diff --git a/src/physcom/db/repository.py b/src/physcom/db/repository.py
index c1f937c..acbcb9a 100644
--- a/src/physcom/db/repository.py
+++ b/src/physcom/db/repository.py
@@ -9,7 +9,7 @@ from datetime import datetime, timezone
 from typing import Sequence
 
 from physcom.models.entity import Dependency, Entity
-from physcom.models.domain import Domain, MetricBound
+from physcom.models.domain import Domain, DomainConstraint, MetricBound
 from physcom.models.combination import Combination
 
 
@@ -249,9 +249,25 @@ class Repository:
                 (domain.id, metric_id, mb.weight, mb.norm_min, mb.norm_max,
                  int(mb.lower_is_better)),
             )
+        for dc in domain.constraints:
+            for val in dc.allowed_values:
+                self.conn.execute(
+                    "INSERT OR IGNORE INTO domain_constraints (domain_id, key, value) VALUES (?, ?, ?)",
+                    (domain.id, dc.key, val),
+                )
         self.conn.commit()
         return domain
 
+    def _load_domain_constraints(self, domain_id: int) -> list[DomainConstraint]:
+        rows = self.conn.execute(
+            "SELECT key, value FROM domain_constraints WHERE domain_id = ? ORDER BY key, value",
+            (domain_id,),
+        ).fetchall()
+        by_key: dict[str, list[str]] = {}
+        for r in rows:
+            by_key.setdefault(r["key"], []).append(r["value"])
+        return [DomainConstraint(key=k, allowed_values=v) for k, v in by_key.items()]
+
     def get_domain(self, name: str) -> Domain | None:
         row = self.conn.execute("SELECT * FROM domains WHERE name = ?", (name,)).fetchone()
         if not row:
@@ -277,6 +293,7 @@ class Repository:
                 )
                 for w in weights
             ],
+            constraints=self._load_domain_constraints(row["id"]),
         )
 
     def list_domains(self) -> list[Domain]:
@@ -308,6 +325,7 @@ class Repository:
                 )
                 for w in weights
             ],
+            constraints=self._load_domain_constraints(row["id"]),
         )
 
     def update_domain(self, domain_id: int, name: str, description: str) -> None:
@@ -359,9 +377,28 @@ class Repository:
         self.conn.execute("DELETE FROM combination_results WHERE domain_id = ?", (domain_id,))
         self.conn.execute("DELETE FROM combination_scores WHERE domain_id = ?", (domain_id,))
         self.conn.execute("DELETE FROM domain_metric_weights WHERE domain_id = ?", (domain_id,))
+        self.conn.execute("DELETE FROM domain_constraints WHERE domain_id = ?", (domain_id,))
         self.conn.execute("DELETE FROM domains WHERE id = ?", (domain_id,))
         self.conn.commit()
 
+    def replace_domain_constraints(self, domain: Domain) -> None:
+        """Delete and re-insert domain constraints. Used by seed backfill."""
+        if not domain.id:
+            existing = self.conn.execute(
+                "SELECT id FROM domains WHERE name = ?", (domain.name,)
+            ).fetchone()
+            if not existing:
+                return
+            domain.id = existing["id"]
+        self.conn.execute("DELETE FROM domain_constraints WHERE domain_id = ?", (domain.id,))
+        for dc in domain.constraints:
+            for val in dc.allowed_values:
+                self.conn.execute(
+                    "INSERT OR IGNORE INTO domain_constraints (domain_id, key, value) VALUES (?, ?, ?)",
+                    (domain.id, dc.key, val),
+                )
+        self.conn.commit()
+
     def reset_domain_results(self, domain_name: str) -> int:
         """Delete all pipeline results for a domain so it can be re-run from scratch.
 
