diff --git a/crates/miroir-core/src/api_error.rs b/crates/miroir-core/src/api_error.rs
index 5bcb1c2..9e0a88d 100644
--- a/crates/miroir-core/src/api_error.rs
+++ b/crates/miroir-core/src/api_error.rs
@@ -55,8 +55,7 @@ pub enum MiroirCode {
 
 impl MiroirCode {
     /// All variants, used for iteration in tests.
-    #[cfg(test)]
-    const ALL: [MiroirCode; 14] = [
+    pub const ALL: [MiroirCode; 14] = [
         MiroirCode::PrimaryKeyRequired,
         MiroirCode::NoQuorum,
         MiroirCode::ShardUnavailable,
@@ -157,7 +156,7 @@ impl MiroirCode {
 ///
 /// Both Miroir-specific and forwarded Meilisearch-native errors use this shape
 /// so that existing SDK error handling branches remain functional.
-#[derive(Debug, Clone, thiserror::Error, Serialize)]
+#[derive(Debug, Clone, thiserror::Error, Serialize, serde::Deserialize)]
 #[error("{message}")]
 pub struct MeilisearchError {
     pub message: String,
diff --git a/crates/miroir-core/src/scatter.rs b/crates/miroir-core/src/scatter.rs
index 92ccc9f..f7f36b9 100644
--- a/crates/miroir-core/src/scatter.rs
+++ b/crates/miroir-core/src/scatter.rs
@@ -444,7 +444,7 @@ pub async fn plan_search_scatter_with_version_floor(
     version_checker: &impl Fn(&str, &str) -> u64,
     replica_selector: Option<&ReplicaSelector>,
 ) -> Option<ScatterPlan> {
-    let chosen_group = query_group(query_seq, topology.replica_group_count());
+    let chosen_group = crate::router::query_group_active(query_seq, topology);
 
     let group = topology.group(chosen_group)?;
 
@@ -551,7 +551,7 @@ pub async fn plan_search_scatter_adaptive(
     shard_count: u32,
     replica_selector: &ReplicaSelector,
 ) -> ScatterPlan {
-    let chosen_group = query_group(query_seq, topology.replica_group_count());
+    let chosen_group = crate::router::query_group_active(query_seq, topology);
 
     let group = match topology.group(chosen_group) {
         Some(g) => g,
diff --git a/crates/miroir-core/tests/p28_api_compatibility.rs b/crates/miroir-core/tests/p28_api_compatibility.rs
new file mode 100644
index 0000000..e61bec2
--- /dev/null
+++ b/crates/miroir-core/tests/p28_api_compatibility.rs
@@ -0,0 +1,256 @@
+//! P2.8 API compatibility tests — Phase 2 Definition of Done.
+//!
+//! Tests that verify:
+//! 1. Error format parity with Meilisearch (plan §5)
+//! 2. GET /_miroir/topology matches plan §10 JSON shape
+
+use miroir_core::api_error::{ErrorType, MeilisearchError, MiroirCode};
+use serde_json::json;
+
+/// Test 1: All Miroir error codes produce the correct Meilisearch-compatible shape.
+///
+/// Per plan §5, every error must match the shape:
+/// {"message": "...", "code": "...", "type": "...", "link": "..."}
+#[test]
+fn test_all_miroir_error_codes_have_correct_shape() {
+    for code in MiroirCode::ALL {
+        let err = MeilisearchError::new(code, "test error message");
+
+        // Serialize to JSON
+        let json_val = serde_json::to_value(&err).expect("failed to serialize error");
+
+        // Verify all required fields exist
+        assert!(json_val.get("message").is_some(), "message field missing for {:?}", code);
+        assert!(json_val.get("code").is_some(), "code field missing for {:?}", code);
+        assert!(json_val.get("type").is_some(), "type field missing for {:?}", code);
+        assert!(json_val.get("link").is_some(), "link field missing for {:?}", code);
+
+        // Verify field types
+        assert_eq!(json_val["message"], "test error message");
+        assert_eq!(json_val["code"], code.as_str());
+        assert_eq!(json_val["type"], code.error_type().to_string());
+        assert!(json_val["link"].as_str().unwrap().starts_with("https://github.com/jedarden/miroir"));
+    }
+}
+
+/// Test 2: Error code strings match the expected miroir_ prefix pattern.
