refactor(compression): Astra native-compaction gate reuses is_astra_model

The -900k alias fix hand-rolled a second copy of the Astra slug set and its
vendor-prefix normalization. agent/reasoning_effort.py::is_astra_model is the
documented single home for that set (picker, effort vocabulary and request
sanitizer already key off it), so the gate now calls it and a future Astra
alias stays a one-line edit. The gpt-5.6 marker check is back to main's exact
form.

Tests move into the existing parametrized Astra gate table, which checks both
the capability resolver and the per-request gate: -900k on official Codex OAuth
is eligible; -900k through a relay or on provider openai is not. Docs and the
config example no longer say "exact gpt-6-astra".

agent/native_compaction.py

@@ -2,8 +2,8 @@
 
 ``context_management=[{"type": "compaction", "compact_threshold": N}]`` makes the server
 summarize older context into an opaque ``compaction`` item once the input crosses N tokens.
-Deliberately narrow: gpt-5.6 on api.openai.com or the ChatGPT Codex backend, plus exact
-gpt-6-astra on official Codex OAuth. The local compressor
+Deliberately narrow: gpt-5.6 on api.openai.com or the ChatGPT Codex backend, plus gpt-6-astra
+(and its ``-900k`` picker alias) on official Codex OAuth. The local compressor
 stays armed as fallback (native threshold clamped below the local trigger); compaction items
 ride the ``codex_reasoning_items`` sidecar. No transport imports (shared gate, no cycles).
 """
@@ -17,6 +17,7 @@ from urllib.parse import urlsplit
 from agent.codex_headers import is_official_codex_base_url
 from agent.context_compressor import is_compaction_summary_message
 from agent.message_content import flatten_message_text
+from agent.reasoning_effort import is_astra_model
 
 logger = logging.getLogger(__name__)
 
@@ -26,22 +27,15 @@ LOCAL_TRIGGER_SAFETY_MARGIN = 8_192
 DEFAULT_COMPACT_THRESHOLD = 200_000
 # Substring match so dated snapshots and variants (gpt-5.6-mini) stay eligible.
 _ELIGIBLE_MODEL_MARKER = "gpt-5.6"
-_ASTRA_NATIVE_COMPACTION_MODELS = frozenset({"gpt-6-astra", "gpt-6-astra-900k"})
 
 
 def is_native_compaction_model(
     model: Optional[str], *, provider: Optional[str] = None, base_url: Optional[str] = None,
 ) -> bool:
-    """Preserve gpt-5.6 eligibility; Astra additionally requires official Codex OAuth.
-
-    ``-900k`` is Hermes' local picker alias and is stripped before the request is
-    sent.  It must retain the same native-compaction eligibility as the bare
-    Astra wire model; otherwise enabling the feature in a 900K session is a
-    silent no-op.
-    """
-    model_name = (model or "").strip().lower().rsplit("/", 1)[-1]
-    return _ELIGIBLE_MODEL_MARKER in model_name or (
-        model_name in _ASTRA_NATIVE_COMPACTION_MODELS
+    """Preserve gpt-5.6 eligibility; Astra (``-900k`` is a picker alias of the same wire slug)
+    additionally requires official Codex OAuth."""
+    return _ELIGIBLE_MODEL_MARKER in (model or "").lower() or (
+        is_astra_model(model)
         and (provider or "").strip().lower() == "openai-codex"
         and is_official_codex_base_url(base_url or "")
     )

cli-config.yaml.example

@@ -723,7 +723,7 @@ compression:
 
   # Native OpenAI Responses server-side compaction (default: false). When true,
   # gpt-5.6-family models on the DIRECT OpenAI API (api.openai.com) or a ChatGPT
-  # Codex subscription, plus exact gpt-6-astra on official Codex OAuth, compact
+  # Codex subscription, plus gpt-6-astra (and its -900k alias) on official Codex OAuth, compact
   # server-side: OpenAI prunes older context into an
   # encrypted checkpoint that Hermes replays on later turns. No other provider,
   # route, or model is affected. Hermes' local compression stays armed as the

tests/agent/test_astra_oauth_native_compaction.py

@@ -1,4 +1,5 @@
-"""Exact gpt-6-astra is native-compaction eligible only on official Codex OAuth (#103720).
+"""gpt-6-astra (and its ``-900k`` picker alias) is native-compaction eligible only on official
+Codex OAuth (#103720).
 
