diff --git a/docs/MODEL-PRICING-AUDIT.md b/docs/MODEL-PRICING-AUDIT.md
index b5c09f97..4ab96e8d 100644
--- a/docs/MODEL-PRICING-AUDIT.md
+++ b/docs/MODEL-PRICING-AUDIT.md
@@ -37,8 +37,9 @@ which the estimator does not model.
   GPT-6 models first and labels GPT-5.6 "Older"; OpenAI's Codex docs make Sol the
   default preset ([Codex models](https://learn.chatgpt.com/docs/models)). GPT-5.6
   models remain listed for pins.
-- Cross-host tiers pair by role and per-token price: GPT-6 Sol ↔ Sonnet 5
-  ($2/$10), GPT-6 Astra ↔ Fable 5.1 ($10/$50), GPT-6 Luna ↔ Haiku 4.5.
+- Cross-host tiers pair by role: GPT-6 Sol ↔ Sonnet 5 (balanced, $2/$10),
+  GPT-6 Astra ↔ Opus 5.5 (reasoning; Astra also pairs with Fable 5.1), GPT-6
+  Luna ↔ Haiku 4.5 (fast). A Codex-driven seed runs reasoning work on Astra.
 - Routes seeded before this change are reported as diverged and keep their model
   until `ak x host refresh`.
 - Not added as retirements: OpenAI's Codex docs retire GPT-5.4 / 5.4 Mini
diff --git a/docs/PROVIDERS.md b/docs/PROVIDERS.md
index bf0adaf0..d9f2c19f 100644
--- a/docs/PROVIDERS.md
+++ b/docs/PROVIDERS.md
@@ -460,8 +460,19 @@ ak host pick --primary-host codex       # make codex the lead; claude becomes th
 **Which host leads.** `--primary-host claude|codex` (default `claude`) chooses the primary.
 Codex-primary **mirrors** the default table below — codex takes the reasoning/review lead
 and claude becomes the alternate/escalation target — so the experience is ambidextrous
-regardless of which CLI drives. `ak status` marks the primary and fails (not warns) if the
-primary host is missing.
+regardless of which CLI drives. Models pair by tier, so each host leads reasoning work on its
+reasoning model and escalation steps up a tier either way:
+
+| Tier | Claude | Codex |
+|---|---|---|
+| reasoning | `claude-opus-5-5` | `gpt-6-astra` |
+| balanced | `claude-sonnet-5` | `gpt-6-sol` |
+| fast | `claude-haiku-4-5-20251001` | `gpt-6-luna` |
+
+With Codex leading, architecture, design, security-analysis and debugging run on `gpt-6-astra`,
+and implementation and testing run on `claude-sonnet-5`, escalating to `gpt-6-astra`. Astra costs
+more per token than Opus 5.5 ($10/$50 vs $4/$20), so Codex-led reasoning uses more Codex allowance.
+`ak status` marks the primary and fails (not warns) if the primary host is missing.
 
 **OpenCode is explicit, not seeded or AQE-projected.** Enable it, then use `ak run` with either
 a persisted route or a run-local override:
diff --git a/docs/adr/0002-activity-vocabulary-defaults-from-ruv-templates.md b/docs/adr/0002-activity-vocabulary-defaults-from-ruv-templates.md
index 82c149d3..4fec7745 100644
--- a/docs/adr/0002-activity-vocabulary-defaults-from-ruv-templates.md
+++ b/docs/adr/0002-activity-vocabulary-defaults-from-ruv-templates.md
@@ -48,13 +48,14 @@ as such** wherever surfaced (an `ak` tag in the UI, a comment in the defaults ta
 Default *models* map to the host's appropriate tier (Opus for deep reasoning, Sonnet for review, a Codex
 model for execution) and are treated as **soft defaults** — see the "open question" on pinning live model IDs.
 
