diff --git a/headroom/proxy/helpers.py b/headroom/proxy/helpers.py
index ce3c93a8b..94d0768f3 100644
--- a/headroom/proxy/helpers.py
+++ b/headroom/proxy/helpers.py
@@ -26,6 +26,7 @@ from headroom import paths as _paths
 from headroom._subprocess import run
 from headroom.proxy import (
     diagnostic_decode_policy,
+    memory_injection_mode_policy,
     query_log_policy,
     request_limit_policy,
     sse_byte_buffer_policy,
@@ -232,9 +233,9 @@ def capture_codex_wire_debug(
 # Configurable via HEADROOM_MEMORY_INJECTION_MODE env var. There is no
 # "system_prompt" option — that path is permanently retired by I2 (cache hot
 # zone never modified). See REALIGNMENT/02-architecture.md §2.2.
-_MEMORY_INJECTION_MODE_ENV = "HEADROOM_MEMORY_INJECTION_MODE"
-_MEMORY_INJECTION_MODE_DEFAULT: Literal["live_zone_tail", "disabled"] = "live_zone_tail"
-MemoryInjectionMode = Literal["live_zone_tail", "disabled"]
+_MEMORY_INJECTION_MODE_ENV = memory_injection_mode_policy.MEMORY_INJECTION_MODE_ENV
+_MEMORY_INJECTION_MODE_DEFAULT = memory_injection_mode_policy.MEMORY_INJECTION_MODE_DEFAULT
+MemoryInjectionMode = memory_injection_mode_policy.MemoryInjectionMode
 
 
 def get_memory_injection_mode() -> MemoryInjectionMode:
@@ -243,13 +244,8 @@ def get_memory_injection_mode() -> MemoryInjectionMode:
     Read at request time so the env var can be flipped without restart for
     smoke tests. Unknown values are rejected loudly (no silent fallback).
     """
-    raw = os.environ.get(_MEMORY_INJECTION_MODE_ENV, "").strip().lower()
-    if not raw:
-        return _MEMORY_INJECTION_MODE_DEFAULT
-    if raw in ("live_zone_tail", "disabled"):
-        return cast(MemoryInjectionMode, raw)
-    raise ValueError(
-        f"Invalid {_MEMORY_INJECTION_MODE_ENV}={raw!r}; expected 'live_zone_tail' or 'disabled'"
+    return memory_injection_mode_policy.resolve_memory_injection_mode(
+        os.environ.get(_MEMORY_INJECTION_MODE_ENV)
     )
 
 
diff --git a/headroom/proxy/memory_injection_mode_policy.py b/headroom/proxy/memory_injection_mode_policy.py
new file mode 100644
index 000000000..016312198
--- /dev/null
+++ b/headroom/proxy/memory_injection_mode_policy.py
@@ -0,0 +1,22 @@
+"""Memory-injection mode resolution policy."""
+
+from __future__ import annotations
+
+from typing import Literal, cast
+
+MEMORY_INJECTION_MODE_ENV = "HEADROOM_MEMORY_INJECTION_MODE"
+MEMORY_INJECTION_MODE_DEFAULT: Literal["live_zone_tail", "disabled"] = "live_zone_tail"
+MemoryInjectionMode = Literal["live_zone_tail", "disabled"]
+
+
+def resolve_memory_injection_mode(raw: str | None) -> MemoryInjectionMode:
+    """Resolve the active memory-injection routing mode from an optional value."""
+    normalized = (raw or "").strip().lower()
+    if not normalized:
+        return MEMORY_INJECTION_MODE_DEFAULT
+    if normalized in ("live_zone_tail", "disabled"):
+        return cast(MemoryInjectionMode, normalized)
+    raise ValueError(
+        f"Invalid {MEMORY_INJECTION_MODE_ENV}={normalized!r}; "
+        "expected 'live_zone_tail' or 'disabled'"
+    )
diff --git a/tests/test_memory_injection_mode_policy.py b/tests/test_memory_injection_mode_policy.py
new file mode 100644
index 000000000..a0d5661b6
--- /dev/null
+++ b/tests/test_memory_injection_mode_policy.py
@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+import pytest
+
+from headroom.proxy.memory_injection_mode_policy import (
+    MEMORY_INJECTION_MODE_DEFAULT,
+    resolve_memory_injection_mode,
+)
+
+
+def test_resolve_memory_injection_mode_defaults_for_missing_value() -> None:
+    assert resolve_memory_injection_mode(None) == MEMORY_INJECTION_MODE_DEFAULT
+    assert resolve_memory_injection_mode("") == MEMORY_INJECTION_MODE_DEFAULT
+    assert resolve_memory_injection_mode("   ") == MEMORY_INJECTION_MODE_DEFAULT
+
+
+def test_resolve_memory_injection_mode_accepts_known_values_case_insensitively() -> None:
+    assert resolve_memory_injection_mode("live_zone_tail") == "live_zone_tail"
+    assert resolve_memory_injection_mode(" DISABLED ") == "disabled"
+
+
+def test_resolve_memory_injection_mode_rejects_unknown_values() -> None:
+    with pytest.raises(ValueError, match="HEADROOM_MEMORY_INJECTION_MODE"):
+        resolve_memory_injection_mode("system_prompt")