 Both the destination capability (``resolve_native_compaction_capabilities``) and the
 per-request gate (``native_compaction_context_management``) must agree, and the request
@@ -20,6 +21,9 @@ _CODEX = "https://chatgpt.com/backend-api/codex"
 @pytest.mark.parametrize("model,provider,base_url,eligible", [
     ("gpt-6-astra", "openai-codex", _CODEX, True),
     ("GPT-6-ASTRA", "openai-codex", "https://chatgpt.com:443/backend-api/codex/", True),
+    ("gpt-6-astra-900k", "openai-codex", _CODEX, True),
+    ("gpt-6-astra-900k", "openai-codex", "https://relay.example/v1", False),
+    ("gpt-6-astra-900k", "openai", "https://api.openai.com/v1", False),
     ("gpt-6-astra", "openai", "https://api.openai.com/v1", False),
     ("gpt-6-astra", "openai", _CODEX, False),
     ("gpt-6-astra", "openai-codex", "https://relay.example/v1", False),

tests/agent/test_native_compaction.py

@@ -46,13 +46,6 @@ class TestModelGate:
         assert is_native_compaction_model("gpt-5.6-mini")
         assert is_native_compaction_model("GPT-5.6-2026-07-15")
 
-    def test_astra_900k_picker_alias_is_eligible_on_official_codex_oauth(self):
-        assert is_native_compaction_model(
-            "gpt-6-astra-900k",
-            provider="openai-codex",
-            base_url="https://chatgpt.com/backend-api/codex",
-        )
-
     def test_other_models_ineligible(self):
         # gpt-5.1/5.2 fail server-side on context_management (live-verified);
         # gpt-5.3-codex works upstream but is outside the supported set.

website/docs/developer-guide/context-compression-and-caching.md

@@ -266,7 +266,7 @@ auxiliary:
 | `codex_gpt55_autoraise` | `true` | bool | Raise the trigger to 85% for gpt-5.4/5.5/5.6 and gpt-6 Astra on the ChatGPT Codex OAuth route (see below). Set `false` to keep the global `threshold` |
 | `codex_gpt55_autoraise_notice` | `true` | bool | Show the one-time Codex gpt-5.5 autoraise notice. Set `false` to keep the 85% autoraise but suppress the banner |
 | `codex_app_server_auto` | `native` | `native`, `hermes`, `off` | Thread-compaction mode for Codex app-server sessions (see below) |
-| `codex_responses_native` | `false` | bool | Opt in to OpenAI's server-side compaction on the Responses API. Engages for gpt-5.6-family models on the direct OpenAI API or a ChatGPT Codex subscription, and exact `gpt-6-astra` on official Codex OAuth (see below) |
+| `codex_responses_native` | `false` | bool | Opt in to OpenAI's server-side compaction on the Responses API. Engages for gpt-5.6-family models on the direct OpenAI API or a ChatGPT Codex subscription, and `gpt-6-astra` (including its `-900k` picker alias) on official Codex OAuth (see below) |
 | `codex_responses_compact_threshold` | `null` | `null` or positive integer | Server-side compaction trigger, read **only when `codex_responses_native: true`** — it never changes when local compression fires; the local trigger is `threshold` (ratio) capped by `threshold_tokens`. `null` follows the resolved local compression trigger with an 8,192 token safety margin. A positive integer remains absolute and only clamps downward when required. Invalid values use automatic behavior. Automatic mode falls back to `200000` when no usable local trigger exists |
 | `in_place` | `true` | bool | Compact on the same session id instead of rotating to a new one (see below) |
 
@@ -429,9 +429,9 @@ client-side summary pass, and ZDR-friendly (`store: false`, no
 Opt in with `compression.codex_responses_native: true`. The gate is deliberately
 narrow, re-checked on every request:
 
-- **Models:** the gpt-5.6 family, plus exact `gpt-6-astra` on official Codex
-  subscription OAuth. Astra on the direct API, Astra variants and other GPT-6
-  models are excluded. gpt-5.1/5.2 return HTTP 500 or stall the stream when the
+- **Models:** the gpt-5.6 family, plus `gpt-6-astra` (and its `-900k` picker
+  alias) on official Codex subscription OAuth. Astra on the direct API, other
+  Astra variants and other GPT-6 models are excluded. gpt-5.1/5.2 return HTTP 500 or stall the stream when the
   field is present (no structured rejection to downgrade on, verified live Aug 2026).
 - **Routes:** `api.openai.com` (OpenAI API key) or the ChatGPT Codex backend
   (Codex subscription OAuth) only. xAI, GitHub/Copilot, OpenRouter, relays, and