diff --git a/aai_cli/app/context.py b/aai_cli/app/context.py
index 9b8cb6dc..6508c538 100644
--- a/aai_cli/app/context.py
+++ b/aai_cli/app/context.py
@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-import sys
 from collections.abc import Callable
 from dataclasses import dataclass
 from typing import NoReturn, Protocol
@@ -8,7 +7,7 @@
 import keyring.errors
 import typer
 
-from aai_cli.core import config, debuglog, env, environments, telemetry
+from aai_cli.core import config, debuglog, env, environments, stdio, telemetry
 from aai_cli.core.environments import Environment
 from aai_cli.core.errors import APIError, CLIError, NotAuthenticated
 from aai_cli.ui import output, update_check
@@ -143,7 +142,7 @@ def _fail(err: CLIError, *, json_mode: bool) -> NoReturn:
 def _interactive_session() -> bool:
     """True only when a human can complete a browser login: stdin and stderr are both
     real TTYs and no agent/CI context is detected (`output.is_agentic`)."""
-    return sys.stdin.isatty() and sys.stderr.isatty() and not output.is_agentic()
+    return stdio.stdin_is_tty() and stdio.stderr_is_tty() and not output.is_agentic()
 
 
 def _should_auto_login(err: NotAuthenticated) -> bool:
diff --git a/aai_cli/commands/login.py b/aai_cli/commands/login.py
index 21fca3b6..e5e61f33 100644
--- a/aai_cli/commands/login.py
+++ b/aai_cli/commands/login.py
@@ -1,14 +1,12 @@
 from __future__ import annotations
 
-import sys
-
 import typer
 from rich.markup import escape
 from rich.table import Table
 
 from aai_cli import command_registry, help_panels, options
 from aai_cli.app.context import AppState, persist_browser_login, run_command
-from aai_cli.core import client, config, environments
+from aai_cli.core import client, config, environments, stdio
 from aai_cli.core.errors import STDIN_KEY_RECIPE, APIError, CLIError, UsageError, mutually_exclusive
 from aai_cli.ui import output
 from aai_cli.ui.help_text import examples_epilog
@@ -28,13 +26,15 @@ def _read_stdin_key() -> str:
     Stdin-only on purpose (the Codex-CLI pattern): a key passed as an argv value
     lands in shell history and ``ps`` output; a piped key does not.
     """
-    if sys.stdin.isatty():
+    if not stdio.stdin_is_piped():
         raise UsageError(
             "--with-api-key reads the key from stdin, but stdin is a terminal.",
             suggestion=f"Pipe the key in: {STDIN_KEY_RECIPE}",
         )
-    key = sys.stdin.read().strip()
-    if not key:
+    # stdin is piped, so a None here means an empty/blank pipe (not a terminal) —
+    # distinct from the terminal case above, so the recipe hint stays accurate.
+    key = stdio.piped_stdin_text()
+    if key is None:
         raise UsageError(
             "--with-api-key found no key on stdin.",
             suggestion=(
@@ -42,7 +42,7 @@ def _read_stdin_key() -> str:
                 "(check that the variable you piped is set)."
             ),
         )
-    return key
+    return key.strip()
 
 
 @app.command(
diff --git a/aai_cli/core/stdio.py b/aai_cli/core/stdio.py
index 48268f01..05db2ef3 100644
--- a/aai_cli/core/stdio.py
+++ b/aai_cli/core/stdio.py
@@ -24,13 +24,31 @@ def silence_stdout() -> None:
             os.close(devnull_fd)
 
 
+def stdin_is_tty() -> bool:
+    """True when stdin is an interactive terminal. The single raw-``isatty`` chokepoint
+    for stdin, so higher layers compose this rather than re-reaching for ``sys.stdin``."""
+    return sys.stdin.isatty()
+
+
+def stdout_is_tty() -> bool:
+    """True when stdout is an interactive terminal. The single raw-``isatty`` chokepoint
+    for stdout (e.g. `output.is_agentic`/`print_code` compose it)."""
+    return sys.stdout.isatty()
+
+
+def stderr_is_tty() -> bool:
+    """True when stderr is an interactive terminal. The single raw-``isatty`` chokepoint
+    for stderr (the browser-login interactivity probe composes it)."""
+    return sys.stderr.isatty()
+
+
 def interactive_stdio() -> bool:
     """True only when stdin and stdout are both real TTYs — i.e. a human can answer
     a prompt and see it. The shared "may we prompt here?" predicate for the bare-`assembly`
     setup offer, the onboarding prompter, and the `assembly init` template picker, so the
     three can't drift on what counts as interactive.
     """
-    return sys.stdin.isatty() and sys.stdout.isatty()
+    return stdin_is_tty() and stdout_is_tty()
 
