diff --git a/.env.template b/.env.template
index 71f1b969..3573637d 100644
--- a/.env.template
+++ b/.env.template
@@ -113,6 +113,7 @@ LLM_OPENAI_API_KEY=your-api-key-here
 # DERIVER_MODEL_CONFIG__MAX_OUTPUT_TOKENS=4096
 # DERIVER_LOG_OBSERVATIONS=false
 # DERIVER_MAX_INPUT_TOKENS=25000
+# Required for non-blank reasoning.custom_instructions; unset disables non-blank custom instructions
 # DERIVER_MAX_CUSTOM_INSTRUCTIONS_TOKENS=2000
 # DERIVER_WORKING_REPRESENTATION_MAX_OBSERVATIONS=100
 # DERIVER_REPRESENTATION_BATCH_MAX_TOKENS=1024
diff --git a/tests/test_config.py b/tests/test_config.py
index 6b38ba5b..f18efb4b 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -36,11 +36,10 @@ def test_effective_custom_instructions_tokens_uses_explicit_limit() -> None:
     assert settings.effective_max_custom_instructions_tokens == 2000
 
 
-def test_deriver_defaults_allow_larger_custom_instruction_budget() -> None:
-    settings = _make_deriver_settings(MAX_CUSTOM_INSTRUCTIONS_TOKENS=2000)
+def test_deriver_default_input_budget_accommodates_custom_instruction_cap() -> None:
+    settings = _make_deriver_settings()
 
     assert settings.MAX_INPUT_TOKENS == 25000
-    assert settings.effective_max_custom_instructions_tokens == 2000
 
 
 def test_custom_instructions_tokens_cannot_exceed_input_budget() -> None:
diff --git a/tests/utils/test_config_helpers.py b/tests/utils/test_config_helpers.py
index 7ad602f3..fb1551cc 100644
--- a/tests/utils/test_config_helpers.py
+++ b/tests/utils/test_config_helpers.py
@@ -23,9 +23,7 @@ def _session(configuration: dict[str, Any]) -> models.Session:
 def test_preserves_workspace_custom_instructions(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
-    monkeypatch.setattr(
-        settings.DERIVER, "MAX_CUSTOM_INSTRUCTIONS_TOKENS", 100, raising=False
-    )
+    monkeypatch.setattr(settings.DERIVER, "MAX_CUSTOM_INSTRUCTIONS_TOKENS", 100)
 
     workspace = _workspace(
         {
@@ -46,9 +44,7 @@ def test_preserves_workspace_custom_instructions(
 def test_message_custom_instructions_override_session_and_workspace(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
-    monkeypatch.setattr(
-        settings.DERIVER, "MAX_CUSTOM_INSTRUCTIONS_TOKENS", 100, raising=False
-    )
+    monkeypatch.setattr(settings.DERIVER, "MAX_CUSTOM_INSTRUCTIONS_TOKENS", 100)
 
     workspace = _workspace(
         {