diff --git a/app/src/components/settings/panels/EmbeddingsPanel.tsx b/app/src/components/settings/panels/EmbeddingsPanel.tsx
index 8485ffb5b..b2849af2a 100644
--- a/app/src/components/settings/panels/EmbeddingsPanel.tsx
+++ b/app/src/components/settings/panels/EmbeddingsPanel.tsx
@@ -321,10 +321,20 @@ const EmbeddingsPanel = ({ embedded = false }: EmbeddingsPanelProps = {}) => {
         result.error === 'EMBEDDINGS_NO_MODEL_LOADED' ||
         result.error === 'EMBEDDINGS_VERIFICATION_FAILED'
       ) {
-        setSetupError(
+        // `result.message`/`result.detail` are backend-emitted (already
+        // context-specific); only the generic fallback is frontend-owned UI
+        // text, so route just that through useT() (#4056 CodeRabbit).
+        const baseMessage =
           typeof result.message === 'string'
             ? result.message
-            : "Couldn't verify the embeddings endpoint. Make sure it's running and serving an embedding model, then save again."
+            : t('settings.embeddings.verifyFallback');
+        // Append the underlying probe failure (HTTP status / server error body)
+        // so the user can self-diagnose instead of seeing only the generic
+        // message (#4056).
+        setSetupError(
+          typeof result.detail === 'string' && result.detail.trim()
+            ? `${baseMessage} (${result.detail})`
+            : baseMessage
         );
         setStatus({ kind: 'idle' });
         return;
diff --git a/app/src/components/settings/panels/__tests__/EmbeddingsPanel.test.tsx b/app/src/components/settings/panels/__tests__/EmbeddingsPanel.test.tsx
index b1380447e..265183aa8 100644
--- a/app/src/components/settings/panels/__tests__/EmbeddingsPanel.test.tsx
+++ b/app/src/components/settings/panels/__tests__/EmbeddingsPanel.test.tsx
@@ -522,6 +522,37 @@ describe('EmbeddingsPanel', () => {
     expect(screen.getByPlaceholderText(/https:\/\/your-endpoint/i)).toBeInTheDocument();
   });
 
+  it('appends the underlying probe detail to the verification error so the user can self-diagnose (#4056)', async () => {
+    // The issue asks for the underlying HTTP status / error body, not just the
+    // generic message. When the backend supplies `detail`, the panel appends it.
+    const settings = makeSettings({
+      providers: [
+        makeProvider('managed', { requires_api_key: false }),
+        makeProvider('custom', { requires_api_key: false, requires_endpoint: true }),
+      ],
+    });
+    vi.mocked(loadEmbeddingsSettings).mockResolvedValue(settings);
+    vi.mocked(updateEmbeddingsSettings).mockResolvedValue({
+      error: 'EMBEDDINGS_VERIFICATION_FAILED',
+      message: "Couldn't verify the embeddings endpoint — the test embed failed.",
+      detail: 'Embedding API error (401 Unauthorized): invalid api key',
+    });
+
+    renderWithProviders(<EmbeddingsPanel />);
+    await screen.findByText('Custom');
+
+    fireEvent.click(screen.getByRole('radio', { name: /custom/i }));
+    await screen.findByPlaceholderText(/https:\/\/your-endpoint/i);
+    fireEvent.change(screen.getByPlaceholderText(/https:\/\/your-endpoint/i), {
+      target: { value: 'https://api.example.com/v1' },
+    });
+    fireEvent.click(screen.getByRole('button', { name: /save.*switch/i }));
+
+    // The detail (HTTP status + body) is shown alongside the generic message.