-Tier is the pairing key, not the model id: `MODEL_CATALOG` spells `flagship`/`balanced`/`fast`
+Tier is the pairing key, not the model id: `MODEL_CATALOG` spells `reasoning`/`balanced`/`fast`
 identically on both hosts so primary-host mirroring can map a route to its counterpart's equivalent.
 As of 2026-09-23 execution routes to `gpt-6-sol` (balanced) and mechanical work to `gpt-6-luna`
 (fast), and deep reasoning routes to `claude-opus-5-5`. These are agentic-kit default preferences,
 following each vendor's recommended starting model, not evidence that earlier models were withdrawn.
 Cross-host tiers pair by role and per-token price: `gpt-6-sol` ↔ `claude-sonnet-5`,
-`gpt-6-astra` ↔ `claude-fable-5-1`, `gpt-6-luna` ↔ Haiku. Withdrawn ids are handled by the retirement
+`gpt-6-astra` ↔ `claude-opus-5-5` (reasoning; Astra also pairs with `claude-fable-5-1`),
+`gpt-6-luna` ↔ Haiku. See [ADR-0006](0006-primary-host-and-ambidextrous-mirroring.md) for tier parity. Withdrawn ids are handled by the retirement
 mechanism in [ADR-0003](0003-auto-seed-dual-host-provenance.md), not by editing this table alone.
 
 ## Consequences
diff --git a/docs/adr/0006-primary-host-and-ambidextrous-mirroring.md b/docs/adr/0006-primary-host-and-ambidextrous-mirroring.md
index ad7451ef..58c56f90 100644
--- a/docs/adr/0006-primary-host-and-ambidextrous-mirroring.md
+++ b/docs/adr/0006-primary-host-and-ambidextrous-mirroring.md
@@ -2,9 +2,12 @@
 
 - **Status:** Amended by [ADR-0020](0020-ga-stable-surfaces.md)
 - **Date:** 2026-07-23
-- **Updated:** 2026-07-30
-- **Update note:** Preserved primary-host mirroring while removing the compatibility executor and
-  moving persisted intent to `routing.primaryHost`.
+- **Updated:** 2026-09-23
+- **Update note:** Tier parity: both hosts now have a `reasoning` tier (`claude-opus-5-5` ↔
+  `gpt-6-astra`), so a Codex-driven seed gets reasoning-tier models and an escalation that steps up a
+  tier, as a Claude-driven seed does. A catalog entry may `pairs` with an extra tier.
+- **Earlier update (2026-07-30):** Preserved primary-host mirroring while removing the compatibility
+  executor and moving persisted intent to `routing.primaryHost`.
 - **Deciders:** agentic-kit maintainers
 
 > **GA amendment:** primary-host mirroring remains. The old command spelling, adapter dependency,
@@ -51,9 +54,14 @@ orchestrator stays symmetric underneath.
 - Codex-primary is expressible in one flag at setup or pick time, with the whole experience
   (routing, escalation, status severity, dashboard indicator) following symmetrically.
 - The default (claude-primary) is unchanged, so existing repos see no difference.
-- The mirror is coarse where host model tiers don't line up (claude tiers ≠ codex tiers): a
-  swapped route falls back to the counterpart host's recommended model. Users tune per activity
-  with `--route`.
+- The mirror pairs models by tier. Both hosts carry `reasoning`, `balanced` and `fast` tiers
+  (`claude-opus-5-5` ↔ `gpt-6-astra`, `claude-sonnet-5` ↔ `gpt-6-sol`, Haiku ↔ `gpt-6-luna`), so
+  reasoning work and escalation land on the same tier whichever host drives. A model with no
+  same-tier twin uses an explicit `pairs` entry (`claude-fable-5-1` → `gpt-6-astra`), else the
+  counterpart host's recommended model. Users tune per activity with `--route`.
+- Tier parity is not price parity: `gpt-6-astra` costs more per token than `claude-opus-5-5`
+  ($10/$50 vs $4/$20). Codex-driven reasoning therefore costs more at API rates, and uses more
+  Codex plan allowance, than Claude-driven reasoning.
 - `status`/dashboard must read `primaryHost` to render the correct severity + primary marker
   (done: `status.mjs` host rows, `dashboard-server.mjs` routing matrix).
 