@@ -560,14 +597,15 @@ class Repository:
         novelty_flag: str | None = None,
         llm_review: str | None = None,
         human_notes: str | None = None,
+        domain_block_reason: str | None = None,
     ) -> None:
         self.conn.execute(
             """INSERT OR REPLACE INTO combination_results
                (combination_id, domain_id, composite_score, novelty_flag,
-                llm_review, human_notes, pass_reached)
-               VALUES (?, ?, ?, ?, ?, ?, ?)""",
+                llm_review, human_notes, pass_reached, domain_block_reason)
+               VALUES (?, ?, ?, ?, ?, ?, ?, ?)""",
             (combo_id, domain_id, composite_score, novelty_flag,
-             llm_review, human_notes, pass_reached),
+             llm_review, human_notes, pass_reached, domain_block_reason),
         )
         self.conn.commit()
 
@@ -667,6 +705,7 @@ class Repository:
                 "human_notes": r["human_notes"],
                 "pass_reached": r["pass_reached"],
                 "domain_id": r["domain_id"],
+                "domain_block_reason": r["domain_block_reason"],
             }
             for r in rows
         ]
@@ -814,6 +853,7 @@ class Repository:
         self.conn.execute("DELETE FROM dependencies")
         self.conn.execute("DELETE FROM entities")
         self.conn.execute("DELETE FROM domain_metric_weights")
+        self.conn.execute("DELETE FROM domain_constraints")
         self.conn.execute("DELETE FROM domains")
         self.conn.execute("DELETE FROM metrics")
         self.conn.execute("DELETE FROM dimensions")
diff --git a/src/physcom/db/schema.py b/src/physcom/db/schema.py
index 9bc5eee..d929fe0 100644
--- a/src/physcom/db/schema.py
+++ b/src/physcom/db/schema.py
@@ -81,14 +81,15 @@ CREATE TABLE IF NOT EXISTS combination_scores (
 );
 
 CREATE TABLE IF NOT EXISTS combination_results (
-    id              INTEGER PRIMARY KEY AUTOINCREMENT,
-    combination_id  INTEGER NOT NULL REFERENCES combinations(id),
-    domain_id       INTEGER NOT NULL REFERENCES domains(id),
-    composite_score REAL,
-    novelty_flag    TEXT,
-    llm_review      TEXT,
-    human_notes     TEXT,
-    pass_reached    INTEGER,
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT,
+    combination_id      INTEGER NOT NULL REFERENCES combinations(id),
+    domain_id           INTEGER NOT NULL REFERENCES domains(id),
+    composite_score     REAL,
+    novelty_flag        TEXT,
+    llm_review          TEXT,
+    human_notes         TEXT,
+    pass_reached        INTEGER,
+    domain_block_reason TEXT,
     UNIQUE(combination_id, domain_id)
 );
 
@@ -109,6 +110,14 @@ CREATE TABLE IF NOT EXISTS pipeline_runs (
     created_at      TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 );
 
+CREATE TABLE IF NOT EXISTS domain_constraints (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    domain_id   INTEGER NOT NULL REFERENCES domains(id),
+    key         TEXT NOT NULL,
+    value       TEXT NOT NULL,
+    UNIQUE(domain_id, key, value)
+);
+
 CREATE INDEX IF NOT EXISTS idx_deps_entity ON dependencies(entity_id);
 CREATE INDEX IF NOT EXISTS idx_deps_category_key ON dependencies(category, key);
 CREATE INDEX IF NOT EXISTS idx_combo_status ON combinations(status);
@@ -126,6 +135,26 @@ def _migrate(conn: sqlite3.Connection) -> None:
             "ALTER TABLE domain_metric_weights ADD COLUMN lower_is_better INTEGER NOT NULL DEFAULT 0"
         )
 
+    # Create domain_constraints table if missing (added after initial schema)
+    tables = {r[0] for r in conn.execute(
+        "SELECT name FROM sqlite_master WHERE type='table'"
+    ).fetchall()}
+    if "domain_constraints" not in tables:
+        conn.execute("""CREATE TABLE IF NOT EXISTS domain_constraints (
+            id          INTEGER PRIMARY KEY AUTOINCREMENT,
+            domain_id   INTEGER NOT NULL REFERENCES domains(id),
+            key         TEXT NOT NULL,
+            value       TEXT NOT NULL,
+            UNIQUE(domain_id, key, value)
+        )""")
+
+    # Add domain_block_reason to combination_results if missing
+    result_cols = {r[1] for r in conn.execute("PRAGMA table_info(combination_results)").fetchall()}
+    if "domain_block_reason" not in result_cols:
+        conn.execute(
+            "ALTER TABLE combination_results ADD COLUMN domain_block_reason TEXT"
+        )
+
     # Backfill: cost_efficiency is lower-is-better in all domains
     conn.execute(
         """UPDATE domain_metric_weights SET lower_is_better = 1
diff --git a/src/physcom/engine/constraint_resolver.py b/src/physcom/engine/constraint_resolver.py
index ee19f6b..3b2a120 100644
--- a/src/physcom/engine/constraint_resolver.py
+++ b/src/physcom/engine/constraint_resolver.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 from dataclasses import dataclass, field
 
 from physcom.models.combination import Combination
+from physcom.models.domain import DomainConstraint
 from physcom.models.entity import Dependency
 