+    await screen.findByText(/401 Unauthorized/i);
+    expect(screen.getByPlaceholderText(/https:\/\/your-endpoint/i)).toBeInTheDocument();
+  });
+
   // ─── Confirm wipe dialog ──────────────────────────────────────────────────
 
   it('shows confirm-wipe dialog when updateEmbeddingsSettings returns DIMENSION_CHANGE error', async () => {
diff --git a/app/src/lib/i18n/ar.ts b/app/src/lib/i18n/ar.ts
index 1b9eae62b..5560c9ecb 100644
--- a/app/src/lib/i18n/ar.ts
+++ b/app/src/lib/i18n/ar.ts
@@ -1264,6 +1264,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'جارٍ الاختبار…',
   'settings.embeddings.testSuccess': 'متصل — {dims} بُعد',
   'settings.embeddings.connectionTestFailed': 'فشل الاختبار',
+  'settings.embeddings.verifyFallback':
+    'تعذّر التحقق من نقطة نهاية التضمين. تأكد من أنها قيد التشغيل وتوفّر نموذج تضمين، ثم احفظ مرة أخرى.',
   'settings.embeddings.testFailed': 'فشل: {error}',
   'settings.embeddings.saving': 'جارٍ الحفظ…',
   'settings.embeddings.saved': 'تم الحفظ.',
diff --git a/app/src/lib/i18n/bn.ts b/app/src/lib/i18n/bn.ts
index 178d893b3..f24cf480f 100644
--- a/app/src/lib/i18n/bn.ts
+++ b/app/src/lib/i18n/bn.ts
@@ -1285,6 +1285,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'পরীক্ষা হচ্ছে…',
   'settings.embeddings.testSuccess': 'সংযুক্ত — {dims} মাত্রা',
   'settings.embeddings.connectionTestFailed': 'পরীক্ষা ব্যর্থ হয়েছে',
+  'settings.embeddings.verifyFallback':
+    'এমবেডিংস এন্ডপয়েন্ট যাচাই করা যায়নি। নিশ্চিত করুন এটি চলছে এবং একটি এমবেডিং মডেল পরিবেশন করছে, তারপর আবার সংরক্ষণ করুন।',
   'settings.embeddings.testFailed': 'ব্যর্থ: {error}',
   'settings.embeddings.saving': 'সংরক্ষণ হচ্ছে…',
   'settings.embeddings.saved': 'সংরক্ষিত।',
diff --git a/app/src/lib/i18n/de.ts b/app/src/lib/i18n/de.ts
index 79b108b86..e79e704dd 100644
--- a/app/src/lib/i18n/de.ts
+++ b/app/src/lib/i18n/de.ts
@@ -1320,6 +1320,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Wird getestet…',
   'settings.embeddings.testSuccess': 'Verbunden — {dims} Dimensionen',
   'settings.embeddings.connectionTestFailed': 'Test fehlgeschlagen',
+  'settings.embeddings.verifyFallback':
+    'Der Embeddings-Endpunkt konnte nicht überprüft werden. Stelle sicher, dass er läuft und ein Embedding-Modell bereitstellt, und speichere erneut.',
   'settings.embeddings.testFailed': 'Fehlgeschlagen: {error}',
   'settings.embeddings.saving': 'Wird gespeichert…',
   'settings.embeddings.saved': 'Gespeichert.',
diff --git a/app/src/lib/i18n/en.ts b/app/src/lib/i18n/en.ts
index e4e07e1db..ab344f082 100644
--- a/app/src/lib/i18n/en.ts
+++ b/app/src/lib/i18n/en.ts
@@ -1638,6 +1638,8 @@ const en: TranslationMap = {
   'settings.embeddings.testing': 'Testing…',
   'settings.embeddings.testSuccess': 'Connected — {dims} dimensions',
   'settings.embeddings.connectionTestFailed': 'Test failed',
+  'settings.embeddings.verifyFallback':
+    "Couldn't verify the embeddings endpoint. Make sure it's running and serving an embedding model, then save again.",
   'settings.embeddings.testFailed': 'Failed: {error}',
   'settings.embeddings.saving': 'Saving…',
   'settings.embeddings.saved': 'Saved.',
diff --git a/app/src/lib/i18n/es.ts b/app/src/lib/i18n/es.ts
index c422a1c99..987669776 100644
--- a/app/src/lib/i18n/es.ts
+++ b/app/src/lib/i18n/es.ts
@@ -1313,6 +1313,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Probando…',
   'settings.embeddings.testSuccess': 'Conectado — {dims} dimensiones',
   'settings.embeddings.connectionTestFailed': 'La prueba falló',
+  'settings.embeddings.verifyFallback':
+    'No se pudo verificar el endpoint de embeddings. Asegúrate de que esté en ejecución y sirviendo un modelo de embeddings, luego guarda de nuevo.',
