diff --git a/tests/python/test_construct_chat_template_validation.py b/tests/python/test_construct_chat_template_validation.py
new file mode 100644
index 0000000000..9ab68639c4
--- /dev/null
+++ b/tests/python/test_construct_chat_template_validation.py
@@ -0,0 +1,77 @@
+"""Negative-path validation tests for unsloth.chat_templates.construct_chat_template.
+
+Regression coverage for the str.find() / regex no-match guards added in
+PR #5763 follow-up: missing placeholders or unrecoverable two-example
+structures must raise RuntimeError with a clear message, not IndexError
+or AttributeError, and must never silently drop the last character via
+s[:-1].
+
+Uses a minimal fake tokenizer so the cases run on CPU-only CI without
+HF_TOKEN and without downloading a gated model. The validation paths
+exercised here fail before construct_chat_template reaches any heavy
+tokenizer interaction, so the stub stays small.
+"""
+
+import pytest
+
+from unsloth.chat_templates import construct_chat_template
+
+
+class _FakeTokenizer:
+    """Minimum surface construct_chat_template touches before the
+    validation guards fire."""
+
+    name_or_path = "fake/tokenizer"
+    eos_token = "</s>"
+
+    def get_vocab(self):
+        return {"</s>": 0}
+
+
+@pytest.mark.parametrize(
+    "template, expected_in_message",
+    [
+        ("only {INPUT} here, no output marker", "{OUTPUT}"),
+        ("only {OUTPUT} here, no input marker", "{INPUT}"),
+        ("neither sentinel here, just literal text", "{INPUT}"),
+        ("neither sentinel here, just literal text", "{OUTPUT}"),
+    ],
+)
+def test_missing_placeholder_in_chat_template_raises(template, expected_in_message):
+    with pytest.raises(RuntimeError) as exc_info:
+        construct_chat_template(
+            tokenizer = _FakeTokenizer(),
+            chat_template = template,
+            extra_eos_tokens = ["</s>"],
+        )
+    assert expected_in_message in str(exc_info.value)
+
+
+def test_single_pair_template_raises_clear_error_not_attribute_error():
+    """One {INPUT}/{OUTPUT} pair (rather than the required two) used to
+    crash with AttributeError on `found.group(1)` after the for-loop
+    broke without setting `found`. Must raise RuntimeError now."""
+    template = "user: {INPUT}\nassistant: {OUTPUT}\n"
+    with pytest.raises(RuntimeError):
+        construct_chat_template(
+            tokenizer = _FakeTokenizer(),
+            chat_template = template,
+            extra_eos_tokens = ["</s>"],
+        )
+
+
+def test_error_message_excerpt_is_bounded():
+    """Error messages must include a bounded excerpt of the offending
+    template, not dump arbitrarily large content into the traceback."""
+    huge = ("garbage " * 5000) + "{INPUT}"  # ~40 KB, missing {OUTPUT}
+    with pytest.raises(RuntimeError) as exc_info:
+        construct_chat_template(
+            tokenizer = _FakeTokenizer(),
+            chat_template = huge,
+            extra_eos_tokens = ["</s>"],
+        )
+    msg = str(exc_info.value)
+    # Excerpt is repr-quoted and capped; total message should stay well
+    # under the template length.
+    assert len(msg) < 1000
+    assert "{OUTPUT}" in msg
diff --git a/unsloth/chat_templates.py b/unsloth/chat_templates.py
index e8a34cbc60..956fcb2392 100644
--- a/unsloth/chat_templates.py
+++ b/unsloth/chat_templates.py
@@ -2461,17 +2461,40 @@ extra_eos_tokens = None,
                 f"{left_changed}"
             )
     except:
-        ending = chat_template[chat_template.find("{OUTPUT}") + len("{OUTPUT}"):]
+        output_pos = chat_template.find("{OUTPUT}")
+        input_pos  = chat_template.find("{INPUT}")
+        if output_pos == -1 or input_pos == -1:
+            missing = []
+            if input_pos  == -1: missing.append("{INPUT}")
+            if output_pos == -1: missing.append("{OUTPUT}")
+            raise RuntimeError(
+                f"Unsloth: chat_template must contain {' and '.join(missing)} "
+                f"placeholder(s). Got: {chat_template[:200]!r}"
+            )
+        ending = chat_template[output_pos + len("{OUTPUT}"):]
 
         ending = re.escape(ending)
         find_text = "{INPUT}" + ending + "(.+?{OUTPUT}" + ending + ")"
         response_part = re.findall(find_text, chat_template, flags = re.DOTALL | re.MULTILINE)
+        if len(response_part) == 0:
+            raise RuntimeError(
+                "Unsloth: Could not recover a two-example structure from chat_template. "
+                "Provide exactly two {INPUT}/{OUTPUT} pairs (and optionally {SYSTEM}). "
+                f"Got: {chat_template[:200]!r}"
+            )
         response_part = response_part[0]
 
+        found = None
         for j in range(1, len(response_part)):
             try_find = re.escape(response_part[:j])
             try: found = next(re.finditer("(" + try_find + ").+?\\{INPUT\\}", chat_template, flags = re.DOTALL | re.MULTILINE))
             except: break
+        if found is None:
+            raise RuntimeError(
+                "Unsloth: Could not locate a separator between examples in chat_template. "
+                "Provide exactly two {INPUT}/{OUTPUT} pairs (and optionally {SYSTEM}). "
+                f"Got: {chat_template[:200]!r}"
+            )
         separator = found.group(1)
 
         response_start = chat_template.find(response_part)
@@ -2607,8 +2630,20 @@ extra_eos_tokens = None,
             jinja_template = "{{ bos_token }}" + jinja_template
 
     # Get instruction and output parts for train_on_inputs = False
-    input_part  = input_part [:input_part .find("{INPUT}")]
-    output_part = output_part[:output_part.find("{OUTPUT}")]
+    input_idx  = input_part .find("{INPUT}")
+    output_idx = output_part.find("{OUTPUT}")
+    if input_idx == -1:
+        raise RuntimeError(
+            f"Unsloth: The instruction section of the template must contain the "
+            f"'{{INPUT}}' placeholder. Section: {input_part[:200]!r}"
+        )
+    if output_idx == -1:
+        raise RuntimeError(
+            f"Unsloth: The response section of the template must contain the "
+            f"'{{OUTPUT}}' placeholder. Section: {output_part[:200]!r}"
+        )
+    input_part  = input_part [:input_idx ]
+    output_part = output_part[:output_idx]
     return modelfile, jinja_template, input_part, output_part