diff --git a/reflexio/server/README.md b/reflexio/server/README.md
index b4b8d957..875deac2 100644
--- a/reflexio/server/README.md
+++ b/reflexio/server/README.md
@@ -462,11 +462,12 @@ Key files:
 - `shadow_comparison/judge.py`: Per-turn regular-vs-shadow judge
 - `shadow_comparison/dispatcher.py` and `shadow_comparison/worker.py`: Publish-time dispatch and bounded background execution for shadow verdict writes
 - `shadow_comparison/outcome.py`: Verdict outcome model helpers
-- `evaluation_overview/service.py`: Aggregates evaluation-page metrics
+- `evaluation_overview/service.py`: Bulk-loads evaluation-page metrics, first-request sources, citations, Braintrust scores, and optional shadow verdicts; source-set cohorts are computed server-side so the dashboard does not re-aggregate.
 - `evaluation_overview/components/hero_state.py`, `evaluation_overview/components/distribution.py`, `evaluation_overview/components/rule_attribution.py`, `evaluation_overview/components/shadow_aggregation.py`: Focused aggregation helpers
 - `evaluation_overview/eval_sampler.py`: Evaluation sampling helpers that remain root-level
+- `models/api_schema/eval_overview_schema.py`: `GetEvaluationOverviewRequest/Response`, including optional `source_sets` input and `source_set_comparison` output keyed by `(user_id, session_id)`.
 
-**Pattern**: Session-level agent success evaluation remains in `agent_success_evaluation/`; dashboard-facing rollups and per-turn shadow verdict analysis live in these companion directories.
+**Pattern**: Session-level agent success evaluation remains in `agent_success_evaluation/`; dashboard-facing rollups, first-source cohort comparisons, and per-turn shadow verdict analysis live in these companion directories.
 
 ### Playbook Optimizer and Braintrust
 
diff --git a/reflexio/server/services/evaluation_overview/README.md b/reflexio/server/services/evaluation_overview/README.md
index 264da11e..1e3eebc7 100644
--- a/reflexio/server/services/evaluation_overview/README.md
+++ b/reflexio/server/services/evaluation_overview/README.md
@@ -2,14 +2,18 @@
 
 Read-side aggregation module for `POST /api/get_evaluation_overview`.
 
-- `service.py` is the request-path entry point. It loads evaluation, citation, Braintrust, and optional shadow verdict data, then composes `GetEvaluationOverviewResponse`.
+- `service.py` is the request-path entry point. It bulk-loads evaluation results (without embeddings), first-request sources, citations, Braintrust scores, and optional shadow verdicts, then composes `GetEvaluationOverviewResponse` for the dashboard.
 - `components/` contains pure read-side aggregation helpers used by the service and focused tests.
 - `eval_sampler.py` stays at the package root because regenerate jobs also use it to sample evaluation sessions.
+- `reflexio/models/api_schema/eval_overview_schema.py` defines the public request/response contract, including optional `source_sets` cohorts and `source_set_comparison` output.
 
 The overview reports task success across every evaluated session and a separate
 behavior-success metric that excludes `failure_type=system_error` rows from
 both numerator and denominator. A window with no behavior-evaluable rows
 returns a null behavior rate plus eligible/excluded counts for honest UI
-rendering.
+rendering. Source-set comparison groups sessions by the first request source,
+returns collision-safe `(user_id, session_id)` identities, and rejects duplicate
+labels or overlapping source values so the frontend can compare cohorts without
+client-side re-aggregation.
 
 This module mutates no core state. Keep response-shape changes in API schema tests and service integration tests.