   'settings.embeddings.testFailed': 'Fallido: {error}',
   'settings.embeddings.saving': 'Guardando…',
   'settings.embeddings.saved': 'Guardado.',
diff --git a/app/src/lib/i18n/fr.ts b/app/src/lib/i18n/fr.ts
index ebd75c358..408b578c1 100644
--- a/app/src/lib/i18n/fr.ts
+++ b/app/src/lib/i18n/fr.ts
@@ -1320,6 +1320,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Test en cours…',
   'settings.embeddings.testSuccess': 'Connecté — {dims} dimensions',
   'settings.embeddings.connectionTestFailed': 'Test échoué',
+  'settings.embeddings.verifyFallback':
+    "Impossible de vérifier le point de terminaison d'embeddings. Vérifiez qu'il fonctionne et qu'il propose un modèle d'embedding, puis enregistrez à nouveau.",
   'settings.embeddings.testFailed': 'Échec : {error}',
   'settings.embeddings.saving': 'Enregistrement…',
   'settings.embeddings.saved': 'Enregistré.',
diff --git a/app/src/lib/i18n/hi.ts b/app/src/lib/i18n/hi.ts
index 6fa21abac..37c332f32 100644
--- a/app/src/lib/i18n/hi.ts
+++ b/app/src/lib/i18n/hi.ts
@@ -1284,6 +1284,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'परीक्षण हो रहा है…',
   'settings.embeddings.testSuccess': 'कनेक्ट — {dims} आयाम',
   'settings.embeddings.connectionTestFailed': 'परीक्षण विफल',
+  'settings.embeddings.verifyFallback':
+    'एम्बेडिंग एंडपॉइंट सत्यापित नहीं किया जा सका। सुनिश्चित करें कि यह चल रहा है और एक एम्बेडिंग मॉडल प्रदान कर रहा है, फिर दोबारा सहेजें।',
   'settings.embeddings.testFailed': 'विफल: {error}',
   'settings.embeddings.saving': 'सहेजा जा रहा है…',
   'settings.embeddings.saved': 'सहेजा गया।',
diff --git a/app/src/lib/i18n/id.ts b/app/src/lib/i18n/id.ts
index ecc1f270f..a64e001e7 100644
--- a/app/src/lib/i18n/id.ts
+++ b/app/src/lib/i18n/id.ts
@@ -1293,6 +1293,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Menguji…',
   'settings.embeddings.testSuccess': 'Terhubung — {dims} dimensi',
   'settings.embeddings.connectionTestFailed': 'Pengujian gagal',
+  'settings.embeddings.verifyFallback':
+    'Tidak dapat memverifikasi endpoint embeddings. Pastikan endpoint berjalan dan menyediakan model embedding, lalu simpan lagi.',
   'settings.embeddings.testFailed': 'Gagal: {error}',
   'settings.embeddings.saving': 'Menyimpan…',
   'settings.embeddings.saved': 'Tersimpan.',
diff --git a/app/src/lib/i18n/it.ts b/app/src/lib/i18n/it.ts
index 67cc88975..404c3363c 100644
--- a/app/src/lib/i18n/it.ts
+++ b/app/src/lib/i18n/it.ts
@@ -1311,6 +1311,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Test in corso…',
   'settings.embeddings.testSuccess': 'Connesso — {dims} dimensioni',
   'settings.embeddings.connectionTestFailed': 'Test non riuscito',
+  'settings.embeddings.verifyFallback':
+    "Impossibile verificare l'endpoint di embedding. Assicurati che sia in esecuzione e fornisca un modello di embedding, poi salva di nuovo.",
   'settings.embeddings.testFailed': 'Fallito: {error}',
   'settings.embeddings.saving': 'Salvataggio…',
   'settings.embeddings.saved': 'Salvato.',
diff --git a/app/src/lib/i18n/ko.ts b/app/src/lib/i18n/ko.ts
index 261f9dad1..62c418771 100644
--- a/app/src/lib/i18n/ko.ts
+++ b/app/src/lib/i18n/ko.ts
@@ -1281,6 +1281,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': '테스트 중…',
   'settings.embeddings.testSuccess': '연결됨 — {dims} 차원',
   'settings.embeddings.connectionTestFailed': '테스트 실패',
+  'settings.embeddings.verifyFallback':
+    '임베딩 엔드포인트를 확인할 수 없습니다. 엔드포인트가 실행 중이고 임베딩 모델을 제공하는지 확인한 후 다시 저장하세요.',
   'settings.embeddings.testFailed': '실패: {error}',
   'settings.embeddings.saving': '저장 중…',
   'settings.embeddings.saved': '저장됨.',
diff --git a/app/src/lib/i18n/pl.ts b/app/src/lib/i18n/pl.ts
index 9b30cdb3f..16dc3cb13 100644
