diff --git a/CHANGELOG.md b/CHANGELOG.md
index e13f3f0..6b90dd5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,14 @@ names the commit's date and the pull request it came from.
 
 ## November 2026
 
+- A fallback stays inside the reviewer's model family. A review whose model was parked or
+  unroutable continued on `REDLINE_FALLBACK_MODEL` whatever that model was, so a host that read on
+  one vendor's models could have a review finished by another vendor's without anyone choosing
+  that. `review.mjs` now uses the fallback only when it is of the stopped model's family
+  (`modelFamily`: `claude`, `gpt`, or an unlisted id's leading name); otherwise the run logs why
+  and fails, as it does with no fallback. A host that reads on a family other than Claude sets
+  `REDLINE_FALLBACK_MODEL` to a model of that family: the default, `claude-opus-5-5`, no longer
+  serves it. The hosted quickstart says how to choose the reviewer's family.
 - A starter rubric for hosted mode is public at
   [askalf/redline-rubric](https://github.com/askalf/redline-rubric), linked from the README and
   the hosted quickstart. This repository still ships no prompt.
diff --git a/README.md b/README.md
index 5136383..609f2d1 100644
--- a/README.md
+++ b/README.md
@@ -15,11 +15,15 @@ On our hosts, routine and deep reviews run on OpenAI models through a ChatGPT su
 `REDLINE_MODEL=gpt-5.6-terra` reads routine pull requests and `REDLINE_DEEP_MODEL=gpt-6-astra` reads
 large or sensitive ones. Two paths have their own settings: a pull request that only moves Redline
 pins is read on `REDLINE_PIN_MODEL`, and a review whose model the endpoint has parked or cannot
-route moves to `REDLINE_FALLBACK_MODEL` (any other model error fails the review). When a setting is
-unset, the default in
-[`scripts/redline/review.mjs`](scripts/redline/review.mjs) applies: `claude-fable-5-1` for
-`REDLINE_MODEL`, and `claude-opus-5-5` for the pin and fallback models, so those two paths run on
-Claude unless a host sets them.
+route moves to `REDLINE_FALLBACK_MODEL` when that is a model of the same family (any other model
+error fails the review). A fallback of another family is never used. Which family reads a repository
+is a choice: where a coding agent writes the pull requests, another vendor's model reads them, so
+the reviewer does not share the author's blind spots. A parked model does not undo that choice: the
+review fails, and a new run reads the pull request once its model is back. When a setting is unset,
+the default in [`scripts/redline/review.mjs`](scripts/redline/review.mjs) applies: `claude-fable-5-1` for
+`REDLINE_MODEL`, and `claude-opus-5-5` for the pin and fallback models. So a pull request that only
+moves pins is read on Claude unless a host sets another model, and a host that reads on another
+family has no fallback until it names one of that family.
 
 **Who this is for.** The askalf repositories call it today. Another repository can call the review
 on a GitHub-hosted runner with a model API key and a rubric of its own, and no runner host
diff --git a/docs/hosted-quickstart.md b/docs/hosted-quickstart.md
index 50dc2a0..5ae5f15 100644
--- a/docs/hosted-quickstart.md
+++ b/docs/hosted-quickstart.md
@@ -95,6 +95,11 @@ jobs:
 The endpoint defaults to `https://api.anthropic.com`; pass `model-url` for another one that
 answers the Anthropic Messages API.
 
+Choose the model by who writes your pull requests. If a coding agent writes them, a model from
+another vendor is the stronger reviewer, since one of the author's own family tends to share its
+blind spots; `model-url` takes any endpoint that serves such a model over the Anthropic Messages
+API. Hosted mode sets no fallback model, so a review is never read on a model you did not name.
+
 ## 4. `.github/workflows/redline-post.yml`
 
 The same `<sha>`, the same `reviewer-login` and the same `dispatch-actor` as above.
diff --git a/scripts/redline/review.mjs b/scripts/redline/review.mjs
index 3decf13..53f33a5 100644
--- a/scripts/redline/review.mjs
+++ b/scripts/redline/review.mjs
@@ -64,6 +64,11 @@
 //   - A model dario refuses as unroutable (a 400 marked model_unroutable: no provider lists it) goes
 //     to REDLINE_FALLBACK_MODEL the same way: the same turn is sent again, with the history kept.
