock answers only for test code (#2455): whatever
   // pass paired them, production code never links into a `__mocks__/` file.