diff --git a/src/specify_cli/workflows/engine.py b/src/specify_cli/workflows/engine.py
index aff5e92e29..6c100d11bf 100644
--- a/src/specify_cli/workflows/engine.py
+++ b/src/specify_cli/workflows/engine.py
@@ -296,6 +296,40 @@ def _validate_steps(
                     f"boolean, got {type(coe).__name__}."
                 )
 
+        # Fan-in: every wait_for id must reference a step declared at or before
+        # this point. An id not yet seen is either a typo (unknown step) or a
+        # forward reference (the target runs after this fan-in, so its results
+        # cannot exist yet) — both are wiring errors that previously surfaced as
+        # a silent empty result + COMPLETED. A step that is declared but only
+        # conditionally executed (e.g. inside an if/switch branch) is still
+        # "seen" here, so a legitimately-empty result at runtime stays valid.
+        if step_type == "fan-in":
+            wait_for = step_config.get("wait_for")
+            if isinstance(wait_for, list):
+                for wid in wait_for:
+                    if not isinstance(wid, str):
+                        # A non-string entry (e.g. YAML `wait_for: [123]`) can
+                        # never match a real step id, so the join is silently
+                        # empty at runtime — surface it as a wiring error.
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' entries must "
+                            f"be step-id strings, got {type(wid).__name__} "
+                            f"({wid!r})."
+                        )
+                    elif wid == step_id:
+                        # The fan-in's own id is already in seen_ids by now, so
+                        # a self-reference would pass the membership check below
+                        # while still producing an empty join at runtime.
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' references "
+                            f"itself; a fan-in cannot wait for its own results."
+                        )
+                    elif wid not in seen_ids:
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' references "
+                            f"unknown or not-yet-declared step id {wid!r}."
+                        )
+
         # Recursively validate nested steps
         for nested_key in ("then", "else", "steps"):
             nested = step_config.get(nested_key)
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index 988730d783..2906fdb30a 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -1971,6 +1971,128 @@ def test_validate_wait_for_not_list(self):
         assert any("non-empty list" in e for e in errors)
 
 
+class TestFanInWaitForValidation:
+    """fan-in wait_for must reference a declared step (no silent empty join)."""
+
+    @staticmethod
+    def _errors(yaml_text):
+        from specify_cli.workflows.engine import (
+            WorkflowDefinition,
+            validate_workflow,
+        )
+
+        return validate_workflow(WorkflowDefinition.from_string(yaml_text))
+
+    def test_unknown_wait_for_id_is_rejected(self):
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: collect
+    type: fan-in
+    wait_for: [ghost]
+""")
+        assert any(
+            "unknown or not-yet-declared step id 'ghost'" in e for e in errors
+        )
+
+    def test_wait_for_declared_earlier_step_passes(self):
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: produce
+    type: command
+    command: speckit.implement
+  - id: collect
+    type: fan-in
+    wait_for: [produce]
+""")
+        assert not any("wait_for" in e for e in errors)
+
+    def test_wait_for_conditionally_declared_step_passes(self):
+        # A step declared inside an if-branch may be skipped at runtime, but it is
+        # still "declared", so referencing it must validate — a legitimately-empty
+        # runtime join stays valid.
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: maybe
+    type: if
+    condition: "{{ inputs.flag }}"
+    then:
+      - id: branch_task
+        type: command
+        command: speckit.implement
+  - id: collect
+    type: fan-in
+    wait_for: [branch_task]
+""")
+        assert not any("wait_for" in e for e in errors)
+
+    def test_forward_reference_is_rejected(self):
+        # wait_for points at a step declared AFTER the fan-in; its results cannot
+        # exist when the fan-in runs, so it is flagged.
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: collect
+    type: fan-in
+    wait_for: [later]
+  - id: later
+    type: command
+    command: speckit.implement
+""")
+        assert any(
+            "unknown or not-yet-declared step id 'later'" in e for e in errors
+        )
+
+    def test_self_reference_is_rejected(self):
+        # A fan-in's own id is in scope by the time it is validated, so a
+        # self-reference slips past the membership check while still producing
+        # an empty join at runtime.
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: collect
+    type: fan-in
+    wait_for: [collect]
+""")
+        assert any(
+            "references itself" in e and "collect" in e for e in errors
+        )
+
+    def test_non_string_wait_for_entry_is_rejected(self):
+        # A non-string entry (e.g. YAML `wait_for: [123]`) can never match a
+        # real step id, so it must be flagged rather than silently ignored.
+        errors = self._errors("""
+workflow:
+  id: wf
+  name: wf
+  version: "1.0.0"
+steps:
+  - id: collect
+    type: fan-in
+    wait_for: [123]
+""")
+        assert any(
+            "must be step-id strings" in e and "int" in e for e in errors
+        )
+
+
 # ===== Workflow Definition Tests =====
 
 class TestWorkflowDefinition: