diff --git a/src/workflow/planner.py b/src/workflow/planner.py
index f3e1cb91..c6cf2dcf 100644
--- a/src/workflow/planner.py
+++ b/src/workflow/planner.py
@@ -77,22 +77,37 @@ def parse_plan(raw: str) -> Plan:
         except json.JSONDecodeError:
             args[n] = {}
 
-    steps = [
-        PlanStep(
-            step_number=n,
-            task=tasks[n],
-            server=servers.get(n, ""),
-            tool=tools.get(n, ""),
-            tool_args=args.get(n, {}),
-            dependencies=(
-                []
-                if deps_raw.get(n, "None").strip().lower() == "none"
-                else [int(x) for x in _DEP_NUM_RE.findall(deps_raw.get(n, ""))]
-            ),
-            expected_output=outputs.get(n, ""),
+    steps = []
+    for n in sorted(tasks):
+        raw_dep = deps_raw.get(n, "None").strip()
+
+        if raw_dep.lower() == "none":
+            dependencies = []
+        else:
+            dependencies = [int(x) for x in _DEP_NUM_RE.findall(raw_dep)]
+
+            # Make sure dependency references only point to earlier valid steps.
+            if not dependencies:
+                raise ValueError(f"Invalid dependency format for step {n}: {raw_dep}")
+
+            for dep in dependencies:
+                if dep < 1 or dep >= n:
+                    raise ValueError(
+                        f"Invalid dependency reference for step {n}: #S{dep}"
+                    )
+
+        steps.append(
+            PlanStep(
+                step_number=n,
+                task=tasks[n],
+                server=servers.get(n, ""),
+                tool=tools.get(n, ""),
+                tool_args=args.get(n, {}),
+                dependencies=dependencies,
+                expected_output=outputs.get(n, ""),
+            )
         )
-        for n in sorted(tasks)
-    ]
+
     return Plan(steps=steps, raw=raw)
 
 
@@ -121,4 +136,4 @@ def generate_plan(
         )
         prompt = _PLAN_PROMPT.format(servers=servers_text, question=question)
         raw = self._llm.generate(prompt)
-        return parse_plan(raw)
+        return parse_plan(raw)
\ No newline at end of file