diff --git a/src/backend/models/api.py b/src/backend/models/api.py
new file mode 100644
index 0000000..cdca9c9
--- /dev/null
+++ b/src/backend/models/api.py
@@ -0,0 +1,59 @@
+from pydantic import BaseModel, field_validator
+from typing import Any
+
+
+class ApiError(BaseModel):
+    code: str
+    message: str
+
+
+class ApiResponse(BaseModel):
+    success: bool
+    data: Any | None = None
+    error: ApiError | None = None
+
+
+def ok(data: Any) -> dict:
+    return {"success": True, "data": data, "error": None}
+
+
+def fail(code: str, message: str) -> dict:
+    return {
+        "success": False,
+        "data": None,
+        "error": {"code": code, "message": message},
+    }
+
+
+def _validate_model(v: str | None) -> str | None:
+    if v is not None and not v.startswith("claude-"):
+        raise ValueError("Model must start with 'claude-' or be null")
+    return v
+
+
+class TopicCreate(BaseModel):
+    name: str
+    system_prompt: str
+    summary: str = ""
+    model: str | None = None
+
+    validate_model = field_validator("model")(_validate_model)
+
+
+class TopicUpdate(BaseModel):
+    name: str | None = None
+    system_prompt: str | None = None
+    summary: str | None = None
+    model: str | None = None
+
+    validate_model = field_validator("model")(_validate_model)
+
+
+class RequestCreate(BaseModel):
+    prompt: str
+    topic_id: str
+    session_id: str | None = None
+    model: str | None = None
+    summarize: bool = False
+
+    validate_model = field_validator("model")(_validate_model)