--- a/app/src/lib/i18n/pl.ts
+++ b/app/src/lib/i18n/pl.ts
@@ -1303,6 +1303,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Testowanie…',
   'settings.embeddings.testSuccess': 'Połączono — wymiarów: {dims}',
   'settings.embeddings.connectionTestFailed': 'Test nie powiódł się',
+  'settings.embeddings.verifyFallback':
+    'Nie można zweryfikować punktu końcowego osadzeń. Upewnij się, że działa i udostępnia model osadzeń, a następnie zapisz ponownie.',
   'settings.embeddings.testFailed': 'Niepowodzenie: {error}',
   'settings.embeddings.saving': 'Zapisywanie…',
   'settings.embeddings.saved': 'Zapisano.',
diff --git a/app/src/lib/i18n/pt.ts b/app/src/lib/i18n/pt.ts
index 576fc1a76..2dc21b516 100644
--- a/app/src/lib/i18n/pt.ts
+++ b/app/src/lib/i18n/pt.ts
@@ -1317,6 +1317,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Testando…',
   'settings.embeddings.testSuccess': 'Conectado — {dims} dimensões',
   'settings.embeddings.connectionTestFailed': 'Teste falhou',
+  'settings.embeddings.verifyFallback':
+    'Não foi possível verificar o endpoint de embeddings. Verifique se ele está em execução e fornecendo um modelo de embedding e salve novamente.',
   'settings.embeddings.testFailed': 'Falhou: {error}',
   'settings.embeddings.saving': 'Salvando…',
   'settings.embeddings.saved': 'Salvo.',
diff --git a/app/src/lib/i18n/ru.ts b/app/src/lib/i18n/ru.ts
index 7b24cd579..f7c1d95ed 100644
--- a/app/src/lib/i18n/ru.ts
+++ b/app/src/lib/i18n/ru.ts
@@ -1301,6 +1301,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': 'Проверка…',
   'settings.embeddings.testSuccess': 'Подключено — {dims} измерений',
   'settings.embeddings.connectionTestFailed': 'Проверка не удалась',
+  'settings.embeddings.verifyFallback':
+    'Не удалось проверить конечную точку эмбеддингов. Убедитесь, что она запущена и предоставляет модель эмбеддингов, затем сохраните снова.',
   'settings.embeddings.testFailed': 'Ошибка: {error}',
   'settings.embeddings.saving': 'Сохранение…',
   'settings.embeddings.saved': 'Сохранено.',
diff --git a/app/src/lib/i18n/zh-CN.ts b/app/src/lib/i18n/zh-CN.ts
index b928db921..caad352a7 100644
--- a/app/src/lib/i18n/zh-CN.ts
+++ b/app/src/lib/i18n/zh-CN.ts
@@ -1224,6 +1224,8 @@ const messages: TranslationMap = {
   'settings.embeddings.testing': '测试中…',
   'settings.embeddings.testSuccess': '已连接 — {dims} 维度',
   'settings.embeddings.connectionTestFailed': '测试失败',
+  'settings.embeddings.verifyFallback':
+    '无法验证嵌入端点。请确认其正在运行并提供嵌入模型，然后重新保存。',
   'settings.embeddings.testFailed': '失败：{error}',
   'settings.embeddings.saving': '保存中…',
   'settings.embeddings.saved': '已保存。',
diff --git a/app/src/services/api/embeddingsApi.ts b/app/src/services/api/embeddingsApi.ts
index a11f11771..29b6045d6 100644
--- a/app/src/services/api/embeddingsApi.ts
+++ b/app/src/services/api/embeddingsApi.ts
@@ -44,6 +44,8 @@ export interface EmbeddingsUpdateResult {
   /** Present when confirm_wipe was required but not supplied */
   error?: string;
   message?: string;
+  /** Underlying probe failure (HTTP status / server error body) for diagnosis (#4056) */
+  detail?: string;
   old_signature?: string;
 }
 
diff --git a/src/openhuman/embeddings/factory.rs b/src/openhuman/embeddings/factory.rs
index 27cbf9940..8eec9f635 100644
--- a/src/openhuman/embeddings/factory.rs
+++ b/src/openhuman/embeddings/factory.rs
@@ -16,7 +16,7 @@ use super::{NoopEmbedding, OllamaEmbedding, OpenAiEmbedding};
 /// arbitrary OpenAI-compatible servers (vLLM, text-embeddings-inference,
 /// stricter LocalAI builds) makes those servers 400 on an unknown field, so we
 /// gate on the model id rather than the provider kind. (Reviewer sanil-23, #3076.)