 
 def stdin_is_piped() -> bool:
diff --git a/aai_cli/ui/output.py b/aai_cli/ui/output.py
index 2c3d68cd..2c67e37b 100644
--- a/aai_cli/ui/output.py
+++ b/aai_cli/ui/output.py
@@ -12,7 +12,7 @@
 from rich.text import Text
 
 from aai_cli import __version__
-from aai_cli.core import choices, env, jsonshape
+from aai_cli.core import choices, env, jsonshape, stdio
 from aai_cli.ui import theme
 
 if TYPE_CHECKING:
@@ -27,7 +27,7 @@
 
 
 def _stdout_is_tty() -> bool:
-    return sys.stdout.isatty()
+    return stdio.stdout_is_tty()
 
 
 def is_agentic() -> bool:
diff --git a/tests/test_output.py b/tests/test_output.py
index 641045e8..0b014702 100644
--- a/tests/test_output.py
+++ b/tests/test_output.py
@@ -5,6 +5,23 @@
 from aai_cli.ui import output
 
 
+class _StdoutProbe:
+    def __init__(self, tty):
+        self._tty = tty
+
+    def isatty(self):
+        return self._tty
+
+
+def test_stdout_is_tty_seam_reflects_real_stdout(monkeypatch):
+    # The seam every other test patches delegates to the stdio chokepoint; exercise
+    # the real body so the delegation is covered and a negated isatty mutant dies.
+    monkeypatch.setattr("sys.stdout", _StdoutProbe(True))
+    assert output._stdout_is_tty() is True
+    monkeypatch.setattr("sys.stdout", _StdoutProbe(False))
+    assert output._stdout_is_tty() is False
+
+
 def test_resolve_json_true_only_when_explicit():
     # JSON is opt-in: the flag is the single source of truth.
     assert output.resolve_json(explicit=True) is True
diff --git a/tests/test_stdio.py b/tests/test_stdio.py
index e1285f7f..9b0e2fc0 100644
--- a/tests/test_stdio.py
+++ b/tests/test_stdio.py
@@ -30,6 +30,39 @@ def test_stdin_is_piped(monkeypatch):
     assert stdio.stdin_is_piped() is False
 
 
+def test_stdin_is_tty(monkeypatch):
+    monkeypatch.setattr("sys.stdin", _Tty(""))
+    assert stdio.stdin_is_tty() is True
+    monkeypatch.setattr("sys.stdin", _Pipe(""))
+    assert stdio.stdin_is_tty() is False
+
+
+def test_stdout_is_tty(monkeypatch):
+    monkeypatch.setattr("sys.stdout", _Tty(""))
+    assert stdio.stdout_is_tty() is True
+    monkeypatch.setattr("sys.stdout", _Pipe(""))
+    assert stdio.stdout_is_tty() is False
+
+
+def test_stderr_is_tty(monkeypatch):
+    monkeypatch.setattr("sys.stderr", _Tty(""))
+    assert stdio.stderr_is_tty() is True
+    monkeypatch.setattr("sys.stderr", _Pipe(""))
+    assert stdio.stderr_is_tty() is False
+
+
+def test_interactive_stdio_requires_both_stdin_and_stdout_tty(monkeypatch):
+    # Both must be terminals; either one piped flips it false (kills the and->or mutant).
+    monkeypatch.setattr("sys.stdin", _Tty(""))
+    monkeypatch.setattr("sys.stdout", _Tty(""))
+    assert stdio.interactive_stdio() is True
+    monkeypatch.setattr("sys.stdout", _Pipe(""))
+    assert stdio.interactive_stdio() is False
+    monkeypatch.setattr("sys.stdin", _Pipe(""))
+    monkeypatch.setattr("sys.stdout", _Tty(""))
+    assert stdio.interactive_stdio() is False
+
+
 def test_piped_stdin_text_returns_none_on_tty(monkeypatch):
     monkeypatch.setattr("sys.stdin", _Tty("ignored\n"))
     assert stdio.piped_stdin_text() is None