+#[test]
+fn test_error_code_strings_have_miroir_prefix() {
+    for code in MiroirCode::ALL {
+        let code_str = code.as_str();
+        assert!(
+            code_str.starts_with("miroir_"),
+            "Error code {:?} ({}) does not start with 'miroir_'",
+            code,
+            code_str
+        );
+    }
+}
+
+/// Test 3: HTTP status codes match Meilisearch conventions.
+#[test]
+fn test_http_status_codes_match_meilisearch_conventions() {
+    // Invalid request errors → 400
+    assert_eq!(MiroirCode::PrimaryKeyRequired.http_status(), 400);
+    assert_eq!(MiroirCode::ReservedField.http_status(), 400);
+
+    // Auth errors → 401
+    assert_eq!(MiroirCode::JwtInvalid.http_status(), 401);
+    assert_eq!(MiroirCode::InvalidAuth.http_status(), 401);
+    assert_eq!(MiroirCode::MissingCsrf.http_status(), 401);
+
+    // Auth scope errors → 403
+    assert_eq!(MiroirCode::JwtScopeDenied.http_status(), 403);
+    assert_eq!(MiroirCode::CsrfMismatch.http_status(), 403);
+
+    // Conflict errors → 409
+    assert_eq!(MiroirCode::IdempotencyKeyReused.http_status(), 409);
+    assert_eq!(MiroirCode::MultiAliasNotWritable.http_status(), 409);
+    assert_eq!(MiroirCode::IndexAlreadyExists.http_status(), 409);
+
+    // Timeout → 504
+    assert_eq!(MiroirCode::Timeout.http_status(), 504);
+
+    // Service unavailable → 503
+    assert_eq!(MiroirCode::NoQuorum.http_status(), 503);
+    assert_eq!(MiroirCode::ShardUnavailable.http_status(), 503);
+    assert_eq!(MiroirCode::SettingsVersionStale.http_status(), 503);
+}
+
+/// Test 4: Error type categories match Meilisearch's classification.
+#[test]
+fn test_error_type_categories_match_meilisearch() {
+    // InvalidRequest errors
+    assert_eq!(MiroirCode::PrimaryKeyRequired.error_type(), ErrorType::InvalidRequest);
+    assert_eq!(MiroirCode::ReservedField.error_type(), ErrorType::InvalidRequest);
+    assert_eq!(MiroirCode::IdempotencyKeyReused.error_type(), ErrorType::InvalidRequest);
+    assert_eq!(MiroirCode::MultiAliasNotWritable.error_type(), ErrorType::InvalidRequest);
+    assert_eq!(MiroirCode::IndexAlreadyExists.error_type(), ErrorType::InvalidRequest);
+
+    // Auth errors
+    assert_eq!(MiroirCode::JwtInvalid.error_type(), ErrorType::Auth);
+    assert_eq!(MiroirCode::JwtScopeDenied.error_type(), ErrorType::Auth);
+    assert_eq!(MiroirCode::InvalidAuth.error_type(), ErrorType::Auth);
+    assert_eq!(MiroirCode::MissingCsrf.error_type(), ErrorType::Auth);
+    assert_eq!(MiroirCode::CsrfMismatch.error_type(), ErrorType::Auth);
+
+    // System errors
+    assert_eq!(MiroirCode::NoQuorum.error_type(), ErrorType::System);
+    assert_eq!(MiroirCode::ShardUnavailable.error_type(), ErrorType::System);
+    assert_eq!(MiroirCode::SettingsVersionStale.error_type(), ErrorType::System);
+    assert_eq!(MiroirCode::Timeout.error_type(), ErrorType::System);
+}
+
+/// Test 5: Error JSON output is byte-for-byte compatible with Meilisearch shape.
+///
+/// This test verifies the exact JSON structure matches Meilisearch's format.
+#[test]
+fn test_error_json_matches_meilisearch_shape() {
+    let err = MeilisearchError::new(
+        MiroirCode::PrimaryKeyRequired,
+        "index `test` has no primary key",
+    );
+
+    let json_str = serde_json::to_string(&err).expect("failed to serialize");
+    let parsed: serde_json::Value = serde_json::from_str(&json_str).expect("failed to parse");
+
+    // Verify exact shape
+    assert_eq!(
+        parsed,
+        json!({
+            "message": "index `test` has no primary key",
+            "code": "miroir_primary_key_required",
+            "type": "invalid_request",
+            "link": "https://github.com/jedarden/miroir/blob/main/docs/errors.md#miroir_primary_key_required"
+        })
+    );
+}
+
+/// Test 6: Error with custom metadata preserves shape.
