diff --git a/codex-rs/app-server/src/request_processors/background_agent_live.rs b/codex-rs/app-server/src/request_processors/background_agent_live.rs
index be01f7a3f..15c46c81b 100644
--- a/codex-rs/app-server/src/request_processors/background_agent_live.rs
+++ b/codex-rs/app-server/src/request_processors/background_agent_live.rs
@@ -4208,10 +4208,7 @@ async fn resolve_background_agent_config(
         })
         .and_then(Value::as_str)
         .map(str::to_string);
-    let service_tier = payload
-        .and_then(|payload| payload.get("serviceTier"))
-        .and_then(Value::as_str)
-        .map(|value| Some(value.to_string()));
+    let service_tier = background_agent_snapshot_service_tier(payload)?;
     let approval_policy = payload
         .and_then(|payload| payload.get("approvalPolicy"))
         .cloned()
@@ -4319,6 +4316,28 @@ async fn resolve_background_agent_config(
     })
 }
 
+fn background_agent_snapshot_service_tier(
+    payload: Option<&serde_json::Map<String, Value>>,
+) -> anyhow::Result<Option<Option<String>>> {
+    let Some(payload) = payload else {
+        return Ok(None);
+    };
+    let Some(service_tier) = payload.get("serviceTier") else {
+        return Ok(None);
+    };
+    // Workflow admission snapshots always include this field, using null to
+    // represent the absence of a requested tier. Preserve that absence so the
+    // loaded config cannot invent the explicit default tier and diverge from
+    // the immutable admission receipt. Other snapshots retain explicit null
+    // as the ConfigOverrides reset-to-default signal.
+    if service_tier.is_null() && payload.contains_key("routeReceipt") {
+        return Ok(None);
+    }
+    serde_json::from_value::<Option<String>>(service_tier.clone())
+        .map(Some)
+        .map_err(anyhow::Error::from)
+}
+
 fn validate_background_agent_initial_execution_snapshot(
     run: &BackgroundAgentRun,
     snapshot: &BackgroundAgentExecutionSnapshot,
@@ -6451,6 +6470,39 @@ mod tests {
         );
     }
 
+    #[test]
+    fn background_agent_snapshot_service_tier_preserves_explicit_null() {
+        assert_eq!(
+            background_agent_snapshot_service_tier(/*payload*/ None).unwrap(),
+            None
+        );
+
+        let explicit_null_payload = json!({
+            "serviceTier": null,
+        });
+        assert_eq!(
+            background_agent_snapshot_service_tier(explicit_null_payload.as_object()).unwrap(),
+            Some(None)
+        );
+
+        let explicit_value_payload = json!({
+            "serviceTier": "default",
+        });
+        assert_eq!(
+            background_agent_snapshot_service_tier(explicit_value_payload.as_object()).unwrap(),
+            Some(Some("default".to_string()))
+        );
+
+        let workflow_null_payload = json!({
+            "serviceTier": null,
+            "routeReceipt": {},
+        });
+        assert_eq!(
+            background_agent_snapshot_service_tier(workflow_null_payload.as_object()).unwrap(),
+            None
+        );
+    }
+
     #[test]
     fn launch_capability_requests_merge_server_defaults_and_reject_caller_duplicates() {
         let mut requests = vec![
diff --git a/codex-rs/state/src/runtime/workflow_orchestrator.rs b/codex-rs/state/src/runtime/workflow_orchestrator.rs
index 6739206f4..7aff71fa4 100644
--- a/codex-rs/state/src/runtime/workflow_orchestrator.rs
+++ b/codex-rs/state/src/runtime/workflow_orchestrator.rs
@@ -929,7 +929,6 @@ async fn admit_ready_workflow_branches_in_tx(
             candidate: &candidate,
             attempt: branch_attempt,
             route_receipt: &route_receipt,
-            workspace_json: workspace_json.as_ref(),
             provisioned_workspace: &provisioned_workspace,
             review_context: review_context.as_ref(),
             background_agent_run_id: background_agent_run_id.as_str(),
@@ -1340,7 +1339,6 @@ struct BackgroundBranchRunCreate<'a> {
     candidate: &'a ReadyBranchCandidate,
     attempt: i64,
     route_receipt: &'a WorkflowRouteReceipt,
-    workspace_json: Option<&'a Value>,
     provisioned_workspace: &'a ProvisionedWorkflowWorkspace,
     review_context: Option<&'a WorkflowReviewBranchContext>,
     background_agent_run_id: &'a str,
@@ -1725,7 +1723,6 @@ async fn create_background_branch_run_if_missing_in_tx(
         candidate,
         attempt,
         route_receipt,
-        workspace_json: _,
         provisioned_workspace,
         review_context: _,
         background_agent_run_id,
@@ -2267,6 +2264,10 @@ fn branch_execution_payload(
     let workspace_root = crate::runtime::managed_worktrees::path_to_db_string(
         provisioned_workspace.worktree_path.as_path(),
     );
+    let workspace_mode = match provisioned_workspace.mode {
+        WorkflowWorkspaceMode::IsolatedWorktree => "isolated_worktree",
+        WorkflowWorkspaceMode::SharedRepository => "shared_repository",
+    };
     json!({
         "snapshotSource": "workflow/branch_admission",
         "workflowRunId": run.run_id.as_str(),
@@ -2293,7 +2294,7 @@ fn branch_execution_payload(
             .auth_profile
             .as_deref()
             .map(|profile| StateRuntime::background_agent_identity_sha256(profile.as_bytes())),
-        "workspace": branch.workspace_json,
+        "workspace": json!({"mode": workspace_mode}),
         "workflowBootstrap": &candidate.bootstrap_json,
         "reviewContext": branch.review_context.map(|context| &context.state_json),
         "envSnapshotPolicy": "inherit-minimal",
@@ -6913,6 +6914,15 @@ WHERE worktree_id = ?
             .await
             .expect("execution snapshot should load")
             .expect("execution snapshot should exist");
+        assert_eq!(
+            Some("shared_repository"),
+            execution_snapshot
+                .payload_json
+                .get("workspace")
+                .and_then(|workspace| workspace.get("mode"))
+                .and_then(Value::as_str),
+            "shared non-Git branch must preserve the effective shared-repository workspace mode"
+        );
         assert_eq!(
             Some(
                 crate::runtime::managed_worktrees::path_to_db_string(source_cwd.as_path()).as_str()