diff --git a/plugin/external/adapter.go b/plugin/external/adapter.go
index 3c304cc9..82a0a4e0 100644
--- a/plugin/external/adapter.go
+++ b/plugin/external/adapter.go
@@ -303,6 +303,21 @@ func createTypedConfigRequest(descriptor *pb.ContractDescriptor, cfg map[string]
 		}
 		return s, nil, nil
 	}
+	// Contracts that declare a typed Mode (STRICT_PROTO or
+	// PROTO_WITH_LEGACY_STRUCT) but leave ConfigMessage empty have no
+	// per-instance config schema — primarily input-only steps like
+	// step.eventbus.ack/publish/consume where data flows through the
+	// InputMessage proto, but also applies to any contract Kind that
+	// legitimately omits a config schema. Encode cfg as legacy struct
+	// only; typed payload is nil. The plugin's typed factory reads data
+	// from the input message (or other typed payload), not from config.
+	if descriptor.ConfigMessage == "" {
+		s, err := mapToStruct(cfg)
+		if err != nil {
+			return nil, nil, fmt.Errorf("encode config as Struct (no typed config schema): %w", err)
+		}
+		return s, nil, nil
+	}
 	// Strip engine-internal "_"-prefix keys before proto decode. STRICT_PROTO
 	// and PROTO_WITH_LEGACY_STRUCT modules use protojson with DiscardUnknown
 	// = false (convert.go:62), which rejects engine internals like
diff --git a/plugin/external/adapter_test.go b/plugin/external/adapter_test.go
index 8f1f54e4..2d723f81 100644
--- a/plugin/external/adapter_test.go
+++ b/plugin/external/adapter_test.go
@@ -1022,6 +1022,48 @@ func TestCreateTypedConfigRequestStripsInternalKeysForStrictProtoStep(t *testing
 	}
 }
 
+// TestCreateTypedConfigRequestEmptyConfigMessageStrictProto covers
+// contracts that declare STRICT_PROTO with InputMessage + OutputMessage but
+// no ConfigMessage (input-only steps like step.eventbus.ack /
+// step.eventbus.publish). The engine must NOT attempt to encode an
+// unnamed typed proto; typed payload is nil, legacy struct mirrors cfg
+// (nil cfg → nil legacy via mapToStruct(nil); non-nil cfg → populated
+// struct).
+func TestCreateTypedConfigRequestEmptyConfigMessageStrictProto(t *testing.T) {
+	descriptor := &pb.ContractDescriptor{
+		Kind:          pb.ContractKind_CONTRACT_KIND_STEP,
+		StepType:      "step.eventbus.ack",
+		Mode:          pb.ContractMode_CONTRACT_MODE_STRICT_PROTO,
+		InputMessage:  "workflow.plugin.eventbus.v1.AckRequest",
+		OutputMessage: "workflow.plugin.eventbus.v1.AckResponse",
+		// ConfigMessage intentionally empty — step has no per-instance
+		// config schema; data flows via the input message.
+	}
+	// nil cfg — mapToStruct(nil) returns nil; legacy is permitted to be nil.
+	legacy, typed, err := createTypedConfigRequest(descriptor, nil, nil)
+	if err != nil {
+		t.Fatalf("createTypedConfigRequest with nil cfg + empty ConfigMessage: %v", err)
+	}
+	if typed != nil {
+		t.Fatalf("expected nil typed *anypb.Any for input-only step contract; got %v", typed)
+	}
+	if legacy != nil {
+		t.Fatalf("expected nil legacy struct for nil cfg; got %v", legacy.Fields)
+	}
+	// Non-nil cfg — fields populated into legacy struct; typed still nil.
+	cfg := map[string]any{"timeout_ms": float64(5000)}
+	legacy2, typed2, err := createTypedConfigRequest(descriptor, cfg, nil)
+	if err != nil {
+		t.Fatalf("createTypedConfigRequest with cfg + empty ConfigMessage: %v", err)
+	}
+	if typed2 != nil {
+		t.Fatalf("expected nil typed *anypb.Any for input-only step contract; got %v", typed2)
+	}
+	if legacy2 == nil || legacy2.Fields["timeout_ms"] == nil {
+		t.Fatalf("expected legacy struct with timeout_ms populated; got %v", legacy2)
+	}
+}
+
 // TestCreateTypedConfigRequestRetainsInternalKeysInLegacyStruct asserts the
 // legacy-struct path keeps "_"-prefix keys on its *structpb.Struct payload.
 // Legacy modules consume "_config_dir" at the plugin side to resolve filesystem-