diff --git a/crates/at-blob/src/store.rs b/crates/at-blob/src/store.rs
index bf2f379..a355831 100644
--- a/crates/at-blob/src/store.rs
+++ b/crates/at-blob/src/store.rs
@@ -1,7 +1,10 @@
 use anyhow::Result;
 use async_trait::async_trait;
+use at_crypto::cid::{cid_for_raw, sha256};
 use bytes::Bytes;
 use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+use tokio::sync::RwLock;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct BlobInfo {
@@ -19,20 +22,116 @@ pub trait BlobStore: Send + Sync {
     async fn public_url(&self, key: &str) -> Result<String>;
 }
 
-pub struct InMemoryBlobStore;
+/// In-process blob store backed by a `HashMap` guarded by a
+/// [`tokio::sync::RwLock`]. Used in tests and any dev/single-node
+/// deployment that doesn't need a real object store.
+///
+/// Behaves like [`crate::s3::S3BlobStore`] with respect to `BlobInfo`
+/// (same CID computation over the raw bytes, same field semantics) so
+/// tests can swap one for the other transparently. Data does not
+/// survive process restarts.
+#[derive(Default)]
+pub struct InMemoryBlobStore {
+    blobs: RwLock<HashMap<String, (Bytes, String /* mime */)>>,
+}
+
+impl InMemoryBlobStore {
+    pub fn new() -> Self {
+        Self::default()
+    }
+}
 
 #[async_trait]
 impl BlobStore for InMemoryBlobStore {
-    async fn put(&self, _key: &str, _data: Bytes, _mime: &str) -> Result<BlobInfo> {
-        unimplemented!("in-memory blob store placeholder")
+    async fn put(&self, key: &str, data: Bytes, mime: &str) -> Result<BlobInfo> {
+        let hash = sha256(&data);
+        let cid = cid_for_raw(0x55, hash)?;
+        let size = data.len() as u64;
+        self.blobs
+            .write()
+            .await
+            .insert(key.to_string(), (data, mime.to_string()));
+        Ok(BlobInfo {
+            cid: cid.to_string(),
+            mime_type: mime.to_string(),
+            size,
+            storage_key: key.to_string(),
+        })
     }
-    async fn get(&self, _key: &str) -> Result<Option<Bytes>> {
-        unimplemented!()
+
+    async fn get(&self, key: &str) -> Result<Option<Bytes>> {
+        Ok(self
+            .blobs
+            .read()
+            .await
+            .get(key)
+            .map(|(data, _mime)| data.clone()))
     }
-    async fn delete(&self, _key: &str) -> Result<()> {
-        unimplemented!()
+
+    async fn delete(&self, key: &str) -> Result<()> {
+        self.blobs.write().await.remove(key);
+        Ok(())
     }
-    async fn public_url(&self, _key: &str) -> Result<String> {
-        unimplemented!()
+
+    async fn public_url(&self, key: &str) -> Result<String> {
+        Ok(format!("/blob/{key}"))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn put_get_roundtrip() {
+        let store = InMemoryBlobStore::new();
+        let data = Bytes::from_static(b"hello world");
+        let info = store.put("k1", data.clone(), "text/plain").await.unwrap();
+        assert_eq!(info.storage_key, "k1");
+        assert_eq!(info.mime_type, "text/plain");
+        assert_eq!(info.size, data.len() as u64);
+
+        let got = store.get("k1").await.unwrap();
+        assert_eq!(got, Some(data));
+    }
+
+    #[tokio::test]
+    async fn get_missing_key_returns_none() {
+        let store = InMemoryBlobStore::new();
+        let got = store.get("does-not-exist").await.unwrap();
+        assert_eq!(got, None);
+    }
+
+    #[tokio::test]
+    async fn delete_then_get_returns_none() {
+        let store = InMemoryBlobStore::new();
+        store
+            .put("k2", Bytes::from_static(b"data"), "application/octet-stream")
+            .await
+            .unwrap();
+        store.delete("k2").await.unwrap();
+        let got = store.get("k2").await.unwrap();
+        assert_eq!(got, None);
+    }
+
+    #[tokio::test]
+    async fn delete_missing_key_is_ok() {
+        let store = InMemoryBlobStore::new();
+        store.delete("never-existed").await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn same_bytes_produce_same_cid() {
+        let store = InMemoryBlobStore::new();
+        let data = Bytes::from_static(b"identical payload");
+        let info_a = store
+            .put("key-a", data.clone(), "application/octet-stream")
+            .await
+            .unwrap();
+        let info_b = store
+            .put("key-b", data.clone(), "application/octet-stream")
+            .await
+            .unwrap();
+        assert_eq!(info_a.cid, info_b.cid);
     }
 }