diff --git a/docs/architecture/06-agent-core.md b/docs/architecture/06-agent-core.md
index ba4c2eeb8..94a4987b9 100644
--- a/docs/architecture/06-agent-core.md
+++ b/docs/architecture/06-agent-core.md
@@ -319,6 +319,12 @@ engine, not an Agent Run. `LLMClient` dispatches sync and async calls through
 `llm_claudexor.py` before OpenAI-compatible filtering/retries. Ouroboros retains
 its SYSTEM, BIBLE, canonical messages, tool selection and execution. The raw-model adapter is Codex; connected Claude/Cursor and other harnesses keep
 their existing Agent capabilities. Direct API keys keep their existing routes.
+Every main-loop execution of one install sends the same Codex cache key
+(`llm_claudexor.cache_key_for_model`: per data root and model, projected to
+`prompt_cache_key` and the `session_id` header), so a new task, child or
+consciousness cycle is served the governance prefix its predecessors already
+cached on its first round; per-execution turn states ride `nativeContinuation`
+unchanged under that shared session.
 
 Before creating a model operation, the host discovers the existing operation
 catalog's `captureFailureEvidence=true` query and freezes that choice for the
diff --git a/docs/development/06-rules-by-change-class.md b/docs/development/06-rules-by-change-class.md
index 5f7139877..e8dc31dfe 100644
--- a/docs/development/06-rules-by-change-class.md
+++ b/docs/development/06-rules-by-change-class.md
@@ -1042,7 +1042,13 @@ by "Provider Independence" above. Call-site imperatives:
   direct-Anthropic lane by `_anthropic_blocks_from_content` and on
   OpenRouter by `supports_message_cache_control`, and pinned by
   `tests/test_review_prompt_caching.py`. The main loop declares an
-  execution-scoped cache affinity only for subscription transport; API-compatible
+  install-scoped cache affinity for the subscription transport
+  (`llm_claudexor.cache_key_for_model`: one Codex `prompt_cache_key`, and so one
+  `session_id`, per data root and model, shared by every task, child and
+  consciousness cycle), because the Codex backend reuses a cached prefix across
+  conversations only under the same session and per-conversation turn states stay
+  valid under a shared one (measured 2026-09-17); a per-execution key paid the
+  shared governance prefix cold on every task start. API-compatible
   lanes retain their prefix-derived session identity. A consciousness wake-up is one
   of those main-loop executions: its schema array and cached system prefix are
   byte-identical to an owner turn's, so everything the level or the wake reason
diff --git a/ouroboros/llm_claudexor.py b/ouroboros/llm_claudexor.py
index 21e8cb0d2..3d9891407 100644
--- a/ouroboros/llm_claudexor.py
+++ b/ouroboros/llm_claudexor.py
@@ -42,7 +42,9 @@ import asyncio
 import copy
 import contextvars
 from dataclasses import replace
+import hashlib
 import json
+import re
 import logging
 import threading
 import time
@@ -301,6 +303,27 @@ def _remember_failed_profile(target: dict, parameters: dict, error: ClaudexorMod
         _FAILED_PROFILE.set((*key, route["credentialProfileId"]))
 
 
+def cache_key_for_model(model: str) -> str:
+    """The Codex prompt-cache key every main-loop execution of this install shares.
+
+    The Codex backend reuses a cached prefix across conversations only when both
+    ``prompt_cache_key`` and the ``session_id`` header match (the adapter sets
+    both from ``cacheKey``), and per-conversation turn states stay valid under a
+    shared session (measured 2026-09-17). One key per data root and model
+    therefore lets a new task, child or consciousness cycle be served the
+    governance prefix it shares with its predecessors on its very first round,
+    instead of paying it cold under a per-execution key. Empty for every other
+    provider: API-compatible lanes keep their prefix-derived session identity.
+    """
+    from ouroboros.provider_models import provider_for_model
+
+    if provider_for_model(model) != "claudexor":
+        return ""
+    label = re.sub(r"[^A-Za-z0-9._-]+", "-", str(model).rsplit("=", 1)[-1]).strip("-")[:40] or "model"
+    digest = hashlib.sha256(f"{config.DATA_DIR}\0{model}".encode("utf-8")).hexdigest()[:16]
+    return f"ouroboros-{label}-{digest}"
+
+
 def _request(target: dict, messages: list, tools: list | None, parameters: dict) -> dict:
     from ouroboros.llm_messages import _MessageShapingMixin
 
