diff --git a/src/agentrust_trace/provenance.py b/src/agentrust_trace/provenance.py
index a548deb..55441cd 100644
--- a/src/agentrust_trace/provenance.py
+++ b/src/agentrust_trace/provenance.py
@@ -66,6 +66,13 @@ def _as_object(value: Any, field: str) -> dict[str, Any]:
     return value
 
 
+def _nonempty_string(value: Any, field: str) -> str:
+    """Return a required textual locator, rejecting truthy non-string JSON values."""
+    if not isinstance(value, str) or not value:
+        raise ProvenanceError(f"{field} must be a non-empty string")
+    return value
+
+
 def _tool_count(catalog: dict[str, Any]) -> int:
     """Return the required catalog count as a JSON integer."""
     value = catalog.get("tool_count")
@@ -153,8 +160,7 @@ def _check_structure(
             raise ProvenanceError(
                 f"identity.artifact must be an object, got {type(artifact).__name__}"
             )
-        if not artifact.get("package"):
-            raise ProvenanceError("artifact.package is required (a Package URL)")
+        _nonempty_string(artifact.get("package"), "artifact.package")
         if not _DIGEST_RE.match(str(artifact.get("digest", ""))):
             raise ProvenanceError(
                 "artifact.digest must be a sha256: digest of the entrypoint. For an "
@@ -166,8 +172,7 @@ def _check_structure(
             raise ProvenanceError(
                 f"identity.endpoint must be an object, got {type(endpoint).__name__}"
             )
-        if not endpoint.get("url"):
-            raise ProvenanceError("endpoint.url is required when endpoint is present")
+        _nonempty_string(endpoint.get("url"), "endpoint.url")
         if not _DIGEST_RE.match(str(endpoint.get("spki_sha256", ""))):
             raise ProvenanceError(
                 "endpoint.spki_sha256 must be a sha256: digest of the Subject Public Key "
diff --git a/tests/test_provenance_locator_types.py b/tests/test_provenance_locator_types.py
new file mode 100644
index 0000000..c8006e3
--- /dev/null
+++ b/tests/test_provenance_locator_types.py
@@ -0,0 +1,94 @@
+"""Regression tests for MCP provenance identity locator primitive boundaries."""
+
+from __future__ import annotations
+
+import time
+from typing import Any
+
+import pytest
+
+from agentrust_trace.provenance import (
+    FORMAT,
+    ProvenanceError,
+    build_record,
+    sign_record,
+    tool_catalog_hash,
+    verify_record,
+)
+from agentrust_trace.sign import generate_key, key_to_jwk
+
+DIGEST = "sha256:" + "a" * 64
+SPKI = "sha256:" + "b" * 64
+TOOLS = [{"name": "search", "description": "search", "input_schema": {"type": "object"}}]
+BAD_LOCATORS: tuple[Any, ...] = (True, 1, [1], {"x": 1})
+
+
+def _base_record(identity: dict[str, Any]) -> dict[str, Any]:
+    return {
+        "format": FORMAT,
+        "kind": "publisher-asserted",
+        "issued_at": int(time.time()),
+        "identity": identity,
+        "publisher": "did:web:acme.example",
+        "tool_catalog": {"hash": tool_catalog_hash(TOOLS), "tool_count": len(TOOLS)},
+        "attestation": None,
+    }
+
+
+@pytest.mark.parametrize("package", BAD_LOCATORS, ids=repr)
+def test_builder_refuses_non_string_artifact_package(package: Any) -> None:
+    with pytest.raises(ProvenanceError, match="artifact.package must be a non-empty string"):
+        build_record(
+            kind="publisher-asserted",
+            publisher="did:web:acme.example",
+            tools=TOOLS,
+            artifact={"package": package, "digest": DIGEST},  # type: ignore[dict-item]
+        )
+
+
+@pytest.mark.parametrize("url", BAD_LOCATORS, ids=repr)
+def test_builder_refuses_non_string_endpoint_url(url: Any) -> None:
+    with pytest.raises(ProvenanceError, match="endpoint.url must be a non-empty string"):
+        build_record(
+            kind="publisher-asserted",
+            publisher="did:web:acme.example",
+            tools=TOOLS,
+            endpoint={"url": url, "spki_sha256": SPKI},  # type: ignore[dict-item]
+        )
+
+
+@pytest.mark.parametrize("package", BAD_LOCATORS, ids=repr)
+def test_verifier_refuses_signed_non_string_artifact_package(package: Any) -> None:
+    key = generate_key()
+    signed = sign_record(
+        _base_record({"artifact": {"package": package, "digest": DIGEST}}), key
+    )
+    with pytest.raises(ProvenanceError, match="artifact.package must be a non-empty string"):
+        verify_record(signed, key_to_jwk(key))
+
+
+@pytest.mark.parametrize("url", BAD_LOCATORS, ids=repr)
+def test_verifier_refuses_signed_non_string_endpoint_url(url: Any) -> None:
+    key = generate_key()
+    signed = sign_record(
+        _base_record({"endpoint": {"url": url, "spki_sha256": SPKI}}), key
+    )
+    with pytest.raises(ProvenanceError, match="endpoint.url must be a non-empty string"):
+        verify_record(signed, key_to_jwk(key))
+
+
+def test_textual_locator_controls_still_pass() -> None:
+    artifact = build_record(
+        kind="publisher-asserted",
+        publisher="did:web:acme.example",
+        tools=TOOLS,
+        artifact={"package": "pkg:npm/%40acme/mcp-search@2.1.0", "digest": DIGEST},
+    )
+    endpoint = build_record(
+        kind="publisher-asserted",
+        publisher="did:web:acme.example",
+        tools=TOOLS,
+        endpoint={"url": "https://mcp.acme.example/", "spki_sha256": SPKI},
+    )
+    assert artifact["identity"]["artifact"]["package"].startswith("pkg:")
+    assert endpoint["identity"]["endpoint"]["url"].startswith("https://")