diff --git a/bin/__pycache__/inject-hashed-filenames.cpython-313.pyc b/bin/__pycache__/inject-hashed-filenames.cpython-313.pyc
new file mode 100644
index 0000000..4a92e14
Binary files /dev/null and b/bin/__pycache__/inject-hashed-filenames.cpython-313.pyc differ
diff --git a/bin/inject-hashed-filenames.py b/bin/inject-hashed-filenames.py
index 0afdddc..2cb2d21 100755
--- a/bin/inject-hashed-filenames.py
+++ b/bin/inject-hashed-filenames.py
@@ -26,9 +26,29 @@ containing the config file's parent chain → `$PWD`).
 """
 import json
 import os
+import re
 import sys
 
 
+def _build_pattern(old_src: str) -> "re.Pattern[str]":
+    # Match `src` on <script> and `href` on <link>, single- or double-quoted.
+    # We anchor to the tag name so an `old_src` substring sitting inside an
+    # HTML comment, a JSON literal, or a `data-…` attribute is not rewritten.
+    return re.compile(
+        r'(<(?:script|link)\b[^>]*?\b(?:src|href)\s*=\s*["\'])'
+        + re.escape(old_src)
+        + r'(["\'])',
+        re.IGNORECASE,
+    )
+
+
+def rewrite(html: str, old_src: str, new_src: str) -> "tuple[str, int]":
+    """Return (new_html, count). Tag-aware: only rewrites <script src> /
+    <link href> attributes, never substring matches in comments or JSON."""
+    pattern = _build_pattern(old_src)
+    return pattern.subn(lambda m: m.group(1) + new_src + m.group(2), html)
+
+
 def inject(manifest_path: str, html_path: str, old_src: str) -> None:
     if not os.path.exists(manifest_path):
         print(f"skip: no manifest at {manifest_path}")
@@ -45,12 +65,18 @@ def inject(manifest_path: str, html_path: str, old_src: str) -> None:
         new_src = f"{os.path.dirname(old_src)}/{hashed}"
         with open(html_path) as f:
             html = f.read()
-        if old_src not in html:
-            print(f"skip: {old_src!r} not in {html_path}")
+        new_html, count = rewrite(html, old_src, new_src)
+        if count == 0:
+            # Loud warning — silent skip used to mask typos in `old_src`.
+            print(
+                f"WARN: no <script src> or <link href> matching {old_src!r} "
+                f"in {html_path} — leaving file unchanged",
+                file=sys.stderr,
+            )
             return
         with open(html_path, "w") as f:
-            f.write(html.replace(old_src, new_src))
-        print(f"{old_src} -> {new_src}")
+            f.write(new_html)
+        print(f"{old_src} -> {new_src} ({count} occurrence{'s' if count != 1 else ''})")
         return
     print(f"skip: no isEntry row in {manifest_path}")
 
diff --git a/tests/__pycache__/test_inject_hashed_filenames.cpython-313-pytest-8.3.5.pyc b/tests/__pycache__/test_inject_hashed_filenames.cpython-313-pytest-8.3.5.pyc
new file mode 100644
index 0000000..ca48ab9
Binary files /dev/null and b/tests/__pycache__/test_inject_hashed_filenames.cpython-313-pytest-8.3.5.pyc differ
diff --git a/tests/test_inject_hashed_filenames.py b/tests/test_inject_hashed_filenames.py
new file mode 100644
index 0000000..579f368
--- /dev/null
+++ b/tests/test_inject_hashed_filenames.py
@@ -0,0 +1,77 @@
+"""Tests for bin/inject-hashed-filenames.py rewrite() (#3).
+
+Pinned behaviour: the rewrite is tag-aware — only `<script src="…">` and
+`<link href="…">` attribute values are replaced. Inert occurrences of the
+old src in HTML comments, JSON literals, or unrelated attributes must be
+left alone (the previous `html.replace` was substring-blind).
+"""
+import importlib.util
+import os
+import sys
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+SCRIPT = os.path.join(os.path.dirname(HERE), "bin", "inject-hashed-filenames.py")
+
+spec = importlib.util.spec_from_file_location("ihf", SCRIPT)
+ihf = importlib.util.module_from_spec(spec)
+sys.modules["ihf"] = ihf
+spec.loader.exec_module(ihf)
+
+
+def test_rewrites_script_src_double_quoted():
+    html = '<script src="/static/dist/app.js"></script>'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 1
+    assert out == '<script src="/static/dist/app.abc123.js"></script>'
+
+
+def test_rewrites_script_src_single_quoted():
+    html = "<script src='/static/dist/app.js'></script>"
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 1
+    assert "/static/dist/app.abc123.js" in out
+
+
+def test_rewrites_link_href():
+    html = '<link rel="stylesheet" href="/static/dist/app.css">'
+    out, n = ihf.rewrite(html, "/static/dist/app.css", "/static/dist/app.abc123.css")
+    assert n == 1
+    assert '/static/dist/app.abc123.css' in out
+
+
+def test_does_not_rewrite_inside_html_comment():
+    html = '<!-- old script was at /static/dist/app.js --><script src="/other.js"></script>'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 0
+    assert "/static/dist/app.js" in out
+    assert "/static/dist/app.abc123.js" not in out
+
+
+def test_does_not_rewrite_inside_json_literal():
+    html = '<pre>{ "src": "/static/dist/app.js" }</pre>'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 0
+    assert out == html
+
+
+def test_does_not_rewrite_unrelated_attribute():
+    html = '<img data-bundle="/static/dist/app.js">'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 0
+    assert out == html
+
+
+def test_does_not_rewrite_anchor_href():
+    # Even though <a href="…"> is a `href` attribute, it isn't a <link>.
+    html = '<a href="/static/dist/app.js">debug link</a>'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 0
+    assert out == html
+
+
+def test_rewrites_with_extra_attributes_around_src():
+    html = '<script type="module" src="/static/dist/app.js" defer></script>'
+    out, n = ihf.rewrite(html, "/static/dist/app.js", "/static/dist/app.abc123.js")
+    assert n == 1
+    assert '/static/dist/app.abc123.js' in out
+    assert 'type="module"' in out and 'defer' in out