feat: model taxonomy gains gpt-5.6-sol + per-host generation defaults

Adds 'gpt-5.6-sol' to the model taxonomy with exact-match-only resolution
(Terra/Luna/suffixed IDs deliberately fall back to generic gpt) and replaces
the hardcoded 'claude' generation default with a validated
HostConfig.defaultModel: codex renders the gpt profile when --model is
absent, every other host keeps claude. Codex ship golden regenerated
accordingly; ADDING_A_HOST documents the new field.

docs/ADDING_A_HOST.md

@@ -60,6 +60,7 @@ That expands to the full `HostConfig` with these defaults:
 
 - `cliCommand: 'myhost'` (the name; binary for `command -v` detection)
 - `cliAliases: []`
+- `defaultModel: 'claude'` (model overlay used when generation gets no explicit `--model`; codex overrides to `'gpt'`)
 - `globalRoot` / `localSkillRoot`: `.myhost/skills/gstack`, `hostSubdir`: `.myhost`
 - `usesEnvVars: true` (false only for Claude, which uses literal `~` paths)
 - `frontmatter`: allowlist keeping `name` + `description`, no description limit
@@ -173,6 +174,7 @@ Key fields:
 The `validateHostConfig()` function in `scripts/host-config.ts` checks:
 - Name: lowercase alphanumeric with hyphens
 - CLI command: alphanumeric with hyphens/underscores
+- `defaultModel`: must be a known model family from `scripts/models.ts` `ALL_MODEL_NAMES`
 - Paths: safe characters only (alphanumeric, `.`, `/`, `$`, `{}`, `~`, `-`, `_`)
 - No duplicate names, hostSubdirs, or globalRoots across configs
 

hosts/codex.ts

@@ -4,6 +4,7 @@ const codex = defineHost({
   name: 'codex',
   displayName: 'OpenAI Codex CLI',
   cliAliases: ['agents'],
+  defaultModel: 'gpt',
 
   localSkillRoot: '.agents/skills/gstack',
   hostSubdir: '.agents',

hosts/define-host.ts

@@ -86,6 +86,7 @@ export function defineHost<const N extends string>(overrides: HostOverrides<N>):
     displayName,
     cliCommand = name,
     cliAliases = [],
+    defaultModel = 'claude',
     globalRoot = `.${name}/skills/gstack`,
     localSkillRoot = `.${name}/skills/gstack`,
     hostSubdir = `.${name}`,
@@ -140,6 +141,7 @@ export function defineHost<const N extends string>(overrides: HostOverrides<N>):
     displayName,
     cliCommand,
     cliAliases,
+    defaultModel,
     globalRoot,
     localSkillRoot,
     hostSubdir,

scripts/gen-skill-docs.ts

@@ -92,12 +92,13 @@ const HOST_ARG_VAL: HostArg = (() => {
 let HOST: Host = HOST_ARG_VAL === 'all' ? 'claude' : HOST_ARG_VAL;
 
 // ─── Model Overlay Selection ────────────────────────────────
-// --model is explicit. We do NOT auto-detect from host (host ≠ model).
-// Default is 'claude'. Missing overlay file → empty string (graceful).
+// --model is explicit. Without it, each host uses HostConfig.defaultModel.
+// Host defaults are generation fallbacks, not claims that host === model.
+// Missing overlay file → empty string (graceful).
 import { ALL_MODEL_NAMES, resolveModel, type Model } from './models';
 const MODEL_ARG = process.argv.find(a => a.startsWith('--model'));
-const MODEL_ARG_VAL: Model = (() => {
-  if (!MODEL_ARG) return 'claude';
+const MODEL_ARG_VAL: Model | null = (() => {
+  if (!MODEL_ARG) return null;
   const val = MODEL_ARG.includes('=') ? MODEL_ARG.split('=')[1] : process.argv[process.argv.indexOf(MODEL_ARG) + 1];
   const resolved = resolveModel(val);
   if (!resolved) {
@@ -106,6 +107,10 @@ const MODEL_ARG_VAL: Model = (() => {
   return resolved;
 })();
 
+function generationModelForHost(host: Host): Model {
+  return MODEL_ARG_VAL ?? getHostConfig(host).defaultModel;
+}
+
 // ─── Catalog Mode (v1.45.0.0 T4) ────────────────────────────
 // 'trim' (default): shorten frontmatter description to lead sentence and
 // move routing/voice prose into a "## When to invoke" body section.
@@ -753,7 +758,7 @@ function buildContext(
   const interactive = interactiveMatch ? interactiveMatch[1] === 'true' : undefined;
   return {
     skillName, tmplPath, benefitsFrom, host, paths: HOST_PATHS[host],
-    preambleTier, model: MODEL_ARG_VAL, interactive, explainLevel: EXPLAIN_LEVEL,
+    preambleTier, model: generationModelForHost(host), interactive, explainLevel: EXPLAIN_LEVEL,
   };
 }
 

scripts/host-config.ts

@@ -14,6 +14,9 @@
  *                                              platform-detect, uninstall
  */
 
