fix: an empty REPL line no longer quits the chapter silently (#569)

Every chapter REPL treated an empty line as a quit command and exited with
status 0 without a word, while the banner only promises "Type q to quit".
Under VS Code's debugger the Python extension's environment-activation line
is often injected into stdin after the program started, so the first thing
the REPL read was a blank line and the chapter vanished right after its
banner.

An empty line is now ignored and the prompt comes back; q and exit still
quit, and EOF/Ctrl-C still end the loop. s17 already behaved this way. A
static test keeps every chapter's quit check free of the empty string.

Fixes #567

s01_agent_loop/code.py

@@ -138,7 +138,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s01 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         history.append({"role": "user", "content": query})
         agent_loop(history)

s02_tool_use/code.py

@@ -196,7 +196,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s02 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         history.append({"role": "user", "content": query})
         agent_loop(history)

s03_permission/code.py

@@ -257,7 +257,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s03 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         history.append({"role": "user", "content": query})
         agent_loop(history)

s04_hooks/code.py

@@ -269,7 +269,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s04 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s05_todo_write/code.py

@@ -357,7 +357,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s05 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s06_subagent/code.py

@@ -372,7 +372,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s06 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s07_skill_loading/code.py

@@ -388,7 +388,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s07 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s08_context_compact/code.py

@@ -591,7 +591,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s08 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s09_memory/code.py

@@ -773,7 +773,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s09 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s10_task_system/code.py

@@ -589,7 +589,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s10 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s11_background_tasks/code.py

@@ -531,7 +531,9 @@ if __name__ == "__main__":
             query = input("\001\033[36m\002s11 >> \001\033[0m\002")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s12_cron_scheduler/code.py

@@ -782,7 +782,9 @@ if __name__ == "__main__":
                 query = input("\001\033[36m\002s12 >> \001\033[0m\002")
             except (EOFError, KeyboardInterrupt):
                 break
-            if query.strip().lower() in ("q", "exit", ""):
+            if not query.strip():
+                continue
+            if query.strip().lower() in ("q", "exit"):
                 break
             with agent_lock:
                 run_agent_turn_locked(query)

s13_agent_teams/code.py

@@ -1862,7 +1862,11 @@ if __name__ == "__main__":
         if kind == "quit":
             break
         if kind == "user":
-            if payload is None or payload.strip().lower() in {"q", "exit", ""}:
+            if payload is None:
+                break
+            if not payload.strip():
+                continue
+            if payload.strip().lower() in {"q", "exit"}:
                 break
             trigger_hooks("UserPromptSubmit", payload)
             history.append({"role": "user", "content": payload})

s14_mcp_plugin/code.py

@@ -536,7 +536,9 @@ if __name__ == "__main__":
             query = input("s14 >> ")
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in {"q", "exit", ""}:
+        if not query.strip():
+            continue
+        if query.strip().lower() in {"q", "exit"}:
             break
         trigger_hooks("UserPromptSubmit", query)
         history.append({"role": "user", "content": query})

s15_integrated_harness/code.py

@@ -3313,7 +3313,9 @@ if __name__ == "__main__":
             query = CONSOLE.ask()
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         with agent_lock:
             trigger_hooks("UserPromptSubmit", query)

s16_workflow_runtime/code.py

@@ -868,7 +868,9 @@ def run_cli():
             query = host.CONSOLE.ask()
         except (EOFError, KeyboardInterrupt):
             break
-        if query.strip().lower() in ("q", "exit", ""):
+        if not query.strip():
+            continue
+        if query.strip().lower() in ("q", "exit"):
             break
         with host.agent_lock:
             host.trigger_hooks("UserPromptSubmit", query)

tests/test_repl_quit_commands.py

@@ -0,0 +1,57 @@
+"""Every chapter REPL quits only on an explicit command (issue #567).
+
+An empty line used to be a silent exit with status 0, which contradicted the
+"Type q to quit" banner and, combined with VS Code's environment-activation
+line being injected into stdin, made a debugged chapter vanish right after its
+banner. An empty line is now ignored and the prompt comes back.
+"""
+
+import ast
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+CHAPTER_FILES = sorted(ROOT.glob("s*/code.py"))
+
+
+def _quit_checks(source: str) -> list[ast.Compare]:
+    """Return every `X.strip().lower() in (...)` / `in {...}` comparison."""
+    found = []
+    for node in ast.walk(ast.parse(source)):
+        if not isinstance(node, ast.Compare) or len(node.ops) != 1 or not isinstance(node.ops[0], ast.In):
+            continue
+        left = node.left
+        if (
+            isinstance(left, ast.Call)
+            and isinstance(left.func, ast.Attribute)
+            and left.func.attr == "lower"
+            and isinstance(left.func.value, ast.Call)
+            and isinstance(left.func.value.func, ast.Attribute)
+            and left.func.value.func.attr == "strip"
+        ):
+            found.append(node)
+    return found
+
+
+def test_every_chapter_has_a_repl_quit_check():
+    assert len(CHAPTER_FILES) >= 17
+    for path in CHAPTER_FILES:
+        assert _quit_checks(path.read_text(encoding="utf-8")), f"{path}: no quit command check found"
+
+
+def test_empty_line_is_never_a_quit_command():
+    for path in CHAPTER_FILES:
+        for check in _quit_checks(path.read_text(encoding="utf-8")):
+            container = check.comparators[0]
+            assert isinstance(container, (ast.Tuple, ast.Set)), f"{path}:{check.lineno}"
+            values = [c.value for c in container.elts if isinstance(c, ast.Constant)]
+            assert "" not in values, f"{path}:{check.lineno}: empty input still quits the REPL"
+            assert "q" in values, f"{path}:{check.lineno}: q must still quit"
+
+
+def test_blank_input_is_skipped_before_the_quit_check():
+    pattern = re.compile(r"if not (\w+)\.strip\(\):\n\s+continue\n\s+if \1\.strip\(\)\.lower\(\) in", re.M)
+    for path in CHAPTER_FILES:
+        if path.parent.name.startswith("s17"):
+            continue  # s17 already handled blank input before this change
+        assert pattern.search(path.read_text(encoding="utf-8")), f"{path}: blank input is not skipped"