diff --git a/inputs/org.osbuild.tree b/inputs/org.osbuild.tree
index 69999c50..f5be2f83 100755
--- a/inputs/org.osbuild.tree
+++ b/inputs/org.osbuild.tree
@@ -2,8 +2,10 @@
 """
 Tree inputs
 
-Resolve the given pipeline `id` to a path and return that. If
-`id` is `null` or the empty string it returns an empty tree.
+Open the tree produced by the pipeline supplied via the
+first and only entry in `references`. The tree is opened
+in read only mode. If the id is `null` or the empty 
+string it returns an empty tree.
 """
 
 
@@ -15,30 +17,53 @@ from osbuild.objectstore import StoreClient
 
 SCHEMA = """
 "additionalProperties": false,
-"required": ["pipeline"],
+"required": ["origin", "references"],
 "properties": {
-  "pipeline": {
-    "description": "The Pipeline that built the desired tree",
-    "type": "object",
-    "required": ["id"],
-    "additionalProperties": false,
-    "properties": {
-      "id": {
-        "description": "Identifier for the pipeline",
+  "origin": {
+    "description": "The origin of the input (must be 'org.osbuild.pipeline')",
+    "type": "string",
+    "enum": ["org.osbuild.pipeline"]
+  },
+  "references": {
+    "description": "Exactly one pipeline identifier to ues as tree input",
+    "oneOf": [{
+      "type": "array",
+      "additionalItems": false,
+      "items": [{
         "type": "string"
-      }
-    }
+      }]
+    }, {
+      "type": "object",
+      "additionalProperties": false,
+      "patternProperties": {
+         ".*": {
+           "type": "object",
+           "additionalProperties": false
+         }
+       },
+       "minProperties": 1,
+       "maxProperties": 1
+    }]
   }
 }
 """
 
 
+def error(msg):
+    json.dump({"error": msg}, sys.stdout)
+    sys.exit(1)
+
+
 def main():
     args = json.load(sys.stdin)
-    options = args["options"]
+    refs = args["refs"]
+
+    # input verification *must* have been done via schema
+    # verification. It is expected that origin is a pipeline
+    # and we have exactly one reference, i.e. a pipeline id
+    pid, _ = refs.popitem()
 
     store = StoreClient(connect_to=args["api"]["store"])
-    pid = options["pipeline"]["id"]
 
     if not pid:
         path = store.mkdtemp(prefix="empty")
@@ -46,8 +71,7 @@ def main():
         path = store.read_tree(pid)
 
     if not path:
-        json.dump({"error": "Could find target"}, sys.stdout)
-        return 1
+        error(f"Could not find pipeline with id '{pid}'")
 
     json.dump({"path": path}, sys.stdout)
     return 0
diff --git a/osbuild/formats/v1.py b/osbuild/formats/v1.py
index 5a0644c6..1cf34e59 100644
--- a/osbuild/formats/v1.py
+++ b/osbuild/formats/v1.py
@@ -64,10 +64,9 @@ def load_assembler(description: Dict, index: Index, manifest: Manifest):
 
     stage = pipeline.add_stage(info, options, {})
     info = index.get_module_info("Input", "org.osbuild.tree")
-    stage.inputs = {
-        "tree": Input(info, {"pipeline": {"id": base}})
-    }
-
+    ip = Input(info, "org.osbuild.pipeline", {})
+    ip.add_reference(base)
+    stage.inputs = {"tree": ip}
     return pipeline
 
 
diff --git a/osbuild/inputs.py b/osbuild/inputs.py
index 7503de34..12900445 100644
--- a/osbuild/inputs.py
+++ b/osbuild/inputs.py
@@ -23,7 +23,7 @@ import json
 import os
 import subprocess
 
-from typing import Dict, Tuple
+from typing import Dict, Optional, Tuple
 
 from .meta import ModuleInfo
 from .objectstore import StoreServer
@@ -34,14 +34,22 @@ class Input:
     A single input with its corresponding options.
     """
 
-    def __init__(self, info: ModuleInfo, options: Dict):
+    def __init__(self, info: ModuleInfo, origin: str, options: Dict):
         self.info = info
+        self.origin = origin
+        self.refs = {}
         self.options = options or {}
         self.id = self.calc_id()
 
+    def add_reference(self, ref, options: Optional[Dict] = None):
+        self.refs[ref] = options or {}
+        self.id = self.calc_id()
+
     def calc_id(self):
         m = hashlib.sha256()
         m.update(json.dumps(self.name, sort_keys=True).encode())
+        m.update(json.dumps(self.origin, sort_keys=True).encode())
+        m.update(json.dumps(self.refs, sort_keys=True).encode())
         m.update(json.dumps(self.options, sort_keys=True).encode())
         return m.hexdigest()
 
@@ -52,8 +60,14 @@ class Input:
     def run(self, storeapi: StoreServer) -> Tuple[str, Dict]:
         name = self.info.name
         msg = {
+            # mandatory bits
+            "origin": self.origin,
+            "refs": self.refs,
+
+            # global options
             "options": self.options,
-            "origin": name,
+
+            # API endpoints
             "api": {
                 "store": storeapi.socket_address
             }