 //     A 400 without that marker still fails the run.
+//   - A fallback stays inside the reviewer's model family (modelFamily). Which family reads a
+//     repository is the host's choice: where one vendor's coding agent writes the pull requests,
+//     another vendor's model reads them, so the reviewer does not share the author's blind spots.
+//     A parked or unroutable model must not undo that choice, so a REDLINE_FALLBACK_MODEL of
+//     another family is not used: the run says why and fails as it does with no fallback.
 //
 // CLI (the workflow's review step):
 //   REPO=owner/name PR=<n> HEAD_SHA=<sha> CHECKOUT=<dir> GH_READ_TOKEN=... [REDLINE_REREAD=1] \
@@ -71,8 +76,9 @@
 // The env file holds DARIO_API_KEY, or DARIO_SOCKET (a dario key socket: the socket is the key, and
 // no key is stored or sent), and optionally REDLINE_GITHUB_TOKEN (or GITHUB_PAT_REVIEWER),
 // DARIO_URL (default http://127.0.0.1:3456), REDLINE_MODEL and REDLINE_FALLBACK_MODEL (default
-// claude-opus-5-5; set it empty for no fallback), and REDLINE_PIN_MODEL (default claude-opus-5-5,
-// the model for a PR that only moves Redline pins; set it empty to read those on REDLINE_MODEL), and
+// claude-opus-5-5, used only for a model of its own family; set it empty for no fallback), and
+// REDLINE_PIN_MODEL (default claude-opus-5-5, the model for a PR that only moves Redline pins; set
+// it empty to read those on REDLINE_MODEL), and
 // REDLINE_DEEP_MODEL (default empty, which turns it off: the model for a PR that deepReview() picks,
 // one touching workflows, auth, crypto, secrets or the like, or a large one), and REDLINE_DEEP_API_KEY
 // (default empty: the deep model's reads go on DARIO_API_KEY; set, they go on this dario key, so a
@@ -134,6 +140,23 @@ export function metaPhrase(text) {
 }
 export const DEFAULT_MODEL = 'claude-fable-5-1';
 export const DEFAULT_FALLBACK_MODEL = 'claude-opus-5-5';
+// The ids that name a vendor family outright. `o3` and `o4-mini` are OpenAI ids with no `gpt-`.
+const MODEL_FAMILIES = [['claude', /^claude-/], ['gpt', /^(?:gpt-|chatgpt-|codex-|o[0-9]+(?![a-z0-9]))/]];
+/**
+ * The family of a model id, lowercase: `claude` or `gpt` for an id MODEL_FAMILIES lists, else the
+ * id's leading run of letters and digits (`gemini-3-pro` is `gemini`). A provider path before the
+ * id (`vendor/claude-opus-5-5`) is not part of it. Empty when nothing is left to name one.
+ */
+export function modelFamily(model) {
+  const id = String(model ?? '').trim().toLowerCase().split('/').pop();
+  for (const [family, re] of MODEL_FAMILIES) if (re.test(id)) return family;
+  return /^[a-z0-9]+/.exec(id)?.[0] ?? '';
+}
+/** Whether two model ids are of one family. An id with no family is of none, not even its own. */
+export function sameModelFamily(a, b) {
+  const family = modelFamily(a);
+  return family !== '' && family === modelFamily(b);
+}
 // A PR that only moves Redline pins (redline-pin-bump.yml opens one in every caller) is read on
 // this model instead of REDLINE_MODEL: it changes one commit sha per caller file, and every bump
 // wave is a review in each caller, so it does not spend the reviewer model's seat.