 
@@ -158,6 +159,25 @@ class ConstraintResolver:
                         f"(under-density)"
                     )
 
+    def check_domain_constraints(
+        self, combination: Combination, constraints: list[DomainConstraint]
+    ) -> ConstraintResult:
+        """Check if a combo's entity requirements fall within domain-allowed values."""
+        result = ConstraintResult()
+        for dc in constraints:
+            allowed = set(dc.allowed_values)
+            for entity in combination.entities:
+                for dep in entity.dependencies:
+                    if dep.key == dc.key and dep.constraint_type == "requires":
+                        if dep.value not in allowed:
+                            result.violations.append(
+                                f"{entity.name} requires {dc.key}={dep.value} "
+                                f"but domain only allows {dc.allowed_values}"
+                            )
+        if result.violations:
+            result.status = "p1_fail"
+        return result
+
     def _check_unmet_requirements(
         self, all_deps: list[tuple[str, Dependency]], result: ConstraintResult
     ) -> None:
diff --git a/src/physcom/engine/pipeline.py b/src/physcom/engine/pipeline.py
index 1b20627..c32eeaf 100644
--- a/src/physcom/engine/pipeline.py
+++ b/src/physcom/engine/pipeline.py
@@ -164,6 +164,26 @@ class Pipeline:
                     else:
                         combo.status = "valid"
                         self.repo.update_combination_status(combo.id, "valid")
+
+                        # Domain constraint check (per-domain block only)
+                        if domain.constraints:
+                            dc_result = self.resolver.check_domain_constraints(
+                                combo, domain.constraints
+                            )
+                            if dc_result.status == "p1_fail":
+                                self.repo.save_result(
+                                    combo.id, domain.id,
+                                    composite_score=0.0, pass_reached=1,
+                                    domain_block_reason="; ".join(
+                                        dc_result.violations
+                                    ),
+                                )
+                                result.pass1_failed += 1
+                                self._update_run_counters(
+                                    run_id, result, current_pass=1
+                                )
+                                continue
+
                         if cr.status == "conditional":
                             result.pass1_conditional += 1
                         else:
@@ -175,8 +195,11 @@ class Pipeline:
                     if combo.status.endswith("_fail"):
                         result.pass1_failed += 1
                         continue
-                    else:
-                        result.pass1_valid += 1
+                    # Check if domain-blocked from a prior run
+                    if existing_result and existing_result["pass_reached"] == 1:
+                        result.pass1_failed += 1
+                        continue
+                    result.pass1_valid += 1
                 else:
                     # Pass 1 not requested; check if failed from a prior run
                     if combo.status.endswith("_fail"):
diff --git a/src/physcom/models/domain.py b/src/physcom/models/domain.py
index d02670f..b58b133 100644
--- a/src/physcom/models/domain.py
+++ b/src/physcom/models/domain.py
@@ -18,6 +18,14 @@ class MetricBound:
     metric_id: int | None = None
 
 
+@dataclass
+class DomainConstraint:
+    """Whitelist constraint: only these values are allowed for a dependency key."""
+
+    key: str  # dependency key, e.g. "medium"
+    allowed_values: list[str] = field(default_factory=list)  # e.g. ["ground", "air"]
+
+
 @dataclass
 class Domain:
     """A context frame that defines what 'good' means (e.g., urban_commuting)."""
@@ -25,4 +33,5 @@ class Domain:
     name: str
     description: str = ""
     metric_bounds: list[MetricBound] = field(default_factory=list)
+    constraints: list[DomainConstraint] = field(default_factory=list)
     id: int | None = None
diff --git a/src/physcom/seed/transport_example.py b/src/physcom/seed/transport_example.py
index 71e8f61..a36fb6d 100644
--- a/src/physcom/seed/transport_example.py
+++ b/src/physcom/seed/transport_example.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from physcom.models.entity import Entity, Dependency
-from physcom.models.domain import Domain, MetricBound
+from physcom.models.domain import Domain, DomainConstraint, MetricBound
 
