diff --git a/packages/python/openproblems/CHANGELOG.md b/packages/python/openproblems/CHANGELOG.md
index d87b1ff..4354b69 100644
--- a/packages/python/openproblems/CHANGELOG.md
+++ b/packages/python/openproblems/CHANGELOG.md
@@ -33,6 +33,10 @@
 * `render_component_spec`: Include non-file arguments (e.g. `--seed`) in the arguments table,
   and fall back to an argument's `description` when it has no `summary`.
 
+* `resolve_path`: Resolve a path starting with a `/` relative to the project root, the way
+  Viash does. `os.path.join()` treats such a path as absolute and silently dropped the project
+  root, so a config containing e.g. `__merge__: /src/api/file_dataset.yaml` failed to read.
+
 # openproblems core Python v0.1.1
 
 ## NEW FUNCTIONALITY
diff --git a/packages/python/openproblems/src/openproblems/project/resolve_path.py b/packages/python/openproblems/src/openproblems/project/resolve_path.py
index 20b1e12..6ec2a71 100644
--- a/packages/python/openproblems/src/openproblems/project/resolve_path.py
+++ b/packages/python/openproblems/src/openproblems/project/resolve_path.py
@@ -1,4 +1,7 @@
-def resolve_path(path: str, project_path: str, parent_path: str) -> str:
+from __future__ import annotations
+
+
+def resolve_path(path: str, project_path: str | None, parent_path: str) -> str:
     """
     Resolve a path relative to a parent path or project path
 
@@ -32,6 +35,12 @@ def resolve_path(path: str, project_path: str, parent_path: str) -> str:
     import os
 
     if path.startswith("/"):
-        return os.path.join(project_path, path)
+        if project_path is None:
+            raise ValueError(
+                f"Cannot resolve '{path}' relative to the project root: "
+                "no project root (_viash.yaml) was found"
+            )
+        # note: os.path.join() discards project_path if path starts with a "/"
+        return os.path.abspath(os.path.join(project_path, path.lstrip("/")))
     else:
         return os.path.abspath(os.path.join(parent_path, path))
diff --git a/packages/python/openproblems/tests/data/example_project/api/comp_method.yaml b/packages/python/openproblems/tests/data/example_project/api/comp_method.yaml
index 3a93846..958e17a 100644
--- a/packages/python/openproblems/tests/data/example_project/api/comp_method.yaml
+++ b/packages/python/openproblems/tests/data/example_project/api/comp_method.yaml
@@ -8,7 +8,8 @@ info:
       A method to predict the task effects.
 arguments:
   - name: --input_train
-    __merge__: file_train.yaml
+    # project-absolute, i.e. relative to the _viash.yaml, unlike the merges below
+    __merge__: /api/file_train.yaml
     required: true
     direction: input
   - name: "--input_test"
diff --git a/packages/python/openproblems/tests/test_project_resolve_path.py b/packages/python/openproblems/tests/test_project_resolve_path.py
new file mode 100644
index 0000000..256a831
--- /dev/null
+++ b/packages/python/openproblems/tests/test_project_resolve_path.py
@@ -0,0 +1,27 @@
+import pytest
+from openproblems.project.resolve_path import resolve_path
+
+PROJECT = "/path/to/project"
+PARENT = "/path/to/project/src/api"
+
+
+def test_resolve_path_relative_to_parent():
+    assert resolve_path("file.yaml", PROJECT, PARENT) == PARENT + "/file.yaml"
+    assert resolve_path("./file.yaml", PROJECT, PARENT) == PARENT + "/file.yaml"
+    assert (
+        resolve_path("../file.yaml", PROJECT, PARENT)
+        == "/path/to/project/src/file.yaml"
+    )
+
+
+def test_resolve_path_absolute_is_relative_to_project():
+    # a leading slash means "relative to the _viash.yaml", not to the filesystem
+    assert (
+        resolve_path("/src/api/file_dataset.yaml", PROJECT, PARENT)
+        == PROJECT + "/src/api/file_dataset.yaml"
+    )
+
+
+def test_resolve_path_absolute_without_project_root():
+    with pytest.raises(ValueError, match="no project root"):
+        resolve_path("/src/api/file_dataset.yaml", None, PARENT)