diff --git a/src/lib/routing.mjs b/src/lib/routing.mjs
index 3b3a8b9a..5beec44a 100644
--- a/src/lib/routing.mjs
+++ b/src/lib/routing.mjs
@@ -81,8 +81,10 @@ export const MODEL_CATALOG_VERIFIED = '2026-09-23';
 export const COST_AXIS_NOTE = 'per-token price ≠ per-task cost — compare total tokens, cache use and agentic turns on representative tasks';
 // Tier names are the pairing key for swapHostModel(): a codex tier only mirrors
 // to a claude model (and back) when BOTH catalogs use the same tier string.
-// Keep `flagship`/`balanced`/`fast` spelled identically on both hosts —
-// renaming one side silently degrades every mirrored route to cat[0].
+// Keep `reasoning`/`balanced`/`fast` spelled identically on both hosts —
+// renaming one side silently degrades every mirrored route to cat[0]. A model
+// may also list `pairs`: extra tiers it answers for when the other host has no
+// model of that exact tier (Astra stands in for claude's `flagship`).
 export const MODEL_CATALOG = {
   claude: [
     // Opus 5.5 (released 2026-09-22) is the reasoning preset: Anthropic's
@@ -105,12 +107,17 @@ export const MODEL_CATALOG = {
     // catalog ranks these first and labels every GPT-5.6 model "Older", and
     // OpenAI's Codex docs make Sol the default preset
     // (learn.chatgpt.com/docs/models, verified 2026-09-23). Tier pairing with the
-    // claude catalog follows role and per-token price: Sol ↔ Sonnet 5 ($2/$10
-    // both), Astra ↔ Fable 5.1 ($10/$50 both), Luna ↔ Haiku. Sol is first, so a
-    // claude tier with no codex twin (reasoning) mirrors to the workhorse.
+    // claude catalog follows role: Sol ↔ Sonnet 5 (balanced, $2/$10 both),
+    // Luna ↔ Haiku (fast), and Astra ↔ Opus 5.5 (reasoning). Astra is the
+    // reasoning tier so a Codex-driven seed gets what a Claude-driven one does:
+    // reasoning work on the host's reasoning model and an escalation that steps
+    // UP a tier (Sonnet → Astra), not sideways to an equal-priced workhorse.
+    // Astra is priced above Opus 5.5 ($10/$50 vs $4/$20); OpenAI positions it for
+    // the hardest end-to-end work, and Anthropic reports Opus 5.5 at Fable 5.1
+    // level, which is Astra's class. It also answers for claude's `flagship`.
     { id: 'gpt-6-sol', tier: 'balanced', note: 'workhorse preset for coding, testing and everyday work' },
     { id: 'gpt-6-luna', tier: 'fast', note: 'fast-tier preset for mechanical work, documentation and packaging' },
-    { id: 'gpt-6-astra', tier: 'flagship', note: 'frontier preset for the hardest end-to-end work; compare per-task cost before selection' },
+    { id: 'gpt-6-astra', tier: 'reasoning', pairs: ['flagship'], note: 'reasoning preset for the hardest end-to-end work; compare per-task cost before selection' },
     // Still served, retained for user pins. Codex labels them "Older".
     { id: 'gpt-5.6-sol', tier: 'prior', note: 'prior coding preset retained for user pins; compare measured per-task results before switching' },
     { id: 'gpt-5.6-terra', tier: 'prior', note: 'prior balanced preset retained for user pins' },
@@ -224,14 +231,14 @@ const MODEL_TIER = Object.fromEntries(
   HOSTS.flatMap((h) => (MODEL_CATALOG[h] ?? []).map((m) => [m.id, m.tier])),
 );
 
