diff --git a/extension/scripts/build-injection-patterns.ts b/extension/scripts/build-injection-patterns.ts
index 7f4bcd9..84a652c 100644
--- a/extension/scripts/build-injection-patterns.ts
+++ b/extension/scripts/build-injection-patterns.ts
@@ -18,6 +18,7 @@ import { readFileSync, writeFileSync } from "node:fs";
 import { join, relative } from "node:path";
 import { load } from "js-yaml";
 import { z } from "zod";
+import { fileHasContent } from "./file-has-content";
 
 const ROOT = join(import.meta.dir, "..");
 const INPUT = join(ROOT, "data", "injection-patterns.yaml");
@@ -100,7 +101,13 @@ export function generateInjectionPatterns(): void {
     return { name: entry.name, source, flags: entry.flags };
   });
 
-  writeFileSync(OUTPUT, buildOutput(entries));
+  // Skip the write when nothing changed: the output lives under `src/`, which
+  // `build.ts --watch` watches recursively, so an unconditional write would
+  // re-trigger the watcher and loop forever. See ./file-has-content.
+  const output = buildOutput(entries);
+  if (!fileHasContent(OUTPUT, output)) {
+    writeFileSync(OUTPUT, output);
+  }
   console.log(
     `Generated ${relative(ROOT, OUTPUT)} from ${entries.length} patterns.`,
   );
diff --git a/extension/scripts/build-site-data.ts b/extension/scripts/build-site-data.ts
index a462ae1..c8ebda5 100644
--- a/extension/scripts/build-site-data.ts
+++ b/extension/scripts/build-site-data.ts
@@ -24,6 +24,7 @@ import {
   SiteFileSchema,
   toEntries,
 } from "../data/site-rules.schema";
+import { fileHasContent } from "./file-has-content";
 
 const ROOT = join(import.meta.dir, "..");
 const SITES_DIR = join(ROOT, "data", "sites");
@@ -313,7 +314,13 @@ export function generateSiteData(): void {
 
   const parsed = loadSites();
   const output = buildOutput(parsed);
-  writeFileSync(OUTPUT, output);
+  // Only write when the content actually changes. The output lives inside
+  // `src/`, which `build.ts --watch` watches recursively; an unconditional
+  // write would re-trigger the watcher on every build and spin into an
+  // infinite rebuild loop.
+  if (!fileHasContent(OUTPUT, output)) {
+    writeFileSync(OUTPUT, output);
+  }
   console.log(
     `Generated ${relative(ROOT, OUTPUT)} from ${parsed.length} site YAML files.`,
   );
diff --git a/extension/scripts/file-has-content.ts b/extension/scripts/file-has-content.ts
new file mode 100644
index 0000000..f768a65
--- /dev/null
+++ b/extension/scripts/file-has-content.ts
@@ -0,0 +1,16 @@
+// Copyright (c) 2026 PixieBrix, Inc.
+// Licensed under PolyForm Shield 1.0.0 — see LICENSE.
+
+import { readFileSync } from "node:fs";
+
+// Returns true when `path` already holds exactly `expected`. Codegen writes
+// into `src/`, which `build.ts --watch` watches recursively; guarding each
+// `writeFileSync` with this check keeps an unchanged output from re-arming the
+// watcher and spinning into an infinite rebuild loop.
+export function fileHasContent(path: string, expected: string): boolean {
+  try {
+    return readFileSync(path, "utf8") === expected;
+  } catch {
+    return false;
+  }
+}