diff --git a/tests/AGENTS.md b/tests/AGENTS.md
index ba88180a..0d28760a 100644
--- a/tests/AGENTS.md
+++ b/tests/AGENTS.md
@@ -40,6 +40,14 @@ Lessons that cost iterations getting the patch-coverage and mutation tail gates
 - **Help text and docstrings are pinned by the syrupy snapshots, not unit asserts** — a
   mutated help string is killed by the regenerated `.ambr`, so `--snapshot-update` and commit
   rather than adding redundant `--help` substring asserts.
+- **Render width is pinned suite-wide (`COLUMNS=80`), so a `--help` substring assert is
+  deterministic** — the autouse `fixed_render_size` fixture (`conftest.py`) sets `COLUMNS`/`LINES`
+  for *every* test, because the no-clip help table ellipsizes a long flag name
+  (`--end-of-turn-c…`) once its column overflows. Without the pin a `--with-api-key`-style
+  substring assert passes at a wide local terminal and fails at CI's narrower width — that gap
+  cost a PR three CI rounds. Don't fight it: a local green is now a CI green for output tests.
+  A test that genuinely needs a different width passes it on the call
+  (`runner.invoke(app, argv, env={"COLUMNS": "300"})`), which overrides the default.
 - **Typer's `CliRunner` merges stderr into `result.output`, and not in call order**, so don't
   assume `splitlines()[-1]` is the command payload. In `--json` mode the env-mismatch warning
   is its own `{"warning": …}` line, so filter parsed lines by a key the payload carries
diff --git a/tests/conftest.py b/tests/conftest.py
index 39a33c8c..f115411b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -87,12 +87,23 @@ def pin_timezone(monkeypatch):
         time.tzset()
 
 
-@pytest.fixture
+@pytest.fixture(autouse=True)
 def fixed_render_size(monkeypatch):
-    # Pin the render width/height so the CLI-surface goldens (the
-    # test_snapshots_* modules) are byte-identical across machines and CI.
-    # Named (not autouse): only the snapshot modules opt in via
-    # `pytestmark = pytest.mark.usefixtures("fixed_render_size")`.
+    # Pin the render width/height for the *whole* suite so anything that renders
+    # CLI/help output is byte-identical across machines and CI. Rich/Click derive
+    # the help width from COLUMNS (falling back to the inherited terminal size),
+    # and the no-clip table ellipsizes long flag names ("--end-of-turn-c…") once
+    # the column overflows — so a `--help` substring assertion that passes at a
+    # contributor's wide local terminal can silently fail at CI's narrower width.
+    # That exact gap (a hidden `--with-api-key` flag asserted present, clipped away
+    # in CI) burned three CI rounds on one PR; autouse closes it so a local green
+    # means a CI green.
+    #
+    # Autouse, not opt-in: every test gets the pinned size. A test that needs a
+    # different width passes it explicitly to the invocation (the canonical pattern
+    # in test_help_rendering.py: `runner.invoke(app, argv, env={"COLUMNS": "80"})`),
+    # which CliRunner merges over this default. The snapshot modules still carry an
+    # explicit `usefixtures("fixed_render_size")` marker to document the dependency.
     monkeypatch.setenv("COLUMNS", "80")
     monkeypatch.setenv("LINES", "40")