diff --git a/backend/app/services/ai_tree_builder.py b/backend/app/services/ai_tree_builder.py
new file mode 100644
index 00000000..c9f44c4b
--- /dev/null
+++ b/backend/app/services/ai_tree_builder.py
@@ -0,0 +1,154 @@
+"""Constrained, node-by-node L1 decision-tree generation (spec §4/§5/§6.1).
+
+Each call produces ONE node given the problem, category, and full walked path.
+Generation is constrained to safe/reversible L1 steps and biased to escalate
+early. normalize_walked_path() turns a resolved walk into a valid tree object
+for flywheel capture.
+"""
+import logging
+from typing import Any, Optional
+
+from app.core.ai_provider import get_ai_provider
+from app.core.config import settings
+from app.services.l1_category_service import HARD_FLOOR_TEXT_PATTERNS
+from app.services.llm_utils import parse_llm_json
+
+logger = logging.getLogger(__name__)
+
+MAX_DEPTH = 12
+VALID_NODE_TYPES = {"question", "instruction", "resolved", "escalate"}
+
+
+class UnsafeNodeError(ValueError):
+    """Raised when a generated node violates the hard floor or is malformed."""
+
+
+SYSTEM_PROMPT = """\
+You are an L1 helpdesk troubleshooting guide builder. Given a problem and the
+steps already tried, produce the SINGLE next node of a yes/no decision tree.
+
+HARD RULES:
+- Only safe, reversible, observe-or-restart-class steps: checking status, toggling,
+  restarting, reconnecting, re-entering credentials the USER already knows.
+- NEVER produce steps that: edit the registry/system files/boot config; delete or
+  format data/disks; change credentials/MFA/security/firewall/AV; run elevated or
+  admin scripts; touch domain controllers/DNS/DHCP or production servers; or have
+  billing/license impact. These are out of L1 scope.
+- When you run out of safe in-scope steps, DO NOT GUESS. Emit an "escalate" node.
+
+Return ONLY a JSON object for ONE node, one of:
+{"node_type":"question","text":"<yes/no question>"}
+{"node_type":"instruction","text":"<one safe reversible action>"}
+{"node_type":"resolved","text":"<confirmation the issue is fixed>"}
+{"node_type":"escalate","reason_category":"exhausted_safe_steps","text":"<why>"}
+No prose, no markdown fences.
+"""
+
+
+def _build_context(problem_text: str, category: str, walked_path: list[dict]) -> str:
+    lines = [f"PROBLEM: {problem_text}", f"CATEGORY: {category}", "STEPS SO FAR:"]
+    if not walked_path:
+        lines.append("(none yet — produce the first diagnostic question)")
+    for i, step in enumerate(walked_path, 1):
+        ans = step.get("answer")
+        suffix = f" -> {ans}" if ans else ""
+        lines.append(f"{i}. [{step.get('node_type','?')}] {step.get('text','')}{suffix}")
+    return "\n".join(lines)
+
+
+def validate_node(node: dict[str, Any]) -> dict[str, Any]:
+    """Shape + hard-floor validation. Raises UnsafeNodeError on violation."""
+    if not isinstance(node, dict) or node.get("node_type") not in VALID_NODE_TYPES:
+        raise UnsafeNodeError(f"invalid node_type: {node!r}")
+    text = (node.get("text") or "").lower()
+    for pat in HARD_FLOOR_TEXT_PATTERNS:
+        if pat in text:
+            raise UnsafeNodeError(f"hard-floor pattern '{pat}' in node text")
+    return node
+
+
+def escalate_if_depth_exceeded(walked_path: list[dict]) -> Optional[dict[str, Any]]:
+    if len(walked_path) >= MAX_DEPTH:
+        return {
+            "node_type": "escalate",
+            "reason_category": "depth_cap",
+            "text": "Reached the L1 troubleshooting depth limit — escalating to engineering.",
+        }
+    return None
+
+
+async def generate_next_node(
+    problem_text: str, category: str, walked_path: list[dict]
+) -> dict[str, Any]:
+    """Generate + validate the next node. Regenerate once on failure, then escalate."""
+    capped = escalate_if_depth_exceeded(walked_path)
+    if capped:
+        return capped
+
+    provider = get_ai_provider(settings.get_model_for_action("l1_realtime_build"))
+    context = _build_context(problem_text, category, walked_path)
+
+    for attempt in range(2):
+        try:
+            raw, _, _ = await provider.generate_json(
+                system_prompt=SYSTEM_PROMPT,
+                messages=[{"role": "user", "content": context}],
+                max_tokens=1024,
+            )
+            node = parse_llm_json(raw)
+            return validate_node(node)
+        except Exception as e:
+            logger.warning("ai_tree_builder node attempt %d failed: %s", attempt + 1, e)
+            continue
+
+    return {
+        "node_type": "escalate",
+        "reason_category": "generation_failed",
+        "text": "Could not generate a safe next step — escalating to engineering.",
+    }
+
+
+def normalize_walked_path(walked_path: list[dict]) -> dict[str, Any]:
+    """Turn a resolved walk into a valid troubleshooting tree (spec §6.1).
