fix(ai): move the Codex slug ladder onto the gpt-5.6 line

The ChatGPT Codex backend now rejects every slug from gpt-5.5 down, so a
deploy without a CODEX_MODEL_FALLBACK_CHAIN override walked the whole
default ladder into "all configured Codex slugs were rejected". The
default now starts at gpt-5.6-terra, with its luna and sol siblings and
gpt-5.5 as the last rung.

The gpt-6 line stays out of the default: the reasoning, sampling and
vision gates do not recognise that family yet, and a test now holds every
default slug to a family the reasoning gate knows.

docs/codex-protocol-spec.md

@@ -630,7 +630,8 @@ Confirmed-rejected slugs (despite the source-tree references in the table above)
 
 Confirmed-accepted slugs on a ChatGPT Plus subscription (per `available_in_plans` in `models.json`):
 
-- `gpt-5.5` — HealthLog's default (first chain entry) as of 2026-06
+- `gpt-5.6-terra` — HealthLog's default (first chain entry) as of 2026-10-24, with `gpt-5.6-luna` and `gpt-5.6-sol` behind it; the backend rejected `gpt-5.5` and everything below it that day
+- `gpt-5.5` — the default from 2026-06 until 2026-10-24
 - `gpt-5.4` / `gpt-5.4-mini` / `gpt-5.2` — accepted per plan
 - `gpt-5.3-codex` — was the default (verified 2026-05-09); the backend rotated off the `-codex` line on 2026-06-02 and this slug is now only accepted intermittently, so it sits on a late chain rung
 
@@ -646,11 +647,10 @@ To eliminate that risk class, HealthLog's `CodexClient` walks an ordered chain o
 
 ```ts
 const DEFAULT_SLUG_FALLBACK_CHAIN = [
-  "gpt-5.5", // current default across paid tiers as of 2026-06-02
-  "gpt-5.4", // documented fallback
-  "gpt-5.4-mini", // documented lighter-weight fallback
-  "gpt-5.3-codex", // late rung — still accepted intermittently
-  "gpt-5.2", // legacy floor
+  "gpt-5.6-terra", // current default, verified against the backend 2026-10-24
+  "gpt-5.6-luna", // lighter sibling on the same line
+  "gpt-5.6-sol", // heavier sibling on the same line
+  "gpt-5.5", // previous default, kept as the last rung while plans differ
 ];
 ```
 

src/__tests__/baselines/source-file-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production files over 800 lines and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 89,
-  "total": 99598,
+  "total": 99593,
   "entries": {
     "app/medications/page-client.tsx": 839,
     "components/charts/mood-chart.tsx": 1166,
@@ -29,7 +29,7 @@
     "lib/ai/coach/stream-events.ts": 817,
     "lib/ai/coach/turn/model.ts": 820,
     "lib/ai/coach/types.ts": 1011,
-    "lib/ai/codex-client.ts": 1094,
+    "lib/ai/codex-client.ts": 1089,
     "lib/ai/local-client.ts": 880,
     "lib/ai/prompts/insight-generator.ts": 1110,
     "lib/ai/provider.ts": 1632,

src/lib/ai/__tests__/codex-slug-fallback.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { CodexClient, __test } from "../codex-client";
 import { singleUserTurn } from "../types";
+import { openAIReasoningFamily } from "../reasoning/support";
 import {
   clearCodexSlugCache,
   getCachedCodexSlug,
@@ -341,17 +342,26 @@ describe("Codex slug fallback chain", () => {
   });
 
   it("DEFAULT_SLUG_FALLBACK_CHAIN tracks the current ChatGPT-auth ladder", () => {
-    // The backend rotated its ChatGPT-auth slugs onto the gpt-5.x line on
-    // 2026-06-02; the in-code default must match the accepted set, in order.
+    // The backend rejected every slug from gpt-5.5 down on 2026-10-24 and
+    // served the gpt-5.6 line; the in-code default must match the accepted
+    // set, in order.
     expect([...__test.DEFAULT_SLUG_FALLBACK_CHAIN]).toEqual([
+      "gpt-5.6-terra",
+      "gpt-5.6-luna",
+      "gpt-5.6-sol",
       "gpt-5.5",
-      "gpt-5.4",
-      "gpt-5.4-mini",
-      "gpt-5.3-codex",
-      "gpt-5.2",
     ]);
   });
 
+  it("DEFAULT_SLUG_FALLBACK_CHAIN holds only slugs the reasoning gate recognises", () => {
+    // A slug whose family the reasoning gate does not know goes out with the
+    // wrong parameters. A new line (gpt-6) joins the default only together
+    // with its family support.
+    for (const slug of __test.DEFAULT_SLUG_FALLBACK_CHAIN) {
+      expect(openAIReasoningFamily(slug), slug).toBe("gpt-5.1+");
+    }
+  });
+
   it("loadFallbackChain falls back to the default ladder with no env override", () => {
     const origModel = process.env.CODEX_MODEL;
     const origChain = process.env.CODEX_MODEL_FALLBACK_CHAIN;
@@ -359,11 +369,10 @@ describe("Codex slug fallback chain", () => {
     delete process.env.CODEX_MODEL_FALLBACK_CHAIN;
     try {
       expect(__test.loadFallbackChain()).toEqual([
+        "gpt-5.6-terra",
+        "gpt-5.6-luna",
+        "gpt-5.6-sol",
         "gpt-5.5",
-        "gpt-5.4",
-        "gpt-5.4-mini",
-        "gpt-5.3-codex",
-        "gpt-5.2",
       ]);
     } finally {
       if (origModel === undefined) delete process.env.CODEX_MODEL;

src/lib/ai/codex-client.ts

@@ -51,23 +51,18 @@ import {
 const CODEX_ENDPOINT = "https://chatgpt.com/backend-api/codex/responses";
 
 /**
- * Default fallback chain — most-current-first as of 2026-06-02. Override
- * via `CODEX_MODEL_FALLBACK_CHAIN` (comma-separated) on apps01 if a
- * specific plan ladder needs different ordering. `CODEX_MODEL` is
- * folded into position 0 of the chain when set; duplicates are dropped.
- *
- * The ChatGPT Codex backend rotated its ChatGPT-auth slugs off the
- * `-codex` specialist line onto the plain `gpt-5.x` line on 2026-06-02;
- * the former `-codex` defaults are now rejected with a 503 "all
- * configured Codex slugs were rejected". The ladder below tracks the
- * current accepted set.
+ * Default fallback chain, most-current-first as of 2026-10-24. Override via
+ * `CODEX_MODEL_FALLBACK_CHAIN` (comma-separated); `CODEX_MODEL` is folded
+ * into position 0 when set, duplicates dropped. The backend rotates its
+ * ChatGPT-auth slugs without notice: by 2026-10-24 it rejected everything
+ * from `gpt-5.5` down and served the `gpt-5.6-*` line. The `gpt-6` line is
+ * left out until the reasoning, sampling and vision gates recognise it.
  */
 const DEFAULT_SLUG_FALLBACK_CHAIN = [
-  "gpt-5.5", // current default across paid tiers as of 2026-06-02
-  "gpt-5.4", // documented fallback
-  "gpt-5.4-mini", // documented lighter-weight fallback
-  "gpt-5.3-codex", // late rung — still accepted intermittently
-  "gpt-5.2", // legacy floor
+  "gpt-5.6-terra", // current default, verified against the backend 2026-10-24
+  "gpt-5.6-luna", // lighter sibling on the same line
+  "gpt-5.6-sol", // heavier sibling on the same line
+  "gpt-5.5", // previous default, kept as the last rung while plans differ
 ] as const;
 
 /**