@@ -1297,9 +1320,15 @@ export async function reviewChange(ctx, { pr, files, commits, reviews = [], prot
       });
     } catch (e) {
       if (!(e instanceof ModelParked || e instanceof ModelUnroutable) || !ctx.fallbackModel || ctx.fallbackModel === ctx.model) throw e;
-      ctx.log?.(e instanceof ModelParked
-        ? `${ctx.model} is parked in dario for ${Math.ceil(e.retryAfterMs / 1000)}s; the review continues on ${ctx.fallbackModel}`
-        : `no provider in dario lists ${ctx.model}; the review continues on ${ctx.fallbackModel}`);
+      const why = e instanceof ModelParked
+        ? `${ctx.model} is parked in dario for ${Math.ceil(e.retryAfterMs / 1000)}s`
+        : `no provider in dario lists ${ctx.model}`;
+      // The reviewer's family is the host's choice; a fallback does not change it.
+      if (!sameModelFamily(ctx.fallbackModel, ctx.model)) {
+        ctx.log?.(`${why}; the fallback ${ctx.fallbackModel} is of another model family, so it is not used and the review stops`);
+        throw e;
+      }
+      ctx.log?.(`${why}; the review continues on ${ctx.fallbackModel}`);
       ctx.model = ctx.fallbackModel;
       turn--;
       continue;
diff --git a/scripts/redline/review.test.mjs b/scripts/redline/review.test.mjs
index 380f72c..9efc3cc 100644
--- a/scripts/redline/review.test.mjs
+++ b/scripts/redline/review.test.mjs
@@ -636,6 +636,19 @@ console.log('\n  captured data (redline-protected)');
   const r = await runReview(ctx);
   check('429 and 5xx from the model are retried with backoff', r.verdict === 'APPROVE' && calls.sleeps === 2);
 }
+{
+  const f = reviewModule.modelFamily;
+  const same = reviewModule.sameModelFamily;
+  check('modelFamily names the vendor family of a listed id, whatever its case, date or effort suffix',
+    f('claude-fable-5-1') === 'claude' && f(' Claude-Opus-5-5-20261001 ') === 'claude' && f('gpt-5.6-terra') === 'gpt' && f('gpt-6-astra:high') === 'gpt'
+      && f('chatgpt-5') === 'gpt' && f('codex-2') === 'gpt' && f('o3') === 'gpt' && f('o4-mini') === 'gpt' && f('o3:high') === 'gpt');
+  check('an unlisted id is its leading run of letters and digits, and a provider path is not part of the family',
+    f('gemini-3-pro') === 'gemini' && f('opus') === 'opus' && f('o3x-1') === 'o3x' && f('m') === 'm' && f('vendor/claude-opus-5-5') === 'claude' && f('a/b/gpt-6-astra') === 'gpt');
+  check('an id that names nothing has no family', f('') === '' && f(null) === '' && f(undefined) === '' && f('vendor/') === '' && f('-x') === '');
+  check('sameModelFamily: one family is the same, two are not, and no family is never the same',
+    same('claude-fable-5-1', 'claude-opus-4-6') && same('gpt-5.6-terra', 'o3') && same('gemini-3-pro', 'gemini-3-flash')
+      && !same('gpt-6-astra', 'claude-opus-5-5') && !same('claude-opus-5-5', 'gpt-6-astra') && !same('', '') && !same('claude-opus-5-5', '') && !same(null, 'claude-opus-5-5'));
+}
 {
   const parked = (retryAfter) => [429, { 'x-dario-upstream-rejection': 'pool_parked', 'retry-after': String(retryAfter) }];
   {
@@ -669,18 +682,55 @@ console.log('\n  captured data (redline-protected)');
     const e = await throws(() => runReview(ctx));
     check('with no fallback a park fails the run at once', e instanceof reviewModule.ModelParked && calls.model.length === 1 && calls.sleeps === 0);
   }
+  {
+    const { ctx, calls } = world({ modelStatus: [parked(351838)], turns: [submit(APPROVE)] });
+    ctx.model = 'gpt-5.6-terra'; ctx.fallbackModel = 'claude-opus-5-5';
+    const lines = [];
+    ctx.log = (l) => lines.push(l);
+    const e = await throws(() => runReview(ctx));