-fn model_supports_dimensions(model: &str) -> bool {
+pub(crate) fn model_supports_dimensions(model: &str) -> bool {
     model.starts_with("text-embedding-3-")
 }
 
diff --git a/src/openhuman/embeddings/mod.rs b/src/openhuman/embeddings/mod.rs
index 0721057fb..86c02db6d 100644
--- a/src/openhuman/embeddings/mod.rs
+++ b/src/openhuman/embeddings/mod.rs
@@ -40,6 +40,10 @@ pub use factory::{
     create_embedding_provider, create_embedding_provider_with_credentials,
     default_embedding_provider, default_local_embedding_provider,
 };
+// `pub(crate)` helper — reused by the memory-tree OpenAI-compat adapter to gate
+// configs whose dimension the fixed-1024 tree can't store (#4056). Not part of
+// the public surface, so it can't ride the `pub use` above (E0364).
+pub(crate) use factory::model_supports_dimensions;
 // #002 FR-015: the memory-tree OpenAI-compat embedder reuses the same key
 // resolution the embeddings RPC uses, so there is one source of truth.
 pub use noop::NoopEmbedding;
diff --git a/src/openhuman/embeddings/openai_tests.rs b/src/openhuman/embeddings/openai_tests.rs
index ee26243e9..ba906ce2f 100644
--- a/src/openhuman/embeddings/openai_tests.rs
+++ b/src/openhuman/embeddings/openai_tests.rs
@@ -525,6 +525,28 @@ async fn embed_dimension_mismatch() {
     assert!(err.to_string().contains("dimension mismatch"));
 }
 
+/// Issue #4056: a `dims == 0` provider is the dimension-agnostic verification
+/// probe — it must NOT enforce any length, so an endpoint returning its own
+/// native size passes instead of being rejected. This is what lets a Custom
+/// endpoint verify when the user's guessed `dimensions` differs from the
+/// model's native output; the caller then adopts the returned length.
+#[tokio::test]
+async fn embed_dims_zero_skips_dimension_guard() {
+    let app = Router::new().route(
+        "/v1/embeddings",
+        post(|| async {
+            Json(serde_json::json!({
+                "data": [{ "embedding": [1.0, 2.0, 3.0, 4.0, 5.0] }]
+            }))
+        }),
+    );
+    let url = start_mock(app).await;
+    let p = OpenAiEmbedding::new(&url, "k", "m", 0);
+
+    let result = p.embed(&["hi"]).await.unwrap();
+    assert_eq!(result[0].len(), 5, "dims=0 must accept the native length");
+}
+
 #[tokio::test]
 async fn embed_malformed_json() {
     let app = Router::new().route(
diff --git a/src/openhuman/embeddings/rpc.rs b/src/openhuman/embeddings/rpc.rs
index 54fbdc811..609f54545 100644
--- a/src/openhuman/embeddings/rpc.rs
+++ b/src/openhuman/embeddings/rpc.rs
@@ -7,10 +7,51 @@ use crate::openhuman::credentials::AuthService;
 use crate::rpc::RpcOutcome;
 
 use super::catalog;
-use super::factory::create_embedding_provider_with_credentials;
+use super::factory::{create_embedding_provider_with_credentials, model_supports_dimensions};
 
 const LOG_PREFIX: &str = "[embeddings::rpc]";
 
+/// Dimension to run a Custom (OpenAI-compatible) verification probe at.
+///
+/// The user-entered `dimensions` field is a guess: for any model outside the
+/// `text-embedding-3-*` family we never send the OpenAI `dimensions` request
+/// param (see [`model_supports_dimensions`]), so the endpoint returns its own
+/// native vector length. Forcing the probe to enforce the guessed length makes
+/// every reachable, valid embedding endpoint fail verification whenever the
+/// guess (default 1024) differs from the native size — the root cause of
+/// issue #4056.
+///
+/// So we probe a `text-embedding-3-*` model at the configured size (the server
+/// honours the param and returns exactly that), but probe every other model at
+/// `0`, which disables both the request param and the post-response length
+/// guard in `OpenAiEmbedding::embed` — the probe then only has to prove the
+/// endpoint can embed, and we learn the real dimension from the returned
+/// vector (see [`final_probe_dims`]).
+fn probe_dims_for(model: &str, configured: usize) -> usize {
+    if model_supports_dimensions(model) {
+        configured
+    } else {
+        0
+    }
+}
+
+/// Dimension to persist after a successful Custom verification probe.
+///
+/// For a `text-embedding-3-*` model the endpoint honoured the requested size,
+/// so keep the user's `configured` value (Matryoshka). For every other model we
+/// probed dimension-agnostically, so adopt the endpoint's actual returned
+/// length (`actual`) — the user can't be expected to know it, and storing the
+/// real size is what lets the live embed path's length guard pass afterwards.