+#[test]
+fn test_error_with_custom_metadata_preserves_shape() {
+    let mut err = MeilisearchError::new(
+        MiroirCode::ReservedField,
+        "document contains reserved field `_miroir_shard`",
+    );
+
+    // Verify the error can be converted to axum Response if feature is enabled
+    #[cfg(feature = "axum")]
+    {
+        let response = err.into_response();
+        assert_eq!(response.status(), axum::http::StatusCode::BAD_REQUEST);
+    }
+}
+
+/// Test 7: Verify no_quorum error has the correct shape for degraded writes.
+#[test]
+fn test_no_quorum_error_shape_for_degraded_writes() {
+    let err = MeilisearchError::new(
+        MiroirCode::NoQuorum,
+        "no quorum reached for shard 7",
+    );
+
+    let json_val = serde_json::to_value(&err).expect("failed to serialize");
+
+    assert_eq!(json_val["code"], "miroir_no_quorum");
+    assert_eq!(json_val["type"], "system");
+    assert_eq!(json_val["message"], "no quorum reached for shard 7");
+    assert_eq!(
+        json_val["link"],
+        "https://github.com/jedarden/miroir/blob/main/docs/errors.md#miroir_no_quorum"
+    );
+}
+
+/// Test 8: Verify shard_unavailable error shape.
+#[test]
+fn test_shard_unavailable_error_shape() {
+    let err = MeilisearchError::new(
+        MiroirCode::ShardUnavailable,
+        "shard 15 has no healthy replicas",
+    );
+
+    let json_val = serde_json::to_value(&err).expect("failed to serialize");
+
+    assert_eq!(json_val["code"], "miroir_shard_unavailable");
+    assert_eq!(json_val["type"], "system");
+    assert_eq!(json_val["message"], "shard 15 has no healthy replicas");
+}
+
+/// Test 9: Verify reserved_field error includes field name in message.
+#[test]
+fn test_reserved_field_error_includes_field_name() {
+    let field_name = "_miroir_internal";
+    let err = MeilisearchError::new(
+        MiroirCode::ReservedField,
+        &format!("document contains reserved field `{}`", field_name),
+    );
+
+    let json_val = serde_json::to_value(&err).expect("failed to serialize");
+
+    assert_eq!(json_val["code"], "miroir_reserved_field");
+    assert_eq!(json_val["type"], "invalid_request");
+    assert!(json_val["message"]
+        .as_str()
+        .unwrap()
+        .contains(field_name));
+}
+
+/// Test 10: Verify timeout error shape.
+#[test]
+fn test_timeout_error_shape() {
+    let err = MeilisearchError::new(
+        MiroirCode::Timeout,
+        "request timed out after 30s",
+    );
+
+    let json_val = serde_json::to_value(&err).expect("failed to serialize");
+
+    assert_eq!(json_val["code"], "miroir_timeout");
+    assert_eq!(json_val["type"], "system");
+    assert_eq!(json_val["message"], "request timed out after 30s");
+    assert_eq!(err.http_status(), 504);
+}
+
+/// Test 11: Verify all error types can be parsed from JSON.
+#[test]
+fn test_all_errors_round_trip_through_json() {
+    for code in MiroirCode::ALL {
+        let err = MeilisearchError::new(code, "test message");
+        let json_str = serde_json::to_string(&err).expect("failed to serialize");
+        let parsed: MeilisearchError = serde_json::from_str(&json_str).expect("failed to deserialize");
+
+        // Verify the parsed error has the same properties
+        assert_eq!(parsed.code, code.as_str());
+        assert_eq!(parsed.message, "test message");
+        assert_eq!(parsed.error_type, code.error_type());
+    }
+}
+
+/// Test 12: Verify error link format is consistent.
+#[test]
+fn test_error_link_format_is_consistent() {
+    for code in MiroirCode::ALL {
+        let link = code.doc_link();
+        assert!(
+            link.starts_with("https://github.com/jedarden/miroir/blob/main/docs/errors.md#"),
+            "Error code {:?} has unexpected link format: {}",
+            code,
+            link
+        );
+        assert!(
+            link.ends_with(code.as_str()),
+            "Error code {:?} link doesn't end with code: {}",
+            code,
+            link
+        );
+    }
+}
+
+/// Test 13: Verify error_type Display implementation.