 
 # ── Platforms — Ground ──────────────────────────────────────────
@@ -726,6 +726,7 @@ URBAN_COMMUTING = Domain(
         MetricBound("availability", weight=0.15, norm_min=0.0, norm_max=1.0, unit="0-1"),
         MetricBound("range_fuel", weight=0.10, norm_min=5000, norm_max=500000, unit="m"),
     ],
+    constraints=[DomainConstraint("medium", ["ground", "air"])],
 )
 
 INTERPLANETARY = Domain(
@@ -738,6 +739,7 @@ INTERPLANETARY = Domain(
         MetricBound("cost_efficiency", weight=0.10, norm_min=1.0, norm_max=1e6, unit="$/m", lower_is_better=True),
         MetricBound("range_degradation", weight=0.10, norm_min=8640000, norm_max=3.1536e9, unit="s"),
     ],
+    constraints=[DomainConstraint("medium", ["space"])],
 )
 
 MARITIME_SHIPPING = Domain(
@@ -750,6 +752,7 @@ MARITIME_SHIPPING = Domain(
         MetricBound("safety", weight=0.20, norm_min=0.0, norm_max=1.0, unit="0-1"),
         MetricBound("range_fuel", weight=0.15, norm_min=100000, norm_max=40000000, unit="m"),
     ],
+    constraints=[DomainConstraint("medium", ["water"])],
 )
 
 LAST_MILE_DELIVERY = Domain(
@@ -762,6 +765,7 @@ LAST_MILE_DELIVERY = Domain(
         MetricBound("safety", weight=0.15, norm_min=0.0, norm_max=1.0, unit="0-1"),
         MetricBound("environmental_impact", weight=0.10, norm_min=0, norm_max=5e-4, unit="kg/m", lower_is_better=True),
     ],
+    constraints=[DomainConstraint("medium", ["ground", "air"])],
 )
 
 CROSS_COUNTRY_FREIGHT = Domain(
@@ -774,6 +778,7 @@ CROSS_COUNTRY_FREIGHT = Domain(
         MetricBound("range_fuel", weight=0.20, norm_min=100000, norm_max=5000000, unit="m"),
         MetricBound("reliability", weight=0.10, norm_min=0.0, norm_max=1.0, unit="0-1"),
     ],
+    constraints=[DomainConstraint("medium", ["ground"])],
 )
 
 ALL_DOMAINS = [
@@ -831,5 +836,7 @@ def load_transport_seed(repo) -> dict:
             repo.ensure_metric(mb.metric_name, unit=mb.unit)
             if mb.lower_is_better:
                 repo.backfill_lower_is_better(domain.name, mb.metric_name)
+        # Backfill domain constraints
+        repo.replace_domain_constraints(domain)
 
     return counts
diff --git a/src/physcom_web/templates/results/detail.html b/src/physcom_web/templates/results/detail.html
index cb8851b..46e11e3 100644
--- a/src/physcom_web/templates/results/detail.html
+++ b/src/physcom_web/templates/results/detail.html
@@ -10,7 +10,17 @@
 <div class="card">
   <dl>
     <dt>Domain</dt><dd>{{ domain.name }}</dd>
-    <dt>Status</dt><dd><span class="badge badge-{{ combo.status }}">{{ combo.status }}</span></dd>
+    <dt>Status</dt>
+    <dd>
+      {% if result and result.domain_block_reason %}
+        <span class="badge badge-p1_fail">domain_blocked</span>
+      {% else %}
+        <span class="badge badge-{{ combo.status }}">{{ combo.status }}</span>
+      {% endif %}
+    </dd>
+    {% if result and result.domain_block_reason %}
+    <dt>Domain Block Reason</dt><dd>{{ result.domain_block_reason }}</dd>
+    {% endif %}
     {% if combo.block_reason %}
     <dt>Block Reason</dt><dd>{{ combo.block_reason }}</dd>
     {% endif %}
diff --git a/src/physcom_web/templates/results/list.html b/src/physcom_web/templates/results/list.html
index e5c7a87..18e51a3 100644
--- a/src/physcom_web/templates/results/list.html
+++ b/src/physcom_web/templates/results/list.html
@@ -62,9 +62,17 @@
         <td>{{ loop.index }}</td>
         <td class="score-cell">{{ "%.4f"|format(r.composite_score) if r.composite_score is not none else '—' }}</td>
         <td>{{ r.combination.entities|map(attribute='name')|join(' + ') }}</td>
-        <td><span class="badge badge-{{ r.combination.status }}">{{ r.combination.status }}</span></td>
+        <td>
+          {%- if r.domain_block_reason -%}
+            <span class="badge badge-p1_fail">domain_blocked</span>
+          {%- else -%}
+            <span class="badge badge-{{ r.combination.status }}">{{ r.combination.status }}</span>
+          {%- endif -%}
+        </td>
         <td class="block-reason-cell">
-          {%- if r.combination.status.endswith('_fail') and r.combination.block_reason -%}
+          {%- if r.domain_block_reason -%}
+            {{ r.domain_block_reason }}
+          {%- elif r.combination.status.endswith('_fail') and r.combination.block_reason -%}
             {{ r.combination.block_reason }}
           {%- elif r.novelty_flag -%}
             {{ r.novelty_flag }}
diff --git a/tests/conftest.py b/tests/conftest.py
index fbfcd6a..b1a93d7 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -7,7 +7,7 @@ import pytest
 from physcom.db.schema import init_db
 from physcom.db.repository import Repository
 from physcom.models.entity import Entity, Dependency
-from physcom.models.domain import Domain, MetricBound
+from physcom.models.domain import Domain, DomainConstraint, MetricBound
 from physcom.models.combination import Combination
 