diff --git a/ouroboros/loop_llm_call.py b/ouroboros/loop_llm_call.py
index 682cab52d..79513e5e6 100644
--- a/ouroboros/loop_llm_call.py
+++ b/ouroboros/loop_llm_call.py
@@ -29,7 +29,7 @@ from ouroboros.deadline_utils import (
     main_transport_timeout_sec as _main_transport_timeout,
 )
 from ouroboros.llm import LLMClient, LocalContextTooLargeError, add_usage
-from ouroboros.llm_claudexor import propagate_model_error
+from ouroboros.llm_claudexor import cache_key_for_model, propagate_model_error
 from ouroboros.model_wait import propagate_model_control
 from ouroboros.openai_chat_dispatch import CUSTOM_RECEIPTS_USAGE_KEY, pop_custom_validation_receipts
 from ouroboros.llm_attempt import PROVIDER_POLICY_REFUSAL, _is_provider_policy_refusal  # typed-refusal contract owner
@@ -1382,7 +1382,7 @@ def call_llm_with_retry(
                 "max_tokens": MAIN_LOOP_MAX_TOKENS,
                 "stream": True, "caller_deadline_ts": (None if deadline_ts is None
                     else float(deadline_ts) - float(transport_reserve_sec or 0.0)),
-                "use_local": use_local, "cache_affinity": execution_id if provider_for_model(model) == "claudexor" else "",
+                "use_local": use_local, "cache_affinity": cache_key_for_model(model),
                 "allow_server_web_search": bool(allow_server_web_search) and provider_for_model(model) != "claudexor",
                 "bypass_response_cache": response_cache_bypass_requested and provider_for_model(model) != "claudexor",
                 "timeout": _main_transport_timeout(model, deadline_ts, reserve_sec=transport_reserve_sec),
diff --git a/ouroboros/loop_model_call.py b/ouroboros/loop_model_call.py
index f52b6355f..e3663ace9 100644
--- a/ouroboros/loop_model_call.py
+++ b/ouroboros/loop_model_call.py
@@ -555,11 +555,9 @@ def _reprepare_waiting_main(ctx: _RoundModelCallContext, kwargs: dict):
                 _loop()._run_main_reclaim(ctx, disposition)
         disposition = _measure_after_reclaim(ctx)
     kwargs["messages"] = ctx.messages
+    from ouroboros.llm_claudexor import cache_key_for_model
     from ouroboros.provider_models import provider_for_model
-    kwargs["cache_affinity"] = (
-        str(ctx.accumulated_usage.get("execution_id") or "")
-        if not use_local and provider_for_model(model) == "claudexor" else ""
-    )
+    kwargs["cache_affinity"] = "" if use_local else cache_key_for_model(model)
     kwargs["allow_server_web_search"] = (_loop()._server_web_allowed_by_task(ctx.tools._ctx)
                                          and not use_local and provider_for_model(model) != "claudexor")
     if provider_for_model(model) == "claudexor":
diff --git a/ouroboros/task_pacing.py b/ouroboros/task_pacing.py
index 7dfeb1ae4..baedc6673 100644
--- a/ouroboros/task_pacing.py
+++ b/ouroboros/task_pacing.py
@@ -718,10 +718,10 @@ def prepared_wrapup_candidate(
 
     The forced send this candidate admits continues the loop's active transport
     turn, so the candidate is built from that same owner slot."""
+    from ouroboros.llm_claudexor import cache_key_for_model
     from ouroboros.loop_llm_call import _prepare_main_messages
     from ouroboros.model_slots import task_model_binding, task_processing_preference
     from ouroboros.model_wait import current_model_wait
-    from ouroboros.observability import new_execution_id
 
     owner_ctx = getattr(getattr(ctx, "tools", None), "_ctx", None)
     waiter = current_model_wait()
@@ -750,9 +750,9 @@ def prepared_wrapup_candidate(
         model_account_override=account,
         model_turn_state=getattr(owner_ctx, "model_turn_state", None),
         # The admitted candidate must be the payload the send will produce: the
-        # main loop declares the same execution-scoped cache affinity, so this
-        # prepared copy binds the execution id exactly as that dispatch does.
-        cache_affinity=str(ctx.accumulated_usage.setdefault("execution_id", new_execution_id())),
+        # main loop declares the same install-scoped cache affinity, so this
+        # prepared copy binds the same key as that dispatch does.
+        cache_affinity="" if getattr(ctx, "active_use_local", False) else cache_key_for_model(ctx.active_model),
         processing_preference=task_processing_preference(
             {"task_metadata": getattr(owner_ctx, "task_metadata", {})}, model_role=role),
     )
diff --git a/tests/test_review_prompt_caching_p3.py b/tests/test_review_prompt_caching_p3.py
index 91f23302d..927b7ee5e 100644
--- a/tests/test_review_prompt_caching_p3.py
+++ b/tests/test_review_prompt_caching_p3.py
@@ -2,7 +2,8 @@
 
 import queue
 
-from ouroboros.llm_claudexor import _request
+from ouroboros import config
+from ouroboros.llm_claudexor import _request, cache_key_for_model
 from ouroboros.loop_llm_call import call_llm_with_retry
 
