Studio: r14 fixes - longer CommonMark closing fence, explicit-plan header standalone, use-python tool wording

- _HAS_ANSWER_ARTIFACT closing fence now accepts strictly more delimiters
  than the opener (CommonMark rule). The opener stays anchored on both
  sides so a 4-open / 3-close payload still does not match, but a
  legitimate 3-open / 4-close (and 3-tilde / 4-tilde) answer is now
  recognised as a completed artifact.

- _EXPLICIT_PLAN_HEADER triggers the plan classification by itself when
  the response contains \"Here's my plan\" / \"Here's my approach\" /
  \"Here's the plan\". Numbered stalls like \"Here's my plan:\n1. Analyze\n
  2. Draft\" re-prompt again without needing a freshness-gated verb.
  Plain \"Plan:\" / \"My weekly plan:\" stay valid answers because they
  lack the possessive first-person header.

- _TOOL_ACTION_VERBS adds \"use python (tool) to ...\", \"use the python
  tool\", \"invoke the python tool\", and \"use the search tool\" so
  numbered plans that route through these phrasings still re-prompt.

studio/backend/core/inference/llama_cpp.py

@@ -74,6 +74,8 @@ _TOOL_ACTION_VERBS = (
     rf"{_TOOL_LOOKUP_TARGET}|"
     r"(?:research|investigate|find|check|verify) (?:for )?(?:the |a |an )?"
     rf"{_TOOL_LOOKUP_TARGET}|"
+    r"(?:use|invoke|call) (?:the )?(?:python|search) tool|"
+    r"use python(?: tool)? to|"
     r"call (?:a |the )?tool|run (?:python|the code)|execute (?:python|the code)"
 )
 
@@ -122,11 +124,11 @@ _HAS_ANSWER_ARTIFACT = re.compile(
     # closing fence must have at least as many delimiters, and the line
     # must end cleanly (only trailing whitespace before newline / EOS),
     # so spam like ``` ```not actually closed ``` does not count.
-    r"(?<!`)(?P<bf>`{3,})(?!`)[^\r\n]{0,200}\r?\n[\s\S]{1,4000}?\r?\n[ \t]*(?P=bf)(?!`)[ \t]*(?:\r?\n|\Z)"
+    r"(?<!`)(?P<bf>`{3,})(?!`)[^\r\n]{0,200}\r?\n[\s\S]{1,4000}?\r?\n[ \t]*(?P=bf)`*[ \t]*(?:\r?\n|\Z)"
     # Closed tilde code fence; same 3+ rule (several models emit ~~~ when
-    # the body itself contains backticks). Anchored to the full run of
-    # tildes on both sides so a 4-tilde open cannot match a 3-tilde close.
-    r"|(?<!~)(?P<tf>~{3,})(?!~)[^\r\n]{0,200}\r?\n[\s\S]{1,4000}?\r?\n[ \t]*(?P=tf)(?!~)[ \t]*(?:\r?\n|\Z)"
+    # the body itself contains backticks). Opener anchored to the full
+    # run of tildes; closer accepts >= opener length per CommonMark.
+    r"|(?<!~)(?P<tf>~{3,})(?!~)[^\r\n]{0,200}\r?\n[\s\S]{1,4000}?\r?\n[ \t]*(?P=tf)~*[ \t]*(?:\r?\n|\Z)"
     # Complete HTML page; doctype prefix is optional.
     r"|(?:<!doctype\b[\s\S]{0,200}?)?<html\b[\s\S]{0,4000}?</html>"
     # Complete SVG document.
@@ -158,6 +160,16 @@ _PLAN_LIST_FRAMING = re.compile(
     re.IGNORECASE,
 )
 
+# "Here's my plan" / "Here's my approach" are strong stand-alone plan
+# signals: a possessive, first-person framing where the model is
+# announcing what it WILL do. Treat the following numbered list as a
+# plan regardless of the specific verbs each item uses, so stalls like
+# ``Here's my plan: 1. Analyze 2. Draft`` still re-prompt.
+_EXPLICIT_PLAN_HEADER = re.compile(
+    r"\bhere['’]?s (?:my |the |a )?(?:plan|approach)\b",
+    re.IGNORECASE,
+)
+
 
 _FENCE_LINE_RE = re.compile(r"^[ \t]*(?P<fence>`{3,}|~{3,})(?P<trailing>[^\r\n]*)$")
 