-/** The opposite host's best model when swapping. Tier names differ between claude
- *  and codex, so an exact-tier match is best-effort; else fall back to that host's
- *  first (recommended) model. Pure. */
+/** The opposite host's best model when swapping: the model of the same tier,
+ *  else one that `pairs` with that tier, else that host's first (recommended)
+ *  model. Pure. */
 export function swapHostModel(host, model) {
   const other = host === 'claude' ? 'codex' : 'claude';
   const cat = MODEL_CATALOG[other] ?? [];
   const tier = MODEL_TIER[model];
-  const pick = (tier && cat.find((m) => m.tier === tier)) || cat[0];
+  const pick = (tier && (cat.find((m) => m.tier === tier) || cat.find((m) => m.pairs?.includes(tier)))) || cat[0];
   return { host: other, model: pick?.id };
 }
 
diff --git a/tests/kit/routing-primary.test.mjs b/tests/kit/routing-primary.test.mjs
index 7b48933a..5385331f 100644
--- a/tests/kit/routing-primary.test.mjs
+++ b/tests/kit/routing-primary.test.mjs
@@ -38,6 +38,40 @@ test('seedActivityRoutes with codex primary mirrors host assignments', () => {
   assert.equal(policy.implementation.host, 'claude'); // claude becomes the alternate
 });
 
+// Tier parity: a Codex-driven seed must get the same tier benefits a
+// Claude-driven one does — reasoning work on each host's reasoning-tier model,
+// and an escalation that steps UP a tier rather than sideways.
+const REASONING_ACTIVITIES = ['architecture', 'design', 'security-analysis', 'debugging'];
+
+test('each host leads reasoning work on its own reasoning-tier model', () => {
+  const claudeLed = seedActivityRoutes({ hosts: ['claude', 'codex'] });
+  const codexLed = seedActivityRoutes({ hosts: ['claude', 'codex'], primary: 'codex' });
+  for (const act of REASONING_ACTIVITIES) {
+    assert.deepEqual([claudeLed[act].host, claudeLed[act].model], ['claude', 'claude-opus-5-5'], act);
+    assert.deepEqual([codexLed[act].host, codexLed[act].model], ['codex', 'gpt-6-astra'], act);
+  }
+});
+
+test('escalation steps up to the reasoning tier whichever host drives', () => {
+  const claudeLed = seedActivityRoutes({ hosts: ['claude', 'codex'] });
+  const codexLed = seedActivityRoutes({ hosts: ['claude', 'codex'], primary: 'codex' });
+  for (const act of ['implementation', 'testing']) {
+    assert.deepEqual(claudeLed[act].escalation, [{ host: 'claude', model: 'claude-opus-5-5' }], act);
+    assert.equal(codexLed[act].model, 'claude-sonnet-5', act);
+    assert.deepEqual(codexLed[act].escalation, [{ host: 'codex', model: 'gpt-6-astra' }], act);
+  }
+});
+
+test('tier pairs mirror both ways, and flagship pins reach Astra through an explicit pair', () => {
+  assert.equal(swapHostModel('claude', 'claude-opus-5-5').model, 'gpt-6-astra');
+  assert.equal(swapHostModel('codex', 'gpt-6-astra').model, 'claude-opus-5-5');
+  assert.equal(swapHostModel('claude', 'claude-fable-5-1').model, 'gpt-6-astra');
+  assert.equal(swapHostModel('claude', 'claude-sonnet-5').model, 'gpt-6-sol');
+  assert.equal(swapHostModel('codex', 'gpt-6-sol').model, 'claude-sonnet-5');
+  assert.equal(swapHostModel('claude', 'claude-haiku-4-5-20251001').model, 'gpt-6-luna');
+  assert.equal(swapHostModel('codex', 'gpt-6-luna').model, 'claude-haiku-4-5-20251001');
+});
+
 test('seedActivityRoutes stamps every seeded entry with provenance:seeded', () => {
   const policy = seedActivityRoutes({ hosts: ['claude', 'codex'], primary: 'codex' });
   assert.ok(Object.values(policy).every((r) => r.provenance === 'seeded'));