diff --git a/src/agentrust_trace/adapters/sandbox.py b/src/agentrust_trace/adapters/sandbox.py
index cbf805fc..6210e9df 100644
--- a/src/agentrust_trace/adapters/sandbox.py
+++ b/src/agentrust_trace/adapters/sandbox.py
@@ -96,6 +96,10 @@ class SandboxAttestation:
     nonce: str | None = None
 
     def __post_init__(self) -> None:
+        if not isinstance(self.platform, str):
+            raise ValueError(
+                f"SandboxAttestation.platform must be a string, got {type(self.platform).__name__}"
+            )
         if self.platform == "software-only":
             raise ValueError(
                 "SandboxAttestation.platform must not be 'software-only'. Omit the "
@@ -108,6 +112,11 @@ def __post_init__(self) -> None:
                 f"SandboxAttestation.platform {self.platform!r} is not an accepted "
                 f"platform. Accepted: {', '.join(sorted(_PLATFORMS))}."
             )
+        if not isinstance(self.measurement, str):
+            raise ValueError(
+                "SandboxAttestation.measurement must be a string, got "
+                f"{type(self.measurement).__name__}"
+            )
         if not _DIGEST_RE.match(self.measurement):
             raise ValueError(
                 f"SandboxAttestation.measurement {self.measurement!r} is not a sha256: or "
@@ -151,12 +160,18 @@ class SandboxSessionResult:
     """Issuance timestamp. Defaults to now."""
 
     def __post_init__(self) -> None:
+        if not isinstance(self.sandbox_id, str):
+            raise ValueError(f"sandbox_id must be a string, got {type(self.sandbox_id).__name__}")
         if not _SUBJECT_RE.match(self.sandbox_id):
             raise ValueError(
                 f"sandbox_id {self.sandbox_id!r} must be a SPIFFE URI "
                 "('spiffe://<trust-domain>/<path>') or a DID ('did:<method>:<id>'). "
                 "It becomes the record subject, which is what a verifier keys on."
             )
+        if not isinstance(self.image_digest, str):
+            raise ValueError(
+                f"image_digest must be a string, got {type(self.image_digest).__name__}"
+            )
         if not _DIGEST_RE.match(self.image_digest):
             raise ValueError(
                 f"image_digest {self.image_digest!r} must be a sha256: or sha384: digest."
diff --git a/tests/test_sandbox_required_string_fields.py b/tests/test_sandbox_required_string_fields.py
new file mode 100644
index 00000000..ea7d10c6
--- /dev/null
+++ b/tests/test_sandbox_required_string_fields.py
@@ -0,0 +1,59 @@
+"""Regression tests for sandbox adapter required-string primitive boundaries."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from agentrust_trace.adapters.sandbox import SandboxAttestation, SandboxSessionResult
+
+DIGEST = "sha256:" + "a" * 64
+BAD_VALUES: tuple[Any, ...] = (True, 1, [1], {"x": 1})
+
+
+@pytest.mark.parametrize("platform", BAD_VALUES, ids=repr)
+def test_attestation_platform_refuses_non_string_values(platform: Any) -> None:
+    with pytest.raises(ValueError, match="SandboxAttestation.platform"):
+        SandboxAttestation(platform=platform, measurement=DIGEST)
+
+
+@pytest.mark.parametrize("measurement", BAD_VALUES, ids=repr)
+def test_attestation_measurement_refuses_non_string_values(measurement: Any) -> None:
+    with pytest.raises(ValueError, match="SandboxAttestation.measurement"):
+        SandboxAttestation(platform="tpm2", measurement=measurement)
+
+
+@pytest.mark.parametrize("sandbox_id", BAD_VALUES, ids=repr)
+def test_session_sandbox_id_refuses_non_string_values(sandbox_id: Any) -> None:
+    with pytest.raises(ValueError, match="sandbox_id"):
+        SandboxSessionResult(
+            sandbox_id=sandbox_id,
+            image_digest=DIGEST,
+            policy_bundle_bytes=b"policy",
+            decisions=[],
+        )
+
+
+@pytest.mark.parametrize("image_digest", BAD_VALUES, ids=repr)
+def test_session_image_digest_refuses_non_string_values(image_digest: Any) -> None:
+    with pytest.raises(ValueError, match="image_digest"):
+        SandboxSessionResult(
+            sandbox_id="spiffe://runtime.example.org/sandbox/test",
+            image_digest=image_digest,
+            policy_bundle_bytes=b"policy",
+            decisions=[],
+        )
+
+
+def test_textual_controls_still_construct() -> None:
+    attestation = SandboxAttestation(platform="tpm2", measurement=DIGEST)
+    session = SandboxSessionResult(
+        sandbox_id="spiffe://runtime.example.org/sandbox/test",
+        image_digest=DIGEST,
+        policy_bundle_bytes=b"policy",
+        decisions=[],
+        attestation=attestation,
+    )
+    assert attestation.platform == "tpm2"
+    assert session.sandbox_id.startswith("spiffe://")