diff --git a/backend/app/schemas/ai_chat.py b/backend/app/schemas/ai_chat.py
new file mode 100644
index 00000000..35ae66b7
--- /dev/null
+++ b/backend/app/schemas/ai_chat.py
@@ -0,0 +1,80 @@
+"""Pydantic schemas for the AI Chat Builder."""
+from typing import Any, Literal, Optional
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+# ── Requests ──
+
+
+class AIChatStartRequest(BaseModel):
+    """Start a new chat builder session."""
+
+    flow_type: Literal["troubleshooting", "procedural"] = Field(
+        ..., description="Type of flow to build"
+    )
+
+
+class AIChatMessageRequest(BaseModel):
+    """Send a user message in a chat session."""
+
+    content: str = Field(..., min_length=1, max_length=5000)
+
+
+class AIChatImportRequest(BaseModel):
+    """Import generated tree with optional metadata overrides."""
+
+    name: Optional[str] = Field(None, min_length=1, max_length=255)
+    description: Optional[str] = Field(None, max_length=2000)
+    category_id: Optional[UUID] = None
+    tags: list[str] = Field(default_factory=list)
+
+
+# ── Responses ──
+
+
+class AIChatStartResponse(BaseModel):
+    """Response after creating a chat session."""
+
+    session_id: UUID
+    greeting: str
+    current_phase: str
+
+
+class AIChatMessageResponse(BaseModel):
+    """Response after sending a message."""
+
+    content: str
+    current_phase: str
+    working_tree: Optional[dict[str, Any]] = None
+    tree_metadata: Optional[dict[str, Any]] = None
+
+
+class AIChatSessionResponse(BaseModel):
+    """Full session state for resume."""
+
+    session_id: UUID
+    status: str
+    current_phase: str
+    flow_type: str
+    conversation_history: list[dict[str, Any]]
+    working_tree: Optional[dict[str, Any]] = None
+    tree_metadata: Optional[dict[str, Any]] = None
+    message_count: int
+    generated_tree: Optional[dict[str, Any]] = None
+
+
+class AIChatGenerateResponse(BaseModel):
+    """Response with the final generated tree."""
+
+    tree_structure: dict[str, Any]
+    tree_metadata: dict[str, Any]
+    status: str
+
+
+class AIChatImportResponse(BaseModel):
+    """Response after importing tree to editor."""
+
+    tree_id: UUID
+    tree_type: str