diff --git a/hc_agent/__init__.py b/hc_agent/__init__.py
new file mode 100644
index 0000000000..af24bc8c23
--- /dev/null
+++ b/hc_agent/__init__.py
@@ -0,0 +1 @@
+"""hc_agent package."""
diff --git a/hc_agent/core/__init__.py b/hc_agent/core/__init__.py
new file mode 100644
index 0000000000..8fa0a5e540
--- /dev/null
+++ b/hc_agent/core/__init__.py
@@ -0,0 +1,10 @@
+"""Core helpers for hc_agent."""
+
+from hc_agent.core.contracts import RunSummary, RunnerEnvelope, StructuredError, build_structured_error
+
+__all__ = [
+    "RunSummary",
+    "RunnerEnvelope",
+    "StructuredError",
+    "build_structured_error",
+]
diff --git a/hc_agent/core/contracts.py b/hc_agent/core/contracts.py
new file mode 100644
index 0000000000..bb23910321
--- /dev/null
+++ b/hc_agent/core/contracts.py
@@ -0,0 +1,48 @@
+"""Shared contracts for runner payloads and summaries."""
+
+from __future__ import annotations
+
+from typing import Any, Dict, Optional, TypedDict
+
+
+class StructuredError(TypedDict, total=False):
+    """Structured error details used in runner payloads."""
+
+    type: str
+    message: str
+    details: Dict[str, Any]
+
+
+def build_structured_error(
+    error_type: str,
+    message: str,
+    details: Optional[Dict[str, Any]] = None,
+) -> StructuredError:
+    """Build a StructuredError dictionary with stable JSON field names."""
+
+    payload: StructuredError = {
+        "type": error_type,
+        "message": message,
+    }
+    if details is not None:
+        payload["details"] = details
+    return payload
+
+
+class RunSummary(TypedDict, total=False):
+    """Summary payload for a completed run."""
+
+    status: str
+    started_at: str
+    finished_at: str
+    duration_ms: int
+    metadata: Dict[str, Any]
+
+
+class RunnerEnvelope(TypedDict, total=False):
+    """Envelope payload for runner communication."""
+
+    run_id: str
+    summary: RunSummary
+    error: StructuredError
+    metadata: Dict[str, Any]