diff --git a/docs/research/pre_work_error_ledger.md b/docs/research/pre_work_error_ledger.md
index a3cd6569..e7e5d79c 100644
--- a/docs/research/pre_work_error_ledger.md
+++ b/docs/research/pre_work_error_ledger.md
@@ -57,6 +57,7 @@ For large architecture or release work, also read:
 | ERR-024 | VedAstro gateway status can choose `local_fallback` before `.env` is loaded, even when official endpoint/network settings are present. | mitigated 2026-07-08 | `gateway_status()` must load official readiness before resolving active backend; `run_gateway_packet()` must expose `official_closure_state` separately from legacy `status`. |
 | ERR-025 | VedAstro gateway can report legacy `status=ok` from catalog availability even when no official raw response is present. | mitigated 2026-07-08 | `official_closure_state=official_verified` requires `official_raw_response`; otherwise expose `official_closure_reason=official_raw_response_missing`. |
 | ERR-026 | VedAstro service adapter can obtain an official full-snapshot raw response while the user entrypoint drops it, leaving gateway official closure permanently blocked. | mitigated 2026-07-09 | `vedastro_user_entrypoint` must expose `vedastro_official_full_snapshot.raw_response_available` and root `official_raw_response` when explicitly requested; gateway tests must prove raw propagation reaches `official_verified`. |
+| ERR-027 | External engine readiness diagnostics can be mistaken for a completed same-chart parity comparison. | mitigated 2026-07-09 | `diagnose_external_engine_adapters.py` must expose `same_chart_parity_contract.required_outputs`, per-engine expected oracle fields, and `tested=false` until a real same-chart comparison runs. |
 
 ## Fragment Sweep Command Set
 
diff --git a/scripts/diagnose_external_engine_adapters.py b/scripts/diagnose_external_engine_adapters.py
index 7ee0ef05..1b6284bc 100644
--- a/scripts/diagnose_external_engine_adapters.py
+++ b/scripts/diagnose_external_engine_adapters.py
@@ -16,6 +16,50 @@ except Exception:  # pragma: no cover - import path varies in tests/CLI
     from scripts.diagnose_jyotishganit_adapter import build_report as build_jyotishganit_report
 
 
+REQUIRED_PARITY_OUTPUTS = ["D1", "D9", "D10", "D2", "D4", "Vimshottari", "Shadbala", "Ashtakavarga"]
+
+
+def _same_chart_parity_contract(engines: dict) -> dict:
+    engine_states = {}
+    for name, engine in engines.items():
+        available = engine["status"] == "available"
+        engine_states[name] = {
+            "available": available,
+            "tested": False,
+            "blocked": not available,
+            "blocking_reason": "" if available else engine["status"],
+        }
+    return {
+        "status": "ready" if all(state["available"] for state in engine_states.values()) else "blocked",
+        "required_outputs": REQUIRED_PARITY_OUTPUTS,
+        "expected_oracle_fields": {
+            "VedAstro": [
+                "official_raw_response",
+                "official_chart",
+                "section_statuses",
+                "request_manifest",
+                "source_metadata.artifact_path",
+            ],
+            "PyJHora/JHora": [
+                "raw_output_path",
+                "settings.ayanamsa",
+                "settings.node_mode",
+                "D1",
+                "D9",
+                "D10",
+                "D2",
+                "D4",
+                "Vimshottari",
+                "Shadbala",
+                "Ashtakavarga",
+            ],
+            "jyotishganit": ["raw_output_path", "panchanga", "tithi", "nakshatra", "yoga", "karana"],
+        },
+        "engine_states": engine_states,
+        "boundary": "This is a parity contract, not proof that the same-chart comparison has run.",
+    }
+
+
 def build_report() -> dict:
     vedastro = build_vedastro_report()
     pyjhora = build_pyjhora_report()
@@ -45,6 +89,7 @@ def build_report() -> dict:
         "scope": "external_engine_adapter_diagnostics",
         "status": "complete" if all(engine["status"] == "available" for engine in engines.values()) else "partial",
         "engines": engines,
+        "same_chart_parity_contract": _same_chart_parity_contract(engines),
         "boundary": "Readiness diagnostics only; this does not run a three-engine consultation comparison.",
     }
 
diff --git a/tests/test_external_engine_adapter_diagnostics.py b/tests/test_external_engine_adapter_diagnostics.py
index e98df1fa..a5e48e81 100644
--- a/tests/test_external_engine_adapter_diagnostics.py
+++ b/tests/test_external_engine_adapter_diagnostics.py
@@ -30,4 +30,11 @@ def test_external_engine_adapter_diagnostics_aggregates_three_engines() -> None:
     assert report["engines"]["PyJHora/JHora"]["install_hint"]["package"] == "PyJHora"
     assert report["engines"]["PyJHora/JHora"]["license_boundary"].startswith("AGPL external benchmark")
     assert report["engines"]["jyotishganit"]["license"] == "MIT"
+    contract = report["same_chart_parity_contract"]
+    assert contract["status"] in {"ready", "blocked"}
+    assert contract["required_outputs"] == ["D1", "D9", "D10", "D2", "D4", "Vimshottari", "Shadbala", "Ashtakavarga"]
+    assert "official_raw_response" in contract["expected_oracle_fields"]["VedAstro"]
+    assert "raw_output_path" in contract["expected_oracle_fields"]["PyJHora/JHora"]
+    assert contract["engine_states"]["jyotishganit"]["available"] is True
+    assert contract["engine_states"]["PyJHora/JHora"]["tested"] is False
     assert report["status"] in {"complete", "partial"}