+/// Falls back to `configured` if the probe somehow reported a zero-length
+/// vector (defensive — `classify_embed_probe` already rejects empty vectors).
+fn final_probe_dims(model: &str, configured: usize, actual: usize) -> usize {
+    if model_supports_dimensions(model) || actual == 0 {
+        configured
+    } else {
+        actual
+    }
+}
+
 /// Returns the current embedding settings plus the provider catalog.
 pub async fn get_settings(config: &Config) -> Result<RpcOutcome<serde_json::Value>, String> {
     let provider = &config.memory.embedding_provider;
@@ -102,12 +143,15 @@ pub async fn update_settings(
     let new_model = model
         .clone()
         .unwrap_or_else(|| config.memory.embedding_model.clone());
-    let new_dims = dimensions.unwrap_or(config.memory.embedding_dimensions);
-    let new_sig = format_embedding_signature(&new_provider, &new_model, new_dims);
+    // `new_dims`/`new_sig`/`dims_changed` are recomputed after the Custom
+    // verification probe auto-detects the endpoint's real vector length
+    // (issue #4056), so they must be mutable.
+    let mut new_dims = dimensions.unwrap_or(config.memory.embedding_dimensions);
+    let mut new_sig = format_embedding_signature(&new_provider, &new_model, new_dims);
 
     let old_dims = config.memory.embedding_dimensions;
-    let dims_changed = new_dims != old_dims;
-    let sig_changed = new_sig != old_sig;
+    let mut dims_changed = new_dims != old_dims;
+    let mut sig_changed = new_sig != old_sig;
 
     // Setup-time verification gate (TAURI-RUST-5JR / 4P4): a Custom
     // (OpenAI-compatible) embeddings endpoint — e.g. LM Studio — must prove it
@@ -131,11 +175,16 @@ pub async fn update_settings(
         _ => new_provider.clone(),
     };
     if effective_provider.starts_with("custom:") {
-        match build_embedder(&config, &effective_provider, &new_model, new_dims) {
+        // Probe dimension-agnostically for non-`text-embedding-3-*` models so the
+        // user's guessed `dimensions` can't fail an otherwise-valid endpoint; the
+        // real length is detected from the returned vector below (issue #4056).
+        let probe_dims = probe_dims_for(&new_model, new_dims);
+        match build_embedder(&config, &effective_provider, &new_model, probe_dims) {
             Ok(embedder) => {
                 // Time-box the probe so a black-hole host can't hang the RPC.
                 tracing::debug!(
                     provider = effective_provider.as_str(),
+                    probe_dims,
                     "{LOG_PREFIX} update_settings verifying embeddings endpoint with a test embed"
                 );
                 let probe = tokio::time::timeout(
@@ -150,6 +199,12 @@ pub async fn update_settings(
                     Ok(Err(e)) => EmbedProbe::Failed(e.to_string()),
                     Err(_elapsed) => EmbedProbe::TimedOut,
                 };
+                // Peek the actual vector length before the policy consumes the
+                // outcome — on a pass this is the endpoint's real dimension.
+                let probe_actual_dims = match &outcome {
+                    EmbedProbe::Returned(vectors) => vectors.first().map(|v| v.len()).unwrap_or(0),
+                    _ => 0,
+                };
                 if let Some(reject) = classify_embed_probe(outcome) {
                     tracing::warn!(
                         provider = effective_provider.as_str(),
@@ -207,8 +262,28 @@ pub async fn update_settings(
                     }
                     return Ok(reject);
                 }
+                // Passed. Adopt the endpoint's real vector length for every model
+                // we probed dimension-agnostically — the user can't be expected to
+                // know it, and storing the actual size is what keeps the live embed
+                // path's length guard from rejecting future embeds (issue #4056).
+                // `text-embedding-3-*` keeps the requested size (server honoured it).