+
+    Root = first node's id; question nodes' traversed branch points to the next
+    node, the untraversed branch to a needs_review stub; terminal node ends it.
+    Returns {id, nodes: {id: node}} — a dict with an id (passes the proposal
+    approval guard).
+    """
+    nodes: dict[str, Any] = {}
+    if not walked_path:
+        root_id = "root"
+        nodes[root_id] = {"id": root_id, "node_type": "needs_review",
+                          "text": "Empty walk — needs authoring."}
+        return {"id": root_id, "nodes": nodes}
+
+    stub_seq = 0
+    for i, step in enumerate(walked_path):
+        nid = step.get("id") or f"n{i+1}"
+        ntype = step.get("node_type", "question")
+        nxt = walked_path[i + 1].get("id", f"n{i+2}") if i + 1 < len(walked_path) else None
+        node: dict[str, Any] = {"id": nid, "node_type": ntype, "text": step.get("text", "")}
+        if step.get("reason_category"):
+            node["reason_category"] = step["reason_category"]
+        if ntype == "question":
+            answer = (step.get("answer") or "").lower()
+            stub_seq += 1
+            stub_id = f"review-{stub_seq}"
+            nodes[stub_id] = {"id": stub_id, "node_type": "needs_review",
+                              "text": "Branch not explored during the originating call."}
+            traversed_next = nxt
+            if traversed_next is None:
+                # Walk ended on this question (no terminal recorded) — stub the
+                # branch the tech actually took so the tree has no dangling edge.
+                stub_seq += 1
+                traversed_next = f"review-{stub_seq}"
+                nodes[traversed_next] = {"id": traversed_next, "node_type": "needs_review",
+                                         "text": "Walk ended here before a terminal step was reached."}
+            node["yes_next"] = traversed_next if answer == "yes" else stub_id
+            node["no_next"] = traversed_next if answer == "no" else stub_id
+        elif ntype == "instruction":
+            node["next"] = nxt
+        nodes[nid] = node
+
+    return {"id": walked_path[0].get("id", "n1"), "nodes": nodes}
diff --git a/backend/tests/test_ai_tree_builder.py b/backend/tests/test_ai_tree_builder.py
new file mode 100644
index 00000000..49d4854a
--- /dev/null
+++ b/backend/tests/test_ai_tree_builder.py
@@ -0,0 +1,58 @@
+import pytest
+from app.services import ai_tree_builder as atb
+
+
+def test_validate_node_rejects_hard_floor_text():
+    node = {"node_type": "instruction", "id": "n1", "text": "Open regedit and change the key", "next": "generate"}
+    with pytest.raises(atb.UnsafeNodeError):
+        atb.validate_node(node)
+
+
+def test_validate_node_accepts_safe_instruction():
+    node = {"node_type": "instruction", "id": "n1", "text": "Restart the printer.", "next": "generate"}
+    assert atb.validate_node(node)["node_type"] == "instruction"
+
+
+def test_depth_cap_forces_escalate():
+    walked = [{"node_type": "question", "id": f"n{i}", "text": "?", "answer": "no"} for i in range(atb.MAX_DEPTH)]
+    node = atb.escalate_if_depth_exceeded(walked)
+    assert node is not None and node["node_type"] == "escalate"
+
+
+def test_normalize_walked_path_builds_valid_tree():
+    walked = [
+        {"node_type": "question", "id": "n1", "text": "Powered on?", "answer": "no"},
+        {"node_type": "instruction", "id": "n2", "text": "Power it on.", "answer": "ack"},
+        {"node_type": "resolved", "id": "n3", "text": "Fixed."},
+    ]
+    tree = atb.normalize_walked_path(walked)
+    assert isinstance(tree, dict) and tree.get("id") == "n1"
+    # untraversed 'yes' branch of n1 became a needs_review stub
+    assert any(n["node_type"] == "needs_review" for n in tree["nodes"].values())
+
+
+def test_normalize_walk_ending_on_question_has_no_none_branches():
+    walked = [
+        {"node_type": "question", "id": "n1", "text": "Powered on?", "answer": "no"},
+    ]
+    tree = atb.normalize_walked_path(walked)
+    n1 = tree["nodes"]["n1"]
+    assert n1["yes_next"] is not None and n1["no_next"] is not None
+    # both branches must reference real nodes present in the tree
+    assert n1["yes_next"] in tree["nodes"] and n1["no_next"] in tree["nodes"]
+
+
+def test_normalize_preserves_escalate_reason_category():
+    walked = [
+        {"node_type": "question", "id": "n1", "text": "On?", "answer": "no"},
+        {"node_type": "escalate", "id": "n2", "text": "Beyond L1.",
+         "reason_category": "exhausted_safe_steps"},
+    ]
+    tree = atb.normalize_walked_path(walked)
+    assert tree["nodes"]["n2"]["reason_category"] == "exhausted_safe_steps"
+
+
+def test_normalize_empty_walk_returns_needs_review_root():
+    tree = atb.normalize_walked_path([])
+    assert tree["id"] in tree["nodes"]
+    assert tree["nodes"][tree["id"]]["node_type"] == "needs_review"