 
@@ -225,4 +225,5 @@ def urban_domain():
             MetricBound("availability", weight=0.15, norm_min=0.0, norm_max=1.0),
             MetricBound("range_fuel", weight=0.10, norm_min=5000, norm_max=500000),
         ],
+        constraints=[DomainConstraint("medium", ["ground", "air"])],
     )
diff --git a/tests/test_constraint_resolver.py b/tests/test_constraint_resolver.py
index e1e62fb..eec4975 100644
--- a/tests/test_constraint_resolver.py
+++ b/tests/test_constraint_resolver.py
@@ -2,6 +2,7 @@
 
 from physcom.engine.constraint_resolver import ConstraintResolver
 from physcom.models.combination import Combination
+from physcom.models.domain import DomainConstraint
 from physcom.models.entity import Entity, Dependency
 
 
@@ -140,3 +141,22 @@ def test_energy_density_no_constraint_if_no_provider():
     result = resolver.resolve(combo)
     density_violations = [v for v in result.violations if "energy density" in v]
     assert len(density_violations) == 0
+
+
+def test_domain_constraint_blocks_wrong_medium(spaceship, solar_sail, solar_radiation):
+    """Spaceship (space medium) should be blocked in a ground-only domain."""
+    resolver = ConstraintResolver()
+    combo = Combination(entities=[spaceship, solar_sail, solar_radiation])
+    constraints = [DomainConstraint("medium", ["ground", "air"])]
+    result = resolver.check_domain_constraints(combo, constraints)
+    assert result.status == "p1_fail"
+    assert any("medium" in v for v in result.violations)
+
+
+def test_domain_constraint_allows_matching_medium(bicycle, human_pedalling, food_calories):
+    """Bicycle (ground medium) should pass a ground+air domain constraint."""
+    resolver = ConstraintResolver()
+    combo = Combination(entities=[bicycle, human_pedalling, food_calories])
+    constraints = [DomainConstraint("medium", ["ground", "air"])]
+    result = resolver.check_domain_constraints(combo, constraints)
+    assert result.status == "valid"
diff --git a/tests/test_pipeline_async.py b/tests/test_pipeline_async.py
index a35ba9f..5e588c8 100644
--- a/tests/test_pipeline_async.py
+++ b/tests/test_pipeline_async.py
@@ -251,10 +251,10 @@ def test_blocked_combos_have_results(seeded_repo):
     assert total_with_results == result.pass1_failed + result.pass3_scored
 
     # Failed combos should have pass_reached=1 and composite_score=0.0
-    failed_results = [r for r in all_results if r["combination"].status == "p1_fail"]
+    # (includes both entity-blocked and domain-blocked combos)
+    failed_results = [r for r in all_results if r["pass_reached"] == 1]
     assert len(failed_results) == result.pass1_failed
     for br in failed_results:
-        assert br["pass_reached"] == 1
         assert br["composite_score"] == 0.0