+                let detected_dims = final_probe_dims(&new_model, new_dims, probe_actual_dims);
+                if detected_dims != new_dims {
+                    tracing::info!(
+                        provider = effective_provider.as_str(),
+                        model = new_model.as_str(),
+                        requested = new_dims,
+                        detected = detected_dims,
+                        "{LOG_PREFIX} update_settings auto-detected custom embedding dimension from probe"
+                    );
+                    new_dims = detected_dims;
+                    new_sig = format_embedding_signature(&new_provider, &new_model, new_dims);
+                    dims_changed = new_dims != old_dims;
+                    sig_changed = new_sig != old_sig;
+                }
                 tracing::debug!(
                     provider = effective_provider.as_str(),
+                    new_dims,
                     "{LOG_PREFIX} update_settings test embed passed — accepting config"
                 );
             }
@@ -262,9 +337,11 @@ pub async fn update_settings(
     if let Some(m) = &model {
         config.memory.embedding_model = m.clone();
     }
-    if let Some(d) = dimensions {
-        config.memory.embedding_dimensions = d;
-    }
+    // Persist `new_dims`, not the raw `dimensions` arg: the Custom verification
+    // probe may have auto-detected the endpoint's real length (issue #4056), and
+    // `new_dims` already defaults to the stored value when neither a new arg nor
+    // detection changed it — so this is a no-op for the unchanged case.
+    config.memory.embedding_dimensions = new_dims;
     if let Some(rl) = rate_limit_per_min {
         config.memory.embedding_rate_limit_per_min = rl;
     }
@@ -443,10 +520,21 @@ pub async fn test_connection(
         slug
     };
 
+    // Probe a Custom endpoint dimension-agnostically (issue #4056): the user's
+    // `dims` is a guess, so enforcing it here would make a valid endpoint fail
+    // the Test-connection button whenever the guess differs from the native
+    // size. Catalog providers keep their fixed `dims`. We still report the
+    // requested vs actual dimensions in the payload below.
+    let probe_dims = if provider_tag == "custom" {
+        probe_dims_for(model, dims)
+    } else {
+        dims
+    };
+
     let embedder = create_embedding_provider_with_credentials(
         provider_tag,
         model,
-        dims,
+        probe_dims,
         &api_key,
         custom_endpoint.as_deref(),
     )
@@ -456,6 +544,7 @@ pub async fn test_connection(
         provider = provider_tag,
         model,
         dims,
+        probe_dims,
         "{LOG_PREFIX} test_connection starting"
     );
 
@@ -790,6 +879,39 @@ mod tests {
         );
     }
 
+    /// Issue #4056: a Custom endpoint is probed dimension-agnostically for any
+    /// model that doesn't honour the OpenAI `dimensions` request param, so the
+    /// user's guessed size can't fail an otherwise-valid endpoint. Only the
+    /// `text-embedding-3-*` family (which honours the param) is probed at the
+    /// requested size.
+    #[test]
+    fn probe_dims_for_zeroes_non_matryoshka_models() {
+        // text-embedding-3-* honours the param → probe at the requested size.
+        assert_eq!(probe_dims_for("text-embedding-3-large", 1024), 1024);
+        assert_eq!(probe_dims_for("text-embedding-3-small", 512), 512);
+        // Everything else → 0 (no param sent, no length guard).
+        assert_eq!(probe_dims_for("bge-m3", 1024), 0);
+        assert_eq!(probe_dims_for("nomic-embed-text", 768), 0);
+        assert_eq!(probe_dims_for("gpt-5-mini", 1024), 0);
+    }
+
+    /// Issue #4056: after a successful probe we adopt the endpoint's real
+    /// returned length for auto-detected models, but keep the requested size for
+    /// `text-embedding-3-*` (the server returned exactly that). A zero actual
+    /// (defensive — empty vectors are already rejected upstream) falls back to
+    /// the configured value.
+    #[test]
+    fn final_probe_dims_adopts_actual_for_auto_detected_models() {
+        // Auto-detected model → adopt the real length, ignoring the guess.
+        assert_eq!(final_probe_dims("bge-m3", 1024, 1024), 1024);
+        assert_eq!(final_probe_dims("bge-m3", 1024, 768), 768);
+        assert_eq!(final_probe_dims("nomic-embed-text", 1024, 768), 768);
+        // text-embedding-3-* → keep the requested size (param was honoured).
+        assert_eq!(final_probe_dims("text-embedding-3-large", 1024, 3072), 1024);
+        // Defensive: zero actual falls back to the configured value.