+    check('a parked model does not hand the review to a fallback of another family: the run fails on the one call',
+      e instanceof reviewModule.ModelParked && calls.model.map((b) => b.model).join() === 'gpt-5.6-terra' && calls.sleeps === 0 && calls.posted.length === 0);
+    check('the refusal is logged with the park',
+      lines.some((l) => l.includes('gpt-5.6-terra is parked in dario for 351838s; the fallback claude-opus-5-5 is of another model family, so it is not used and the review stops'))
+        && !lines.some((l) => l.includes('the review continues on')));
+  }
+  {
+    const { ctx, calls } = world({ modelStatus: [parked(351838)], turns: [submit(APPROVE)] });
+    ctx.model = 'claude-fable-5-1'; ctx.fallbackModel = 'gpt-6-astra';
+    const e = await throws(() => runReview(ctx));
+    check('the rule holds the other way round: a parked Claude model does not fall back to a GPT one',
+      e instanceof reviewModule.ModelParked && calls.model.map((b) => b.model).join() === 'claude-fable-5-1' && calls.posted.length === 0);
+  }
+  {
+    const { ctx, calls } = world({ modelStatus: [parked(351838)], turns: [submit(APPROVE)] });
+    ctx.model = 'gpt-5.6-terra'; ctx.fallbackModel = 'gpt-6-astra';
+    const r = await runReview(ctx);
+    check('a fallback of the parked model\'s own family is used, whatever the family',
+      r.verdict === 'APPROVE' && calls.sleeps === 0 && calls.model.map((b) => b.model).join() === 'gpt-5.6-terra,gpt-6-astra');
+  }
   {
     const unroutable = [400, { 'x-dario-upstream-rejection': 'model_unroutable' }];
     {
       const { ctx, calls } = world({ modelStatus: [200, unroutable], turns: [use('redline_list', {}), submit(APPROVE)] });
-      ctx.model = 'gpt-6-astra'; ctx.fallbackModel = 'claude-opus-5-5';
+      ctx.model = 'gpt-6-astra'; ctx.fallbackModel = 'gpt-5.6-terra';
       const lines = [];
       ctx.log = (l) => lines.push(l);
       const r = await runReview(ctx);
       check('a model dario stops listing mid-review is not retried: the turn goes again on the fallback',
-        r.verdict === 'APPROVE' && calls.sleeps === 0 && calls.model.map((b) => b.model).join() === 'gpt-6-astra,gpt-6-astra,claude-opus-5-5'
+        r.verdict === 'APPROVE' && calls.sleeps === 0 && calls.model.map((b) => b.model).join() === 'gpt-6-astra,gpt-6-astra,gpt-5.6-terra'
           && JSON.stringify(calls.model[2].messages) === JSON.stringify(calls.model[1].messages));
-      check('the switch is logged with the reason', lines.some((l) => l.includes('no provider in dario lists gpt-6-astra; the review continues on claude-opus-5-5')));
+      check('the switch is logged with the reason', lines.some((l) => l.includes('no provider in dario lists gpt-6-astra; the review continues on gpt-5.6-terra')));
+    }
+    {
+      const { ctx, calls } = world({ modelStatus: [200, unroutable], turns: [use('redline_list', {}), submit(APPROVE)] });
+      ctx.model = 'gpt-6-astra'; ctx.fallbackModel = 'claude-opus-5-5';
+      const lines = [];
+      ctx.log = (l) => lines.push(l);
+      const e = await throws(() => runReview(ctx));
+      check('an unroutable model does not hand the review to a fallback of another family either',
+        e instanceof reviewModule.ModelUnroutable && calls.model.map((b) => b.model).join() === 'gpt-6-astra,gpt-6-astra' && calls.sleeps === 0 && calls.posted.length === 0);
+      check('the refusal is logged with the reason',
+        lines.some((l) => l.includes('no provider in dario lists gpt-6-astra; the fallback claude-opus-5-5 is of another model family, so it is not used and the review stops')));
     }
     {
       const { ctx, calls } = world({ modelStatus: [unroutable], turns: [submit(APPROVE)] });