diff --git a/src/embed.rs b/src/embed.rs
index 4b0fd9d..c80f7e3 100644
--- a/src/embed.rs
+++ b/src/embed.rs
@@ -1 +1,84 @@
-// implemented in a later task
+use crate::ionos::IonosClient;
+use crate::model::AppError;
+use sha2::{Digest, Sha256};
+use std::fs;
+use std::path::PathBuf;
+
+pub struct EmbeddingStore {
+    dir: PathBuf,    // <index_dir>/embeddings/<model-sanitized>/
+    model: String,
+}
+
+pub struct Estimate { pub items: usize, pub chars: usize }
+
+pub fn estimate(pending: &[String]) -> Estimate {
+    Estimate { items: pending.len(), chars: pending.iter().map(|s| s.len()).sum() }
+}
+
+impl EmbeddingStore {
+    pub fn key(model: &str, text: &str) -> String {
+        let mut h = Sha256::new();
+        h.update(model.as_bytes());
+        h.update([0u8]);
+        h.update(text.as_bytes());
+        hex(&h.finalize())
+    }
+
+    pub fn open(index_dir: &str, model: &str) -> Result<Self, AppError> {
+        let safe = model.replace('/', "_");
+        let dir = PathBuf::from(index_dir).join("embeddings").join(safe);
+        fs::create_dir_all(&dir)
+            .map_err(|e| AppError::Io(format!("{}: {e}", dir.display())))?;
+        Ok(Self { dir, model: model.to_string() })
+    }
+
+    fn path(&self, text: &str) -> PathBuf {
+        self.dir.join(format!("{}.f32", Self::key(&self.model, text)))
+    }
+
+    pub fn has(&self, text: &str) -> bool { self.path(text).exists() }
+
+    pub fn get(&self, text: &str) -> Option<Vec<f32>> {
+        let bytes = fs::read(self.path(text)).ok()?;
+        Some(bytes.chunks_exact(4)
+            .map(|c| f32::from_le_bytes([c[0], c[1], c[2], c[3]]))
+            .collect())
+    }
+
+    /// Atomic write (temp + rename) so an interrupted run never leaves
+    /// a half-written vector — the next run simply re-embeds that key.
+    pub fn put(&self, text: &str, v: &[f32]) -> Result<(), AppError> {
+        let mut bytes = Vec::with_capacity(v.len() * 4);
+        for f in v { bytes.extend_from_slice(&f.to_le_bytes()); }
+        let final_path = self.path(text);
+        let tmp = final_path.with_extension("tmp");
+        fs::write(&tmp, &bytes).map_err(|e| AppError::Io(format!("{e}")))?;
+        fs::rename(&tmp, &final_path).map_err(|e| AppError::Io(format!("{e}")))?;
+        Ok(())
+    }
+}
+
+fn hex(b: &[u8]) -> String {
+    b.iter().map(|x| format!("{:02x}", x)).collect()
+}
+
+/// Embed a batch via IONOS, persisting each result. Only `texts` not
+/// already cached should be passed (caller filters via `has`).
+pub fn embed_batch(
+    client: &IonosClient, store: &EmbeddingStore, model: &str, texts: &[String],
+) -> Result<usize, AppError> {
+    if texts.is_empty() { return Ok(0); }
+    let body = serde_json::json!({ "model": model, "input": texts });
+    let resp: serde_json::Value = client.post_json("/embeddings", &body)?;
+    let data = resp["data"].as_array()
+        .ok_or_else(|| AppError::Ionos("embeddings: no data[]".into()))?;
+    let mut n = 0;
+    for (i, item) in data.iter().enumerate() {
+        let v: Vec<f32> = item["embedding"].as_array()
+            .ok_or_else(|| AppError::Ionos("embeddings: no embedding[]".into()))?
+            .iter().map(|x| x.as_f64().unwrap_or(0.0) as f32).collect();
+        store.put(&texts[i], &v)?;
+        n += 1;
+    }
+    Ok(n)
+}
diff --git a/tests/embed_tests.rs b/tests/embed_tests.rs
new file mode 100644
index 0000000..43bbf80
--- /dev/null
+++ b/tests/embed_tests.rs
@@ -0,0 +1,28 @@
+use alpha_id::embed::{EmbeddingStore, estimate};
+
+#[test]
+fn cache_key_is_stable_sha256() {
+    let k1 = EmbeddingStore::key("BAAI/bge-m3", "diabetes");
+    let k2 = EmbeddingStore::key("BAAI/bge-m3", "diabetes");
+    assert_eq!(k1, k2);
+    assert_ne!(k1, EmbeddingStore::key("BAAI/bge-m3", "hypertonie"));
+    assert_eq!(k1.len(), 64); // hex sha256
+}
+
+#[test]
+fn store_persists_and_reloads_vectors_resumably() {
+    let dir = tempfile::tempdir().unwrap();
+    let store = EmbeddingStore::open(dir.path().to_str().unwrap(), "BAAI/bge-m3").unwrap();
+    assert!(store.get("diabetes").is_none());
+    store.put("diabetes", &[0.1, 0.2, 0.3]).unwrap();
+    let reopened = EmbeddingStore::open(dir.path().to_str().unwrap(), "BAAI/bge-m3").unwrap();
+    assert_eq!(reopened.get("diabetes").unwrap(), vec![0.1, 0.2, 0.3]);
+    assert!(reopened.has("diabetes"));            // resume: skip already-embedded
+}
+
+#[test]
+fn estimate_reports_pending_count_and_chars() {
+    let est = estimate(&["abc".to_string(), "de".to_string()]);
+    assert_eq!(est.items, 2);
+    assert_eq!(est.chars, 5);
+}