+import type { Model } from './models';
+import { validateModel } from './models';
+
 export interface HostConfig {
   /** Unique host identifier (e.g., 'opencode'). Must match filename in hosts/. */
   name: string;
@@ -24,6 +27,9 @@ export interface HostConfig {
   /** Alternative binary names (e.g., ['droid'] for factory). */
   cliAliases?: string[];
 
+  /** Model overlay used when generation does not receive an explicit --model. */
+  defaultModel: Model;
+
   // --- Path Configuration ---
   /** Global install path relative to $HOME (e.g., '.config/opencode/skills/gstack'). */
   globalRoot: string;
@@ -119,6 +125,10 @@ export function validateHostConfig(config: HostConfig, validResolverNames?: Read
       }
     }
   }
+  const modelError = validateModel(config.defaultModel);
+  if (modelError) {
+    errors.push(`defaultModel ${modelError}`);
+  }
   if (!PATH_REGEX.test(config.globalRoot)) {
     errors.push(`globalRoot '${config.globalRoot}' contains invalid characters`);
   }

scripts/models.ts

@@ -2,13 +2,17 @@
  * Model taxonomy — neutral module with no imports from hosts/ or resolvers/.
  *
  * Model families supported by model overlays in model-overlays/{family}.md.
- * Host configs can reference these as `defaultModel` strings (validated at
+ * Host configs reference these as `defaultModel` strings (validated at
  * generation time), but the model axis is independent of the host axis.
  *
  * IMPORTANT: host ≠ model. Claude Code can run any Claude model (Opus, Sonnet,
  * Haiku, future). Codex CLI runs GPT/o-series models. Cursor and OpenCode can
- * front multiple providers. We do NOT auto-detect the model from the host —
- * users pass --model explicitly. Default is 'claude'.
+ * front multiple providers. The generator does NOT auto-detect the model from
+ * the host — users can pass --model explicitly, otherwise each host supplies
+ * its own generation default. Exception outside this module: ./setup detects
+ * the Codex model from ${CODEX_HOME:-~/.codex}/config.toml
+ * (scripts/resolve-codex-generation-model.ts) and passes it as an explicit
+ * --model.
  */
 
 export const ALL_MODEL_NAMES = [
@@ -19,6 +23,7 @@ export const ALL_MODEL_NAMES = [
   'sonnet-5',
   'gpt',
   'gpt-5.4',
+  'gpt-5.6-sol',
   'gemini',
   'o-series',
 ] as const;
@@ -29,10 +34,11 @@ export type Model = (typeof ALL_MODEL_NAMES)[number];
  * Resolve a model argument from CLI input to a known Model family.
  *
  * Precedence rules:
- * 1. Exact match against ALL_MODEL_NAMES → return as-is.
+ * 1. Exact match against ALL_MODEL_NAMES → return as-is. This is the ONLY
+ *    path that selects `gpt-5.6-sol` — Sol is intentionally exact-only.
  * 2. Family heuristics for common variants:
  *    - `gpt-5.4-mini`, `gpt-5.4-turbo`, `gpt-5.4-*` → `gpt-5.4`
- *    - `gpt-*` (anything else GPT) → `gpt`
+ *    - `gpt-*` (anything else GPT, including other 5.6 variants) → `gpt`
  *    - `o3`, `o4`, `o4-mini`, `o1`, `o1-mini`, `o1-pro` → `o-series`
  *    - `claude-*` (sonnet, opus, haiku, any version) → `claude`
  *    - `gemini-*` (2.5-pro, flash, etc.) → `gemini`
@@ -52,6 +58,10 @@ export function resolveModel(input: string): Model | null {
   }
 
   // Family heuristics
+  // Sol never reaches here — the exact match above already returned it. Do
+  // not add a Sol family pattern: Terra, Luna, future 5.6 variants, and
+  // suffixed model IDs must NOT inherit Sol's behavioral profile; they fall
+  // through to the generic `gpt` family below.
   if (/^gpt-5\.4(-|$)/.test(s)) return 'gpt-5.4';
   if (/^gpt(-|$)/.test(s)) return 'gpt';
   if (/^o[0-9]+(-|$)/.test(s)) return 'o-series';