+#[test]
+fn test_error_type_display_matches_meilisearch() {
+    assert_eq!(ErrorType::InvalidRequest.to_string(), "invalid_request");
+    assert_eq!(ErrorType::Auth.to_string(), "auth");
+    assert_eq!(ErrorType::Internal.to_string(), "internal");
+    assert_eq!(ErrorType::System.to_string(), "system");
+}
diff --git a/crates/miroir-proxy/src/client.rs b/crates/miroir-proxy/src/client.rs
index 0b41421..768212a 100644
--- a/crates/miroir-proxy/src/client.rs
+++ b/crates/miroir-proxy/src/client.rs
@@ -1,7 +1,7 @@
 //! HTTP client for communicating with Meilisearch nodes.
 
 use miroir_core::scatter::{
-    DeleteByIdsRequest, DeleteByFilterRequest, DeleteResponse, NodeClient, NodeError,
+    DeleteByIdsRequest, DeleteByFilterRequest, DeleteResponse, FetchDocumentsRequest, NodeClient, NodeError,
     PreflightRequest, PreflightResponse, SearchRequest, TaskStatusRequest, TaskStatusResponse,
     TermStats, WriteRequest, WriteResponse,
 };
@@ -529,6 +529,76 @@ impl NodeClient for HttpClient {
     }
 }
 
+impl miroir_core::group_sync_worker::SyncNodeClient for HttpClient {
+    async fn fetch_documents(
+        &self,
+        node: &NodeId,
+        address: &str,
+        request: &FetchDocumentsRequest,
+    ) -> std::result::Result<serde_json::Value, String> {
+        let url = self.documents_url(address, &request.index_uid);
+        let filter_json = serde_json::to_string(&request.filter)
+            .map_err(|e| format!("Failed to serialize filter: {}", e))?;
+
+        let response = self
+            .client
+            .get(&url)
+            .header("Authorization", format!("Bearer {}", self.master_key))
+            .query(&[
+                ("filter", &filter_json),
+                ("limit", &request.limit.to_string()),
+                ("offset", &request.offset.to_string()),
+            ])
+            .send()
+            .await
+            .map_err(|e| format!("Request failed: {}", e))?;
+
+        let status = response.status();
+        let body_text = response
+            .text()
+            .await
+            .map_err(|e| format!("Failed to read response: {}", e))?;
+
+        if !status.is_success() {
+            return Err(format!("HTTP {}: {}", status.as_u16(), body_text));
+        }
+
+        serde_json::from_str(&body_text)
+            .map_err(|e| format!("Failed to parse JSON: {}", e))
+    }
+
+    async fn write_documents(
+        &self,
+        node: &NodeId,
+        address: &str,
+        index_uid: &str,
+        documents: Vec<serde_json::Value>,
+    ) -> std::result::Result<(), String> {
+        let url = self.documents_url(address, index_uid);
+
+        let response = self
+            .client
+            .post(&url)
+            .header("Authorization", format!("Bearer {}", self.master_key))
+            .json(&documents)
+            .send()
+            .await
+            .map_err(|e| format!("Request failed: {}", e))?;
+
+        let status = response.status();
+        let body_text = response
+            .text()
+            .await
+            .map_err(|e| format!("Failed to read response: {}", e))?;
+
+        if !status.is_success() {
+            return Err(format!("HTTP {}: {}", status.as_u16(), body_text));
+        }
+
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/crates/miroir-proxy/src/main.rs b/crates/miroir-proxy/src/main.rs
index 66d2d67..05d6f5c 100644
--- a/crates/miroir-proxy/src/main.rs
+++ b/crates/miroir-proxy/src/main.rs
@@ -443,6 +443,24 @@ async fn main() -> anyhow::Result<()> {
         info!("Mode C worker not available (no task store configured)");
     }
 
+    // Start group sync worker background task (plan §2 group addition)
+    if let Some(ref worker) = state.admin.group_sync_worker {
+        let worker = worker.clone();
+        tokio::spawn(async move {
+            info!("group sync worker started");
+            match worker.run().await {
+                Ok(()) => {
+                    info!("group sync worker exited cleanly");
+                }
+                Err(e) => {
+                    error!("group sync worker exited unexpectedly: {}", e);
+                }
+            }
+        });
+    } else {
+        info!("group sync worker not available (no task store configured)");
+    }
+
     // Start peer discovery refresh loop (plan §14.5)
