diff --git a/src/pf1e_simulator/models.py b/src/pf1e_simulator/models.py
index 1f3420b..d48519d 100644
--- a/src/pf1e_simulator/models.py
+++ b/src/pf1e_simulator/models.py
@@ -410,6 +410,20 @@ class Combatant(_FrozenModel):
             raise ValueError(msg)
         return value
 
+    @field_validator("id")
+    @classmethod
+    def _validate_id(cls, value: str) -> str:
+        # The transcript line format is "round N <id>: <message>"; an id
+        # containing ": " would be mis-split by cli._parse_transcript_line,
+        # and leading/trailing whitespace breaks the "id: " join.
+        if not value or value != value.strip():
+            msg = f"combatant id must be non-empty without surrounding whitespace, got {value!r}"
+            raise ValueError(msg)
+        if ": " in value:
+            msg = f"combatant id must not contain ': ' (breaks transcript parsing), got {value!r}"
+            raise ValueError(msg)
+        return value
+
     @property
     def is_lawful(self) -> bool:
         """Law axis: LG/LN/LE."""
diff --git a/tests/test_models.py b/tests/test_models.py
index e25d5aa..e6dcb25 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -189,6 +189,22 @@ class TestCombatant:
         with pytest.raises(ValidationError):
             Combatant.model_validate(_combatant_dict(hit_dice="1d8"))
 
+    @pytest.mark.parametrize("bad_id", ["a: b", " goblin", "goblin ", ""])
+    def test_rejects_transcript_hostile_id(self, bad_id: str) -> None:
+        # Given an id that would misparse the "round N id: message" transcript
+        # (contains ": ") or is empty / padded with whitespace
+        # When/Then validation rejects it
+        with pytest.raises(ValidationError):
+            Combatant.model_validate(_combatant_dict(id=bad_id))
+
+    def test_accepts_id_with_colon_without_space(self) -> None:
+        # Given an id using ":" as a separator but never ": " (weapon-style
+        # ids are safe for the transcript parser)
+        # When validated
+        combatant = Combatant.model_validate(_combatant_dict(id="goblin:claw"))
+        # Then it is accepted
+        assert combatant.id == "goblin:claw"
+
     def test_frozen(self) -> None:
         # Given a validated combatant
         combatant = Combatant.model_validate(_combatant_dict())