test/fixtures/golden/codex-ship-SKILL.md

@@ -113,7 +113,7 @@ if [ -d ".agents/skills/gstack" ] && [ ! -L ".agents/skills/gstack" ]; then
   fi
 fi
 echo "VENDORED_GSTACK: $_VENDORED"
-echo "MODEL_OVERLAY: claude"
+echo "MODEL_OVERLAY: gpt"
 _CHECKPOINT_MODE=$($GSTACK_BIN/gstack-config get checkpoint_mode 2>/dev/null || echo "explicit")
 _CHECKPOINT_PUSH=$($GSTACK_BIN/gstack-config get checkpoint_push 2>/dev/null || echo "false")
 echo "CHECKPOINT_MODE: $_CHECKPOINT_MODE"
@@ -579,23 +579,45 @@ At skill END before telemetry:
 ```
 
 
-## Model-Specific Behavioral Patch (claude)
+## Model-Specific Behavioral Patch (gpt)
 
-The following nudges are tuned for the claude model family. They are
+The following nudges are tuned for the gpt model family. They are
 **subordinate** to skill workflow, STOP points, AskUserQuestion gates, plan-mode
 safety, and /ship review gates. If a nudge below conflicts with skill instructions,
 the skill wins. Treat these as preferences, not rules.
 
-**Todo-list discipline.** When working through a multi-step plan, mark each task
-complete individually as you finish it. Do not batch-complete at the end. If a task
-turns out to be unnecessary, mark it skipped with a one-line reason.
+**Completion bias.** Do not end your turn with a partial solution when the full
+solution is reachable. If you encounter an error, debug it. If a test fails, fix it.
+If something is ambiguous, make your best judgment and proceed — don't stop and ask
+unless you're genuinely blocked.
 
-**Think before heavy actions.** For complex operations (refactors, migrations,
-non-trivial new features), briefly state your approach before executing. This lets
-the user course-correct cheaply instead of mid-flight.
+**Prefer doing over listing.** When you'd be tempted to write "you could also try X,
+Y, or Z," try the best option yourself. Pick, execute, report results.
 
-**Dedicated tools over Bash.** Prefer Read, Edit, Write, Glob, Grep over shell
-equivalents (cat, sed, find, grep). The dedicated tools are cheaper and clearer.
+**No preamble.** Skip "Great question!", "Let me help with that", and restating the
+user's request. Start with the work.
+
+**AskUserQuestion is NOT preamble.** The "No preamble" and "Prefer doing over listing"
+rules above do NOT apply to AskUserQuestion content. When you invoke AskUserQuestion,
+the user is about to make a decision — they need context, not terseness. Always emit
+the full format from the preamble's AskUserQuestion Format section:
+
+1. **Re-ground** (project + branch + task — 1-2 sentences).
+2. **Simplify (ELI10)** — explain what's happening in plain English a 16-year-old could
+   follow. Concrete stakes, not abstract tradeoffs. Non-negotiable; this is NOT preamble.
+3. **Recommend** — `RECOMMENDATION: Choose [X] because [one-line reason]` on its own
+   line. Never omit this line. Never collapse it into the options list.
+4. **Options** — lettered `A) B) C)` with Completeness scores (coverage-differentiated)
+   or the "options differ in kind" note (kind-differentiated).
+
+If you find yourself about to present an AskUserQuestion without the Simplify/ELI10
+paragraph, without a RECOMMENDATION line, or by just listing options and asking "which
+one?" — stop, back up, and emit the full format. The user will ask you to do it anyway,
+so do it the first time.
+
+**Reminder: subordination applies.** When a skill workflow says STOP, stop. When the
+skill asks via AskUserQuestion, that is the wait-for-user gate, not an ambiguity.
+Completion bias does not override safety gates.
 
 ## Voice
 

test/host-config.test.ts

@@ -112,6 +112,7 @@ describe('validateHostConfig', () => {
       name: 'test-host',
       displayName: 'Test Host',
       cliCommand: 'testcli',
+      defaultModel: 'claude',
       globalRoot: '.test/skills/gstack',
       localSkillRoot: '.test/skills/gstack',
       hostSubdir: '.test',
@@ -165,6 +166,12 @@ describe('validateHostConfig', () => {
     expect(validateHostConfig(c)).toEqual([]);
   });
 
+  test('invalid defaultModel is caught', () => {
+    const c = makeValid();
+    (c as any).defaultModel = 'llama-local';
+    expect(validateHostConfig(c).some(e => e.includes('defaultModel'))).toBe(true);
+  });
+
   test('invalid globalRoot is caught', () => {
     const c = makeValid();
     c.globalRoot = 'path with spaces';
@@ -470,6 +477,13 @@ describe('golden-file regression', () => {
 // ─── Individual host config correctness ─────────────────────
 
 describe('host config correctness', () => {
+  test('Codex defaults to generic GPT while all existing hosts retain Claude', () => {
+    expect(codex.defaultModel).toBe('gpt');
+    for (const host of ALL_HOST_CONFIGS.filter(h => h.name !== 'codex')) {
+      expect(host.defaultModel).toBe('claude');
+    }
+  });
+
   test('claude is the only host with real-dir-symlink strategy', () => {
     for (const config of ALL_HOST_CONFIGS) {
       if (config.name === 'claude') {