@@ -194,14 +206,19 @@ def _has_answer_artifact(text: str) -> bool:
     Code fences, complete HTML, and complete SVG count directly. A
     numbered list counts only when there is no plan framing, so stalls
     like ``Here's my plan:\\n1. search\\n2. summarise`` still re-prompt.
-    An unclosed fence disqualifies the numbered-list fallback so a list
-    INSIDE incomplete code does not look like a final answer.
+    An explicit ``Here's my plan`` / ``Here's my approach`` header is
+    also enough to flag the list as a plan, even when no narrow tool-
+    action verb appears in the items. An unclosed fence disqualifies
+    the numbered-list fallback so a list INSIDE incomplete code does
+    not look like a final answer.
     """
     if _HAS_ANSWER_ARTIFACT.search(text):
         return True
     if _has_unclosed_code_fence(text):
         return False
     if _NUMBERED_LIST_ARTIFACT.search(text):
+        if _EXPLICIT_PLAN_HEADER.search(text):
+            return False
         return _PLAN_LIST_FRAMING.search(text) is None
     return False
 

studio/backend/tests/test_llama_cpp_reprompt_guard.py

@@ -749,6 +749,69 @@ def test_artifact_regex_rejects_shorter_commonmark_closing_fence():
         assert _would_reprompt(content), content
 
 
+def test_artifact_regex_accepts_longer_commonmark_closing_fence():
+    """CommonMark allows the closing fence to have MORE delimiters than
+    the opener. A 3-backtick opener with a 4-backtick close, or a
+    3-tilde opener with a 4-tilde close, is still a complete artifact."""
+    samples = [
+        "First, let me show.\n```python\nprint('hi')\n````",
+        "First, let me show.\n````python\nprint('``` inside')\n`````",
+        "First, let me show.\n~~~python\nprint('hi')\n~~~~",
+    ]
+    for content in samples:
+        assert _has_answer_artifact(content), content
+        assert not _would_reprompt(content), content
+
+
+def test_reprompts_on_explicit_plan_header_numbered_list():
+    """``Here's my plan`` / ``Here's my approach`` is a strong stand-alone
+    plan signal. The following numbered list is the plan itself, not a
+    final answer, even when no narrow tool-action verb appears."""
+    samples = [
+        "Here's my plan:\n1. Analyze the request.\n2. Draft the answer.",
+        "Here's my plan:\n1. Create the Python file.\n2. Add the game loop.\n3. Test.",
+        "Here's my approach:\n1. Outline.\n2. Write.\n3. Review.",
+        "Here's the plan:\n1. Define the variables.\n2. Return the result.",
+    ]
+    for s in samples:
+        assert _would_reprompt(s), s
+
+
+def test_reprompts_on_numbered_plan_with_python_tool_wording():
+    """``use python (tool) to ...`` / ``use the python tool`` / ``use the
+    search tool`` in a numbered plan still re-prompts."""
+    samples = [
+        "Here's my plan:\n1. Use Python to calculate the answer.\n2. Return.",
+        "First, I'll do this:\n1. Use the python tool to parse the file.\n2. Summarize.",
+        "Here's my plan:\n1. Use the search tool.\n2. Summarize.",
+    ]
+    for s in samples:
+        assert _would_reprompt(s), s
+
+
+def test_no_reprompt_on_lesson_plan_answer_without_explicit_header():
+    """A final answer with a ``Plan:`` heading (no ``Here's my``
+    possessive) and no tool framing must STILL count as an answer.
+    Common cases: lesson plan, workout plan, meal plan."""
+    samples = [
+        (
+            "Plan:\n"
+            "1. Warm up for 5 minutes.\n"
+            "2. Run for 20 minutes.\n"
+            "3. Cool down with stretching."
+        ),
+        (
+            "My weekly plan:\n"
+            "1. Monday: rest.\n"
+            "2. Tuesday: jog.\n"
+            "3. Wednesday: swim."
+        ),
+    ]
+    for content in samples:
+        assert _has_answer_artifact(content), content
+        assert not _would_reprompt(content), content
+
+
 def test_open_fence_with_inner_numbered_list_still_reprompts():
     """A response that opens a code fence and emits numbered lines INSIDE
     must NOT count those lines as a completed numbered-list answer."""