+        assert_eq!(final_probe_dims("bge-m3", 1024, 0), 1024);
+    }
+
     #[test]
     fn normalize_embed_model_id_strips_prefix_and_tag() {
         assert_eq!(normalize_embed_model_id("text-embedding-bge-m3"), "bge-m3");
diff --git a/src/openhuman/memory_tree/score/embed/openai_compat.rs b/src/openhuman/memory_tree/score/embed/openai_compat.rs
index 9a5296c07..3a72fc4e0 100644
--- a/src/openhuman/memory_tree/score/embed/openai_compat.rs
+++ b/src/openhuman/memory_tree/score/embed/openai_compat.rs
@@ -139,6 +139,26 @@ impl OpenAiCompatEmbedder {
             }
         };
 
+        // The memory tree's on-disk format is fixed at [`EMBEDDING_DIM`]. Models
+        // that don't honour the OpenAI `dimensions` request param (everything
+        // outside `text-embedding-3-*`) return their own native length, so a
+        // config whose stored dimension isn't `EMBEDDING_DIM` can never satisfy
+        // the tree. Building the adapter anyway would only defer the failure to
+        // the first embed ("expected 1024, got N") — refuse it here with an
+        // actionable message instead (Codex review on #4056). `text-embedding-3-*`
+        // is exempt: we request `EMBEDDING_DIM` below and the server reduces to it.
+        if !crate::openhuman::embeddings::model_supports_dimensions(model)
+            && config.memory.embedding_dimensions != EMBEDDING_DIM
+        {
+            anyhow::bail!(
+                "embeddings provider '{provider}' (model '{model}') produces \
+                 {}-dimensional vectors, but the memory tree requires {EMBEDDING_DIM}. \
+                 Choose a {EMBEDDING_DIM}-dimension model — an OpenAI `text-embedding-3-*` \
+                 model, or a {EMBEDDING_DIM}-dim model such as `mxbai-embed-large` or `bge-large`.",
+                config.memory.embedding_dimensions
+            );
+        }
+
         let inner = crate::openhuman::embeddings::create_embedding_provider_with_credentials(
             slug,
             model,
@@ -329,4 +349,57 @@ mod tests {
             assert!(got.is_none(), "{p} must fall through, got Some");
         }
     }
+
+    /// Codex review on #4056: a custom config whose stored dimension isn't the
+    /// tree's fixed [`EMBEDDING_DIM`] (and whose model can't reduce to it via the
+    /// OpenAI `dimensions` param) must be refused at construction with a clear,
+    /// actionable error — not built and then failed at the first embed with a raw
+    /// "expected 1024, got N". This is what keeps an auto-detected non-1024 custom
+    /// endpoint (which the embeddings RPC still accepts) out of the 1024-only tree.
+    #[test]
+    fn err_for_non_reducible_model_with_incompatible_dimension() {
+        let (_tmp, mut cfg) = cfg_with_provider("custom:https://embed.example/v1");
+        cfg.memory.embedding_model = "nomic-embed-text".to_string(); // not text-embedding-3-*
+        cfg.memory.embedding_dimensions = 768; // != EMBEDDING_DIM (1024)
+                                               // `expect_err` would require the Ok type (the embedder) to impl Debug,
+                                               // which it can't (boxed trait object) — match instead.
+        let err = match OpenAiCompatEmbedder::try_from_config(&cfg) {
+            Err(e) => e,
+            Ok(_) => panic!("768 != tree dim must error, got Ok"),
+        };
+        let msg = format!("{err:#}");
+        assert!(
+            msg.contains("768") && msg.contains(&EMBEDDING_DIM.to_string()),
+            "error must name both the model's dim and the required dim: {msg}"
+        );
+    }
+
+    /// A non-reducible model that natively matches [`EMBEDDING_DIM`] still builds —
+    /// only an incompatible dimension is refused.
+    #[test]
+    fn some_for_non_reducible_model_at_tree_dimension() {
+        let (_tmp, mut cfg) = cfg_with_provider("custom:https://embed.example/v1");
+        cfg.memory.embedding_model = "mxbai-embed-large".to_string();
+        cfg.memory.embedding_dimensions = EMBEDDING_DIM; // 1024
+        let got = OpenAiCompatEmbedder::try_from_config(&cfg).expect("no error");
+        assert!(
+            got.is_some(),
+            "a 1024-native custom model must build the tree adapter"
+        );
+    }
+
+    /// `text-embedding-3-*` is exempt from the dimension guard: the adapter
+    /// requests `EMBEDDING_DIM` and the server reduces to it, so even a config
+    /// stored at a different dimension still builds.
+    #[test]
+    fn some_for_reducible_model_regardless_of_stored_dimension() {
+        let (_tmp, mut cfg) = cfg_with_provider("openai");
+        cfg.memory.embedding_model = "text-embedding-3-large".to_string();
+        cfg.memory.embedding_dimensions = 256; // reducible — tree still requests 1024
+        let got = OpenAiCompatEmbedder::try_from_config(&cfg).expect("no error");
+        assert!(
+            got.is_some(),
+            "reducible model must build regardless of stored dim"
+        );
+    }
 }