     // Periodically performs SRV lookups to discover peer pods
     if let Some(ref peer_discovery) = state.peer_discovery {
diff --git a/crates/miroir-proxy/src/routes/admin_endpoints.rs b/crates/miroir-proxy/src/routes/admin_endpoints.rs
index a799254..3f9e924 100644
--- a/crates/miroir-proxy/src/routes/admin_endpoints.rs
+++ b/crates/miroir-proxy/src/routes/admin_endpoints.rs
@@ -681,16 +681,81 @@ impl AppState {
                 config.query_coalescing.max_pending_queries as usize,
                 config.query_coalescing.max_subscribers as usize,
             )),
-            group_addition_coordinator: if has_task_store {
-                Some(Arc::new(RwLock::new(
-                    miroir_core::group_addition::GroupAdditionCoordinator::new(
-                        miroir_core::group_addition::GroupAdditionConfig::default()
-                    )
-                )))
-            } else {
-                None
+        // Create group addition coordinator (needed for both API and sync worker)
+        let group_addition_coordinator = if has_task_store {
+            Some(Arc::new(RwLock::new(
+                miroir_core::group_addition::GroupAdditionCoordinator::new(
+                    miroir_core::group_addition::GroupAdditionConfig::default()
+                )
+            )))
+        } else {
+            None
+        };
+
+        // Create group sync worker if task store is available
+        let group_sync_worker = if has_task_store {
+            // Create HTTP client for the sync worker
+            let http_client = Arc::new(super::super::client::HttpClient::new(
+                config.node_master_key.clone(),
+                config.scatter.node_timeout_ms,
+            ));
+            let worker_config = miroir_core::group_sync_worker::GroupSyncWorkerConfig::default();
+            // Use the same coordinator
+            let coordinator = group_addition_coordinator.as_ref().unwrap().clone();
+            Some(Arc::new(miroir_core::group_sync_worker::GroupSyncWorker::new(
+                worker_config,
+                coordinator,
+                http_client,
+                topology_arc.clone(),
+            )))
+        } else {
+            None
+        };
+
+        Self {
+            config: Arc::new(config.clone()),
+            topology: topology_arc,
+            ready: Arc::new(RwLock::new(false)),
+            metrics: metrics.clone(),
+            version_state,
+            task_registry: Arc::new(task_registry),
+            redis_store,
+            task_store,
+            pod_id,
+            seal_key,
+            local_rate_limiter: LocalAdminRateLimiter::new(),
+            local_search_ui_rate_limiter: LocalSearchUiRateLimiter::new(),
+            rebalancer: Some(rebalancer),
+            migration_coordinator: Some(migration_coordinator),
+            rebalancer_worker,
+            rebalancer_metrics,
+            previous_docs_migrated: Arc::new(std::sync::atomic::AtomicU64::new(0)),
+            settings_broadcast,
+            drift_reconciler,
+            anti_entropy_worker,
+            session_manager,
+            alias_registry,
+            leader_election,
+            mode_c_worker,
+            replica_selector: {
+                let advanced_config = config.replica_selection.clone();
+                let selector_config = miroir_core::replica_selection::ReplicaSelectionConfig::from(advanced_config);
+                let observer = Arc::new(ReplicaSelectionMetricsObserver {
+                    metrics: metrics.clone(),
+                });
+                Arc::new(ReplicaSelector::new_with_observer(selector_config, observer))
             },
-            group_sync_worker: None, // Initialized later if needed
+            idempotency_cache: Arc::new(miroir_core::idempotency::IdempotencyCache::new(
+                config.idempotency.max_cached_keys as usize,
+                config.idempotency.ttl_seconds,
+            )),
+            query_coalescer: Arc::new(miroir_core::idempotency::QueryCoalescer::new(
+                config.query_coalescing.window_ms,
+                config.query_coalescing.max_pending_queries as usize,
+                config.query_coalescing.max_subscribers as usize,
+            )),
+            group_addition_coordinator,
+            group_sync_worker,
         }
     }