diff --git a/core/mode_resolver.py b/core/mode_resolver.py
new file mode 100644
index 0000000..af863b0
--- /dev/null
+++ b/core/mode_resolver.py
@@ -0,0 +1,94 @@
+"""Nova client-mode resolver (P1, REQ-327, D-226).
+
+Priority: --mode flag → NOVA_CLIENT_MODE env → credential type → TTY.
+No silent fallbacks: every return carries a non-empty selection_reason.
+
+INV-13: invalid env values are ignored + warned, then fall through.
+INV-14: credential_type developer_pat/nova_oidc_token + TTY →
+interactive; + no-TTY → agent. TTY check is sys.stdin.isatty() (D-226).
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import sys
+from pathlib import Path
+from typing import Optional, Tuple
+
+log = logging.getLogger("nova.mode_resolver")
+
+_VALID_MODES = ("agent", "interactive")
+_CRED_MODE_TYPES = ("developer_pat", "nova_oidc_token")
+
+
+def resolve_mode(
+    flag: Optional[str] = None,
+    env_var: Optional[str] = None,
+    credential_type: Optional[str] = None,
+    stdin_isatty: bool = False,
+) -> Tuple[str, str]:
+    """Return (mode, selection_reason) honoring D-226 priority."""
+    if flag is not None and flag in _VALID_MODES:
+        return flag, "flag"
+    if env_var is not None and env_var != "":
+        if env_var in _VALID_MODES:
+            return env_var, "env"
+        log.warning(
+            "NOVA_CLIENT_MODE=%r invalid (expected one of %s); ignoring",
+            env_var,
+            _VALID_MODES,
+        )
+    if credential_type in _CRED_MODE_TYPES:
+        mode = "interactive" if stdin_isatty else "agent"
+        return mode, f"credential:{credential_type}"
+    mode = "interactive" if stdin_isatty else "agent"
+    return mode, "tty"
+
+
+def _read_credential_type(path: Path) -> Optional[str]:
+    """Read the active credential's type from ~/.nova/credentials.json."""
+    try:
+        data = json.loads(path.read_text())
+    except (OSError, json.JSONDecodeError):
+        return None
+    active_jti = data.get("active_credential_jti")
+    for cred in data.get("credentials", []) or []:
+        if cred.get("jti") == active_jti:
+            return cred.get("type")
+    return None
+
+
+def resolve_mode_from_env(credential_type: Optional[str] = None) -> Tuple[str, str]:
+    """Resolve mode using sys.argv, NOVA_CLIENT_MODE, credentials, and TTY.
+
+    Best-effort --mode scan of sys.argv (no full argparse); env var;
+    ~/.nova/credentials.json active credential type; sys.stdin.isatty().
+    """
+    flag: Optional[str] = None
+    argv = sys.argv[1:]
+    for i, tok in enumerate(argv):
+        if tok == "--mode" and i + 1 < len(argv):
+            flag = argv[i + 1]
+            break
+        if tok.startswith("--mode="):
+            flag = tok.split("=", 1)[1]
+            break
+    env_var = os.environ.get("NOVA_CLIENT_MODE")
+    if env_var is not None and env_var == "":
+        env_var = ""
+    if credential_type is None:
+        cred_path = Path.home() / ".nova" / "credentials.json"
+        credential_type = _read_credential_type(cred_path)
+    return resolve_mode(
+        flag=flag,
+        env_var=env_var,
+        credential_type=credential_type,
+        stdin_isatty=sys.stdin.isatty(),
+    )
+
+
+if __name__ == "__main__":
+    mode, reason = resolve_mode_from_env()
+    print(f"mode={mode} reason={reason}")
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