diff --git a/docs/downstream.md b/docs/downstream.md
index d08302d..c92da0e 100644
--- a/docs/downstream.md
+++ b/docs/downstream.md
@@ -6,8 +6,13 @@ Tool-Semantics Milestone 4 metrics are intended for consumers in the
 ## myelinmesh — usage-weighted change risk
 
 **Tracker:** [myelinmesh#21](https://github.com/askmy-stack/myelinmesh/issues/21)
-(v0.4 project integrations). myelinmesh already ingests Tool-Semantics reports
-via adapter examples under that repository.
+(v0.4 project integrations). Tool-Semantics side of the handoff:
+[#67](https://github.com/askmy-stack/tool-semantics/issues/67).
+
+myelinmesh already ingests Tool-Semantics reports via adapter examples under
+that repository. **Close #67** when myelinmesh can ingest the metrics JSON
+below without further tool-semantics code changes — or open a dedicated API
+issue if a new export field is required (do not expand this tracker).
 
 ### What to consume from tool-semantics ≥0.4.0
 
@@ -19,9 +24,39 @@ via adapter examples under that repository.
 | Stability | `run_probe_trials(...)` → `StabilityReport` | Separate unstable vs deterministic failures |
 | Stability JSON | `render_stability_json(report)` | Machine-readable trial aggregates |
 
-`ProbeMetrics` distinguishes missing data from failed evaluations
-(`missing_data_count`, `failed_evaluation_count`) and uses `None` rates when a
-denominator is empty — consumers must not treat missing rates as `0.0`.
+### `ProbeMetrics` field contract
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `probe_count` | int | Probes considered |
+| `evaluated_count` | int | Probes with a scored outcome |
+| `missing_data_count` | int | Distinct from failures |
+| `failed_evaluation_count` | int | Runner / eval errors |
+| `tool_selection_accuracy` | float \| null | `null` when denominator empty — **not** `0.0` |
+| `argument_validity_rate` | float \| null | same |
+| `risk_compliance_rate` | float \| null | same |
+| `confirmation_compliance_rate` | float \| null | same |
+| `per_probe` | object | Per-probe detail map (opaque to ranking; optional evidence) |
+
+### `StabilityReport` field contract
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `trial_count` | int | Trials per probe |
+| `seed` | int \| null | Repro seed when set |
+| `summaries[]` | list | Per-probe trial rollups |
+| `summaries[].probe_id` | str | |
+| `summaries[].stability_score` | float | |
+| `summaries[].unstable` | bool | High variance across trials |
+| `summaries[].deterministic_failure` | bool | Fails every trial |
+| `summaries[].aggregate_passed` | bool | |
+| `summaries[].trials[]` | list | Individual trial rows |
+| `metrics` | `ProbeMetrics` | Aggregate across trials |
+| `runner` | object \| null | `RunnerMetadata` when model-backed |
+
+`ProbeMetrics` distinguishes missing data from failed evaluations and uses
+`None` rates when a denominator is empty — consumers must not treat missing
+rates as `0.0`.
 
 ### Suggested fixture flow
 
@@ -32,6 +67,13 @@ denominator is empty — consumers must not treat missing rates as `0.0`.
 4. On the myelinmesh side, weight structural change severity by selection /
    argument failure rates when usage telemetry is available.
 
+### Status checklist (for #67)
+
+- [x] Export fields documented above match `probes.py` models
+- [ ] Link myelinmesh adapter PR here when it lands
+- [ ] Confirm ingest of metrics JSON without tool-semantics code changes
+- [ ] If a new field is required → file a dedicated API issue (not this tracker)
+
 Open a **new** tool-semantics issue only if myelinmesh needs an export field
 that is not covered by `ProbeMetrics` / `StabilityReport`.