 
@@ -16,7 +17,19 @@ def test_claudexor_request_projects_explicit_affinity_to_cache_key():
     assert payload["options"]["cacheKey"] == "execution-7"
 
 
-def test_main_loop_scopes_execution_affinity_to_claudexor(tmp_path):
+def test_cache_key_is_install_scoped_stable_and_header_safe(monkeypatch, tmp_path):
+    key = cache_key_for_model("claudexor::codex=gpt-6-astra")
+    assert key == cache_key_for_model("claudexor::codex=gpt-6-astra"), "same install + model -> same key"
+    assert key.startswith("ouroboros-gpt-6-astra-")
+    assert all(ch.isalnum() or ch in "._-" for ch in key), "must be a valid HTTP header value"
+    assert key != cache_key_for_model("claudexor::codex=gpt-5.6-sol"), "one shard per model"
+    assert cache_key_for_model("openrouter::openai/model") == ""
+    assert cache_key_for_model("openai/gpt-5.6-sol") == ""
+    monkeypatch.setattr(config, "DATA_DIR", tmp_path / "other-install")
+    assert cache_key_for_model("claudexor::codex=gpt-6-astra") != key, "another data root is another install"
+
+
+def test_main_loop_shares_one_install_affinity_across_executions_on_claudexor(tmp_path):
     captured = []
 
     class LLM:
@@ -29,16 +42,21 @@ def test_main_loop_scopes_execution_affinity_to_claudexor(tmp_path):
 
     logs = tmp_path / "logs"
     logs.mkdir()
-    for model in ("claudexor::codex=model", "openrouter::openai/model"):
-        usage = {"execution_id": "execution-7"}
+    for model, execution in (("claudexor::codex=model", "execution-7"), ("claudexor::codex=model", "execution-8"),
+                             ("openrouter::openai/model", "execution-9")):
+        usage = {"execution_id": execution}
         message, _cost = call_llm_with_retry(
             LLM(), [{"role": "user", "content": "work"}], model, None, "medium", 1,
             logs, "task", 1, queue.Queue(), usage,
         )
         assert message["content"] == "done"
 
-    assert captured[0]["cache_affinity"] == "execution-7"
-    assert captured[1]["cache_affinity"] == ""
+    # Two executions of one install share the key, so the second one's first
+    # round is served the governance prefix the first one already paid for.
+    assert captured[0]["cache_affinity"] == cache_key_for_model("claudexor::codex=model")
+    assert captured[1]["cache_affinity"] == captured[0]["cache_affinity"]
+    assert "execution-7" not in captured[0]["cache_affinity"]
+    assert captured[2]["cache_affinity"] == ""
 
 
 def test_main_loop_projects_claudexor_options_outside_the_route(tmp_path):
diff --git a/tests/test_subscription_main_wait.py b/tests/test_subscription_main_wait.py
index 3b1fe75fb..784b76775 100644
--- a/tests/test_subscription_main_wait.py
+++ b/tests/test_subscription_main_wait.py
@@ -11,6 +11,7 @@ import pytest
 
 from ouroboros import loop, model_wait, usage_accounting as ua
 from ouroboros.llm_attempt import _attempt_request, _candidate_before_dispatch
+from ouroboros.llm_claudexor import cache_key_for_model
 from ouroboros.loop_model_call import _reprepare_waiting_main
 from ouroboros.model_slots import MODEL_ACCOUNTS_KEY
 from tests.test_context_fit_integration import _plan
@@ -539,7 +540,7 @@ def test_live_owner_wait_reprojects_affinity(main_call, monkeypatch, destination
     }
     assert facts["completed_tool_texts"] == ["verified read A", "completed review B"]
     assert ctx.accumulated_usage["execution_id"] == CACHE_REPREPARE_EXECUTION
-    assert prepared[-1]["cache_affinity"] == ("" if use_local or destination != MODEL else CACHE_REPREPARE_EXECUTION), (
+    assert prepared[-1]["cache_affinity"] == ("" if use_local or destination != MODEL else cache_key_for_model(MODEL)), (
         facts
     )
     if not use_local:
@@ -581,4 +582,6 @@ def test_recorded_wait_override_reprojects_affinity_before_send(main_call, initi
     }
     assert facts["completed_tool_texts"] == ["verified read A", "completed review B"]
     assert ctx.accumulated_usage["execution_id"] == CACHE_REPREPARE_EXECUTION
-    assert payload["options"].get("cacheKey") == CACHE_REPREPARE_EXECUTION, facts
+    # The override re-prepared the send for the subscription route, so the wire
+    # carries the install-scoped Codex key that route shares across executions.
+    assert payload["options"].get("cacheKey") == cache_key_for_model(MODEL), facts